132
UNIT 2
Basic Tissues
Figure 7-15A.
Sympathetic ganglion.
H&E,
3
272; inset
3
520
This section from a
sympathetic chain ganglion
shows small
to medium-sized visceromotor neuron cell bodies that give rise
to
postganglionic axons
. These neurons receive synapses from
preganglionic sympathetic axons
that originate in the lateral horn
of the thoracic and upper lumbar spinal cord. The
preganglionic
axons
are myelinated; the
postganglionic axons
are unmyeli-
nated. These
motor neurons
are
multipolar
(in contrast to the
unipolar sensory neurons in the posterior root ganglia), although
the dendrites are not visible in this H&E stain. The sizes of the
cell bodies are more uniform than in the sensory ganglia, and
the cell bodies and axons are distributed more evenly across the
ganglia rather than being grouped into clumps as in the sensory
ganglia. The
satellite cells
are not distributed so evenly around
the neuron cell
bodies as they are in the sensory ganglia.
Nucleolus
Nucleolus
Nucleus
Nucleus
Cytoplasm
Cytoplasm
Axons
Axons
Satellite
Satellite
cell nucleus
cell nucleus
Sympathetic
Sympathetic
motor neuron
motor neuron
Sympathetic
motor neuron
Axons
Satellite
cell nucleus
Cytoplasm
Nucleus
Nucleolus
A
Longitudinal smooth
Longitudinal smooth
muscle layer
muscle layer
Myenteric plexus
Myenteric plexus
Circular smooth
Circular smooth
muscle layer
muscle layer
Circular smooth
muscle layer
Myenteric plexus
P
S
Longitudinal smooth
muscle layer
P
S
B
Figure 7-15B.
Myenteric plexus (Auerbach plexus).
H&E,
3
136; inset
3
300
The enteric division lacks the discrete, encapsulated ganglia that
characterize the sympathetic division. Its visceromotor neurons
are distributed in a network of plexuses that are distributed
within the walls of the gastrointestinal tract. Most neurons in
the enteric division are found in the
myenteric
and
submucosal
plexuses
. The myenteric plexuses lie between the
circular smooth
muscle layer
and the
longitudinal smooth muscle layer
of the
intestine (see Chapter 15 “Digestive Tract”; see also Chapter 6,
“Muscle,” Fig. 6-10B). These plexuses are clusters of parasym-
pathetic postganglionic motor neurons; sensory neurons, which
receive input from chemoreceptors and mechanoreceptors in
the intestinal wall; and
local circuit neurons
(
interneurons
).
Interneurons can process neural signals within a plexus and can
also mediate the coordination of multiple plexuses. (
Inset
: P,
multipolar postganglionic motor neuron; S, satellite cell.)
Figure 7-14.
Overview of the autonomic nervous system (
Continued
).
The ANS is composed of three divisions:
sympathetic
,
parasympathetic
, and
enteric
. The sympathetic and parasympathetic divi-
sions function under direct CNS control; the enteric division functions somewhat more independently. The
sympathetic division
includes
preganglionic neurons
with cell bodies in the
lateral horn
of the thoracic and upper lumbar spinal cord (Fig. 7-4). Some of
these neurons synapse on
postganglionic neurons
in the
sympathetic chain ganglia
(Figs. 7-4 and 7-15A); others continue past these
ganglia and synapse in
prevertebral sympathetic ganglia
(e.g., celiac ganglion, mesenteric ganglia) near the organs to be innervated.
Postganglionic neurons send axons to internal organs, glands, and blood vessels. Effects of sympathetic activity include increas-
ing cardiac output, blood pressure, and bronchial diameter; decreasing gut peristalsis; and, in general, preparing the individual for
strenuous activity, sometimes called the “±
ght-or-fl ight” reaction. The
parasympathetic division
has a markedly different organiza-
tion. Preganglionic ± bers originate in brainstem nuclei associated with cranial nerves III, VII, IX, and X and in the
sacral parasym-
pathetic nucleus
(which occupies a position in the sacral spinal cord similar to that of the lateral horn in the thoracic cord). The
parasympathetic preganglionic ±
bers
that supply the head region synapse in discrete ganglia (Fig. 7-14) and the postganglionic ±
bers
end in glands and smooth muscle. By contrast, parasympathetic preganglionic ±
bers that travel in cranial nerve X (vagus nerve)
and the pelvic splanchnic nerves send signals to the viscera and blood vessels within the body cavity. These ±
bers do not synapse in
discrete ganglia but rather in small
plexuses
of postganglionic cell bodies that lie in or adjacent to the walls of their target organs
(Figs. 7-15B and 7-16A). The general effect of parasympathetic activity is the opposite of sympathetic activity and tends to return
the internal organs and cardiovascular system to a baseline level of function. The
enteric division
consists of a vast number of neu-
rons arranged in a network of plexuses in the walls of the gut. Some of these plexuses are shared with the parasympathetic division.
The activity of the enteric division is modulated in a general way by the sympathetic and parasympathetic divisions, but it is able to
act independently and refl exively to move boluses of food substances through the gastrointestinal tract by peristaltic action and to
control absorption, local blood fl ow, and secretion in response to the chemical composition of the bolus.
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