Medical Terminology Simplified

418 CHAPTER 9• Endocrine and Nervous Systems

and the spinal nerves (emerging from the spinal cord). The PNS connects the CNS to remote body parts to relay and
receive messages, and its autonomic nerves regulate involuntary functions of the internal organs.
Despite the complex organization of the nervous system, it consists of only two principal types of cells, neurons
and neuroglia. Neurons are the basic structural and functional units of the nervous system. (See Fig. 9–7.) They are
specialized to respond to physical and chemical stimuli, conduct electrochemical impulses, and release specific chemical
regulators. Through these activities, neurons perform such functions as perceiving sensory stimuli, learning, remem-
bering, and controlling muscles and glands. Neuroglia do not carry impulses, but perform the functions of support
and protection. Many neuroglial (glial) cells form a supporting network by twining around nerve cells or lining certain
structures in the brain and spinal cord. Others bind nervous tissue to supporting structures and attach the neurons
to their blood vessels. Specialized glial cells are phagocytic. In other words, they protect the CNS from disease by
engulfing invading microbes and clearing away debris. Neuroglia are of clinical interest because they are a common
source of tumors (gliomas) of the nervous system.
As illustrated in Figure 9–7, the (1) dendrites are branching processes (extensions) that transmit impulses toward
the (2) cell body,which contains the (3) nucleus. The integrated signals in the cell body are then transmitted to the
(4) axon, a single projection which extends to the neuron’s target cell or tissue. Surrounding the axon is a fatty coating
called the (5) myelin sheath.The sheath insulates the axon and speeds the transmission of electrical impulses. Due
to their fatty coating, myelinated axons take on a whitish appearance. Because of this, groups of myelinated axons
constitute the “white matter” of the brain and spinal cord, while cell bodies of neurons that are not covered by a myelin
sheath appear gray and are referred to as the “gray matter.”
The axon of one neuron does not actually touch the dendrite of the next neuron. Instead, there is a space or
(6) synapse between the two neurons. The transmission of the impulse across the synapse depends on the release of a
chemical substance called a (7) neurotransmitter. The neurotransmitter crosses the synapse and binds to (8) receptor
sites on the dendrites of the next neuron. Ultimately this creates another electrical impulse that continues through
the next neuron until the target cell is reached. All of this occurs within a fraction of a second.

(2) Cell body

(3) Nucleus

(1) Dendrites

Stimulus begins

an impulse in the

branching fibers

of dendrites

(4) Axon

Axon terminal

(6) Synapse

(7) Neurotransmitter

(8) Receptor sites

(5) Myelin sheath

(4) Axon

A

B

Figure 9-7Neuron. (A)Myelin sheath.

(B)Axon terminal synapse.