8.

1 Functions of the Nervous System

The functions of the nervous system include receiving sensory input, integrating information, controlling
muscles and glands, maintaining homeostasis, and serving as the center of mental activity.
8.2 Divisions of the Nervous System
1. The central nervous system (CNS) consists of the brain and spinal cord, whereas
the peripheral nervous system (PNS) consists of all the nervous tissue outside the CNS.
2. The sensory division of the PNS transmits action potentials to the CNS; the motor division carries
action potentials away from the CNS.
3. The motor division is divided into somatic and autonomic systems. The somatic motor system
innervates skeletal muscle and is mostly under voluntary control. The autonomic nervous system
innervates cardiac muscle, smooth muscle, and glands and is mostly under involuntary control.
4. The autonomic nervous system is divided into sympathetic and parasympathetic divisions.
5. The enteric nervous system contains both sensory and motor neurons, which can function
independently of the CNS.
8.3 Cells of the Nervous System
Neurons
1. Neurons receive stimuli and conduct action potentials. A neuron consists of a cell body, dendrites, and
an axon.
2. Neurons are multipolar, bipolar, or pseudo-unipolar.
Glial Cells
Glial cells are the support cells of the nervous system. They include astrocytes, ependymal cells,
microglia, oligodendrocytes, and Schwann cells.
Myelin Sheaths
Axons are either unmyelinated or myelinated.
Organization of Nervous Tissue
Nervous tissue consists of white matter and gray matter. Gray matter forms the cortex and nuclei in the
brain and ganglia in the PNS. White matter forms nerve tracts in the CNS and nerves in the PNS.
8.4 Electrical Signals and Neural Pathways
Resting Membrane Potential
1. A resting membrane potential results from the charge difference across the
membrane of cells.
2. The resting membrane potential is set by leak ion channels and the sodiumpotassium pump.
Neuron Communication
1. The process of neuron communication can be described in three parts: (1)
generation of action potentials, (2) action potential propagation along the cell
membrane, (3) communication at the synapse.
2. An action potential occurs when the charge across the cell membrane is briefly
reversed.
3. Chemically gated and voltage-gated ion channels generate the action potential.
4. Continuous conduction occurs along unmyelinated axons.
5. Saltatory conduction occurs along myelinated axons.
6. A synapse is a point of contact between two neurons or between a neuron and another cell, such as a
muscle or gland cell.
7. An action potential arriving at the synapse causes the release of a neurotransmitter from the
presynaptic terminal, which diffuses across the synaptic cleft and binds to the receptors of the
postsynaptic membrane.
Neuronal Pathways
1. Neuronal pathways are either converging or diverging.
2. Spatial and temporal summation occur in neuronal pathways.
8.5 Central and Peripheral Nervous Systems
The CNS consists of the brain and spinal cord. The PNS consists of nerves and
ganglia outside the CNS.
8.6 Spinal Cord
1. The spinal cord extends from the foramen magnum to the second lumbar
vertebra; below that is the cauda equina.
2. The spinal cord has a central gray part organized into horns and a peripheral
white part forming nerve tracts.
3. Roots of spinal nerves extend out of the cord.
Reflexes
1. Reflex arcs are the functional units of the nervous system.
2. A complex reflex arc consists of a sensory receptor, a sensory neuron, interneurons, a motor neuron,
and an effector organ; the simplest reflex arcs do not involve interneurons.
3. The knee-jerk reflex occurs when the quadriceps femoris muscle is stretched.
4. The withdrawal reflex removes a body part from a painful stimulus.
8.7 Spinal Nerves
1. The spinal nerves exit the vertebral column at the cervical, thoracic, lumbar, and sacral regions.
2. The nerves are grouped into plexuses.
3. The phrenic nerve, which supplies the diaphragm, is the most important branch of the cervical plexus.
4. The brachial plexus supplies nerves to the upper limb.
5. The lumbosacral plexus supplies nerves to the lower limb.
8.8 Brain
Brainstem
1. The brainstem contains several nuclei.
2. The medulla oblongata contains nuclei that control activities such as heart rate,
breathing, swallowing, and balance.
3. The pons contains relay nuclei between the cerebrum and cerebellum.
4. The midbrain is involved in hearing and in visual reflexes.
5. The reticular formation is scattered throughout the brainstem and is important in
regulating cyclical motor functions. It is also involved in maintaining consciousness and in the sleep-
wake cycle.
Cerebellum
The cerebellum is attached to the brainstem.
Diencephalon
The diencephalon consists of the thalamus (main sensory relay center), the epithalamus (the pineal
gland may play a role in sexual maturation), and the hypothalamus (important in maintaining
homeostasis).
Cerebrum
The cerebrum has two hemispheres, each divided into lobes: the frontal, parietal, occipital, and
temporal lobes.
8.9 Sensory Functions
1. The CNS constantly receives sensory input.
2. We are unaware of much of the input, but it is vital to our survival.
3. Some sensory input results in sensation.
Ascending Tracts
1. Ascending tracts transmit action potentials from the periphery to the brain.
2. Each tract carries a specific type of sensory information.
Sensory Areas of the Cerebral Cortex
1. Ascending tracts project to primary sensory areas of the cerebral cortex.
2. Association areas are involved in recognizing the sensory input.
8.10 Somatic Motor Functions
1. Somatic motor functions include involuntary and voluntary movements.
2. Upper motor neurons in the cerebral cortex connect to lower motor neurons in
the spinal cord or cranial nerve nuclei.
Motor Areas of the Cerebral Cortex
1. Upper motor neurons are located in the primary motor cortex.
2. The premotor and prefrontal areas regulate movements.
Descending Tracts
Descending tracts project directly from upper motor neurons in the cerebralcortex to lower motor
neurons in the spinal cord and brainstem; indirectly, they project from basal nuclei, the cerebellum, or
the cerebral cortex through the brainstem to lower motor neurons in the spinal cord.
Basal Nuclei
1. Basal nuclei help plan, organize, and coordinate motor movements and posture.
2. People with basal nuclei disorders exhibit increased muscle tone and exaggerated, uncontrolled
movements when at rest.
Cerebellum
1. The cerebellum is involved in balance, muscle tone, and muscle coordination.
2. Through its comparator function, the cerebellum compares the intended action to what is occurring
and modifies the action to eliminate differences.
3. If the cerebellum is damaged, muscle tone decreases and fine motor movements become very
clumsy.
8.11 Other Brain Functions
Communication Between the Right and Left Hemispheres
1. Each hemisphere controls the opposite half of the body.
2. Commissures connect the two hemispheres.
3. The left hemisphere is thought to be the dominant analytical hemisphere, and the right hemisphere is
thought to be dominant for spatial perception and musical ability.
Speech
Speech involves the sensory speech area, the motor speech area, and the interactions between them
and other cortical areas.
Brain Waves and Consciousness
An EEG monitors brain waves, which are a summation of the electrical activity of the brain.

Memory
1. The types of memory are working (lasting a few seconds to minutes), short-term (lasting a few
minutes), and long-term (permanent) memory.
2. Long-term memory includes declarative and procedural memories.
Limbic System and Emotions
1. The limbic system includes the olfactory cortex, deep cortical regions, and nuclei.
2. The limbic system is involved with memory, motivation, mood, and other visceral functions. Olfactory
stimulation is a major influence.
8.12 Meninges, Ventricles, and Cerebrospinal Fluid
Meninges
Three connective tissue meninges cover the CNS: the dura mater, arachnoid mater, and pia mater.
Ventricles
1. The brain and spinal cord contain fluid-filled cavities: the lateral ventricles in the cerebral
hemispheres, a third ventricle in the diencephalon, a cerebral aqueduct in the midbrain, a fourth
ventricle at the base of the cerebellum, and a central canal in the spinal cord.
2. The fourth ventricle has openings into the subarachnoid space.
Cerebrospinal Fluid
Cerebrospinal fluid is formed in the choroid plexuses in the ventricles. It exits through the fourth
ventricle and reenters the blood through arachnoid granulations in the superior sagittal sinus.
8.13 Cranial Nerves
1. There are 12 pairs of cranial nerves: 3 with only sensory function (S), 4 with only somatic motor
function (M), 1 with somatic motor (M) and sensory function (S), 1 with somatic motor and
parasympathetic (P) function, and 3 with all three functions. Four of the cranial nerves have
parasympathetic function.
2. The cranial nerves are olfactory (I; S), optic (II; S), oculomotor (III; M, P), trochlear (IV; M), trigeminal
(V; S, M), abducens (VI; M), facial (VII; S, M, P), vestibulocochlear (VIII; S), glossopharyngeal (IX; S, M, P),
vagus (X; S, M, P), accessory (XI; M), and hypoglossal (XII; M).
8.14 Autonomic Nervous System
1. The autonomic nervous system contains preganglionic and postganglionic neurons.
2. The autonomic nervous system has sympathetic and parasympathetic divisions.
Anatomy of the Sympathetic Division
1. Preganglionic cell bodies of the sympathetic division lie in the thoracic and upper lumbar regions of
the spinal cord.
2. Postganglionic cell bodies are located in the sympathetic chain ganglia or in collateral ganglia.
Anatomy of the Parasympathetic Division
1. Preganglionic cell bodies of the parasympathetic division are associated with some of the cranial and
sacral nerves.
2. Postganglionic cell bodies are located in terminal ganglia, either near or within target organs.
Autonomic Neurotransmitters
1. All autonomic preganglionic and parasympathetic postganglionic neurons secrete acetylcholine.
2. Most sympathetic postganglionic neurons secrete norepinephrine.
Functions of the Autonomic Nervous System
1. The sympathetic division prepares a person for action by increasing heart rate, blood pressure,
respiration, and release of glucose for energy.
2. The parasympathetic division is involved in involuntary activities at rest, such as the digestion of food,
defecation, and urination.
8.15 Enteric Nervous System
1. The enteric nervous system forms plexuses in the digestive tract wall.
2. Enteric neurons are sensory, motor, or interneurons; they receive CNS input but can also function
independently.