Page 5 - Medical Terminology

M12_FREM9988_04_SE_C12.qxp 2/18/09 11:46 PM Page 390

390 Chapter 12

Anatomy and Physiology

of the Nervous System

brain nerves
central nervous system peripheral nervous system (per-IF-er-al)
cranial nerves (KRAY-nee-al) sensory receptors
glands spinal cord
muscles spinal nerves
Med Term Tip The nervous system is responsible for coordinating all the activity of the body.
To do this, it first receives information from both external and internal sensory re-
Neuroglial tissue received its name
as a result of its function. This tis- ceptors and then uses that information to adjust the activity of muscles and glands
sue holds neurons together. There- to match the needs of the body.
fore, it was called neuroglial, a term The nervous system can be subdivided into the central nervous system (CNS) and the
literally meaning “nerve glue.”
peripheral nervous system (PNS). The central nervous system consists of the brain and
spinal cord. Sensory information comes into the central nervous system, where it is
processed. Motor messages then exit the central nervous system carrying com-
mands to muscles and glands. The nerves of the peripheral nervous system are cranial
nerves and spinal nerves. Sensory nerves carry information to the central nervous sys-
tem, and motor nerves carry commands away from the central nervous system.
All portions of the nervous system are composed of nervous tissue.

Nervous Tissue

axon (AK-son) neuron (NOO-ron)
dendrites (DEN-drights) neurotransmitter (noo-roh-TRANS-mit-ter)
myelin (MY-eh-lin) synapse (sih-NAPSE)
nerve cell body synaptic cleft (sih-NAP-tik)
neuroglial cells (noo-ROH-glee-all)

Nervous tissue consists of two basic types of cells: neurons and neuroglial cells. Neu-
rons are individual nerve cells. These are the cells that are capable of conducting
electrical impulses in response to a stimulus. Neurons have three basic parts:
dendrites, a nerve cell body, and an axon (see Figure 12.1A ■). Dendrites are highly
branched projections that receive impulses. The nerve cell body contains the nu-
cleus and many of the other organelles of the cell (see Figure 12.1B ■). A neuron
has only a single axon, a projection from the nerve cell body that conducts the elec-
trical impulse toward its destination. The point at which the axon of one neuron
meets the dendrite of the next neuron is called a synapse. Electrical impulses can-
not pass directly across the gap between two neurons, called the synaptic cleft. They
instead require the help of a chemical messenger, called a neurotransmitter.
A variety of neuroglial cells are found in nervous tissue. Each has a different
support function for the neurons. For example, some neuroglial cells produce
myelin, a fatty substance that acts as insulation for many axons so that they con-
duct electrical impulses faster. Neuroglial cells do not conduct electrical impulses.

Central Nervous System

gray matter
tract
meninges (men-IN-jeez)
white matter
myelinated (MY-eh-lih-nayt-ed)
Because the central nervous system is a combination of the brain and spinal
cord, it is able to receive impulses from all over the body, process this informa-