The membranes of the Schwann cell are composed
largely of a lipid-protein called myelin, which forms a
myelin sheath on the outside of an axon. The myelin
sheath has gaps between adjacent Schwann cells called
nodes of Ranvier. Nerve cells without Schwann cells
also lack myelin and neurilemma sheaths.
IMPULSE TRANSMISSION
When dendrites receive a sufficiently strong
stimulus, a short and rapid change in electrical charge,
or polarity, of the neuron is triggered. Sodium ions rush
through the plasma membrane into the cell, potassium
ions leave, and an electrical impulse is formed, which
is conducted toward the cyton. The cyton receives the
impulse and transmits it to the terminal filaments of the
axon. At this point a chemical transmitter such as
acetylcholine is released into the synapse, a space
between the axon of the activated nerve and the
dendrite receptors of another neuron. This chemical
transmitter activates the next nerve. In this manner, the
impulse is passed from neuron to neuron down the
nerve line to a central area at approximately the speed
of a bullet.
Almost immediately after being activated, the
chemical transmitter in the synapse is neutralized by
the enzyme acetylcholinesterase, and the first neuron
returns to its normal state by pumping out the sodium
ions and drawing potassium ions back in through the
plasma membrane. When these actions are completed,
the nerve is ready to be triggered again. A particularly
strong stimulus will cause the nerve to fire in rapid
succession, or will trigger many other neurons, thus
giving a feeling of intensity to the perceived sensation.
NERVES
A nerve is a cordlike bundle of nerve fibers held
together with connective tissue. Each nerve fiber is an
extension of a neuron. Nerves that conduct impulses
into the brain or the spinal cord are called sensory
nerves, and those that carry impulses to muscles and
glands are termed motor nerves. Most nerves,
however, include both sensory and motor fibers, and
they are called mixed nerves.
CENTRAL NERVOUS SYSTEM
The central nervous system (CNS) consists of the
brain and spinal cord. The brain is almost entirely
enclosed in the skull, but it is connected with the spinal
cord, which lies in the canal formed by the vertebral
column.
Brain
The brain has two main divisions, the cerebrum
and the cerebellum. The cerebrum is the largest and
most superiorly situated portion of the brain. It
occupies most of the cranial cavity. The outer surface is
called the cortex. This portion of the brain is also
called gray matter because the nerve fibers are
unmyelinated (not covered by a myelin sheath),
causing them to appear gray. Beneath this layer is the
medulla, often called the white matter of the brain
because the nerves are myelinated (covered with a
myelin sheath), giving them their white appearance.
CEREBRUM.The cortex of the cerebrum is
irregular in shape. It bends on itself in folds called
convolutions, which are separated from each other by
grooves, also known as fissures. The deep sagittal
cleft, a longitudinal fissure, divides the cerebrum into
two hemispheres. Other fissures further subdivide the
cerebrum into lobes, each of which serves a localized,
specific brain function (fig. 1-43). For example, the
frontal lobe is associated with the higher mental
processes such as memory, the parietal lobe is
concerned primarily with general sensations, the
occipital lobe is related to the sense of sight, and the
temporal lobe is concerned with hearing.
CEREBELLUM.The cerebellum is situated
posteriorly to the brain stem (which is made up of the
pons, mid-brain, and medulla oblongata) and inferior
to the occipital lobe. The cerebellum is concerned
chiefly with bringing balance, harmony, and
coordination to the motions initiated by the cerebrum.
PONS AND MEDULLA OBLONGATA. Two
smaller divisions of the brain vital to life are the pons
and the medulla oblongata. Together, the pons and
medulla form the brain stem (fig. 1-43). The pons
consists chiefly of a mass of white fibers connecting
the other three parts of the brain (the cerebrum,
cerebellum, and medulla oblongata).
The medulla oblongata is the inferior portion of the
brain, the last division before the beginning of the
spinal cord. It connects to the spinal cord at the upper
level of the first cervical vertebra (C-1). In the medulla
oblongata are the centers for the control of heart action,
breathing, circulation, and other vital processes such
as blood pressure.
MENINGES.The outer surface of the brain and
spinal cord is covered with three layers of membranes
called the meninges. The dura mater is the strong
outer layer; the arachnoid membrane is the delicate
1-37