Chapter 56: Nervous System
STRUCTURES AND FUNCTIONS
The human nervous system is a highly specialized system responsible for the control and
integration of the body’s many activities.
The nervous system can be divided into the central nervous system (CNS) and parts of the
peripheral nervous system (PNS).
o The central nervous system consists of the brain and spinal cord.
o The peripheral nervous system consists of the cranial and spinal nerves and the
autonomic nervous system (ANS).
The nervous system is made up of two types of cells: neurons and neuroglia.
o The neurons of the nervous system come in many different shapes and sizes, but they
all share common characteristics: (1) excitability, or the ability to generate a nerve
impulse; (2) conductivity, or the ability to transmit the impulse to other portions of the
cell; and (3) the ability to influence other neurons, muscle cells, and glandular cells by
transmitting nerve impulses to them.
o Neuroglia, or glial cells, provide support, nourishment, and protection to neurons.
Nerve impulses originate within a neuron as an action potential that moves along the body of
the cell (axon) until it reaches the end of the nerve fiber. From there, it is transmitted across the
junction between nerve cells by a chemical interaction and then, the impulse will move across
the next neuron as an action potential.
o A synapse is the structural and functional junction between two neurons. It is the point
at which the nerve impulse is transmitted from one neuron to another or from neuron
to glands or muscles.
o A neurotransmitter is a chemical agent involved in the transmission of an impulse
across the synaptic cleft.
CENTRAL NERVOUS SYSTEM
The major structural components of the CNS are the spinal cord and brain.
The spinal cord is continuous with the brainstem and exits from the cranial cavity through the
foramen magnum. A cross section of the spinal cord reveals gray matter that is centrally located
and is surrounded by white matter.
Specific ascending and descending pathways in the white matter can be identified. o In general, the ascending tracts carry specific sensory information to higher levels of the
o Descending tracts carry impulses that are responsible for muscle movement.
Lower motor neurons are the final common pathway through which descending motor tracts
influence skeletal muscle, the effector organ for movement. The cell bodies of these cells are
located in spinal cord and the axons innervate the skeletal muscles.
The brain consists of the cerebral hemispheres, cerebellum, and brainstem.
The cerebrum is composed of the right and left hemispheres. Both hemispheres can be further
divided into four major lobes.
o The frontal lobe controls higher cognitive function, memory retention, voluntary eye
movements, voluntary motor movement, and expressive speech.
o The temporal lobe contains Wernicke’s area, which is responsible for receptive speech
and for integration of somatic, visual, and auditory data.
o The parietal lobe is composed of the sensory cortex, controlling and interpreting spatial
o Processing of sight take place in the occipital lobe.
The basal ganglia, thalamus, hypothalamus, and limbic system are also located in the cerebrum.
The brainstem includes the midbrain, pons, and medulla. The vital centers concerned with
respiratory, vasomotor, and cardiac function are located in the medulla.
The brainstem contains the centers for sneezing, coughing, hiccupping, vomiting, sucking, and
swallowing. Also located in the brainstem is the reticular formation, which relays sensory
information, influences excitatory and inhibitory control of spinal motor neurons, and controls
vasomotor and respiratory activity.
The cerebellum coordinates voluntary movement and to maintains trunk stability and
Cerebrospinal fluid circulates within the subarachnoid space that surrounds the brain,
brainstem, and spinal cord. This fluid provides cushioning for the brain and spinal cord, allows
fluid shifts from the cranial cavity to the spinal cavity, and carries nutrients. PERIPHERAL NERVOUS SYSTEM
The PNS includes all the neuronal structures that lie outside the CNS. It consists of the spinal and
cranial nerves, their associated ganglia (groupings of cell bodies), and portions of the ANS.
The spinal nerve contains a pair of dorsal (afferent) sensory nerve fibers and ventral (efferent)
motor fibers, which innervate a specific region of the neck, trunk, or limbs. This combined
motor-sensory nerve is ca