Chapter 11
Fundamentals of the Nervous System and Nervous Tissue
387
11
We begin this chapter with a brief overview of the functions
and organization of the nervous system. Ten we focus on the
functional anatomy of nervous tissue, especially the nerve cells,
or
neurons
, which are the key to neural communication.
Functions and Divisions
of the Nervous System
List the basic functions of the nervous system.
Explain the structural and functional divisions of the
nervous system.
Te nervous system has three overlapping functions, illustrated
by the example of a thirsty person seeing and then liFing a glass
of water
(Figure 11.1)
:
1.
Sensory input.
Te nervous system uses its millions of
sensory receptors to monitor changes occurring both in-
side and outside the body. Te gathered information is
called
sensory input
.
2.
Integration.
Te nervous system processes and interprets
sensory input and decides what should be done at each
moment—a process called
integration
.
3.
Motor output.
Te nervous system activates
effector
organs
—the muscles and glands—to cause a
response
,
called
motor output.
Here’s another example: You are driving and see a red light
ahead (sensory input). Your nervous system integrates this in-
formation (red light means “stop”), and your foot goes for the
brake (motor output).
We have only one highly integrated nervous system. ±or conve-
nience, it is divided into two principal parts,
central
and
peripheral
.
Te
central nervous system (CNS)
consists of the
brain
and
spinal cord
, which occupy the dorsal body cavity. Te CNS is the
integrating and control center of the nervous system. It inter-
prets sensory input and dictates motor output based on reflexes,
current conditions, and past experience
(Figure 11.2)
.
Te
peripheral nervous system (PNS)
is the part of the
nervous system
outside
the CNS. Te PNS consists mainly of
the nerves (bundles of axons) that extend from the brain and
spinal cord.
Spinal nerves
carry impulses to and from the spinal
cord, and
cranial nerves
carry impulses to and from the brain.
Tese peripheral nerves serve as communication lines that link
all parts of the body to the CNS.
Te PNS has two functional subdivisions, as ±igure 11.2
shows. Te
sensory
, or
afferent
,
division
(af
9
er-ent; “carrying
toward”) consists of nerve fibers (axons) that convey impulses
to
the central nervous system from sensory receptors located
throughout the body (see the blue fibers in ±igure 11.2).
Somatic sensory
fibers
convey impulses from the skin, skel-
etal muscles, and joints (
soma
5
body)
Visceral sensory fibers
transmit impulses from the visceral
organs (organs within the ventral body cavity)
Te sensory division keeps the CNS constantly informed of
events going on both inside and outside the body.
Te
motor
, or
efferent
,
division
(ef
9
er-ent; “carrying
away”) of the PNS transmits impulses
from
the CNS to effector
organs, which are the muscles and glands (see the red fibers in
±igure 11.2). Tese impulses activate muscles to contract and
glands to secrete. In other words, they
effect
(bring about) a
motor response.
Te motor division also has two main parts:
Te
somatic nervous system
is composed of somatic motor
nerve fibers that conduct impulses from the CNS to skeletal
muscles. It is oFen referred to as the
voluntary nervous system
because it allows us to consciously control our skeletal muscles.
Te
autonomic nervous system (ANS)
consists of visceral
motor nerve fibers that regulate the activity of smooth mus-
cles, cardiac muscles, and glands.
Autonomic
means “a law
unto itself,” and because we generally cannot control such
activities as the pumping of our heart or the movement of
food through our digestive tract, the ANS is also called the
involuntary nervous system
.
As we will describe in Chapter 14, the ANS has two func-
tional subdivisions, the
sympathetic division
and the
parasym-
pathetic division
. ²ypically these divisions work in opposition
to each other—whatever one stimulates, the other inhibits.
Check Your Understanding
1.
What is meant by “integration,” and does it primarily occur
in the CNS or the PNS?
2.
Which subdivision of the PNS is involved in (a) relaying the
feeling of a “full stomach” after a meal, (b) contracting the
muscles to lift your arm, and (c) increasing your heart rate?
For answers, see Appendix H.
Histology of Nervous Tissue
Te nervous system consists mostly of nervous tissue, which is
highly cellular. ±or example, less than 20% of the CNS is extra-
cellular space, which means that the cells are densely packed
Sensory input
Motor output
Integration
Figure 11.1
The nervous system’s functions.
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