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U N I T 1 0
Nervous System
Functionally, the nervous system is divided into two
systems: the
somatic
and
autonomic
nervous systems.
The somatic (Greek
soma,
“body”) nervous system
provides sensory and motor innervation for all parts of
the CNS and PNS except viscera, smooth muscle, and
glands. The autonomic nervous system (ANS) provides
efferent innervation to smooth muscle in blood vessels
and visceral structures, the conduction system of the
heart, the sweat glands, and the exocrine glands of the
gastrointestinal tract.
Nervous tissue consists of two principal types of
cells—neurons and neuroglia or supporting cells. The
neurons are the functional cells of the nervous system.
They exhibit membrane excitability and conductivity
and secrete neurotransmitters (signal-transmitting chem-
icals) and hormones, such as epinephrine and antidi-
uretic hormone (ADH). The neuroglial cells, such as the
Schwann cells in the PNS and the oligodendrocytes in
the CNS, protect the nervous system and provide meta-
bolic support for the neurons.
Neurons
A typical neuron has three distinct parts: a cell body,
dendrites, and an axon. The axonal and dendritic pro-
cesses form the functional connections, or synapses,
with other nerve cells, receptor cells, or effector cells.
Afferent, or sensory, neurons of the PNS transmit infor-
mation to the CNS (see Fig. 34-1A), whereas efferent, or
motor, neurons carry information away from the CNS
(see Fig. 34-1B). Interspersed between the afferent and
efferent neurons is a network of interconnecting neurons
(also called
interneurons
or
internuncial
neurons) that
modulate and control the body’s response to changes in
the internal and external environments.
The
cell body
of a neuron, also known as the
soma,
contains a large vesicular nucleus with one or more dis-
tinct nucleoli and a well-developed rough endoplasmic
reticulum. A neuron’s nucleus has the same deoxyri-
bonucleic acid (DNA) and genetic information that is
present in other cells of the body, and its nucleolus pro-
duces the ribonucleic acid (RNA) associated with pro-
tein synthesis. The cytoplasm contains large masses of
ribosomes that are prominent in most neurons. These
RNA masses, which are involved in protein synthesis,
stain as dark Nissl bodies (see Fig. 34-1B). The Nissl
bodies and free ribosomes extend into the dendrites, but
not into the axon. The area of the cell body, called the
axon hillock,
is free of large cytoplasmic organelles and
serves as a landmark to distinguish between axons and
dendrites in microscopic preparations.
The
dendrites
(from the Greek
dendron,
“tree”) are
multiple, branched extensions of the nerve cell body;
they conduct information toward the cell body and are
the main source of information for the neuron. The den-
drites and cell body are studded with synaptic terminals
that communicate with axons and dendrites of other
neurons (see Fig. 34-1A).
Axons
are long efferent processes that project from
the cell body and carry impulses away from the cell.
Most neurons have only one axon; however, axons may
exhibit multiple branching that results in many axonal
terminals. The cytoplasm of the cell body extends to fill
Dendrite
Axon
hillock
Initial segment
CNS
PNS
Axon
Myelin
sheath
Nucleus of
Schwann
cell
Schwann cell myelin
Neuromuscular
junction
Muscle fiber
Nucleus
Nucleolus
Nissl
bodies
PNS
CNS
Synaptic
terminals
Afferent cell body
in dorsal root ganglion
Free nerve
endings in skin
Node of Ranvier
Oligodendroglial
cell myelin
Oligodendroglial
cell myelin
Schwann cell
myelin
Nucleus
Nucleolus
Nissl bodies
A
B
FIGURE 34-1.
Afferent
(A)
and efferent neurons
(B)
, showing the soma or cell
body, dendrites, and axon.
Arrows indicate the direction for
conduction of action potentials.
CNS, central nervous system;
PNS, peripheral nervous system.
