Sympathetic Nervous System Anatomy

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    As I already mentioned, the sympathetic ganglia tend to lie in a chain alongside the spinal cord... why didn't we see that before?  Well, we looked at isolated spinal cords before in lab, so the sympathetic ganglia were gone.  And the diagrams and images we've seen of the spinal cord focused on the spinal cord itself, not what ran along it.  So now it is time to look just beyond the spinal cord in order to see the sympathetic ganglia.  Most of these ganglia are within the chain, called the sympathetic trunk.

    I took the image below from the "Body Works" CD, which is a pretty inexpensive biology CD.  The only problem is that it has many factual errors within it.  In fact, it misidentified many of the items in this specific picture!   But this image is similar to your textbook Figure 11.37 (I just think this one is prettier).

    Running vertically in the center of this picture is the spinal cord.   In red is the pia, in white is the arachnoid, andtrunks.jpg (11654 bytes) in beige is the dura, all of which they show surrounding the spinal cord.  The picture shows the spinal cord running through the vertebral foramen (one, lumbar-ish looking vertebra is depicted).  You can also see the dorsal and ventral roots coming off the spinal cord at three levels (and even the DRGs) and merging for the spinal nerves.

    The other two vertically-running chains (one on either side of the spinal cord) are the sympathetic trunks.  The bulges in the trunks are where the sympathetic ganglia lie (shown about midway between adjacent spinal nerves).  Please note that since there are other sympathetic ganglia that do not lie within the sympathetic trunks, there is a special name given to those within the trunks-- they are called paravertebral ganglia, since they lie next to the vertebral column.

    Another neat thing visible in this picture is that you can even see two little nerves running between the sympathetic ganglia and the spinal nerves.   These are the nerves that carry the axons of the preganglionic neurons to the ganglia, and also carry the axons of the postganglionic neurons back out through the spinal nerves toward the body.

    The only bad thing about this picture is that it looks like the sympathetic trunks run right through the spinal nerves.  Of course, this does not occur.

Details of Sympathetic NS connectivity:

   So how does this entire sympathetic nervous system work?  To understand it, there is some new terminology to learn and a couple of ways that it works.   If I try to gloss over it, I don't think you'll understand it.  If I explain it in detail so that you can follow along, I worry that you'll think you have to memorize it all.  But of these two choices (there really isn't a middle ground here), I've decided to explain it in detail.  You will need to think through it all, but you don't have to memorize every detail.  I hope that you can follow it from beginning to end, learn the bit of new terminology (in bold on this page), and just get a feel for the system.  Let's go!

More background to understand anatomy:

    How does each neuron in the sympathetic nervous system connect to the rest to eventually affect a visceral target?  I have put together some images showing the various way the neurons connect up, but it will require a little explanation.

frame2.gif (26956 bytes)    I can only draw this stuff in 2-D.  I'm not so proficient an artist as to try to make something like the above picture.  However, I needed to add in a few more things that are not visible above.   You should be able to see half of the spinal cord, the spinal roots, the spinal nerve, a couple paravertebral ganglia, and those two little nerves that run between the paravertebral ganglia and the spinal nerve.  Can you?  Those two little nerves are called the white ramus communicans and the gray ramus communicans.  There's more that is shown in the picture, but I'll describe that in a bit.

From the sympathetic trunk:

    Now consider the part of the sympathetic nervous system that includes the postganglionic neurons in the paravertebral ganglia (don't forget, there are other sympathetic ganglia that lie outside the sympathetic trunk).  Here is a more detailed version of how this pathway works:

bulletThe preganglionic neuron projects from the spinal cord out the ventral root (just like motor neurons do).
bulletThe preganglionic neuron sends its axon down one of the two little nerves to reach a ganglion within the sympathetic trunk.  frame7.gif (31486 bytes)The nerve it projects through is called the white ramus communicans.  "White" because all the preganglionic axons that run through it are myelinated.  "Ramus" because it is a nerve branch.   And "communicans" because it provides the path that connects the pre- and post- ganglionic neurons.
bulletWhen the preganglionic axon reaches the paravertebral ganglion, it synapses on a postganglionic neuron.  The preganglionic axon may also send off branches to synapse on other postganglionic neurons in the same or nearby paravertebral ganglia.
bulletThe postganglionic neuron sends its axon out the gray ramus communicans.   It is gray because the postganglionic axons are NOT myelinated.
bulletThe postganglionic axon runs from the gray ramus communicans out through the spinal nerve toward the skin.

The postganglionic neurons in the paravertebral ganglia mainly influence targets in the skin, like the arrector pili muscles and blood vessels and glands.

Through the collateral ganglia:

    What was the rest of the stuff in the picture I drew?  Well, the rest was for if a preganglionic neuron needed to contact postganglionic neurons in sympathetic ganglia that are separate from the sympathetic trunk.  These other sympathetic ganglia, called collateral ganglia, tend to influence the deeper visceral organs, like your heart, kidneys, and digestive system.  Let's go over how this works:

bulletThe preganglionic neuron projectsframe6.gif (30810 bytes) from the spinal cord out the ventral root (just like motor neurons do).
bulletThe preganglionic neuron sends its axon down the white ramus communicans.
bulletOnce the preganglionic axon reaches the paravertebral ganglion, it ignores it, does NOT synapse, and instead bypasses this paravertebral ganglion to continue to project away from the spinal cord in a separate nerve, called a soundicon.gif (538 bytes)splanchnic nerve.
bulletThe preganglionic axon reaches the collateral ganglion via the splanchnic nerve and synapses on a postganglionic neuron within it.  The postganglionic neuron then projects its axon to synapse on its target.

    I put these images together with another that simply reminds you how the somatic nervous system is connected (the voluntary portion) in an animation.  If all goes well, this should help you organize your thoughts on the sympathetic nervous system.

sympns.gif (338375 bytes)

    Another way to view the sympathetic nervous system is to look at the entire rostrocaudal extent of it (rather than focusing on just one or a few spinal levels).  A good way to do that is using Figure 11.39 from your textbook.

Go on to the description of the parasympathetic nervous system anatomy... (it is MUCH simpler!)

2011 STCC Foundation Press
written by Dawn A. Tamarkin, Ph.D.