Sympathetic system is referred to alternatively as the thoracolumbar system because the preganglionic
nerve cell bodies reside in these segments of the spinal cord specifically T1 down to L2
or even as well as L3. Then their output will be to the viscera through preganglionic fibers and
communicating with ganglia. The segments T1-T5, these nerve cell bodies reside in the
intermediate gray horns. They will send axons out to the viscera in the head, neck, and thorax
region. So, this will be the level of the spinal cord innervating those particular structures.
T5 down through L2/3 will supply viscera in the abdominal area and the pelvis via splanchnic
nerves. Those can be seen in this illustration, so we’ll zoom in. The greater splanchnic nerve
is shown in through here. Then its preganglionic fibers will synapse with ganglia that are
associated with the branches of the aorta. So here’s the celiac trunk and you can see
some ganglia associated with it. Another splanchnic nerve is the lesser splanchnic nerve.
There’s also the least splanchnic nerve coming in. Then more inferiorly, we have the lumbar
splanchnics which are shown in through here. Then we have pelvic sympathetic
splanchnic nerves shown here in green. These should not be confused with the pelvic splanchnic
nerves associated with the parasympathetic division which are coming off S2, S3, and S4.
Those are shown in purple. Now, the mystery of the autonomic nervous system resides
in the sympathetic outflow. How do you distribute the sympathetics outwards to the
periphery or to maintain their distribution internally to viscera? We’re going to have
to break this down by the pattern of distribution. So, when we think about sympathetic ,
outflow, we need to think about how that outflow is distributed to the body wall
as well as to the limbs for example. So, this pattern that you see here, has over here
to the far right, the spinal cord will reside over here. We have the anterior nerve root
of the spinal cord and it is distributing motor fibers out toward the periphery. The fibers
that you see here in red are going to be sympathetics that we’re going to highlight
and understand their route. We have this structure here which is a white ramus communicans.
We have a sympathetic ganglion and then sympathetic trunk, sympathetic ganglion
above and these sympathetic chain of ganglia distributed along the vertebral column.
We also have this communication called the gray ramus communication.
Then this more lateral component here is the spinal nerve that’s going out toward the
musculature and the skin of the body wall and the extremities. So our first consideration
for this sympathetic outflow to body wall and limbs is that preganglionic neurons
will reside in the lateral gray horns, again T1-T12 and L1 down to L2 and/or L3. From here,
axons then travel to the sympathetic trunk via the white ramus communicans because
these are myelinated. That imparts that white name to the white ramus communicans.
So here we see some of these preganglionic fibers coming through the anterior root.
They then enter the white ramus communicans and then we see them either synapsing
with a postganglionic neuron at that level with the sympathetic ganglion. However,
the preganglionic neuron may go up and synapse at this level or may go down and
synapse at a more inferior level. So for practical purposes, this preganglionic neuron is
going to synapse at the same level with the ganglion that it entered. From here,
the postganglionic neuron then will leave the sympathetic ganglion via the gray ramus
communicans. So now, we’re going to go from here where the synapse occurred
outwards as a postganglionic nerve fiber, outwards as a gray ramus and then it will
enter the spinal nerve and then can travel through the dorsal ramus of the spinal nerve
or the anterior ramus of the spinal nerve to be distributed to the target structures
of the limb or the body wall. Sympathetic outflow to the cervical and thoracic viscera
should be considered separately from the distribution of the body wall in limbs.
Here again, we have the same illustration. So, we are going to begin with that
preganglionic neuron. The axons are travelling outwards to the anterior nerve root
through the white ramus communicans. Then they’re going to synapse within the ganglion
at or above the level at which they entered. So innervation through cervical and thoracic
viscera, the preganglionic neuron will synapse at the level of ganglion at which it
entered or will all send and synapse at a higher level. The postganglionic neurons then will leave
that ganglion where they synapsed and then will travel outwards to the viscera.
In many cases, they’ll follow arteries but in some cases, they’ll travel more directly to that
structure. So in this case, this preganglionic neuron is synapsing at a higher level and then
it leaves that synaptic ganglion to be distributed to a thoracic visceral structure or a structure
more superior to that. Now, we need to consider separately yet again, sympathetic outflow
to the abdominal pelvic viscera. Here, the preganglionic nerve axons will pass through
the sympathetic trunk. However, when they do so, they will not synapse. Instead, they will
continue and help to form those splanchnic nerves that we identified earlier. So this could
continue on without synapsing in the sympathetic trunk. They’ll enter the greater splanchnic
nerve to go into the lesser splanchnic nerve, could enter the least splanchnic nerve
or this could continue onwards in the lumbar splanchnics or even continue onward
within the pelvic splanchnic. The take home message here is they do not synapse
at all within the sympathetic trunk. They go out more peripherally. They’ll then
synapse with prevertebral ganglia situated at major arteries that branch from the aorta
so they can extend out and branch with the celiac ganglion associated with the celiac trunk
or these preganglionic neurons may synapse with the superior mesenteric ganglion
or renal ganglion or even more inferiorly with the inferior mesenteric ganglion as some examples.
Once they synapse with those peripheral ganglia at major arterial branching points from the aorta
then the postganglionic neurons from then will travel to reach their visceral targets,
perhaps systemic, perhaps the small intestine but that gives you an idea
of how they extend in or distributed.