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McKinley/O’Loughlin
Human Anatomy, 2nd Edition
CHAPTER 18
Answers to “What Did You Learn?”
1.
The autonomic motor pathway involves a series of two neurons in the motor
transmission of impulses, while the somatic motor system transmits impulses
along a single axon from the spinal cord to the effector.
2.
The ANS innervates smooth muscle fibers, cardiac muscle fibers, and glands.
3.
The cell body of a ganglionic neuron is located in an autonomic ganglion.
4.
In the parasympathetic division, the postganglionic axons are relatively short, whereas
in the sympathetic division the postganglionic axons are relatively long. They are
unmyelinated in both divisions.
5.
Terminal ganglia are located close to the target organ, while intramural ganglia
are located within the wall of the target organ. Both ganglia contain
parasympathetic ganglionic cell bodies.
6.
The cranial nerves involved in the parasympathetic division of the ANS include
the oculomotor (CN III), facial (CN VII), glossopharyngeal (CN IX), and vagus
nerve (CN X).
7.
Some of the effects caused by parasympathetic stimulation include: increased secretion
of digestive system glands, increased motility (smooth muscle contraction/movement)
in digestive organs, stimulation of defecation, decreased heart rate, constriction of the
pupil of the eye (reduction in pupil diameter), and erection of the penis or clitoris.
8.
The sympathetic preganglionic neurons originate in the lateral horn of the T1-L2
regions of the spinal cord.
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9.
Human Anatomy, 2nd Edition
Sympathetic trunk ganglia are part of the sympathetic trunks, which are located
lateral to the vertebral column. Prevertebral ganglia are located anterior to the
vertebral column and cluster around the origins of major abdominal organs.
10.
White rami communicantes carry myelinated preganglionic axons to the
sympathetic trunk. White rami communicantes are found only on the T1-L2
spinal nerves. In contrast, gray rami communicantes carry postganglionic axons
and extend from the sympathetic trunk to ALL spinal nerves.
11.
The sympathetic preganglionic axons branch extensively. This broad branching
pattern allows the sympathetic nervous system to display great divergence in the
innervation transmission. A single preganglionic axon innervates as many as 20
or more ganglionic neurons. A greater degree of divergence in this division
provides the means to rapidly activate many visceral organs simultaneously. This
results in a mass activation of almost all the ganglionic sympathetic neurons.
12.
Splanchnic nerves are preganglionic axons that leave the sympathetic trunk
ganglia without synapsing and extend to prevertebral ganglia. Within the
prevertebral ganglia, the preganglionic axon will synapse with a ganglionic
neuron, and the postganglionic axon will travel to the effector organs. Most
abdominal organs and some pelvic organs receive their sympathetic innervation
via this pathway.
13.
The adrenal medullae secretes epinephrine and norepinephrine
14.
Acetylcholine (ACh) is released by all parasympathetic axons, all preganglionic
sympathetic axons, and a few postganglionic sympathetic axons. The remaining
postganglionic sympathetic axons secrete norepinephrine (NE).
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15.
Human Anatomy, 2nd Edition
Dual innervation means that an organ is innervated by postganglionic axons from both
ANS divisions. The actions of the divisions usually oppose each other thus, they are
said to exert antagonistic effects on the same organ.
16.
The hypothalamus is the integration and command center for autonomic
functions. It contains nuclei that control visceral functions in both divisions of the
ANS and communicates with other CNS regions, including: the cerebral cortex,
thalamus, brainstem, cerebellum, and spinal cord.
Answers to “Content Review”
1.
The four CNS regions that control the autonomic function are the hypothalamus,
brainstem, spinal cord, and cerebrum.
2.
The sympathetic trunk ganglia are immediately lateral to the vertebral column (on
both sides) and are a part of the sympathetic trunks. The prevertebral ganglia are
clusters of sympathetic division neuron cell bodies of ganglionic neurons located
anterior to the vertebral column on the anterolateral wall of the abdominal aorta at
the base of major abdominal arteries. The terminal ganglia are a collection of
parasympathetic division neuron cell bodies of ganglionic neurons located very
close to the target organ, while intramural ganglia contain parasympathetic
ganglionic cell bodies within the wall of a target organ.
3.
The sympathetic preganglionic axon is myelinated, relatively short (compared to
the postganglionic axon), and has extensive branching. It releases acetylcholine
as its neurotransmitter. The parasympathetic preganglionic axon is myelinated,
McKinley/O’Loughlin
Human Anatomy, 2nd Edition
relatively long (compared to its postganglionic axon), and has little or no
branching. It also releases acetylcholine as its neurotransmitter.
4.
When preganglionic axons synapse on cells within the adrenal medulla, the
adrenal medulla cells release hormones that are circulated within the bloodstream
and helps prolong the "fight or flight" response. The cells of the adrenal medulla
primarily secrete the hormones epinephrine and, to a lesser degree, the hormone
norepinephrine. Once secreted into the bloodstream, these hormones serve to
potentiate (prolong) the effects of the sympathetic stimulation.
5.
Axons exit the sympathetic trunk ganglia by one of four pathways. These
pathways are: the spinal nerve pathway, the postganglionic sympathetic pathway,
the splanchnic nerve pathway, and the adrenal medulla pathway. In the spinal
nerve pathway, the preganglionic axon synapses in the sympathetic trunk, and the
postganglionic axon leaves the trunk via a gray ramus communicans and through
a spinal nerve. This pathway is used for innervating sweat glands and blood
vessels in the skin. In the postganglionic sympathetic pathway, the preganglionic
axon synapses in the sympathetic trunk and the postganglionic axon leaves the
trunk anteriorly to go directly to the effector organ. In this pathway, the
postganglionic axon does NOT travel back through the spinal nerve, and this
pathway is used to innervate most thoracic viscera. In the splanchnic nerve
pathway, the preganglionic axon does not synapse in the sympathetic trunk, but
leaves as a splanchnic nerve to extend to a prevertebral ganglion. There is a
synapse in this ganglion and the postganglionic axons travel to the abdominal and
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Human Anatomy, 2nd Edition
pelvic organs. Finally, in the adrenal medulla pathway, preganglionic axons
travel to and synapse on cells in the adrenal medulla.
6.
For both divisions, the preganglionic cell bodies are located in the central nervous
system, specifically in the T1-L2 regions of the spinal cord (sympathetic) or
craniosacral regions (parasympathetic) of the CNS. The postganglionic cell
bodies are located in the peripheral nervous system and are given a name
(ganglia) that is indicative of a peripheral nervous system structure.
7.
White rami communicantes carry myelinated preganglionic sympathetic axons
from the T1-L2 spinal nerves from the spinal nerve to the sympathetic trunk.
They are the way preganglionic sympathetic axons enter the sympathetic trunk.
Gray rami communicantes carry postganglionic sympathetic axons from the
sympathetic trunk to the spinal nerve. Gray rami connect to ALL spinal nerves.
This way, the sympathetic information that started out in the thoracolumbar
region now can be dispersed to all parts of the body.
8.
The general functions of the sympathetic division are concerned with fight or
flight, such as preparing the body for emergencies [increase blood pressure and
rate of heart beat, increased release of stored nutrients, increased respiration rate,
dilation of pupils]. The parasympathetic division is primarily involved with
maintaining the body’s internal environment (homeostasis) and has been
nicknamed the “resting and digesting” system, because it concerns itself with
those activities.
9.
In a crisis, many effectors innervated by the sympathetic division respond
together in what is termed sympathetic mass activation. Preganglionic
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Human Anatomy, 2nd Edition
sympathetic axons form numerous collateral branches in order to synapse with a
large number of ganglionic neurons to cause stimulation of many ganglionic
sympathetic neurons and simultaneous activation of many effector organs. Mass
activation of the sympathetic division causes a heightened sense of alertness.
10.
The autonomic nervous system is formed from both the neural tube and the neural
crest cells. The neural tube forms the CNS components of the autonomic nervous
system, including the preganglionic cell bodies, the hypothalamus and the
autonomic nervous system centers in the brainstem. The neural crest cells form
the PNS components of the autonomic nervous system, including the rami
communicantes, all autonomic ganglia, preganglionic axons, the sympathetic
trunk, and the ganglionic neurons.