49

KEY CONCEPTS
Nervous Systems

49.1 Nervous systems consist of circuits

of neurons and supporting cells
p. 1086

49.2 The vertebrate brain is regionally

specialized p. 1091

49.3 The cerebral cortex controls

voluntary movement and cognitive
functions p. 1096

49.4 Changes in synaptic connections

underlie memory and learning
p. 1099

49.5 Many nervous system disorders

can now be explained in molecular
terms p. 1102 Figure 49.1 Neuroscientists engineered mice to express a random combination of
four fluorescent proteins in each brain cell. The result, shown here, is a “brainbow,”
Study Tip with each neuron displaying one of 90 different color combinations. This brainbow
technology holds promise for studying particular pathways within the mouse brain.
Think in pairs: Regional specialization Ultimately, however, we want to understand our own brains, which contain an
involves many examples of paired estimated 1011 (100 billion) neurons and 1014 (100 trillion) connections.
structures or circuits. Fill in the comple-
mentary or reciprocal functions of these
pairs to help you understand their roles
How are billions of neurons organized to
in the brain or nervous system. perform complex tasks?
Regional specialization: Complex tasks, such as responding to a spoken
Structure A /Function Structure B/Function question, involve the stepwise functions of different brain regions.

CNS (brain and PNS (ganglia and Parietal lobe Frontal lobe
spinal cord) peripheral nerves)
Integration of Transfer of information 2 Comprehending 3 Formulating a
information to/from CNS spoken words response (speaking
Sympathetic division Parasympathetic division or other action)
of autonomic nervous of autonomic nervous
system system
Occipital lobe Temporal lobe
Left hemisphere Right hemisphere
of brain of brain Midbrain
Pons
1 Processing sensory
input from the ear Medulla
Hippocampus Cerebral cortex oblongata Hindbrain
(role in memory) (role in memory)
Cerebellum

Memory formation: Information is stored by reinforcing patterns of

active connections (synapses) among particular neurons.
Go to Mastering Biology
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• The Visual Brain: Nervous System
• HHMI Video: The Human
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B
C
For Instructors to Assign (in Item Library)
• Scientific Skills Exercise: Designing an
Experiment Using Genetic Mutants Synapses that are active in synchrony (A and B) are strengthened. Synapses
that are not part of an active circuit (C) are weakened or lost.
• Tutorial: The Vertebrate Nervous System 1085
 CONCEPT 49.1 input and to which it sends signals that control muscle
contraction.
Nervous systems consist Animals with elongated, bilaterally symmetrical bodies
have even more specialized nervous systems. In particular,
of circuits of neurons they exhibit cephalization, an evolutionary trend toward a
and supporting cells clustering of sensory neurons and interneurons at the ante-
rior (front) end of the body. Nerves that extend toward the
The ability to sense and react originated billions of years ago
posterior (rear) end enable these anterior neurons to commu-
in prokaryotes, enhancing survival and reproductive success
nicate with cells elsewhere in the body.
in changing environments. Later in evolution, modifica-
In many animals, neurons that carry out integration
tion of simple recognition and response processes provided a
form a central nervous system (CNS), and neurons that
basis for communication between cells in an animal body. By
carry information into and out of the CNS form a periph-
the time of the Cambrian explosion, more than 500 million
eral nervous system (PNS). In nonsegmented worms,
years ago (see Concept 32.2), specialized nervous systems had
such as the planarian in Figure 49.2c, a small brain and
appeared that enable animals to sense their surroundings and
longitudinal nerve cords constitute the simplest clearly
respond rapidly.
defined CNS. In certain nonsegmented worms, the entire
Hydras, jellies, and other cnidarians are the simplest
nervous system is constructed from only a small number
animals with nervous systems. In most cnidarians, intercon-
of cells, as in the case of the nematode Caenorhabditis
nected neurons form a diffuse nerve net (Figure 49.2a), which
elegans. In this species, an adult worm (hermaphrodite) has
controls the contraction and expansion of the gastrovas-
exactly 302 neurons, no more and no fewer. More complex
cular cavity. In more complex animals, the axons of mul-
invertebrates, such as segmented worms (Figure 49.2d)
tiple neurons are often bundled together, forming nerves.
and arthropods (Figure 49.2e), have many more neurons.
These fibrous structures channel information flow along
Their behavior is regulated by more complicated brains
specific routes through the nervous system. For example,
and by ventral nerve cords containing ganglia, segmen-
sea stars have a set of radial nerves connecting to a central
tally arranged clusters of neurons that act as relay points in
nerve ring (Figure 49.2b). Within each arm of a sea star, the
transmitting information.
radial nerve is linked to a nerve net from which it receives

. Figure 49.2 Nervous system organization. (a) A hydra contains individual neurons (purple) organized in
a diffuse nerve net. (b–h) Animals with more sophisticated nervous systems contain groups of neurons (blue)
organized into nerves and often ganglia and a brain.

Eyespot
Brain
Brain
Radial
nerve Nerve
cords
Nerve Ventral
ring Transverse nerve
Nerve net nerve cord

Segmental
ganglia

(a) Hydra (cnidarian) (b) Sea star (echinoderm) (c) Planarian (flatworm) (d) Leech (annelid)

Brain

Brain Ganglia
Ventral Anterior Spinal
nerve ring Brain
nerve cord cord Sensory
(dorsal ganglia
Longitudinal Ganglia nerve
nerve cords cord)

Segmental
ganglia

(e) Insect (arthropod) (f) Chiton (mollusc) (g) Squid (mollusc) (h) Salamander (vertebrate)

1086 UNIT SEVEN Animal Form and Function

 Within an animal group, nervous system organization of the nerve cord gives rise to the narrow central canal
often correlates with lifestyle. Among the molluscs, for of the spinal cord as well as the ventricles of the brain.
example, sessile and slow-moving species, such as clams and Both the canal and ventricles fill with cerebrospinal fluid,
chitons, have relatively simple sense organs and little or no which is formed in the brain by filtering arterial blood
cephalization (Figure 49.2f). In contrast, active predatory (Figure 49.4). The cerebrospinal fluid supplies the CNS
molluscs, such as octopuses and squids (Figure 49.2g), have with nutrients and hormones and carries away wastes,
the most sophisticated nervous systems of any invertebrate. circulating through the ventricles and central canal before
With their large, image-forming eyes and a brain contain- draining into the veins.
ing millions of neurons, octopuses can learn to discriminate In addition to fluid-filled spaces, the brain and spinal cord
between visual patterns and to perform complex tasks, such have gray and white matter. Gray matter is primarily made
as unscrewing a jar to feed on its contents. up of neuron cell bodies. White matter consists of bundled
In vertebrates such as a salamander (Figure 49.2h) or axons. In the spinal cord, white matter forms the outer layer,
human (Figure 49.3), the brain and the spinal cord form the reflecting its role in linking the CNS to sensory and motor
CNS; nerves and ganglia are the key elements of the PNS. neurons of the PNS. In the brain, white matter is predomi-
Regional specialization is a hallmark of both systems, as we nantly in the interior, where signaling between neurons
will see throughout this chapter. functions in learning, feeling emotions, processing sensory
information, and generating commands.
In vertebrates, the spinal cord runs lengthwise inside
Organization of the Vertebrate
the vertebral column, known as the spine. The spinal cord
Nervous System conveys information to and from the brain and generates
During embryonic development in vertebrates, the central basic patterns of locomotion. It also acts independently of
nervous system develops from the hollow dorsal nerve the brain as part of the simple nerve circuits that produce
cord—a hallmark of chordates (see Figure 34.3). The cavity reflexes, the body’s automatic responses to certain stimuli.
Reflexes protect the body by providing rapid, involuntary
responses to particular stimuli. Reflexes are rapid because
. Figure 49.3 The vertebrate nervous system. The central sensory information is used to activate motor neurons with-
nervous system consists of the brain and spinal cord (yellow). Left- out the information first having to travel from the spinal
right pairs of cranial nerves, spinal nerves, and ganglia make up
cord to the brain and back. If you accidentally put your hand
most of the peripheral nervous system (dark gold).
on a hot burner, a reflex jerks your hand back even before
your brain processes pain. Similarly, the knee-jerk reflex pro-
vides an immediate protective response when you pick up an
Central unexpectedly heavy object. If your legs buckle, the tension
Brain
nervous Cranial across your knees triggers contraction of your thigh muscle
system nerves
Spinal (quadriceps), helping you stay upright and support the load.
(CNS) cord
Peripheral
Ganglia nervous
outside system
CNS (PNS) . Figure 49.4 Ventricles, gray matter, and white matter.
Ventricles deep in the brain’s interior contain cerebrospinal
Spinal
nerves fluid. In the cerebrum, most of the gray matter is on the surface,
surrounding the white matter.

Gray matter

White
matter

Ventricles

CHAPTER 49 Nervous Systems 1087

c Figure 49.5 The knee-jerk reflex. Many 1 The reflex 2 Sensors detect 3 In response to signals from the
neurons are involved in this reflex, but for (shown here in a sudden stretch in sensory neurons, motor neurons
simplicity only a few neurons are shown. the movement of the quadriceps, convey signals to the quadriceps,
the right leg) is and sensory neurons causing it to contract and jerking
MAKE CONNECTIONS Using the nerve signals
initiated artificially convey the information the lower leg forward.
to the hamstring and quadriceps in this reflex as
by tapping the to the spinal cord.
an example, propose a model for regulation of tendon connected
smooth muscle activity in the esophagus during to the quadriceps Cell body of
the swallowing reflex (see Figure 41.9). muscle. sensory neuron in
dorsal root ganglion Gray White
matter matter

Quadriceps
muscle
Spinal cord
(cross section)

During a physical exam, your doctor may

Hamstring
trigger the knee-jerk reflex with a triangu-
muscle
lar mallet to help assess nervous system
function (Figure 49.5).

5 Motor neurons that lead 4 Interneurons in

The Peripheral Nervous to the hamstring muscle
are inhibited by the inter-
the spinal cord also
receive signals from
System neurons. This inhibition sensory neurons.
prevents contraction of the
The PNS transmits information to and hamstring, which would
from the CNS and plays a large role in resist the action of the
regulating an animal’s movement and quadriceps.
its internal environment (Figure 49.6). Key Sensory neuron Motor neuron Interneuron
Sensory information reaches the CNS
along PNS neurons designated as afferent (from the Latin,
meaning “to carry toward”). Following information process-
. Figure 49.6 Functional hierarchy of the vertebrate
ing within the CNS, instructions then travel to muscles,
peripheral nervous system.
glands, and endocrine cells along PNS neurons designated
as efferent (from the Latin, meaning “to carry away”). Note CENTRAL NERVOUS
that most nerves are bundles of both afferent and efferent SYSTEM
(information processing)
neurons.
The PNS has two efferent components: the motor sys-
PERIPHERAL NERVOUS
tem and the autonomic nervous system (see Figure 49.8). SYSTEM
The neurons of the motor system carry signals to skeletal Afferent neurons Efferent neurons
muscles. Motor control can be voluntary, as when you raise
your hand to ask a question, or involuntary, as in the knee-
jerk reflex controlled by the spinal cord. In contrast, regu- Autonomic Motor
nervous system system
lation of smooth and cardiac muscles by the autonomic Sensory
nervous system is generally involuntary. The sympa- receptors
Control of
thetic and parasympathetic divisions of the autonomic
skeletal muscles
nervous system regulate organs of the cardiovascular,
excretory, and endocrine systems. A distinct network of Enteric
neurons now known as the enteric nervous system Internal Sympathetic Parasympathetic nervous
and external division division system
exerts direct and partially independent control over the
stimuli
digestive tract, pancreas, and gallbladder.
Control of smooth muscles,
Homeostasis often relies on cooperation between the
cardiac muscles, glands
motor and autonomic nervous systems. In response to a

1088 UNIT SEVEN Animal Form and Function

c Figure 49.7 The parasympathetic and
Parasympathetic division Sympathetic division
sympathetic divisions of the autonomic
nervous system. Most pathways in each Action on internal organs: Action on internal organs:
division involve two neurons connecting
Constricts pupil Dilates pupil
the CNS to target organs. The axon of the
of eye of eye
first neuron extends from a cell body in
the CNS to a set of PNS neurons whose cell
Stimulates salivary Inhibits salivary
bodies are clustered into a ganglion (plural,
gland secretion gland secretion
ganglia). The axons of these PNS neurons
transmit instructions to internal organs, Sympathetic
where they form synapses with smooth Constricts ganglia Relaxes bronchi
bronchi in lungs Cervical in lungs
muscle, cardiac muscle, or gland cells.

Slows heart Accelerates heart

Stimulates activity Inhibits activity

of stomach and of stomach and
intestines intestines
Thoracic
Stimulates activity Inhibits activity
of pancreas of pancreas

Stimulates Stimulates glucose

gallbladder release from liver;
inhibits gallbladder
Lumbar
Stimulates
adrenal medulla

Promotes emptying Inhibits emptying

of bladder of bladder

Promotes erection Sacral Promotes ejaculation and

of genitalia Synapse vaginal contractions

drop in body temperature, for example, the hypothala- The two divisions differ not only in overall function but
mus signals the motor system to cause shivering, which also in organization and signals released. Parasympathetic
increases heat production. At the same time, the hypothal- nerves exit the CNS at the base of the brain or spinal cord and
amus signals the autonomic nervous system to constrict form synapses in ganglia near or within an internal organ. In
surface blood vessels, reducing heat loss. contrast, sympathetic nerves typically exit the CNS midway
The sympathetic and parasympathetic divisions of the along the spinal cord and form synapses in ganglia located
autonomic nervous system have largely antagonistic (oppo- just outside of the spinal cord.
site) functions in regulating organ function (Figure 49.7). In both the sympathetic and parasympathetic divisions,
Activation of the sympathetic division corresponds to the pathway for information flow typically involves a pre-
arousal and energy generation (the “fight-or-flight” response). ganglionic and a postganglionic neuron. The preganglionic
For example, the heart beats faster, digestion is inhibited, neurons have cell bodies in the CNS and release acetylcho-
the liver converts glycogen to glucose, and the adrenal line as a neurotransmitter (see Concept 48.4). In the case
medulla increases secretion of epinephrine (adrenaline). of the postganglionic neurons, those of the parasympathetic
Activation of the parasympathetic division generally division release acetylcholine, whereas nearly all their coun-
causes opposite responses that promote calming and a return terparts in the sympathetic division release norepinephrine.
to self-maintenance functions (“rest and digest”). Thus, heart It is this difference in neurotransmitters that enables the
rate decreases, digestion is enhanced, and glycogen produc- sympathetic and parasympathetic divisions to bring about
tion increases. However, in regulating reproductive activity, opposite effects in organs such as the lungs, heart, intes-
a function that is not homeostatic, the parasympathetic divi- tines, and bladder. A comparison of these pathways in the
sion complements rather than antagonizes the sympathetic autonomic nervous system, along with a motor system path-
division, as shown at the bottom of Figure 49.7. way, is shown in Figure 49.8.

CHAPTER 49 Nervous Systems 1089

. Figure 49.8 Comparison of pathways in the motor and of glia in the adult vertebrate and the ways in which they
autonomic nervous systems. nourish, support, and regulate the functioning of neurons.
In embryos, two types of glia play essential roles in the
(a) Motor system
development of the nervous system: radial glia and astrocytes.
Target: skeletal Radial glia form tracks along which newly formed neurons
muscle cell
migrate from the neural tube, the structure that gives rise to
Single
neuron the CNS (see Figure 47.14). Later, astrocytes that are adjacent to
brain capillaries participate in the formation of the blood-brain
(b) Autonomic nervous system barrier, a filtering mechanism that prevents many substances
Sympathetic in the blood from entering the CNS. Both radial glia and astro-
division cytes can also act as stem cells, which undergo unlimited cell
Target: divisions to self-renew and to form more specialized cells.
Postganglionic smooth
Preganglionic Ganglia muscle,
neurons CONCEPT CHECK 49.1
neurons cardiac
Parasympathetic muscle,
1. Which division of the autonomic nervous system would likely
division or gland
be activated if a student learned that an exam she had for-
gotten about would start in 5 minutes? Explain your answer.
2. WHAT IF? Suppose a person had an accident that severed
Key to neurotransmitters Acetylcholine Norepinephrine a small nerve required to move some of the fingers of the
right hand. Would you also expect an effect on sensation
from those fingers?
3. MAKE CONNECTIONS Most tissues regulated by the auto-
Glia nomic nervous system receive both sympathetic and para-
The nervous systems of vertebrates and most invertebrates sympathetic input from postganglionic neurons. Responses
are typically local. In contrast, the adrenal medulla receives
include not only neurons but also glial cells, or glia. The
input only from the sympathetic division and only from pre-
Schwann cells that myelinate axons in the PNS are an exam- ganglionic neurons, yet responses are observed throughout
ple of glia, as are oligodendrocytes, their counterparts in the the body. Explain why (see Figure 45.19).
CNS. Figure 49.9 provides an overview of the major types For suggested answers, see Appendix A.

. Figure 49.9 Glia in the vertebrate nervous system.

CNS PNS

VENTRICLE Neuron

Cilia

Ependymal cells
line the ventricles
of the brain and
have cilia that
promote circu-
lation of the
cerebrospinal fluid
that fills these Capillary
compartments.

Astrocytes (from the Greek astron, star) have Oligodendrocytes Microglia are Schwann cells
numerous functions in the CNS. They facilitate myelinate axons in immune cells in the myelinate axons
information transfer, regulate extracellular ion the CNS. Myelination CNS that protect in the PNS.
concentrations, promote blood flow to greatly increases the against pathogens.
neurons, help form the blood-brain barrier, and conduction speed of
act as stem cells to replenish certain neurons. action potentials.

1090 UNIT SEVEN Animal Form and Function

 CONCEPT 49.2 environment using olfaction, vision, and a lateral line system
that detects water currents, electrical stimuli, and body posi-
The vertebrate brain is regionally tion. The olfactory bulb, which detects scents in the water,
is relatively large in these fishes. So is the midbrain, which
specialized processes input from the visual and lateral line systems. In
We turn now to the vertebrate brain, which has three major contrast, the cerebrum, required for complex processing and
regions: the forebrain, midbrain, and hindbrain (shown here learning, is relatively small.
for a ray-finned fish). The correlation between the size and function of brain
regions can also be observed by considering the cerebellum.
Forebrain Midbrain Hindbrain
Free swimming ray-finned fishes, such as the tuna, control
Cerebellum movement in three dimensions in the open water and have
a relatively large cerebellum. In comparison, the cerebel-
lum is much smaller in species that don’t swim actively,
Olfactory such as the lamprey. Evolution has thus resulted in a close
bulb match of structure to function, with the size of particular
Cerebrum brain regions correlating with their importance for that spe-
cies in nervous system function and, hence, species survival
Each region is specialized in function. The forebrain, and reproduction.
which contains the olfactory bulb and cerebrum, has activities If one compares birds and mammals with groups that
that include processing of olfactory input (smells), regula- diverged from the common vertebrate ancestor earlier in evo-
tion of sleep, learning, and any complex processing. The lution, two trends are apparent. First, the forebrain of birds and
midbrain, located centrally in the brain, coordinates routing mammals occupies a larger fraction of the brain than it does in
of sensory input. The hindbrain, part of which forms the amphibians, fishes, and other vertebrates. Second, birds and
cerebellum, controls involuntary activities, such as blood circu- mammals have much larger brains relative to body size than
lation, and coordinates motor activities, such as locomotion. do other groups. Indeed, the ratio of brain size to body weight
EVOLUTION Comparing vertebrates across a phylogenetic for birds and mammals is ten times larger than that for their
tree, we see that the relative sizes of particular brain regions evolutionary ancestors. These differences in both overall brain
vary (Figure 49.10). Furthermore, these size differences reflect size and the relative size of the forebrain reflect the greater
differences in the importance of particular brain functions. capacity of birds and mammals for cognition and higher-order
Consider, for example, ray-finned fishes, which explore their reasoning, traits we will return to later in this chapter.
In the case of humans, the 100 billion
neurons in the brain make 100 trillion
. Figure 49.10 Vertebrate brain structure and evolution. These examples of vertebrate brains
are drawn to the same overall dimensions to highlight differences in the relative size of major
connections. How are so many cells
structures. These differences in relative size, which arose over the course of vertebrate evolution, and links organized into circuits and
correlate with the importance of particular brain functions for particular vertebrate groups. networks that can perform highly
sophisticated information processing,
storage, and retrieval? In addressing this
Lamprey
question, let’s begin with Figure 49.11,
which explores the overall architecture
ANCESTRAL Shark of the human brain. You can use this
VERTEBRATE
figure to trace how brain structures arise
Ray-finned during embryonic development; as a ref-
fish erence for their size, shape, and location
in the adult brain; and as an introduc-
Amphibian
tion to their best-understood functions.
To learn more about how particular
Crocodilian brain structure and brain organization
overall relate to brain function in
Key
humans, we’ll first consider activity
Forebrain Bird
cycles of the brain and the physiological
Midbrain basis of emotion. Then, in Concept 49.3,
Hindbrain Mammal we’ll shift our attention to regional
specialization within the cerebrum.

CHAPTER 49 Nervous Systems 1091

. Figure 49.11 Exploring the Organization of the Human Brain

The brain is the most complex organ in the human body.

Surrounded by the thick bones of the skull, the brain is divided
into a set of distinctive structures, some of which are visible in
the magnetic resonance image (MRI) of an adult’s head shown
at right. The diagram below traces the development of these
structures in the embryo. Their major functions are explained in
the main text of the chapter.

Human Brain Development

As a human embryo develops, the neural tube forms three anterior
bulges—the forebrain, midbrain, and hindbrain—that together
produce the adult brain. The midbrain and portions of the hind-
brain give rise to the brainstem, a stalk that joins with the spinal
cord at the base of the brain. The rest of the hindbrain gives rise
to the cerebellum, which lies behind the brainstem. Meanwhile,
the forebrain develops into the diencephalon, including the
neuroendocrine tissues of the brain, and the telencephalon, which
becomes the cerebrum. Rapid, expansive growth of the telen-
cephalon during the second and third months causes the outer
portion, or cortex, of the cerebrum to extend over and around
much of the rest of the brain.

Embryonic brain regions Brain structures in child and adult

Telencephalon Cerebrum (includes cerebral cortex, basal nuclei)

Forebrain
Diencephalon Diencephalon (thalamus, hypothalamus, epithalamus)

Midbrain Mesencephalon Midbrain (part of brainstem)

Metencephalon Pons (part of brainstem), cerebellum

Hindbrain
Myelencephalon Medulla oblongata (part of brainstem)

Mesencephalon Cerebrum Diencephalon

Metencephalon
Midbrain
Diencephalon Myelencephalon
Hindbrain

Midbrain
Brainstem

Pons
Medulla
Spinal cord oblongata
Forebrain
Telencephalon Cerebellum
Spinal cord

Embryo at 1 month Embryo at 5 weeks Child

1092 UNIT SEVEN Animal Form and Function

 The Cerebrum The Cerebellum
The cerebrum controls skeletal muscle contraction and is the The cerebellum coordinates movement and balance and helps in
center for learning, emotion, memory, and perception. It is divided learning and remembering motor skills. The cerebellum receives
into right and left cerebral hemispheres. The outer layer of the sensory information about the positions of the joints and the
cerebrum is called the cerebral cortex and is vital for lengths of the muscles, as well as input from the auditory
perception, voluntary movement, and learning. (hearing) and visual systems. It also monitors
The left side of the cerebral cortex receives Left cerebral Right cerebral motor commands issued by the cerebrum.
information from, and controls the move- hemisphere hemisphere The cerebellum integrates this
ment of, the right side of the body, and information as it carries out
vice versa. A thick band of axons Cerebral cortex coordination and error
known as the corpus callosum checking during motor and
enables the right and left Corpus callosum perceptual functions.
cerebral cortices to communi- Hand-eye coordination is
cate. Deep within the white Cerebrum Basal nuclei an example of cerebellar
matter, clusters of neurons control; if the cerebellum
called basal nuclei serve as is damaged, the eyes can
centers for planning and follow a moving object,
learning movement sequ- but they will not stop at
ences. Damage to these sites the same place as the
during fetal development can object. Hand movement
result in cerebral palsy, a dis- toward the object will also
order resulting from a disruption be erratic.
Cerebellum
in the transmission of motor
commands to the muscles.
Adult brain viewed from the rear

The Diencephalon The Brainstem

The diencephalon gives rise to the thalamus, hypothalamus, and The brainstem consists of the midbrain, the pons, and the
epithalamus. The thalamus is the main input center for sensory medulla oblongata (commonly called the medulla). The
information going to the cerebrum. Incoming information from all midbrain receives and integrates several types of sensory
the senses, as well as from the cerebral cortex, is information and sends it to specific regions of the
sorted in the thalamus and sent to the forebrain. All sensory axons involved in hearing
appropriate cerebral centers for either terminate in the midbrain or pass
further processing. The thala- through it on their way to the
mus is formed by two masses, cerebrum. In addition, the
each roughly the size and midbrain coordinates visual
Diencephalon reflexes, such as the periph-
shape of a walnut. A much
smaller structure, the Thalamus eral vision reflex: The head
hypothalamus, constitutes Pineal gland Brainstem turns toward an object
a control center that in- Hypothalamus approaching from the
cludes the body’s therm- Midbrain side without the brain
Pituitary gland
ostat as well as the central having formed an image
Pons
biological clock. Through its of the object.
regulation of the pituitary A major function of the
Medulla
gland, the hypothalamus pons and medulla is to
oblongata
regulates hunger and thirst, plays a transfer information
role in sexual and mating behaviors, and Spinal cord between the PNS and the
initiates the fight-or-flight response. The midbrain and forebrain. The pons
hypothalamus is also the source of posterior pituitary and medulla also help coordinate
hormones and of releasing hormones that act on the anterior large-scale body movements, such as
pituitary. The epithalamus includes the pineal gland, the source running and climbing. Most axons that carry instruc-
of melatonin. tions about these movements cross from one side of the CNS to the
other as they pass through the medulla. As a result, the right side of
the brain controls much of the movement of the left side of the
body, and vice versa. An additional function of the medulla is the
control of several automatic, homeostatic functions, including
breathing, heart and blood vessel activity, swallowing, vomiting,
and digestion. The pons also participates in some of these activi-
ties; for example, it regulates the breathing centers in the medulla.
Mastering Biology The Visual Brain: Nervous System

CHAPTER 49 Nervous Systems 1093

Arousal and Sleep . Figure 49.13 The reticular formation. Once thought to
consist of a single diffuse network of neurons, the reticular
If you’ve ever drifted off to sleep while listening to a lecture (or formation is now recognized as many distinct clusters of neurons.
reading a book), you know that your attentiveness and mental These clusters function in part to filter sensory input (blue arrows),
alertness can change rapidly. Such transitions are regulated by the blocking familiar and repetitive information that constantly enters
the nervous system before sending the filtered input to the cerebral
brainstem and cerebrum, which control arousal and sleep. Arousal
cortex (green arrows).
is a state of awareness of the external world. Sleep is a state in
which external stimuli are received but not consciously perceived.
Contrary to appearances, sleep is an active state, at least for
the brain. By placing electrodes at multiple sites on the scalp,
we can record patterns of electrical activity called brain waves
in an electroencephalogram (EEG). These recordings reveal
that brain wave frequencies change as the brain progresses
through distinct stages of sleep.
Some animals have evolutionary adaptations that allow
for substantial activity during sleep. Bottlenose dolphins, for
example, swim while sleeping, rising to the surface to breathe Eye Input from nerves
air on a regular basis. How is this possible? As in other mam- of ears
Reticular formation
mals, the forebrain is physically and functionally divided
into two halves, the right and left hemispheres. Noting that Input from touch,
dolphins sleep with one eye open and one closed, research- pain, and temperature
receptors
ers hypothesized that only one side of the brain is asleep at a
time. EEG recordings from each hemisphere of sleeping dol-
phins support this hypothesis (Figure 49.12).
the midbrain and pons (Figure 49.13). These neurons control
Although sleep is essential for survival, we still know very
the timing of sleep periods characterized by rapid eye move-
little about its function. One hypothesis is that sleep and
ments (REMs) and by vivid dreams. Sleep is also regulated
dreams are involved in consolidating learning and memory.
by the biological clock, discussed next, and by regions of the
Evidence supporting this hypothesis includes the finding that
forebrain that regulate sleep intensity and duration.
test subjects who are kept awake for 36 hours have a reduced
ability to remember when particular events occurred, even if Mastering Biology The Visual Brain: Sleep and Waking
they first “perk up” with caffeine. Other experiments show
that regions of the brain that are activated during a learning
Biological Clock Regulation
task can become active again during sleep.
Arousal and sleep are controlled in part by the reticular Cycles of sleep and wakefulness are an example of a circadian
formation, a diffuse network formed primarily by neurons in rhythm, a daily cycle of biological activity. Such cycles, which
occur in organisms ranging from bacteria to humans, rely
. Figure 49.12 Dolphins can be asleep and awake at the on a biological clock, a molecular mechanism that directs
same time. EEG recordings were made separately for the two periodic gene expression and cellular activity. Although bio-
sides of a dolphin’s brain. At each time point, low-frequency activity logical clocks are typically synchronized to the cycles of light
characteristic of sleep was recorded in one hemisphere while
and dark in the environment, they can maintain a roughly
higher-frequency activity typical of being awake was recorded in
the other hemisphere. 24-hour cycle even in the absence of environmental cues
(see Figure 40.9). For example, in a constant environment
Location Time: 0 hours Time: 1 hour humans exhibit a sleep/wake cycle of 24.2 hours, with very
little variation among individuals.
Left What normally links the biological clock to environmental
hemisphere cycles of light and dark in an animal’s surroundings? In mam-
mals, circadian rhythms are coordinated by clustered neurons
Right in the hypothalamus (see Figure 49.11). These neurons form a
hemisphere structure called the SCN, which stands for suprachiasmatic
nucleus. (Certain clusters of neurons in the CNS are referred
Key to as “nuclei.”) In response to sensory information from the
eyes, the SCN acts as a pacemaker, synchronizing the biologi-
Low-frequency waves characteristic of sleep
cal clock in cells throughout the body to the natural cycles of
High-frequency waves characteristic of wakefulness day length. In the Scientific Skills Exercise, you can interpret

1094 UNIT SEVEN Animal Form and Function

Scientific Skills Exercise v hamster
v hamster with SCN
Wild-type hamster

SCN from v hamster

Wild-type hamster with
Designing an Experiment Using Genetic from wild-type hamster
Mutants
Does the SCN Control the Circadian Rhythm in 24
Hamsters? By surgically removing the SCN from laboratory

Circadian cycle period (hours)

mammals, scientists demonstrated that the SCN is required for
circadian rhythms. Those experiments did not, however, reveal 23
whether circadian rhythms originate in the SCN. To answer this
question, researchers performed an SCN transplant experiment 22
on wild-type and mutant hamsters (Mesocricetus auratus).
Whereas wild-type hamsters have a circadian cycle lasting
about 24 hours in the absence of external cues, hamsters that 21
are homozygous for the τ (tau) mutation have a cycle lasting
only about 20 hours. In this exercise, you will evaluate the de-
20
sign of this experiment and propose additional experiments to
gain further insight.
19
How the Experiment Was Done The researchers surgically Before After surgery
removed the SCN from wild-type and τ hamsters. Several weeks procedures and transplant
later, each of these hamsters received a transplant of an SCN
from a hamster of the opposite genotype. To determine the pe- Data from M. R. Ralph et al., Transplanted suprachiasmatic nucleus determines
circadian period, Science 247:975–978 (1990).
riodicity of rhythmic activity for the hamsters before the surgery
and after the transplants, the researchers measured activity levels 2. For the wild-type hamsters that received τ SCN transplants, what
over a three-week period. They plotted the data collected for would have been an appropriate experimental control?
each day in the manner shown in Figure 40.9a and then calcu-
3. (a) What general trends does the graph above reveal about the
lated the circadian cycle period.
circadian cycle period of the transplant recipients? (b) Do the
Data from the Experiment In 80% of the hamsters in which trends differ for the wild-type and τ recipients? Based on these
the SCN had been removed, transplanting an SCN from another data, what can you conclude about the role of the SCN in deter-
hamster restored rhythmic activity. For hamsters in which an SCN mining the period of the circadian rhythm?
transplant restored a circadian rhythm, the net effect of the two 4. (a) In 20% of the hamsters, there was no restoration of rhythmic
procedures (SCN removal and replacement) on the circadian cycle activity following the SCN transplant. What are some possible rea-
period is graphed at the upper right. Each red line connects the sons for this finding? (b) How confident are you in your conclusion
two data points for an individual hamster. about the role of the SCN based on data from 80% of the hamsters?
5. Suppose that researchers identified a mutant hamster that lacked
INTERPRET THE DATA rhythmic activity; that is, its circadian activity cycle had no regular
pattern. Propose SCN transplant experiments using such a mutant
1. In a controlled experiment, researchers manipulate one variable along with (a) wild-type and (b) τ hamsters. Predict the results of
at a time. (a) What was the variable manipulated in this study? those experiments in light of your conclusion in question 3(b).
(b) Why did the researchers use more than one hamster for each
procedure? (c) What traits of the individual hamsters would Instructors: A version of this Scientific Skills Exercise can be
likely have been held constant among the treatment groups? assigned in Mastering Biology.

data from an experiment and propose experiments to test the Thalamus b Figure 49.14 The
role of the SCN in hamster circadian rhythms. limbic system of the
human brain. This
Mastering Biology HHMI Video: The Human diagram shows the
Suprachiasmatic Nucleus brain and brainstem,
with the left cerebral
hemisphere of the
Emotions brain removed.
Whereas a single structure in the brain controls the biological
clock, the generation and experience of emotions depend on
many brain structures, including the amygdala, hippocampus,
and parts of the thalamus. As shown in Figure 49.14, these
structures border the brainstem in mammals and are therefore Olfactory
bulbs
called the limbic system (from the Latin limbus, border). Hippocampus
One way the limbic system contributes to our emotions Hypothalamus
Amygdala
is by storing emotional experiences as memories that can be
recalled by similar circumstances. This is why, for example,
a situation that causes you to remember a frightening event

CHAPTER 49 Nervous Systems 1095

can trigger a faster heart rate, sweating, or fear, even if there is . Figure 49.15 Functional imaging in the working brain. These
currently nothing scary or threatening in your surroundings. images resulted from using fMRI to reveal brain activity associated
with music that listeners described as happy or sad. (Each view shows
Such storage and recall of emotional memory are especially
activity in a single plane of the brain, as seen from above.)
dependent on function of the amygdala, an almond-shaped
Nucleus accumbens Amygdala
brain structure near the base of the cerebrum.
Often, generating emotion and experiencing emotion
require interactions between different regions of the brain.
For example, both laughing and crying involve the lim-
bic system interacting with sensory areas of the forebrain.
Similarly, structures in the forebrain attach emotional “feel-
ings” to survival-related functions controlled by the brain-
stem, including aggression, feeding, and sexuality.
To study the function of the human amygdala, research-
ers sometimes present adult subjects with an image followed Happy music Sad music
by an unpleasant experience, such as a mild electrical shock. VISUAL SKILLS The two images reveal activity in different horizontal
After several trials, study participants experience autonomic planes through the brain. How can you tell this from the two photographs?
arousal—as measured by increased heart rate or sweating—if What can you conclude about the location of the nucleus accumbens and
the amygdala?
they see the image again. Subjects with brain damage con-
fined to the amygdala can recall the image because their
CONCEPT CHECK 49.2
explicit memory is intact. However, they do not exhibit auto-
nomic arousal, indicating that damage to the amygdala has 1. When you wave your right hand, what part of your brain
initiates the action?
resulted in a reduced capacity for emotional memory.
2. People who are inebriated have difficulty touching their
nose with their eyes closed. Which brain region does this
observation indicate is one of those impaired by alcohol?
Functional Imaging of the Brain 3. WHAT IF? Suppose you examine two groups of individuals
In recent years, scientists have begun studying the amygdala with CNS damage. In one group, the damage has resulted in
a coma (a prolonged state of unconsciousness). In the other
and other brain structures with functional imaging tech-
group, it has caused paralysis (a loss of skeletal muscle func-
niques. By scanning the brain while the subject performs a tion throughout the body). Relative to the position of the
particular function, such as forming a mental image of a per- midbrain and pons, where is the likely site of damage in each
son’s face, researchers are able to match particular functions group? Explain.
with activity in specific brain areas. For suggested answers, see Appendix A.

Several approaches are available for functional imag-

ing. The first widely used technique was positron-emission CONCEPT 49.3
tomography (PET), in which injection of radioactive glucose
enables a display of metabolic activity. Today, the most com- The cerebral cortex controls
monly used approach is functional magnetic resonance imag- voluntary movement and
ing (fMRI). In fMRI, a subject lies with his or her head in the
center of a large, doughnut-shaped magnet. Brain activity is cognitive functions
detected by an increase in the flow of oxygen-rich blood into We turn now to the cerebrum, the part of the brain essential
a particular region. for language, cognition, memory, consciousness, and aware-
In one experiment using fMRI, researchers mapped brain ness of our surroundings. As shown in Figure 49.11, the
activity while subjects listened to music that they described cerebrum is the largest structure in the human brain. Like the
as sad or happy (Figure 49.15). The findings were striking: brain overall, it exhibits regional specialization. For the most
Different regions of the brain were associated with the expe- part, cognitive functions reside in the cortex, the outer layer
rience of each of these contrasting emotions. Subjects who of the cerebrum. Within this cortex, sensory areas receive and
heard sad music had increased activity in the amygdala. In process sensory information, association areas integrate the
contrast, listening to happy music led to increased activity information, and motor areas transmit instructions to other
in the nucleus accumbens, a brain structure important for the parts of the body.
perception of pleasure. In discussing the location of particular functions in the
Functional imaging has an ever-increasing number of cerebral cortex, neurobiologists often use four regions, or lobes,
applications. Hospitals use fMRI to, for example, monitor as physical landmarks. Each lobe—frontal, temporal, occipi-
recovery from stroke, map abnormalities in migraine head- tal, and parietal—is named for a nearby bone of the skull, and
aches, and increase the effectiveness of brain surgery. each is the focus of specific brain activities (Figure 49.16).

1096 UNIT SEVEN Animal Form and Function

. Figure 49.16 The human cerebral cortex. Each of the four lobes of the cerebral cortex combined in a region dedicated to rec-
has specialized functions, some of which are listed here. Some areas on the left side of the brain ognizing complex images, such as faces.
(shown here) have different functions from those on the right side (not shown).
Once processed, sensory information
Motor cortex (control of Somatosensory cortex passes to the prefrontal cortex, which
skeletal muscles) (sense of touch)
helps plan actions and movement.
Frontal lobe Parietal lobe The cerebral cortex may then generate
motor commands that cause particular
Prefrontal cortex behaviors—moving a limb or saying
(decision making, Sensory association
planning) cortex (integration of hello, for example. These commands
sensory information) consist of action potentials produced
by neurons in the motor cortex, which
lies at the rear of the frontal lobe (see
Visual association Figure 49.16). The action potentials travel
Broca’s area cortex (combining along axons to the brainstem and spinal
(forming speech) images and
object recognition)
cord, where they excite motor neurons,
which in turn excite skeletal muscle cells.
Temporal lobe In the somatosensory cortex and
Occipital lobe
motor cortex, neurons are arranged
Auditory cortex (hearing)
according to the part of the body that
Visual cortex (processing
visual stimuli and pattern generates the sensory input or receives
Wernicke’s area Cerebellum recognition) the motor commands (Figure 49.17).
(comprehending language) For example, neurons that process
sensory information from the legs and
Information Processing feet lie in the region of the somatosensory cortex closest to
Broadly speaking, the human cerebral cortex receives sensory the midline. Neurons that control muscles in the legs and
information from two sources. Some sensory input originates feet are located in the corresponding region of the motor
in individual receptors in the hands, scalp, and elsewhere in cortex. Notice in Figure 49.17 that the cortical surface area
the body. These somatic sensory,
or somatosensory, receptors (from . Figure 49.17 Body part representation in the primary motor and primary somatosensory
the Greek soma, body) provide cortices. In these cross-sectional maps of the cortices, the cortical surface area devoted to each body
information about touch, pain, part is represented by the relative size of that part in the cartoons.
pressure, temperature, and the
position of muscles and limbs.
Frontal lobe Parietal lobe
Other sensory input comes from
groups of receptors clustered in
dedicated sensory organs, such as
Upp
Should

the eyes and nose.

Trunk
Head
For

er ar
Elbo m

Knee
Trunk

Neck

Leg
Hip
Elbo
Hip

Most sensory information

Fo
ear
Wr

er
w

rea

m
Ha

ist

coming into the cortex is directed

w
Ha
Fin

rm
nd

Th
nd rs
g

Fin

via the thalamus to primary

er

um
s

b
Th

sensory areas within the brain

um

Ey
Ne e
b

lobes. Information received at the No

Bro ck Fa se
primary sensory areas is passed w c
Eye Lip e
along to nearby association areas, s
Face Toes Genitalia
which process particular fea-
Teeth
tures in the sensory input. In the Gums
occipital lobe, for instance, some Lips Jaw
Jaw Tongue
groups of neurons in the primary
Tongue Pharynx
visual area are specifically sensi-
Primary Primary
tive to rays of light oriented in
motor Abdominal somatosensory
a particular direction. In the cortex organs cortex
visual association area, informa-
tion related to such features is VISUAL SKILLS Why is the hand larger than the forearm in both parts of this figure?

CHAPTER 49 Nervous Systems 1097

devoted to each body part is not proportional to the size of in the recognition of faces and patterns, spatial relations, and
the part. Instead, surface area correlates with the extent of nonverbal thinking. This difference in function between the
neuronal control needed (for the motor cortex) or with the right and left hemispheres is called lateralization.
number of sensory neurons that extend axons to that part The two cerebral hemispheres normally exchange infor-
(for the somatosensory cortex). Thus, the surface area of the mation through the fibers of the corpus callosum (see
motor cortex devoted to the face is proportionately quite Figure 49.11). Severing this connection (a treatment of last
large, reflecting the extensive involvement of facial muscles resort for the most extreme forms of epilepsy, a seizure disor-
in communication. der) results in a “split-brain” effect. In such patients, the two
hemispheres function independently. For example, they can-
not read even a familiar word that appears only in their left
Language and Speech field of vision: The sensory information travels from the left
The mapping of cognitive functions within the cortex began field of vision to the right hemisphere, but cannot then reach
in the 1800s when physicians studied the effects of damage to the language centers in the left hemisphere.
particular regions of the cortex by injuries, strokes, or tumors.
Pierre Broca conducted postmortem (after death) examina-
tions of patients who had been able to understand language Frontal Lobe Function
but unable to speak. He discovered that many had defects in In 1848, a horrific accident . Figure 49.19 Phineas
a small region of the left frontal lobe, now known as Broca’s pointed to the role of the Gage’s skull injury.
area. Karl Wernicke found that damage to a posterior portion prefrontal cortex in tempera-
of the left temporal lobe, now called Wernicke’s area, abol- ment and decision making.
ished the ability to comprehend speech but not the ability Phineas Gage was the foreman
to speak. PET studies have now confirmed activity in Broca’s of a railroad construction crew
area during speech generation and Wernicke’s area when when an explosion drove an
speech is heard (Figure 49.18). iron rod through his head.
The rod, which was more than
3 cm in diameter at one end,
Lateralization of Cortical Function entered his skull just below
Both Broca’s area and Wernicke’s area are located in the left his left eye and exited through
cerebral hemisphere, reflecting a greater role in language for the top of his head, damag-
the left side of the cerebrum than for the right side. The left ing large portions of his fron-
hemisphere is also more adept at math and logical opera- tal lobe (Figure 49.19). Gage recovered, but his personality
tions. In contrast, the right hemisphere appears to dominate changed dramatically. He became emotionally detached, impa-
tient, and erratic in his behavior, providing evidence of the role
. Figure 49.18 Mapping language areas in the cerebral of the prefrontal cortex in temperament and decision making.
cortex. These PET images show activity levels on the left side of one Two further sets of observations support the hypothesis
person’s brain during four activities, all related to speech. Increases in that Gage’s brain injury and personality change inform us
activity are seen in Wernicke’s area when hearing words, Broca’s area about frontal lobe function. First, frontal lobe tumors cause
when speaking words, the visual cortex when seeing words, and the
prefrontal cortex when generating words (without reading them). similar symptoms: Intellect and memory seem intact, but
decision making is flawed and emotional responses are
diminished. Second, the same problems arise when the con-
Max
nection between the prefrontal cortex and the limbic system
is surgically severed. (This procedure, called a frontal lobot-
omy, was once a common treatment for severe behavioral dis-
orders but is no longer in use.) Together, these observations
Hearing Seeing provide evidence that the frontal lobes have a substantial
words words
influence on what are called “executive functions.”

Evolution of Cognition in Vertebrates

EVOLUTION In nearly all vertebrates, the brain has the
Min
same basic structures (see Figure 49.10). Given this uniform
Speaking Generating organization, how did a capacity for advanced cognition,
words words
the perception and reasoning that constitute knowledge,

1098 UNIT SEVEN Animal Form and Function

evolve in certain species? For many years researchers favored How did the bird pallium and human cerebral cortex arise
the hypothesis that higher-order reasoning in vertebrates during evolution? The current consensus is that the common
required evolution of an extensively convoluted cerebral ancestor of birds and mammals had a pallium in which neu-
cortex, as is found in humans, other primates, and cetaceans rons were organized into nuclei, as is still found in birds. Early
(whales, dolphins, and porpoises). In humans, for example, in mammalian evolution, this clustered organization was
the cerebral cortex accounts for about 80% of total brain mass. transformed into a layered one. However, connectivity was
Birds lack a convoluted cerebral cortex and were long maintained such that, for example, the thalamus relays sen-
thought to have much lower intellectual capacity than sory input relating to sights, sounds, and touch to the pallium
primates and cetaceans. However, recent experiments in birds and to the cerebral cortex in mammals.
have refuted this idea: Western scrub jays (Aphelocoma Sophisticated information processing depends not only on
californica) can remember which food items they hid first. the overall organization of a brain but also on the very small-scale
New Caledonian crows (Corvus moneduloides) are highly changes that enable learning and encode memory. We’ll turn to
skilled at making and using tools, an ability otherwise well these changes in the context of humans in the next section.
documented only for humans and some other apes. African
grey parrots (Psittacus erithacus) understand numerical and Mastering Biology
abstract concepts, such as “same” and “different” and “none.” Interview with Erich Jarvis: Studying how
What brain structures enable some birds to have songbirds learn melodies

such sophisticated information processing? The answer

appears to be a nuclear (clustered) organization of neurons
CONCEPT CHECK 49.3
within the pallium, the top or outer portion of the brain
1. How can studying individuals with damage to a particular
(Figure 49.20a). Note that this arrangement is different from
brain region provide insight into the normal function of
that in the human cerebral cortex (Figure 49.20b), where six that region?
parallel layers of neurons are arranged tangential to the sur- 2. How do the functions of Broca’s area and Wernicke’s area
face. Thus, vertebrate evolution has resulted in two different each relate to the activity of the surrounding cortex?
types of outer brain organization that can support complex 3. WHAT IF? If a woman with a severed corpus callosum
and flexible function. viewed a photograph of a familiar face, first in her left field
of vision and then in her right field, why would she find it dif-
ficult to put a name to the face?
. Figure 49.20 Comparison of regions for higher cognition For suggested answers, see Appendix A.
in avian and human brains. Although structurally different, the
(a) pallium of a songbird brain and the (b) cerebral cortex of the
human brain play similar roles in higher cognitive activities and
CONCEPT 49.4
make many similar connections with other brain structures.
Changes in synaptic connections
Cerebrum
(including pallium) underlie memory and learning
Formation of the nervous system occurs stepwise. First, regu-
Cerebellum
lated gene expression and signal transduction determine where
neurons form in the developing embryo. Next, neurons com-
pete for survival. Every neuron requires growth-supporting fac-
tors, which are produced in limited quantities by tissues that
Thalamus Hindbrain
direct neuron growth. Neurons that don’t reach the proper
Midbrain locations fail to receive such factors and undergo programmed
(a) Songbird brain (cross section) cell death. The net effect is the preferential survival of neurons
that are in a proper location. The competition is so severe that
Cerebrum half of the neurons formed in the embryo are eliminated.
(including
cortex) In the final phase of organizing the nervous system, syn-
apse elimination takes place. During development, each
Thalamus neuron forms numerous synapses, more than are required
for its proper function. Once a neuron begins to function, its
Midbrain activity stabilizes some synapses and destabilizes others. By
the time the embryo completes development, more than half
of all synapses have been eliminated. In humans, this elimi-
Hindbrain Cerebellum
nation of unnecessary connections, a process called synaptic
(b) Human brain (cross section) pruning, continues after birth and throughout childhood.

CHAPTER 49 Nervous Systems 1099

 Together, neuron development, neuron death, and syn- shown in Figure 49.21b. In our traffic analogy, this would be
apse elimination establish the basic network of cells and con- equivalent to widening or narrowing an entrance ramp.
nections within the nervous system required throughout life. A defect in neuronal plasticity may underlie autism spec-
trum disorder, which results in impaired communication and
social interaction, as well as stereotyped and repetitive behav-
Neuronal Plasticity iors beginning in early childhood. There is now growing evi-
Although the overall organization of the CNS is established dence that autism spectrum disorder involves a disruption of
during embryonic development, the connections between activity-dependent remodeling at synapses. At the same time,
neurons can be modified. This capacity for the nervous sys- extensive research has ruled out any link to vaccine preserva-
tem to be remodeled, especially in response to its own activ- tives, once proposed as a potential risk factor based on fraudu-
ity, is called neuronal plasticity. lent data.
Much of the reshaping of the nervous system occurs at Although the underlying causes of autism are unknown,
synapses. Synapses belonging to circuits that link informa- there is a strong genetic contribution to this and related
tion in useful ways are maintained, whereas those that disorders. Further understanding of the autism-associated
convey bits of information lacking any context may be lost. disruption in synaptic plasticity may help efforts to better
Specifically, when the activity of a synapse coincides with understand and treat this disorder.
that of other synapses, changes may occur that reinforce that
synaptic connection. Conversely, when the activity of a syn-
apse fails to coincide with that of other synapses, the synaptic Memory and Learning
connection sometimes becomes weaker. Neuronal plasticity is essential to the formation of memo-
Figure 49.21a illustrates how activity-dependent events ries. We are constantly checking what is happening against
can trigger the gain or loss of a synapse. If you think of signals what just happened. We hold information for a time in
in the nervous system as traffic on a highway, such changes short-term memory and then release it if it becomes
are comparable to adding or removing an entrance ramp. irrelevant. If we wish to retain knowledge of a name, phone
The net effect is to increase signaling between particular number, or other fact, the mechanisms of long-term
pairs of neurons and decrease signaling between other pairs. memory are activated. If we later need to recall the name or
Signaling at a synapse can also be strengthen or weakened, as number, we fetch it from long-term memory and return it to
short-term memory.
. Figure 49.21 Neuronal plasticity. Synaptic connections can Short-term and long-term memory both involve the stor-
change over time, strengthening or weakening in response to the level age of information in the cerebral cortex. In short-term mem-
of activity at the synapse. ory, this information is accessed via temporary links formed
in the hippocampus. When memories are made long-term,
N1 N1 the links in the hippocampus are replaced by connections
within the cerebral cortex itself. As discussed earlier, some
of this consolidation of memory is thought to occur during
sleep. Furthermore, the reactivation of the hippocampus that
is required for memory consolidation likely forms the basis
for at least some of our dreams.
N2 N2
According to our current understanding of memory, the
(a) High-level activity at the synapse of neuron N1 with the postsynaptic hippocampus is essential for acquiring new long-term memo-
neuron leads to recruitment of additional axon terminals from that ries but not for maintaining them. This hypothesis readily
neuron. Lack of activity at the synapse with neuron N2 leads to loss explains the symptoms of some individuals who suffer damage
of functional connections with that neuron.
to the hippocampus: They cannot form any new lasting memo-
ries but can freely recall events from before their injury. In
effect, their lack of normal hippocampal function traps them in
their past. Hippocampal damage and memory loss are common
in the early stages of Alzheimer’s disease (see Concept 49.5).
What evolutionary advantage might be offered by orga-
nizing short-term and long-term memories differently? One
hypothesis is that the delay in forming connections in the
cerebral cortex allows long-term memories to be integrated
(b) If two synapses on a postsynaptic cell are often active at the same
gradually into the existing store of knowledge and experi-
time, the strength of both responses may increase. ence, providing a basis for more meaningful associations.

1100 UNIT SEVEN Animal Form and Function

Consistent with this hypothesis, the transfer of information . Figure 49.22 Long-term potentiation in the brain.
from short-term to long-term memory is enhanced by the
association of new data with data previously learned and PRESYNAPTIC NEURON Ca2+
stored in long-term memory. For example, it’s easier to learn a Na+
new card game if you already have “card sense” from playing
other card games.
Motor skills, such as tying your shoes or writing, are
usually learned by repetition. You can perform these skills Mg2+
without consciously recalling the individual steps required Glutamate
to do these tasks correctly. Learning skills and procedures,
such as those required to ride a bicycle, appears to involve NMDA
NMDA receptor receptor
cellular mechanisms very similar to those responsible for (open) Stored (closed)
AMPA
brain growth and development. In such cases, neurons actu-
receptor
ally make new connections. In contrast, memorizing phone POSTSYNAPTIC NEURON
numbers, facts, and places—which can be very rapid and may (a) Synapse prior to long-term potentiation (LTP). The NMDA
require only one exposure to the relevant item—may rely glutamate receptors open in response to glutamate but are blocked
mainly on changes in the strength of existing neuronal con- by Mg2+.
nections. Next we will consider one way that such changes in
strength can take place.

Long-Term Potentiation
In searching for the physiological basis of memory, research-
ers have concentrated their attention on processes that can
1
alter a synaptic connection, making the flow of communica-
tion either more efficient or less efficient. We will focus here
on long-term potentiation (LTP), a lasting increase in the 2
3
strength of synaptic transmission. Data indicate that LTP repre-
sents a fundamental process for memory storage and learning.
First characterized in tissue slices from the hippocampus,
LTP involves a presynaptic neuron that releases the excit- (b) Establishing LTP. Activity at nearby synapses (not shown) depolarizes
atory neurotransmitter glutamate. LTP involves two types of the postsynaptic membrane, causing 1 Mg2+ release from NMDA
receptors. The unblocked receptors respond to glutamate by
glutamate receptors, each named for a molecule—NMDA or allowing 2 an influx of Na+ and Ca2+. The Ca2+ influx triggers
AMPA—that can be used to artificially activate that particular 3 insertion of stored AMPA glutamate receptors into the
postsynaptic membrane.
receptor. As shown in Figure 49.22, the set of receptors present
on the postsynaptic membrane changes when two conditions
are met: a rapid series of action potentials at the presynaptic
neuron and a depolarizing stimulus elsewhere on the postsyn-
aptic cell. The result is LTP—a stable increase in the size of the
postsynaptic potentials at a synapse whose activity coincides
with that of another input.

Mastering Biology The Visual Brain: Learning and Memory 3

1

CONCEPT CHECK 49.4 2 4 Action

1. Outline two mechanisms by which information flow potential
between two neurons in an adult can increase. Depolarization
2. Individuals with localized brain damage have been very use-
ful in the study of many brain functions. Why is this unlikely (c) Synapse exhibiting LTP. Glutamate release activates 1 AMPA
to be true for consciousness? receptors that trigger 2 depolarization. The depolarization
unblocks 3 NMDA receptors. Together, the AMPA and NMDA
3. WHAT IF? Suppose that a person with damage to the hip- receptors trigger postsynaptic potentials strong enough to
pocampus is unable to acquire new long-term memories. Why initiate 4 action potentials without input from other synapses.
might the acquisition of short-term memories also be impaired? Additional mechanisms (not shown) contribute to LTP, including
For suggested answers, see Appendix A. receptor modification by protein kinases.

CHAPTER 49 Nervous Systems 1101

 . Figure 49.23 Genetic contribution to schizophrenia. First
CONCEPT 49.5
cousins, uncles, and aunts of a person with schizophrenia have twice

Many nervous system disorders the risk of unrelated members of the population of developing the
disease. The risks for closer relatives are many times greater.
can now be explained in 50
molecular terms Genes shared with relatives of
person with schizophrenia
Disorders of the nervous system, including schizophrenia, 12.5% (3rd-degree relative)

Risk of developing schizophrenia (%)

depression, drug addiction, Alzheimer’s disease, and 40 25% (2nd-degree relative)
Parkinson’s disease, are a major public health problem. 50% (1st-degree relative)
Together, they result in more hospitalizations in the United 100%
States than do heart disease or cancer. Until recently, hos- 30
pitalization was typically the only available treatment, and
many affected individuals were institutionalized for the rest of
their lives. Today, many disorders that alter mood or behavior 20
can be treated with medication, reducing average hospital
stays for these disorders to only a few weeks. Nevertheless,
many challenges remain with regard to preventing or treating 10
nervous system disorders, especially Alzheimer’s disease and
other disorders that lead to nervous system degeneration.
Major research efforts are under way to identify genes
0
that cause or contribute to disorders of the nervous system.

Individual,
general population
First cousin

Uncle/aunt

Nephew/niece

Grandchild

Half sibling

Parent

Full sibling

Child

Fraternal twin

Identical twin
Identifying such genes offers hope for identifying causes,
predicting outcomes, and developing effective treatments.
For most nervous system disorders, however, genetic con-
tributions only partially account for which individuals are
affected. The other significant contribution to disease comes Relationship to person with schizophrenia
from environmental factors. Unfortunately, such environ-
mental contributions are typically very difficult to identify. INTERPRET THE DATA What is the likelihood of a person
To distinguish between genetic and environmental developing schizophrenia if the disorder affects his or her fraternal twin?
How would the likelihood change if DNA sequencing revealed that the
variables, scientists often carry out family studies. In these twins shared the genetic variants that contribute to the disorder?
studies, researchers track how family members are related
genetically, which individuals are affected, and which schizo, split, and phren, mind) refers to the fragmentation of
family members grew up in the same household. These what are normally integrated brain functions.
studies are especially informative when one of the affected One current hypothesis is that neuronal pathways that use
individuals has either an adopted sibling who is genetically dopamine as a neurotransmitter are disrupted in schizophre-
unrelated or an identical twin, as we’ll see for the disorder nia. Supporting evidence comes from the fact that many drugs
schizophrenia, our next topic. that alleviate the symptoms of schizophrenia block dopamine
receptors. In addition, the drug amphetamine (“speed”),
which stimulates dopamine release, can produce the same set
Schizophrenia of symptoms as schizophrenia. Recent genetic studies sug-
Approximately 1% of the world’s population suffers from gest a link between schizophrenia and particular forms of the
schizophrenia, a severe mental disturbance characterized complement protein C4, an immune system component.
by psychotic episodes in which patients have a distorted per-
ception of reality. People with schizophrenia typically experi-
ence hallucinations (such as “voices” that only they can hear) Depression
and delusions (for example, the idea that others are plotting Depression is a disorder characterized by depressed mood, as well
to harm them). Family studies have revealed a very strong as abnormalities in sleep, appetite, and energy level. Two broad
genetic component for schizophrenia. However, as shown forms of depressive illness are known: major depressive disorder
in Figure 49.23, the disease is also subject to environmental and bipolar disorder. Individuals affected by major depressive
influences, since an individual who shares 100% of his or her disorder undergo periods—often lasting many months—during
genes with a twin with schizophrenia has only a 48% chance which once enjoyable activities provide no pleasure and provoke
of developing the disorder. Despite the commonly held no interest. One of the most common nervous system disorders,
notion, schizophrenia does not necessarily result in multiple major depression affects about one in every seven adults at some
personalities. Rather, the name schizophrenia (from the Greek point, and twice as many women as men.

1102 UNIT SEVEN Animal Form and Function

 Bipolar disorder, or manic-depressive disorder, involves . Figure 49.24 Effects of addictive drugs on the reward system
extreme swings of mood and affects about 1% of the world’s of the mammalian brain. Addictive drugs alter the transmission of
signals in the pathway formed by neurons of the ventral tegmental
population. The manic phase is characterized by high self-
area (VTA), a region located near the base of the brain.
esteem, increased energy, a flow of ideas, overtalkativeness,
and increased risk taking. In its milder forms, this phase
is sometimes associated with great creativity, and some Nicotine
well-known artists, musicians, and literary figures (Vincent stimulates
dopamine- Inhibitory neuron
van Gogh, Robert Schumann, Virginia Woolf, and Ernest releasing
Hemingway, to name a few) have had productive periods dur- VTA neuron.
ing manic phases. The depressive phase comes with lessened
motivation, sense of worth, and ability to feel pleasure, as
well as sleep disturbances. These symptoms can be so severe
that affected individuals attempt suicide. Opioids decrease
Dopamine- activity of
Major depressive and bipolar disorders are among the ner-
releasing inhibitory
vous system disorders for which therapies are available. Many VTA neuron neuron.
drugs used to treat depressive illness, including fluoxetine
(Prozac), increase activity of biogenic amines in the brain.
Cocaine and
amphetamines
The Brain’s Reward System and Drug block removal
of dopamine
Addiction from synaptic
cleft.
Emotions are strongly influenced by a neuronal circuit in the
brain called the reward system. The reward system provides
motivation for activities that enhance survival and reproduc-
tion, such as eating in response to hunger, drinking when Cerebral
thirsty, and engaging in sexual activity when aroused. Inputs neuron of Reward
reward system
to the reward system are received by neurons in the ventral pathway response
tegmental area (VTA), a region located within the midbrain
(see Figure 49.11). When activated, these neurons release the MAKE CONNECTIONS Review depolarization in Concept 48.3. What
neurotransmitter dopamine from their synaptic terminals effect would you expect if you depolarized the neurons in the VTA? Explain.
(Figure 49.24). Targets of this dopamine signaling include the
nucleus accumbens and the prefrontal cortex.
Mastering Biology
The brain’s reward system is dramatically affected by drug
Interview with Ulrike Heberlein: Research
addiction, a disorder characterized by compulsive consump- with drunk flies
tion of a drug and loss of control in limiting intake. Addictive
drugs range from sedatives to stimulants and include alco-
hol, cocaine, and nicotine, as well as opioids, such as heroin, Alzheimer’s Disease
fentanyl and oxycodone. All enhance the activity of the The condition now known as Alzheimer’s disease is a
dopamine pathway (see Figure 49.24). As addiction develops, mental deterioration, or dementia, characterized by confu-
there are also long-lasting changes in the reward circuitry. sion and memory loss. Its incidence is age related, rising
The result is a craving for the drug independent of any plea- from about 10% at age 65 to about 35% at age 85. Overall,
sure associated with consumption. In 2018, the Centers for Alzheimer’s disease accounts for about two of every three
Disease Control and Prevention reported that an average of cases of dementia. It is also the sixth most common cause
130 Americans died from opioid overdose every day. of death among adults in the United States, affecting indi-
Laboratory animals are highly valuable in modeling and viduals including former President Ronald Reagan, author
studying addiction. Rats, for example, will provide themselves E.B. White, and civil rights heroine Rosa Parks.
with heroin, cocaine, or amphetamine when given a dispens- Alzheimer’s disease is progressive; patients gradually
ing system linked to a lever in their cage. Furthermore, they become less able to function and eventually need to be dressed,
exhibit addictive behavior, continuing to self-administer the bathed, and fed by others. Individuals with Alzheimer’s
drug rather than seek food, even to the point of starvation. disease often lose their ability to recognize people and may
As scientists expand their knowledge about the brain’s treat even immediate family members with suspicion and
reward system and the various forms of addiction, there is hostility.
hope that the insights will lead to more effective prevention Examining the brains of individuals who have died
and treatment. of Alzheimer’s disease reveals two characteristic

CHAPTER 49 Nervous Systems 1103

. Figure 49.25 Microscopic signs of Alzheimer’s disease. A the ability of patients to vary their expressions. Cognitive
hallmark of Alzheimer’s disease is the presence in brain tissue of defects may also develop. Like Alzheimer’s disease,
neurofibrillary tangles surrounding plaques made of b-amyloid (LM).

20 om
Parkinson’s disease is a progressive brain illness and is more
Amyloid plaque Neurofibrillary tangle common with advancing age. The incidence of Parkinson’s
disease is about 1% at age 65 and about 5% at age 85. In the
United States population, approximately 1 million people
have Parkinson’s disease.
Parkinson’s disease involves the death of neurons in the
midbrain that normally release dopamine at synapses in the
basal nuclei. As with Alzheimer’s disease, protein aggregates
accumulate. Most cases of Parkinson’s disease lack an identifi-
able cause; however, a rare form of the disease that appears
in relatively young adults has a clear genetic basis. Molecular
studies of mutations linked to this early-onset Parkinson’s
Mastering Biology BBC Video: Searching for a Cure disease reveal disruption of genes required for certain mito-
for Alzheimer’s
chondrial functions. Researchers are investigating whether
features: amyloid plaques and neurofibrillary tangles, as mitochondrial defects also contribute to the more common,
shown in Figure 49.25. There is also often massive shrinkage later-onset form of the disease.
of brain tissue, reflecting the death of neurons in many areas At present, Parkinson’s disease can be treated, but not
of the brain, including the hippocampus and cerebral cortex. cured. Approaches used to manage the symptoms include
The plaques are aggregates of b-amyloid, an insoluble brain surgery, deep-brain stimulation, and a dopamine-
peptide that is cleaved from the extracellular portion of a related drug, l-dopa. Unlike dopamine, l-dopa crosses the
membrane protein found in neurons. Membrane enzymes, blood-brain barrier. Within the brain, the enzyme dopa
called secretases, catalyze the cleavage, causing b-amyloid to decarboxylase converts the drug to dopamine, reducing the
accumulate in plaques outside the neurons. It is these plaques severity of Parkinson’s disease symptoms:
that appear to trigger the death of surrounding neurons.
HO NH2 Dopa HO NH2
The neurofibrillary tangles observed in Alzheimer’s dis- decarboxylase
ease are primarily made up of the tau protein. (This protein is
unrelated to the tau mutation that affects circadian rhythm HO CO2H HO
in hamsters.) The tau protein normally helps assemble and L-dopa Dopamine
maintain microtubules that transport nutrients along axons.
In Alzheimer’s disease, tau undergoes changes that cause it
to bind to itself, resulting in neurofibrillary tangles. There is Future Directions in Brain Research
evidence that changes in tau are associated with the appear- In 2014, the United States government launched a bold
ance of early-onset Alzheimer’s disease, a much less common 12-year project, the BRAIN (Brain Research through
disorder that affects relatively young individuals. At present, Advancing Innovative Neurotechnologies) Initiative.
there are no treatments available that block the progression The aim is to develop innovative technologies and drive
of Alzheimer’s disease. scientific advances, similar to the major projects that
Tau protein accumulation is also characteristic of a degenera- accomplished landing a person on the moon and mapping
tive brain disease found in athletes, military veterans, and others the human genome. Specific BRAIN goals are to map brain
with a history of concussion or other repetitive brain trauma. (A circuits, measure activity within those circuits, and
concussion is a brain injury caused by a blow or jolt to the head discover how this activity is translated into thought
or a hit to the body that shakes the brain.) Known as chronic and behavior.
traumatic encephalopathy (CTE), this disease was first described
in the early 2000s and has now been detected in postmortem
CONCEPT CHECK 49.5
diagnosis of more than 100 men who had played professional
football in the National Football League, among others. 1. Compare Alzheimer’s disease and Parkinson’s disease.
2. How is dopamine activity related to schizophrenia, drug
addiction, and Parkinson’s disease?
Parkinson’s Disease 3. WHAT IF? If you could detect early-stage Alzheimer’s dis-
ease, would you expect to see brain changes that were similar
Symptoms of Parkinson’s disease, a motor disorder, to, although less extensive than, those seen in patients who
include muscle tremors, poor balance, a flexed posture, have died of this disease? Explain.
and a shuffling gait. Facial muscles become rigid, limiting For suggested answers, see Appendix A.

1104 UNIT SEVEN Animal Form and Function

49 Chapter Review Go to Mastering Biology for Assignments, the eText,
the Study Area, and Dynamic Study Modules.

SUMMARY OF KEY CONCEPTS CONCEPT 49.2

The vertebrate brain is regionally specialized
To review key terms, go to the Vocabulary Self-Quiz in the (pp. 1091–1096)
Mastering Biology eText or Study Area, or go to goo.gl/zkjz9t.
Cerebral
CONCEPT 49.1 cortex

Nervous systems consist of circuits of neurons and

supporting cells (pp. 1086–1090)
• Invertebrate nervous systems range in complexity from simple Cerebrum
nerve nets to highly centralized nervous systems having compli- Thalamus
Forebrain
cated brains and ventral nerve cords.
Hypothalamus
Brain
Pituitary gland

Midbrain
Spinal Pons Spinal
cord Sensory
Medulla cord
(dorsal ganglia Hindbrain
nerve oblongata
cord)
Cerebellum
Nerve net
• The cerebrum has two hemispheres, each of which consists of cortical
gray matter overlying white matter and basal nuclei. The basal
nuclei are important in planning and learning movements. The pons
Hydra (cnidarian) Salamander (vertebrate) and medulla oblongata are relay stations for information traveling
between the PNS and the cerebrum. The reticular formation, a net-
• In vertebrates, the central nervous system (CNS), consisting work of neurons within the brainstem, regulates sleep and arousal.
of the brain and the spinal cord, integrates information, while The cerebellum helps coordinate motor, perceptual, and cognitive
the nerves of the peripheral nervous system (PNS) transmit functions. The thalamus is the main center through which sensory
sensory and motor signals between the CNS and the rest of the information passes to the cerebrum. The hypothalamus regulates
body. The simplest circuits control reflex responses, in which homeostasis and basic survival behaviors. Within the hypothalamus,
sensory input is linked to motor output without involvement a group of neurons called the suprachiasmatic nucleus (SCN)
of the brain. acts as the pacemaker for circadian rhythms. The amygdala plays a
key role in recognizing and recalling a number of emotions.
CNS PNS
? What roles do the midbrain, cerebellum, thalamus, and cerebrum
VENTRICLE Astrocyte play in vision and responses to visual input?
Ependy-
mal Oligodendrocyte CONCEPT 49.3
cell
Cilia The cerebral cortex controls voluntary movement and
cognitive functions (pp. 1096–1099)
• Each side of the cerebral cortex has four lobes—frontal, tem-
poral, occipital, and parietal—that contain primary sensory areas
Schwann and association areas. Association areas integrate information
cells
from different sensory areas. Broca’s area and Wernicke’s area are
essential for generating and understanding language. These func-
Capillary Neuron Microglial cell tions are concentrated in the left cerebral hemisphere, as are math
and logic operations. The right hemisphere appears to be stronger
• Afferent neurons carry sensory signals to the CNS. Efferent neu- at pattern recognition and nonverbal thinking.
rons function in either the motor system, which carries signals • In the somatosensory cortex and the motor cortex, neurons are
to skeletal muscles, or the autonomic nervous system, which distributed according to the part of the body that generates sen-
regulates smooth and cardiac muscles. The sympathetic and sory input or receives motor commands.
parasympathetic divisions of the autonomic nervous system • Primates and cetaceans, which are capable of higher cognition,
have antagonistic effects on a diverse set of target organs, while have an extensively convoluted cerebral cortex. In birds, a brain
the enteric nervous system controls the activity of many region called the pallium contains clustered nuclei that carry
digestive organs. out functions similar to those performed by the cerebral cortex
• Vertebrate neurons are supported by glia, including astrocytes, of mammals. Some birds can solve problems and understand ab-
oligodendrocytes, and Schwann cells. Some glia serve as stem cells stractions in a manner indicative of higher cognition.
that can differentiate into mature neurons.
? A patient has trouble with language and has paralysis on one side
? How does the circuitry of a reflex facilitate a rapid response? of the body. Which side would you expect to be paralyzed? Why?

CHAPTER 49 Nervous Systems 1105

 CONCEPT 49.4 Levels 3-4: Applying/Analyzing
Changes in synaptic connections underlie memory 5. After suffering a stroke, a patient can see objects anywhere
and learning (pp. 1099–1101) in front of him but pays attention only to objects in his right
field of vision. When asked to describe these objects, he has
• During development, more neurons and synapses form than will difficulty judging their size and distance. What part of the
exist in the adult. The programmed death of neurons and elimina- brain was likely damaged by the stroke?
tion of synapses in embryos establish the basic structure of the (A) the left frontal lobe (C) the right parietal lobe
nervous system. In the adult, reshaping of the nervous system can (B) the right frontal lobe (D) the corpus callosum
involve the loss or addition of synapses or the strengthening or weak-
6. Injury localized to the hypothalamus would most likely disrupt
ening of signaling at synapses. This capacity for remodeling is termed
neuronal plasticity. Our short-term memory relies on tempo- (A) regulation of body temperature.
rary links in the hippocampus. In long-term memory, these links (B) short-term memory.
are replaced by connections within the cerebral cortex. (C) executive functions, such as decision making.
? Learning multiple languages is typically easier early in childhood than (D) sorting of sensory information.
later in life. How does this fit with our understanding of neural development? 7. DRAW IT The reflex that pulls your hand away when you prick
your finger on a sharp object relies on a neuronal circuit with
CONCEPT 49.5 two synapses in the spinal cord. (a) Using a circle to represent a
cross section of the spinal cord, draw the circuit. Label the types
Many nervous system disorders can now be explained of neurons, the direction of information flow in each, and the
in molecular terms (pp. 1102–1104) locations of synapses. (b) Draw a simple diagram of the brain
indicating where pain would eventually be perceived.
• Schizophrenia, which is characterized by hallucinations, delu-
sions, and other symptoms, affects neuronal pathways that use Levels 5-6: Evaluating/Creating
dopamine as a neurotransmitter. Drugs that increase the activ-
ity of biogenic amines in the brain can be used to treat bipolar 8. EVOLUTION CONNECTION Scientists often use measures of
disorder and major depressive disorder. The compulsive “higher-order thinking” to assess intelligence in other animals. For
drug use that characterizes addiction reflects altered activity of example, birds are judged to have sophisticated thought processes
the brain’s reward system, which normally provides motivation because they can use tools and make use of abstract concepts.
for actions that enhance survival or reproduction. Identify problems you see in defining intelligence in these ways.
• Alzheimer’s disease and Parkinson’s disease are neuro- 9. SCIENTIFIC INQUIRY Consider an individual who had been
degenerative and typically age related. Alzheimer’s disease is a fluent in American Sign Language before suffering an injury
dementia in which neurofibrillary tangles and amyloid plaques to his left cerebral hemisphere. After the injury, he could still
form in the brain. Parkinson’s disease is a motor disorder caused understand that sign language but could not readily generate
by the death of dopamine-secreting neurons and associated with sign language that represented his thoughts. Propose two
the presence of protein aggregates. hypotheses that could explain this finding. How might you
distinguish between them?
? The fact that both amphetamine and PCP have effects similar to the
symptoms of schizophrenia suggests a potentially complex basis for this 10. SCIENCE, TECHNOLOGY, AND SOCIETY With increasingly
disease. Explain. sophisticated methods for scanning brain activity, scientists
are developing the ability to detect an individual’s particular
emotions and thought processes from outside the body. What
TEST YOUR UNDERSTANDING benefits and problems do you envision when such technology
becomes readily available? Explain.
For more multiple-choice questions, go to the Practice Test in the 11. WRITE ABOUT A THEME: INFORMATION In a short
Mastering Biology eText or Study Area, or go to goo.gl/GruWRg. essay (100–150 words), explain how specification of the adult
nervous system by the genome is incomplete.
Levels 1-2: Remembering/Understanding 12. SYNTHESIZE YOUR KNOWLEDGE
1. Activation of the parasympathetic branch of the autonomic Imagine you are standing
nervous system at a microphone in front
(A) increases heart rate. of a crowd. Checking your
(B) enhances digestion. notes, you begin speaking.
(C) triggers release of epinephrine. Using the information in this
(D) causes conversion of glycogen to glucose. chapter, describe the series of
events in particular regions
2. Which of the following structures or regions is correctly paired of the brain that enabled you
with its function? to say the very first word.
(A) limbic system—motor control of speech
(B) medulla oblongata—homeostatic control
(C) cerebrum—coordination of movement and balance
(D) amygdala—short-term memory For selected answers, see Appendix A.
3. Patients with damage to Wernicke’s area have difficulty
(A) coordinating limb (C) recognizing faces. Explore Scientific Papers with Science in the Classroom
movement. (D) understanding language. How does a neuron develop different functions than
(B) generating speech. those of similar neighboring neurons?
4. The cerebral cortex plays a major role in Go to “New Neurons Stand Out from the Crowd”
at www.scienceintheclassroom.org.
(A) emotional memory. (C) circadian rhythm.
(B) hand-eye coordination. (D) breath holding. Instructors: Questions can be assigned in Mastering Biology.

1106 UNIT SEVEN Animal Form and Function