198 Introduction to Anatomy and Physiology
circumstances, such as when we inadvertently
touch a hot surface, the efferent neurons can
trigger involuntary action of the skeletal muscles
through a refl ex arc. The autonomic nervous
system includes sympathetic and parasympathetic
branches, which you will learn about in
Lesson 6.4.
Becoming aware of these various
subdivisions of the nervous system will help you
learn and understand the different functional
capabilities of the nervous system. Keep in mind,
however, that the nervous system as a whole is a
single, remarkably coordinated, functioning unit.
Nervous Tissues
Two categories of tissues exist within the
nervous system. These include specialized
supporting cells called neuroglia and neurons.
Neuroglia
The neuroglia (ner-ROHG-lee-a), also
known as glial (GLIGH-al) cells, are a category of
specialized cells that perform support functions
(Figure 6.2). Within the CNS are four types of
glial cells:
• Astrocytes (AS-troh-sights) are positioned
between neurons and capillaries. Astrocytes
link the nutrient-supplying capillaries
to neurons and control the chemical
environment to protect the neurons from
any harmful substances in the blood. The
astrocytes are so numerous that they account
for nearly half of all neural tissue.
• Microglia (migh-KROHG-lee-a) absorb and
dispose of dead cells and bacteria.
1. Which structures make up the central
nervous system (CNS)?
2. Which structures make up the peripheral
nervous system (PNS)?
3. Which function is the somatic nervous
system responsible for?
4. Which functions is the autonomic
nervous system responsible for?
Check Your Understanding
• Ependymal (eh-PEHN-di-mal) cells form a
protective covering around the spinal cord
and central cavities within the brain.
• Oligodendrocytes (AHL-i-goh-DEHN-droh-
sights) wrap around nerve fi bers and produce
a fatty insulating material called myelin.
The PNS includes two forms of glial cells:
• Schwann (shwahn) cells form the fatty myelin
sheaths around nerve fi bers in the PNS.
• Satellite cells serve as cushioning support
cells.
Neurons
The glial cells provide support and
protection for the nervous system, but it is the
neurons that transmit information in the form of
nerve impulses throughout the body. A typical
neuron, or nerve cell, consists of a cell body
surrounded by branching dendrites. The typical
neuron also has a long, tail-like projection called
an axon (Figure 6.3).
The cell body includes a nucleus and
mitochondria, like all cell bodies, as described in
chapter 2. The dendrites (DEHN-drights) collect
stimuli and transport them to the cell body.
Axons (AK-sahns) transmit impulses away from
the cell body.
Within the PNS, the Schwann cells wrap
around the axon, covering most of it with a
fatty myelin (MIGH-eh-lin) sheath. The myelin
sheaths serve an important purpose: insulating
the axon fi bers, which increases the rate of neural
impulse transmission.
The external covering of the Schwann
cell, outside the myelin sheath, is called the
neurilemma (NOO-ri-LEHM-a). The uninsulated
gaps, where the axon is exposed between the
Schwann cells, are known as the nodes of
Ranvier (rahn-vee-AY). The myelin sheaths are
white, giving rise to the term white matter to
describe tracts of myelinated fi bers within the
CNS. Gray matter is the term for unmyelinated
nerve fi bers.
At the terminal end of each axon, there
can be up to thousands of axon terminals
that connect with other neurons or muscles
(Figure 6.3). The axon terminals are fi lled with