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Learning Modules - Medical Gross Anatomy
Introduction to Autonomics, Part 1 - Page 1 of 8
The nervous system coordinates all the functions of
our bodies, from running, to digesting, to singing, to
breathing. For the sake of discussion and study, the
nervous system is divided functionally into
the somatic nervous system and the autonomic
nervous system, or the ANS. These systems are
not separate, but rather different components of the
nervous system as a whole. The somatic nervous
system may be thought of as the conscious nervous
system, responsible for functions of which we are
aware. It transmits motor signals that cause our
voluntary actions and some reflexive actions, and sensory signals about pain, temperature, touch, and
position. In addition to these conscious functions, there is an entire set of functions that our nervous
system regulates that we rarely, if ever, notice. These unnoticed functions are regulated by the
autonomic nervous system and include all of our homeostatic mechanisms. The autonomic nervous
system may be thought of as the unconscious nervous system, responsible primarily for maintaining
homeostasis. Functions regulated by the autonomic nervous system include heart rate, respiration
rate, blood vessel diameter, sweat gland secretion, digestion, etc.
Fortunately, we do not have to consciously regulate all of our organs' functions; otherwise, a person
could die merely from forgetting to breathe. This module will focus on the autonomic nervous system,
including its functions and its structure. Many students get confused by the autonomic nervous
system, perhaps because it does not follow the patterns of the somatic nervous system or because its
structure varies from one part of the body to the next. It may help to remember that the autonomic
nervous system innervates all the structures that help maintain homeostasis by whatever means
necessary. If you take your time, and go through the autonomic nervous system as a whole until it
makes sense, it will definitely pay off many times in the future.
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Learning Modules - Medical Gross Anatomy
Introduction to Autonomics, Part 1 - Page 2 of 8
The autonomic nervous system regulates
homeostasis via two opposing divisions: the
sympathetic division and the parasympathetic
division. Both the sympathetic and
parasympathetic systems innervate most of the
body's organs and act in opposition to one another
to maintain normal physiology, including blood
pressure, blood oxygen levels, and nutrient levels.
The amount of work that a given organ must do to
maintain homeostasis differs from one situation to
the next. In addition, certain organs need to work to
prepare the body for possible future situations.
These functions are all regulated by the autonomic
nervous system.
The sympathetic division of the autonomic
nervous system is the division that prepares the
body for stressful situations. It is often referred to as
the "fight or flight" system. The effects of this
system are numerous, but generally include
increasing heart rate, constricting blood vessels to
the skin and viscera (thereby increasing blood flow
to muscles), increasing pupil size and decreasing
salivation. These responses all promote survival in a
dangerous situation.
The parasympathetic division of the autonomic
nervous system prepares the body for restful
situations and is often called the "rest and digest" system. Effects of the parasympathetic nervous
system include slowing heart rate, increasing gastric motility, and increasing salivation. These
responses help the body to recover as well as prepare for stressful situations by storing nutrients.
The parasympathetic and sympathetic systems do not work entirely separately, but rather work at the
same time, often in opposition to one another. For example, while relaxing on a hot day after a meal,
your parasympathetic nervous system may predominate as your food digests, but your sympathetic
nervous system will be actively innervating sweat glands. On the other hand, if the building you were in
suddenly caught fire, your sympathetic nervous system would predominate. The sympathetic and
parasympathetic nervous systems may be thought of like a water faucet with hot and cold water
balancing one another to make the perfect water temperature for anything from a cold glass of water to
drink to a relaxing hot bath.
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Learning Modules - Medical Gross Anatomy
Introduction to Autonomics, Part 1 - Page 3 of 8
The parasympathetic and sympathetic components of the autonomic nervous system (ANS) differ in their structure
and in their function, but the two systems have some important features in common. Both the parasympathetic and
sympathetic divisions are two-neuron systems with the first neuron named either presynaptic or preganglionic
(these terms are synonymous), and the second nerve called postsynaptic or postganglionic.
It is important to realize that an autonomic neuron is not called postsynaptic or postganglionic until it has synapsed,
regardless of how many ganglia it has passed through. The cell bodies of the presynaptic neurons of both the
parasympathetic and sympathetic systems are located within the central nervous system. These presynaptic
autonomic neurons synapse only with postsynaptic autonomic neurons. (There is one exception, in the adrenal
gland, which will be discussed later in this module.) The cell bodies of postsynaptic autonomic neurons are located in
ganglia throughout the body. Recall that a ganglion is simply a collection of neuron cell bodies in the peripheral
nervous system. Sensory ganglia, such as dorsal root ganglia, are collections of sensory neuron cell bodies,
but NO synapses occur there. A particular autonomic ganglion will be associated with synapses of either the
parasympathetic nervous system or the sympathetic nervous system, but it may have fibers from both systems
running through it. Remember, a neuron does not necessarily synapse just because it enters a ganglion. Some
fibers pass through without synapsing. (The terms preganglionic and postganglionic may be a bit deceptive, but they
are often used instead of presynaptic and postsynaptic.) Another important similarity between the parasympathetic
and sympathetic divisions of the ANS is that they both use chemical signals to alter the action of the organs they
innervate. The organs innervated by the autonomic nervous system are called effector organs.
The significant differences between the parasympathetic and sympathetic nervous systems include the location of the
presynaptic nerve cell bodies within the central nervous system, and the location of the postsynaptic nerve cell bodies
throughout the body. We will discuss the pre- and postsynaptic neurons in detail in this module. Another important
difference between the parasympathetic nervous system and the sympathetic nervous system are the
neurotransmitters each system uses to effect change. Generally, the sympathetic nervous system releases a
chemical called norepinephrine, which is excitatory to neurons, from its postsynaptic neurons. The parasympathetic
nervous system releases a chemical called acetylcholine from its postsynaptic neurons.
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Learning Modules - Medical Gross Anatomy
Introduction to Autonomics, Part 1 - Page 4 of 8
We will first spend some time discussing the sympathetic division of
the autonomic nervous system. The sympathetic division is
sometimes called the thoracolumbar outflow or division of the
ANS because the cell bodies of the presynaptic sympathetic
neurons are located in the lateral horns (a.k.a. intermediolateral
cell columns) of the spinal cord gray matter, which are found in
spinal cord segments T1 through L2. There are no sympathetic
presynaptic cell bodies above spinal cord level T1 or below spinal
cord level L2. The cell bodies of the postsynaptic sympathetic
neurons are found in either paravertebral or prevertebral ganglia.
The paravertebral ("beside the vertebrae") ganglia are called the
sympathetic chain ganglia and will be discussed in more detail later.
Prevertebral ganglia, also known as preaortic or
collateral ganglia, are located around the major branches of the
abdominal aorta and include the celiac, aorticorenal, superior
mesenteric, and inferior mesenteric ganglia. The sympathetic
nervous system innervates essentially all the organs of the body as
well as blood vessels (smooth muscle in vessel walls), sweat
glands, and arrector pili muscles through out the body. (Arrector pili
muscles make the hairs on your body stand up, resulting in goose
bumps.)
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Learning Modules - Medical Gross Anatomy
Introduction to Autonomics, Part 1 - Page 5 of 8
The paravertebral ganglia consist of the right
and left sympathetic chains or trunks. These ganglia are home
to many postsynaptic sympathetic nerve cell bodies, and the site
of many synapses between presynaptic sympathetic fibers and
postsynaptic sympathetic neuron cell bodies. The sympathetic
chains lie next to the vertebral column throughout its length,
running across the necks of the ribs in the thorax and along the
vertebral bodies in the abdomen. There is approximately one
ganglion associated with each spinal cord segment, except in the
cervical region and the sacral region. Adjacent ganglia of the
sympathetic chain are connected to each
other by interganglionic rami which contain sympathetic nerve
fibers ascending or descending between ganglia.
The presynaptic sympathetic nerve fibers originate
in the lateral horns of spinal cord segments T1-L2.
From the lateral horns, all of these fibers must reach
the sympathetic trunk. Presynaptic sympathetic
fibers exit the spinal cord in the ventral roots
(because they are motor fibers), pass through the
spinal nerves and eventually enter the ventral
primary rami of spinal cord segments T1-L2. Shortly
after entering the ventral primary rami, the
presynaptic sympathetic fibers exit the ventral
primary rami via white rami communicantes which
carry the presynaptic sympathetic fibers to the
sympathetic trunk. The white rami communicantes
are so named because they are collections of
myelinated (therefore white-ish) axons
communicating with the ventral primary rami. White
rami communicantes are only found between spinal
cord segments T1-L2 because there are no
presynaptic sympathetic nerve fibers originating
above or below those levels.
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Learning Modules - Medical Gross Anatomy
Introduction to Autonomics, Part 1 - Page 6 of 8
Once the presynaptic
sympathetic fibers have
arrived in the sympathetic
chain they do one of
three things:
1. The presynaptic
neuron may
synapse
immediately in the
ganglion located at
the level it entered.
2. The presynaptic
neuron may ascend
or descend in the
sympathetic trunk
before synapsing in
a ganglion located
at a different spinal
cord level.
3. The presynaptic
sympathetic neuron
may pass through
the sympathetic
chain ganglia
without synapsing
at all, synapsing
instead in a
prevertebral
ganglion.
In general,
sympathetic innervation
to structures located in
the head, neck, body
wall, limbs and thoracic
cavity follows one of the
first two courses.
Sympathetic innervation
to internal organs in the
abdomen and pelvis
follows the third course,
primarily.
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Learning Modules - Medical Gross Anatomy
Introduction to Autonomics, Part 1 - Page 7 of 8
Once the presynaptic fibers have synapsed, the postsynaptic sympathetic fibers reach their target
structures by the most efficient path for the given region of the body. Postsynaptic sympathetic nerves
have several possible routes depending on the location of their target organs. Many postsynaptic
sympathetic fibers re-enter the spinal nerve via gray rami communicantes and get distributed to the
body with the ventral and dorsal primary rami. Gray rami communicantes carry postsynaptic
sympathetic fibers to every spinal nerve, allowing sympathetic innervation to reach all parts of the
body, especially the blood vessels, sweat glands and arrector pili muscles.
Other postsynaptic sympathetic fibers leave the ganglia where they began and travel directly to
their target organs. This is how postsynaptic sympathetic fibers reach the organs of the thorax. Finally,
some postsynaptic sympathetic neurons form perivascular plexuses along blood vessels that supply a
given area. The nerve fibers then ride along with the blood vessels to reach their targets. This is the
method that postsynaptic sympathetic fibers use to reach target organs in the head as well as in the
abdomen and pelvis.
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Learning Modules - Medical Gross Anatomy
Introduction to Autonomics, Part 1 - Page 8 of 8
So, let's summarize the general plan of the sympathetic nervous system. Presynaptic sympathetic
fibers arise within the lateral horn of the spinal cord between T1 and L2 levels of the cord. ALL of
these fibers leave the spinal cord in ventral rootlets, travel through the spinal nerve, and then enter
the first few millimeters of the ventral primary ramus before leaving to travel to the sympathetic chain
within the white rami communicantes.
The sympathetic chains are bilateral strings of paravertebral sympathetic ganglia, connected by
interganglionic rami, that lie beside or on the vertebral column.
Once a presynaptic sympathetic neuron reaches the sympathetic chain, it faces a choice. It may:
1. synapse at the level it entered and then either:
a. leave within a gray ramus communicans to reach the ventral ramus at the level at which it
synapsed, or
b. leave the chain as a direct branch to organs (thoracic visceral nerve, T1-T4/5).
2. travel up or down the chain to synapse at a higher or lower ganglion, where it may either:
a. leave within a gray ramus communicans to reach a ventral ramus at the level at which it
synapsed, or
b. leave the chain as a direct branch to organs (cervical cardiac nerve or sacral splanchnic
nerve) or to a perivascular plexus (internal or external carotid plexus).
3. leave the chain without synapsing, to travel within a thoracic (T5-T12) or lumbar splanchnic
nerve to either:
a. reach a prevertebral (preaortic) ganglion, where it will synapse and then join a perivascular
plexus to reach an organ, or
b. reach cells of the suprarenal medulla to synapse.
These universal truths must be remembered:
1. White rami communicantes only exist to connect T1 to L2 ventral primary rami to the
sympathetic chain. The terms "white ramus" and "presynaptic" are not synonymous and
interchangeable.
2. Gray rami communicantes exist at ALL spinal nerve levels. EVERY ventral primary ramus is
connected to the sympathetic chain by a gray ramus, which carries postsynaptic sympathetic
fibers to the VPR.
3. The sympathetic chain exists to carry some of the presynaptic fibers from the T1-L2 levels up to
the neck and head and down into the lower abdomen and pelvis.
If you find inner peace with these universal truths, then you have passed the first step on your
journey to become an Autonomic Master.
Copyright© 2002 The University of Michigan. Unauthorized use prohibited.