The spinal cord, the body’s largest nerve, is the
thoroughfare of communication between the
brain and the body. It extends from the second
cervical vertebra to the second lumbar vertebra, a
distance of about 20 inches in an adult. The bones
of the spinal column, the vertebrae, enclose and
protect the spinal cord. The spinal cord controls
the reflexes of URINATIONand defecation as well as
of MUSCLEstretch (the automatic responses of mus-
cle cells that allow movement).
All nervous system structures and functions
outside the central nervous system belong to the
peripheral nervous system: the cranial nerves, the
spinal nerves, and their numerous branches. The
12 paired cranial nerves arise from the base of the
brain and the brainstem. They convey sensory and
motor signals to and from the structures of the
head and face, one of each pair going to each side.
The 31 paired spinal nerves branch from the
spinal cord at each vertebra, carrying sensory and
motor signals to and from the rest of the body.
Within the peripheral nervous system are two
main subdivisions: the somatic nervous system,
which handles voluntary functions such as move-
ment, and the autonomic nervous system, which
handles involuntary functions such as digestion.
The autonomic nervous system has two further
subdivisions. The sympathetic nervous system is
made up of the nerves that serve the structures of
the main trunk (thoracic and lumbar regions). The
parasympathetic nervous system is made up of the
nerves that serve the neck and head and the sacral
region of the trunk.
DIVISIONS OF THE NERVOUS SYSTEM
CENTRAL NERVOUS SYSTEM: BRAINand SPINAL CORD
PERIPHERAL NERVOUS SYSTEM: cranial nerves, SPINAL NERVES, and
their branches
- somatic NERVOUS SYSTEM: voluntary functions
- autonomic nervous system: involuntary functions
† sympathetic nervous system: main trunk
† parasympathetic nervous system: head and sacral region
Nervous system communication: neurons, ions,
and neurotransmitters The basic structure of
function in the nervous system is the neuron, a
specialized cell capable of sending and receiving
electrochemical impulses that initiate or inhibit
actions. What makes the neuron special are the
filaments that extend from its cell body. A single
such filament, the axon, extends from the cell
body to carry nerve impulses from the neuron.
From one to numerous other filamental processes,
the dendrites, branch from other sites on the cell
membrane to capture nerve impulses coming to
the neuron. Like electrical wires, neurons would
“short” if they came into contact with each other.
Microscopic channels—synapses—help neurons
keep safe distance from each other. Axons and
dendrites reach toward, but do not touch, each
other in the synapses.WHITE MATTER AND GRAY MATTER
A fatty substance, myelin, coats most axons to
insulate and protect them. The myelin gives the
axons a whitish color. NERVEtissue in the BRAIN
and SPINAL CORD, which is primarily a collection
of axons, is white matter. The cell bodies of neu-
rons are dark and grayish in color. Nerve tissue
in the brain and spinal cord that is primarily a
collection of NEURONcell bodies is gray matter.Electrically charged chemical molecules—
ions—are within and surround a neuron. Among
the significant ions, sodium, potassium, and cal-
cium are positive ions and chloride is a negative
ion. Microscopic channels in the neuron’s cell
membrane, called ion channels, selectively allow
ions to enter and leave the neuron. Each ion
channel is specific for an ion—that is, calcium ion
channels allow passage only of calcium ions and
sodium ion channels allow passage only of sodium
ions. When a neuron is at rest, the total charge of
the ions within its membrane is negative relative
to the ions outside its membrane. As well, there
are more potassium ions within the cell body and
more sodium ions outside the cell.
When a stimulus triggers an electrical impulse,
the first stage of neuronal communication, the
impulse causes sodium ion channels to open.
Sodium ions rush into the neuron cell body,
changing the neuron’s polarity to become positive
relative to the surrounding environment. The
electrical impulse rides the wave of polarity down
the axon of the sending neuron. About the time
the impulse reaches the presynaptic terminals at
the end of the axon, potassium ion channels open
and potassium enters the cell body. In exchange,216 The Nervous System