CHAPTER 7: Nervous System

Sensory input Integration Motor output

●​ Gathering information ●​ Nervous system ●​ A response, or
●​ Sensory receptors processes and effect, activates
monitor changes, interprets sensory muscles or glands
called stimuli, ●​ Input and decides
occurring inside and whether action is
outside the body needed

Structural Classification

Central nervous system (CNS) Peripheral nervous system (PNS)

●​ Organs ●​ Nerves extending from the brain and spinal

○​ Brain cord
○​ Spinal cord ○​ Spinal nerves—carry impulses to and
●​ Function from the spinal cord
○​ Integration; command center ○​ Cranial nerves—carry impulses to
○​ Interprets incoming sensory and from the brain
information ●​ Functions
○​ Issues outgoing instructions ○​ Serve as communication lines among
sensory organs, the brain and spinal
cord, and glands or muscles

Functional Classification

Sensory (afferent) division Motor (efferent) division

●​ Nerve fibers that carry information to the ●​ Nerve fibers that carry impulses away
central nervous system from the central nervous system organs to
○​ Somatic sensory (afferent) fibers effector organs (muscles and glands)
carry information from the skin, ●​ Two subdivisions
skeletal muscles, and joints ○​ Somatic nervous system = voluntary.
○​ Visceral sensory (afferent) fibers Consciously (voluntarily) controls
carry information from visceral skeletal muscles
organs ○​ Autonomic nervous system =
involuntary. Automatically controls
smooth and cardiac muscles and
glands. Further divided into the
sympathetic and parasympathetic
nervous systems

Nervous Tissue: Support Cells

●​ grouped together as neuroglia
●​ General functions: Support, Insulate, Protect neurons

CNS Glial Cells

Astrocytes Microglia

●​ Abundant, star-shaped cells ●​ Spider Like phagocytes

●​ Brace and anchor neurons to blood ●​ Monitor health of nearby neurons
capillaries ●​ Dispose of debris
●​ Determine permeability and exchanges
between blood capillaries and neurons
●​ Protect neurons from harmful substances
in blood
●​ Control the chemical environment of the
brain

Ependymal cells Oligodendrocytes

●​ Line cavities of the brain and spinal cord ●​ Wrap around nerve fibers in the central
●​ Cilia assist with circulation of nervous system
cerebrospinal fluid ●​ Produce myelin sheaths

PNS Glial Cells

Schwann cells Satellite cells

●​ Form myelin sheath around nerve fibers in ●​ Protect and cushion neuron cell bodies
the PNS

Nervous Tissue: Neurons

Neurons Cell Body

●​ Cells specialized to transmit messages ●​ The metabolic center of the neuron

(nerve impulses) ○​ Nucleus with large nucleolus
●​ Major regions of all neurons ○​ Nissl bodies
○​ Cell body—nucleus and metabolic ■​ Rough endoplasmic reticulum
center of the cell ○​ Neurofibrils
○​ Processes—fibers that extend from ■​ Intermediate filaments that
the cell body maintain cell shape

Processes (fibers) Myelin

●​ Dendrites—conduct impulses toward the ●​ White, fatty material covering axons

cell body ●​ Protects and insulates fibers
○​ Neurons may have hundreds of ●​ Speeds nerve impulse transmission
dendrites
●​ Axons—conduct impulses away from the
 cell body
○​ Neurons have only one axon arising
from the cell body at the axon
hillock
○​ End in axon terminals, which contain
vesicles with neurotransmitters
○​ Axon terminals are separated from
the next neuron by a gap
●​ Synaptic cleft—gap between axon
terminals and the next neuron
●​ Synapse—functional junction between
nerves where a nerve impulse is
transmitted

Myelin Sheaths

●​ Schwann cells—wrap axons in a jelly

roll–like fashion (PNS) to form the myelin
sheath
○​ Neurilemma—part of the Schwann
cell external to the myelin sheath
○​ Nodes of Ranvier—gaps in myelin
sheath along the axon
●​ Oligodendrocytes—produce myelin sheaths
around axons of the CNS
○​ Lack a neurilemma

Terminology Functional Classification

●​ Nuclei—clusters of cell bodies in the CNS ●​ Sensory (afferent) neurons

●​ Ganglia—collections of cell bodies outside ○​ Carry impulses from the sensory
the CNS in the PNS receptors to the CNS
●​ Tracts—bundles of nerve fibers in the ○​ Receptors include:
CNS ■​ Cutaneous sense organs in
●​ Nerves—bundles of nerve fibers in the skin
PNS ■​ Proprioceptors in muscles
●​ White matter—collections of myelinated and tendons
fibers (tracts) ●​ Motor (efferent) neurons
●​ Gray matter—mostly unmyelinated fibers ○​ Carry impulses from the central
and cell bodies from visceral organs nervous system to viscera and/or
muscles and glands
●​ Interneurons (association neurons)
○​ Cell bodies located in the CNS
○​ Connect sensory and motor neurons

Structural Classification Functional Properties

●​ Based on number of processes extending ●​ Irritability
from the cell body ○​ Ability to respond to a stimulus and
●​ Multipolar neurons—many extensions from convert it to a nerve impulse
the cell body ●​ Conductivity
○​ All motor and interneurons are ○​ Ability to transmit the impulse to
multipolar other neurons, muscles, or glands
○​ Most common structural type
●​ Bipolar neurons—one axon and one dendrite
○​ Located in special sense organs,
such as nose and eye
○​ Rare in adults
●​ Unipolar neurons—have a short single
process leaving the cell body
○​ Sensory neurons found in PNS
ganglia
○​ Conduct impulses both toward and
away from the cell body

Electrical conditions of a resting neuron’s Action potential initiation and generation

membrane

●​ The plasma membrane at rest is inactive ●​ A stimulus changes the permeability of the
(polarized) neuron’s membrane to sodium ions
●​ Fewer positive ions are inside the neuron’s ●​ Sodium channels now open, and sodium
plasma membrane than outside (Na+) diffuses into the neuron
○​ K+ is the major positive ion inside ●​ The inward rush of sodium ions changes
the cell the polarity at that site and is called
○​ Na+ is the major positive ion outside depolarization
the cell ●​ A graded potential (localized
●​ As long as the inside of the membrane is depolarization) exists where the inside of
more negative (fewer positive ions) than the membrane is more positive and the
the outside, the cell remains inactive outside is less positive
●​ If the stimulus is strong enough and
sodium influx great enough, local
depolarization activates the neuron to
conduct an action potential (nerve impulse)
●​ Propagation of the action potential
○​ If enough sodium enters the cell,
the action potential (nerve impulse)
starts and is propagated over the
entire axon
○​ All-or-none response means the
nerve impulse either is propagated
or is not
○​ Fibers with myelin sheaths conduct
nerve impulses more quickly

●​ Repolarization
○​ Membrane permeability changes
again—becoming impermeable to
sodium ions and permeable to
potassium ions
○​ Potassium ions rapidly diffuse out
of the neuron, repolarizing the
membrane
○​ Repolarization involves restoring
the inside of the membrane to a
negative charge and the outer
surface to a positive charge
 Transmission of the signal at synapses Reflexes

1.​ When the action potential reaches the ●​ Are rapid, predictable, and involuntary
axon terminal, the electrical charge opens responses to stimuli
calcium channels ●​ Reflexes occur over neural pathways called
2.​ Calcium, in turn, causes the tiny vesicles reflex arcs
containing the neurotransmitter chemical ●​ Somatic reflexes
to fuse with the axonal membrane ○​ Reflexes that stimulate the skeletal
3.​ The entry of calcium into the axon terminal muscles
causes pore like openings to form, releasing ○​ Involuntary, although skeletal
the neurotransmitter into the synaptic muscle is normally under voluntary
cleft control
4.​ The neurotransmitter molecules diffuse ○​ Example: pulling your hand away
across the synaptic cleft and bind to from a hot object
receptors on the membrane of the next ●​ Autonomic reflexes
neuron ○​ Regulate the activity of smooth
5.​ If enough neurotransmitter is released, a muscles, the heart, and glands
graded potential will be generated ○​ Example: regulation of smooth
a.​ Eventually an action potential (nerve muscles, heart and blood pressure,
impulse) will occur in the neuron glands, digestive system
beyond the synapse
6.​ The electrical changes prompted by ●​ Five elements of a reflex arc
neurotransmitter binding are brief 1.​ Sensory receptor—reacts to a stimulus
a.​ The neurotransmitter is quickly 2.​ Sensory neuron—carries message to the
removed from the synapse either by integration center
reuptake or by enzymatic activity ▪ 3.​ Integration center (CNS)—processes
Transmission of an impulse is information and directs motor output
electrochemical 4.​ Motor neuron—carries message to an
b.​ Transmission down neuron is effector
electrical 5.​ 5. Effector organ—is the muscle or gland
c.​ Transmission to next neuron is to be stimulated
chemical

Central Nervous System

Brain (Functional Anatomy)

Cerebral hemispheres Diencephalon

●​ Are paired (left and right) superior parts ●​ Sits on top of the brain stem
of the brain ●​ Enclosed by the cerebral hemispheres
●​ Include more than half of the brain mass ●​ Made of three structures
●​ The surface is made of ridges (gyri) and ○​ Thalamus
grooves (sulci) ■​ Encloses the third ventricle
●​ Fissures are deeper grooves ■​ Relay station for sensory
●​ Lobes are named for the cranial bones that impulses passing upward to
lie over them the cerebral cortex
●​ Three main regions of cerebral hemisphere ■​ Transfers impulses to the
○​ Cortex is superficial gray matter correct part of the cortex
○​ White matter for localization and
○​ Basal nuclei are deep pockets of interpretation
gray matter ○​ Hypothalamus
■​ Makes up the floor of the
●​ Cerebral cortex diencephalon
○​ Primary somatic sensory area ■​ Important autonomic nervous
○​ Located in parietal lobe posterior to system center
central sulcus ■​ Regulates body temperature
○​ Receives impulses from the body’s ■​ Regulates water balance
sensory receptors ■​ Regulates metabolism
■​ Pain, temperature, light ■​ Houses the limbic center for
touch (except for special emotions
senses) ■​ Regulates the nearby
○​ Sensory homunculus is a spatial map pituitary gland
○​ Left side of the primary somatic ■​ Houses mammillary bodies
sensory area receives impulses from for olfaction (smell)
right side (and vice versa) ○​ Epithalamus
○​ Cerebral areas involved in special ■​ Forms the roof of the third
senses ventricle
■​ Visual area (occipital lobe) ■​ Houses the pineal body (an
■​ Auditory area (temporal endocrine gland)
lobe) ■​ Includes the choroid
■​ Olfactory area (temporal plexus—forms cerebrospinal
lobe) fluid
○​ Broca’s area (motor speech area)
■​ Involved in our ability to
speak
■​ Usually in left hemisphere
○​ Other specialized areas
■​ Anterior association area
(frontal lobe)
■​ Posterior association area
(posterior cortex)
■​ Speech area (for sounding
out words)

●​ Basal Nuclei
○​ “Islands” of gray matter buried
deep within the white matter of the
cerebrum
○​ Regulate voluntary motor activities
by modifying instructions sent to
skeletal muscles by the primary
motor cortex
 Brain Stem Cerebrum

●​ Attaches to the spinal cord ●​ Two hemispheres with convoluted surfaces

●​ Parts of the brain stem ●​ Outer cortex of gray matter and inner
○​ Midbrain region of white matter
■​ Extends from the ●​ Controls balance
mammillary bodies to the ●​ Provides precise timing for skeletal muscle
pons inferiorly activity and
■​ Cerebral aqueduct (tiny ●​ coordination of body movements
canal) connects the third and ●​
fourth ventricles
■​ Two bulging fiber tracts,
cerebral peduncles, convey
ascending and descending
impulses
■​ Four rounded protrusions,
corpora quadrigemina, are
visual and auditory reflex
centers
○​ Pons
■​ The rounded structure
protruding just below the
midbrain
■​ Mostly composed of fiber
tracts
■​ Includes nuclei involved in
the control of breathing
○​ Medulla oblongata
 ■​ The most inferior part of
the brain stem that merges
into the spinal cord
■​ Includes important fiber
tracts
■​ Contains important centers
that control:
●​ Heart rate
●​ Blood pressure
●​ Breathing
●​ Swallowing
●​ Vomiting
■​ Fourth ventricle lies
posterior to pons and medulla
○​ Reticular formation
■​ Diffuse mass of gray matter
along the brain stem
■​ Involved in motor control of
visceral organs
■​ Reticular activating system
(RAS)
●​ Plays a role in
awake/sleep cycles
and consciousness
●​ Filter for incoming
sensory information

Protection of the Central Nervous System

Meninges Cerebrospinal Fluid Blood-brain barrier

●​ Dura mater ●​ Similar to blood plasma in ●​ Includes the least

○​ Outermost composition permeable capillaries of
leathery layer ●​ Formed continually by the the body
○​ Double-layered choroid plexuses ●​ Allows water, glucose, and
external covering ○​ Choroid amino acids to pass
■​ Periosteum plexuses—capillari through the capillary
—attached es in the walls
to inner ventricles of the ●​ Excludes many potentially
surface of brain harmful substances from
the skull ●​ CSF forms a watery entering the brain, such
■​ Meningeal cushion to protect the as wastes
layer—oute brain and spinal cord ●​ Useless as a barrier
r covering ●​ Circulated in the against some substances
of the arachnoid space,
brain ventricles, and central
○​ Folds inward in canal of the spinal cord
several areas ○​ Cerebrospinal fluid
■​ Falx circulation
cerebri ○​ CSF is produced
■​ Tentorium by the choroid
cerebelli plexus of each
●​ Arachnoid layer ventricle
○​ Middle layer ○​ CSF flows through
○​ Weblike the ventricles and
extensions span into the
the subarachnoid subarachnoid
 space to attach it space via the
to the pia mater median and lateral
○​ Subarachnoid apertures. Some
space is filled with CSF flows through
cerebrospinal fluid the central canal
○​ Arachnoid of the spinal cord
granulations ○​ CSF flows through
protrude through the subarachnoid
the dura mater space
and absorb ○​ CSF is absorbed
cerebrospinal fluid into the dural
into venous blood venous sinuses via
●​ Pia mater the arachnoid villi
○​ Internal layer
○​ Clings to the
surface of the
brain and spinal
cord

Brain Dysfunctions

Traumatic Brain Injuries Cerebrovascular accident Transient ischemic attack

(CVA)/ Stroke (TIA)

●​ Concussion ●​ Results when blood ●​ Temporary brain ischemia

○​ Slight brain injury circulation to a brain area (restriction of blood
○​ Typically little is blocked and brain flow)
permanent brain tissue dies ●​ Numbness, temporary
damage occurs ●​ Loss of some functions or paralysis, impaired
●​ Contusion death may result speech
○​ Marked nervous ○​ Hemiplegia—one-si
tissue destruction ded paralysis
occurs ○​ Aphasia—damage
○​ Coma may occur to speech center
●​ Death may occur after in left hemisphere
head blows due to:
○​ Intracranial
hemorrhage
○​ Cerebral edema

Spinal Cord

Gray matter of the spinal cord White matter of the spinal

and spinal roots cord

●​ Extends from the ●​ Internal gray matter is ●​ Composed of myelinated

foramen magnum of the mostly cell bodies fiber tracts
skull to the first or ●​ Dorsal (posterior) horns ●​ Three regions: dorsal,
second lumbar vertebra house interneurons lateral, ventral columns
●​ Cauda equina is a ○​ Receive ●​ Sensory (afferent) tracts
collection of spinal nerves information from conduct impulses toward
at the inferior end sensory neurons in brain
●​ Provides a two-way the dorsal root; ●​ Motor (efferent) tracts
conduction pathway to cell bodies housed carry impulses from brain
and from the brain in dorsal root to skeletal muscles
●​ 31 pairs of spinal nerves ganglion
arise from the spinal cord ●​ Anterior (ventral) horns
house motor neurons of
the somatic (voluntary)
nervous system
○​ Send information
out ventral root
●​ Gray matter surrounds
the central canal, which is
filled with cerebrospinal
fluid

Peripheral Nervous System (PNS)

●​ PNS consists of nerves and ganglia outside the CNS

Structure of a Nerve Cranial Nerves Spinal Nerves

●​ Nerves are bundles of ●​ 12 pairs of nerves serve ●​ Spinal nerves

neurons found outside mostly the head and neck ●​ 31 pairs
the CNS ●​ Only the pair of vagus ●​ Formed by the
●​ Endoneurium is a nerves extends to combination of the
connective tissue sheath thoracic and abdominal ventral and dorsal roots
that surrounds each fiber cavities of the spinal cord
●​ Perineurium wraps groups ●​ Most are mixed nerves, ●​ Named for the region of
of fibers bound into a but three are sensory the spinal cord from
fascicle only which they arise
●​ Epineurium binds groups ○​ Optic ●​ Spinal nerves divide soon
of fascicles ○​ Olfactory after leaving the spinal
●​ Mixed nerves ○​ Vestibulocochlear cord into a dorsal ramus
○​ Contain both Mnemonic Device and a ventral ramus
sensory and motor ●​ Oh – Olfactory ○​ Ramus—branch of
fibers ●​ Oh – Optic a spinal nerve;
●​ Sensory (afferent) ●​ Oh – Oculomotor contains both
nerves ●​ To – Trochlear motor and sensory
○​ Carry impulses ●​ Touch – Trigeminal fibers
toward the CNS ●​ And – Abducens ○​ Dorsal rami—serve
●​ Motor (efferent) nerves ●​ Feel – Facial the skin and
○​ Carry impulses ●​ Very – Vestibulocochlear muscles of the
away from the ●​ Green – Glossopharyngeal posterior trunk
CNS ●​ Vegetables – Vagus ○​ Ventral rami
●​ A – Accessory (T1–T12) —form
●​ H – Hypoglossal the intercostal
nerves that supply
muscles and skin
of the ribs and
trunk
○​ Ventral rami
(except
T1–T12)—form a
complex of
networks (plexus)
for the anterior
●​ Plexus—networks of
nerves serving motor and
sensory needs of the
 limbs
●​ Form from ventral rami
of spinal nerves in the
cervical, lumbar, and
sacral regions
●​ Four plexuses
○​ Cervical
○​ Brachial
○​ Lumbar
○​ Sacral

Somatic and Autonomic Nervous Systems Compared

Somatic Autonomic

●​ Motor neuron cell bodies originate inside ●​ Chain of two motor neurons
the CNS ○​ Preganglionic neuron is in the brain
●​ Axons extends to skeletal muscles that are or spinal cord
served ○​ Postganglionic neuron extends to
the organ
●​ Has two arms
●​ Sympathetic division
○​ AKA Thoracolumbar division
○​ Preganglionic neurons originate from
T1 through L2
■​ Axons pass through a ramus
communicans to enter a
sympathetic trunk ganglion
■​ Sympathetic trunk, or chain,
lies near the spinal cord
○​ After synapsing at the ganglion, the
axon may synapse with a second
neuron at the same or different
level
○​ Or, the preganglionic neuron may
pass through the ganglion without
synapsing and form part of the
splanchnic nerves
■​ Splanchnic nerves travel to
the collateral ganglion
■​ Collateral ganglia serve the
abdominal and pelvic organs
●​ Parasympathetic division
○​ AKA Craniosacral division
○​ Preganglionic neurons originate in:
○​ Cranial nerves III, VII, IX, and X
○​ S2 through S4 regions of the spinal
cord
○​ Preganglionic neurons synapse with
terminal ganglia; from there,
postganglionic axons extend to
organs that are served
 Autonomic Functioning
●​ Body organs served by the autonomic nervous system receive fibers from both divisions
●​ Exceptions: blood vessels, structures of the skin, some glands, and the adrenal medulla
●​ These exceptions receive only sympathetic fibers
●​ When body divisions serve the same organ, they cause antagonistic effects due to different
neurotransmitters
○​ Parasympathetic (cholinergic) fibers release acetylcholine
○​ Sympathetic postganglionic (adrenergic) fibers release norepinephrine
○​ Preganglionic axons of both divisions release acetylcholine
●​ Sympathetic—“fight or flight” division
●​ Response to unusual stimulus when emotionally or physically stressed or threatened
●​ Takes over to increase activities
●​ Remember as the “E” division
○​ Exercise
○​ Excitement
○​ Emergency
○​ Embarrassment
●​ Parasympathetic—“housekeeping” activites
●​ “Rest-and-digest” system
●​ Conserves energy
●​ Maintains daily necessary body functions
●​ Remember as the “D” division
○​ Digestion
○​ Defecation
○​ Diuresis
 Developmental Aspects of the Nervous System
●​ The nervous system is formed during the first month of embryonic development
●​ Any maternal infection can have extremely harmful effects
●​ Oxygen deprivation destroys brain cells
●​ The hypothalamus is one of the last areas of the brain to develop
●​ Severe congenital brain diseases include:
○​ Cerebral palsy
○​ Anencephaly
○​ Hydrocephalus
○​ Spina bifida

●​ Premature babies have trouble regulating body temperature because the hypothalamus is one of
the last brain areas to mature prenatally
●​ Development of motor control indicates the progressive myelination and maturation of a child’s
nervous system
●​ Brain growth ends in young adulthood. Neurons die throughout life and are not replaced; thus,
brain mass declines with age
●​ Orthostatic hypotension is low blood pressure due to changes in body position
●​ Healthy aged people maintain nearly optimal intellectual function
●​ Disease—particularly cardiovascular disease—is the major cause of declining mental function with
age
○​ Arteriosclerosis is decreased elasticity of blood vessels