Sympathetic Nervous System (SNS): Output and Thoracolumbar Outflow

by Craig Canby, PhD

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    00:00 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.

    00:55 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.

    01:36 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.

    02:09 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.

    03:23 We have a sympathetic ganglion and then sympathetic trunk, sympathetic ganglion above and these sympathetic chain of ganglia distributed along the vertebral column.

    03:38 We also have this communication called the gray ramus communication.

    03:45 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.

    04:27 So here we see some of these preganglionic fibers coming through the anterior root.

    04:34 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.

    05:53 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.

    06:41 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.

    08:33 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.

    About the Lecture

    The lecture Sympathetic Nervous System (SNS): Output and Thoracolumbar Outflow by Craig Canby, PhD is from the course Autonomic Nervous System (ANS). It contains the following chapters:

    • Sympathetic System
    • Symphatetic Outflow

    Included Quiz Questions

    1. White ramus communicans
    2. Gray ramus communicans
    3. Dorsal column
    4. Central canal
    5. Intermediate horn
    1. Gray ramus communicans
    2. White ramus communicans
    3. Cauda equina
    4. Ventral root
    5. Vertebral body
    1. The level they entered or one level above
    2. The level they entered only
    3. One level below where they entered only
    4. One level above where they entered only
    1. External iliac ganglion
    2. Celiac ganglion
    3. Superior mesenteric ganglion
    4. Renal ganglion
    5. Inferior mesenteric ganglion
    1. They pass through the sympathetic trunk without synapsing and synapse with prevertebral ganglia.
    2. They synapse at the sympathetic trunk and don’t enter the prevertebral ganglia.
    3. They provide innervation to the target organs without a synapse.
    4. The synapse at both prevertebral and sympathetic ganglia.
    5. They don’t enter the sympathetic trunk and synapse with the prevertebral ganglia.

    Author of lecture Sympathetic Nervous System (SNS): Output and Thoracolumbar Outflow

     Craig Canby, PhD

    Craig Canby, PhD

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    By MALI V. on 23. May 2020 for Sympathetic Nervous System (SNS): Output and Thoracolumbar Outflow

    ddnt explain well at all. ddnt explain the exact location of the gray horns or anything.

    By Sona C. on 04. April 2018 for Sympathetic Nervous System (SNS): Output and Thoracolumbar Outflow

    These lectures are great! It's good to know also that a lot of topics that are discussed in one lecture are later elaborated on in another lecture too. Usually if I feel like some topic was not covered in depth, it just means that I was discussed further in another video. This is incredibly helpful for Step 1 prep!

    Odd choice for figure used
    By Kerry T. on 23. January 2018 for Sympathetic Nervous System (SNS): Output and Thoracolumbar Outflow

    I understood SNS input and output quite well prior to watching this video. There are much better figures in textbooks to describe output pathways. I found the figure used a very odd departure from traditional figures and much more confusing.