Sympathetics in the Thorax – Thoracic Nerves

00:01 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.

00:19 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.

00:45 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.

01:51 Along the sympathetic trunks, you will have these dilated areas and those are referred to as sympathetic ganglia.

02:02 Residing within these sympathetic ganglia are your postganglionic nerve cell bodies.

02:10 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 efferent fibers, you will also have your sensory or your afferent fibers.

03:09 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.

03:38 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 postganglionic 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.

04:29 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.

05:08 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.

05:34 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.

06:04 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.

06:47 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.

07:33 Next, we want to look at the splanchnic nerves that are formed from the sympathetic trunk.

07:40 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.

08:00 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.

08:36 We do see two of those splanchnic nerves in the illustration, one is more clearly recognizable.

08:43 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.

08:57 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.

09:13 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.