Alright. Next we want to understand the sympathetics
in the thorax. The sympathetics in the thorax
can be considered to be a relay system of
two neurons. We have preganglionic neurons.
We would have their cell bodies and then,
we would have their axonic processes and those
axons will synapse with postganglionic neurons,
their cell bodies, and then, from those nerve
cell bodies, we will have the post ganglionic
axon that will then reach out to innervate
the appropriate thoracic structures.
The preganglionic neurons, their cell bodies
reside within the spinal cord at the following
levels: T1 down to T12, L1, L2, and may be
as well as L3, but we are within the thorax,
so we were only going to operate at T1 down
to T5 spinal cord levels. So, your preganglionic
nerve cell body will reside in the lateral
grey horn of the spinal cord in about this
location. The axon, then, will extend outwards.
And then, will enter through this communication
here through the sympathetic trunk. This sympathetic
trunk ascends and descends along the vertebral
column. You have a right sympathetic trunk
and you also have a left sympathetic trunk.
Along the sympathetic trunks, you will have
these dilated areas and those are referred
to as sympathetic ganglia.
Residing within these sympathetic ganglia
are your postganglionic nerve cell bodies.
And so, your synapse is going to occur within
here between your preganglionic neuron and
your post ganglionic neuron. Post ganglionic
axons from those neurons then are distributed
to thoracic viscera and in many cases, what’s
going to happen is they will leave the sympathetic
ganglion, perhaps at the same level in which
the preganglionic neuron entered and come
on out or they may go up a level or two or
they may go down a level or two and then,
will have their axons leave at a different
level from which they arrive from the preganglionic
neurons. Also accompanying the sympathetic
afferent fibers, you will also have your sensory
or your afferent fibers.
So, let’s take a look at the specific innervation
of selected thoracic viscera. So, here we
are looking at the sympathetic innervation
of the heart. Sympathetic innervation of the
heart, the preganglionic fibers are going
to come from T1, the T4, may be as well as
T5. So, that’s going to occur here, here,
here, here and may be as well as here.
Many of those fibers from the preganglionic
neurons will enter the sympathetic trunk,
synapse with the postganglionic neuron. Many
of those fibers are going to ascend into cervical
ganglion. And you have a superior cervical
ganglion, you have a middle cervical ganglion
and then, you will have an inferior cervical
ganglion that calmly will join up with the
first thoracic ganglion to form a much larger
ganglion called the stellate ganglion and
then, you have your preganglionic fibers being
distributed to the heart. You will also have
some from the more inferior thoracic spinal
cord segments that are going out directly
to the heart in this direction.
So, there is a question here regarding why
do these fibers ascend in the neck only to
then descend down to innervate the heart?
And the answer to this question, again, involves
embryology. Early on in the embryo, the heart
resides within the neck and then with further
development, the heart will assume a lower
location within the thoracic cavity, thereby
pulling these fibers down so that they descend.
The sympathetics are involved in increasing
heart rate and because they are also well
distributed to the ventricles, they are the
major player in increasing contractility when
they are stimulated and then, when they are
inhibited, they will decrease the contractility
or force of myocardial contraction.
Innervation of the lungs is innervated from
preganglionic sympathetic neurons that are
housed within T1 through T5. So, again, we
are looking at the same levels with the distribution
then out to the lungs. Sympathetic innervation
of the lungs will mildly vasoconstrict the
blood vessels supplying the lungs.
Sympathetics to the respiratory passageways
will cause bronchodilation. And this is very,
very useful in individuals that are asthmatic.
If they are having an asthmatic event, you
want to be able to dilate their respiratory
passageways. We know that sympathetics do
that as a part of their normal activity and
so, you can then administer drugs that mimic
the sympathetic nervous innervations and those
are sympathomimetic drugs and then they will
then produce the efficacious ways of dilation
during an asthmatic attack.
Innervation of the esophagus is provided by
the same spinal cord levels, again, T1 down
to T5. This illustration doesn’t really
show you the esophagus, but again, spinal
cord level T1 through T5 will house a preganglionic
sympathetic nerve bodies. And then the post
ganglionic nerve cell bodies will send out
their fibers from the sympathetic trunks to
innervate the esophagus. Sympathetic innervation
of the esophagus inhibits peristalsis, just
the opposite of what the parasympathetics
do and sympathetics will decrease glandular
secretions that are coming from the esophagus.
Next, we want to look at the splanchnic nerves
that are formed from the sympathetic trunk.
We will see them course within the thorax,
but it’s important to remember that although
they are found in the thorax, they do not
innervate any of the thoracic viscera, instead
they are going to continue on into the abdominal cavity
where their functions are to innervate abdominal viscera.
The splanchnic nerves are known as the greater
splanchnic nerves, the lesser splanchnic nerves,
the least splanchnic nerves and the levels
that contribute to their formation are T5
through T9, may be as well as T10, for your
greater splanchnics. The lesser splanchnics
are from more inferior spinal cord levels,
T10 to T11 and your least splanchnic nerves
are going to be formed from preganglionic
neurons that are housed within T12.
We do see two of those splanchnic nerves in
the illustration, one is more clearly recognizable.
The lesser we just see it forming, the least
we do not see in this illustration, but if
we take a look here, we can see our greater
splanchnic nerve coursing along here.
This is your sympathetic trunk and if we look,
we can see some fibers coming from the sympathetic
trunk at multiple levels that will course
and then ultimately, form this greater splanchnic nerve.
Another term here, and the artist is
using this, is major splanchnic. We also have
the lesser splanchnic starting to form here
and again, T10 and T11 will help from the
lesser splanchnic. Least splanchnic is not
recognizable in this particular illustration.