How the Nervous System Works

The nervous system is divided into the central and peripheral nervous systems.
The central nervous system is comprised of the brain and spinal cord. The periph-
eral nervous system contains the spinal nerves and peripheral nerves.
The basic component of the nervous system is the nerve cell or neuron. Aneuron
is composed of the nucleus (within the cell body), a dendrite, (which receives the sig-
nal), an axon (the extension of the cell that can pass on an impulse to the next nerve
cell), and the axon terminals (which can transmit the signals to other cells). The mes-
sages are sent from one nerve cell to another, crossing a synapse (or gap) between
cells. Neurotransmitters are chemicals released by the presynaptic neuron to enhance
the communication between nerve cells. There are specific receptor sites for the dif-
ferent neurotransmitters on the postsynaptic neuron. Electrically charged ions trans-
mit signals along the cell membranes of the nerve cells. A myelin coating on the
outer surface of the nerve cells helps to speed the transmission along the nerve cells.
This myelin coating also gives a white color to the nerve cells.
Some neurons are afferent neurons. They carry sensory information from the
peripheral areas of the body to the central nervous system. These neurons do not
have dendrites. Motor neurons that transmit information from the central nervous
system to the muscles or glands are efferent neurons.
The brain is protected within the skull. The outermost layer of the brain is the
cerebral cortex, made up primarily of neural cell bodies, giving a gray appearance.
The cerebral cortex is divided into right and left hemispheres and into frontal, pari-
etal, occipital, and temporal lobes. The frontal lobe has motor and pre-motor areas,
as well as Broca’s area, which controls speech articulation, behavior, moral decision-
making, and emotional outburst. The parietal area interprets sensory stimuli, pain,
and touch. The temporal lobe is involved in auditory processing, language interpreta-
tion (Wernicke’s area), and memory formation, and storage. The occipital lobe houses
the visual cortex. The diencephalon includes the thalamus, hypothalamus, and the basal
ganglia. The thalamus relays the sensory information from the body to the appropri-
ate part of the cerebral cortex. Descending messages from the cerebral cortex are
passed through the thalamus to the body. The hypothalamus controls neuroendocrine
function and maintains homeostasis, or constancy, within the body. The basal ganglia
control highly skilled movements that require precision without intentional thought.
The brainstem is comprised of the pons, medulla oblongata, and midbrain.
The spinal column is protected within the vertebral column. Both motor and
sensory fibers are found within the spinal column. Motor nerves are located along
the anterior horns and sensory nerves are located along the posterior horns of the
spinal column. The motor nerve fibers are more protected from traumatic injury
this way. If a patient sustains an external injury to the back that damages the spinal
column, the first area to be impacted will be the sensory nerves, hopefully main-
taining motor function. If enough damage has occurred, then both sensory and
motor function will be lost. Peripheral nerve fibers leave the spinal column to travel
to the rest of the body. Impulses travel from the central nervous system to muscle
fibers to control voluntary motion and involuntary function of organs. Impulses
are also sent from the body to the central nervous system for input.