What’s up, Taim talks med here. In this video we’re gonna talk about the sympathetic nervous system. As you see from this brief diagram, the sympathetic and the parasympathetic parts of our nervous system controls more or less all our internal organs. Sympathetic being the fight or flight response, and parasympathetic being the rest and digest response. And they’re both as you see here a part of the autonomic nervous system, which again is the motor division of our peripheral nervous system. So, in this video again, we’re going detailed into the sympathetic nervous system. And we’re gonna do that by first going through the general structures of the sympathetic, or the autonomic nervous system in general. Just some basic concept that you need to know in order to understand the sympathetic nervous system, like the different paravertebral and prevertebral ganglia, pre and post synaptic neurons, which neurotransmitters are released, etc. Then we’re gonna cover the main highway for the sympathetic nervous system which is the sympathetic trunk, or the sympathetic chain ganglia. And we’re gonna do that in two segments by first covering the innervation of the head, neck and thorax, then the innervation of the abdominal and the pelvic viscera. I’ll show you the relation between the different plexuses in our abdominal and thoracic cavity. This is to understand the relation between ganglia, organs and the plexuses. So, after you’ve watched this video, and understood everything regarding the sympathetic nervous system, I’m gonna test you with a little quiz. You’re not escaping that one, it’s a simple quiz to check if you paid attention. Let’s go ahead and begin with some terms. Now the autonomic nervous system - so both the sympathetic and parasympathetic nervous system is made up of a relay that includes two neurons. And when there’s a group of nerve cell bodies that are next to each other within the actual central nervous system, the whole thing is called a nucleus, while a group of nerve cell bodies that are located outside of the central nervous system is called a ganglion. Alrgiht? That is the first thing I want you to understand. The second thing I want you to understand is that this is pons, alright? Below it is the Medulla, then we got the spinal cord. When we talk about the sympathetic ganglia in general, we divide those into two groups based on their location. They can either be paravertebral ganglia located laterally to the spinal cord. And we got some midline ganglia located in front of the vertebra and the aorta, called prevertebral ganglia. The paravertebral ganglia run alongside the spinal cord, and they’re interconnected forming a sympathetic chain of ganglia as you see here. And this chain receives nerve fibers from the thoracolumbar area of the spinal cord. Some sources write C8-L2, some sources say T1 to L2. Take it with a grain of salt, but you’re never wrong when you say thoracolumbar region. Fibers may go up, down, synapse with nuclei at the same level, or they may leave as splanchnic nerves and go synapse with the prevertebral ganglia. Alright, this is the second thing I want you to understand. The third thing. The third thing I want you to know about the autonomic nervous system is that, you know they can innervate the eye, the salivary glands, cardiovascular, respiratory system, gastrointestinal system, urinary system and so on, right? If we group them all together, and add the brain and the spinal cord here. The third thing I want you to know is actually something I said earlier but the sympathetic nervous system originate from the thoracolumbar region, giving off pre-ganglionic fibers to either the paravertebral or prevertebral ganglia, to then give off postganglionic fibers towards the target organ. Parasympathetic nervous system has two components. A cranial part that consists fo cranial nerves, primarily the oculomotor, facial, glossopharyngeal and the vagus nerve. And it has a sacral part. Notice what’s common for both the sympathetic and the parasympathetic nervous system? They both have pre and post-ganglionic neurons. The preganglionic and postganglionic neurons release different neurotransmitters and this is really important. The preganglionic neurons release acetylcholine, and that’s why the preganglionic neurons are called cholinergic neurons. Acetylcholine binds to nicotinic receptors on the cell membrane of postganglionic neuron. Nicotinic receptors are ion channels that open when acetylcholine binds to them; and they allow positive ions like sodium and calcium to cross the cell membrane, activating the postganglionic neurons and initiating an action potential. Most postganglionic neurons in the sympathetic nervous system are called adrenergic neurons because they release adrenaline and noradrenaline, or catecholamines. There are postganglionic neurons that release acetylcholine as well in the sympathetic nervous system to sweat glands for example, but postganglionic cholinergic neurons are primarily related to the parasympathetic nervous system, while postganglionic adrenergic neurons are primarily for the sympathetic nervous system, releasing catecholamines. Catecholamines activate the adrenergic receptors on the cells of the target organs. And there are two main groups of adrenergic receptors, alpha and beta receptors. So, we have alpha1 and alpha2, and beta1, beta2 and beta3, which are all g-coupled receptors. G-proteins gets activated when catecolamines bind, which ultimately enable cells to change, and that’s how the sympathetic nervous system creates change at the cellular level. I’ll try to cover their function as we go through the scheme for the sympathetic nervous system. So that was the third thing that I wanted you to know. The fourth and last thing I want you to understand before we dive into the actual scheme, is that if we take out a segment of the spinal cord within the thoracolumbar area, you’ll see that the preganglionic fibers come from the lateral horns of the thoracolumbar spinal cord segment. Preganglionic fibers here are denoted in a dotted line. These axons leave the spinal cord through the anterior nerve roots to reach the spinal nerve. Then they enter the white rami communicantes to reach the sympathetic trunk When the preganglionic fibers are within the sympatheic trunk, 4 things can happen. They can descend and synapse in a lower paravertebral ganglion. They can ascend and synapse in a higher paravertebral ganglion, which are usually the cervical parts. They can skip the sympathetic chain completely, and go all the way towards the prevertebral ganglia. Once they pass through the sympathetic trunk, they may combine with fibers from other levels to form something called a splanchnic nerve. So it’s the splanchnic nerves that synapse on a prevertebral ganglia. So they can either go to pre-vertebral ganglia, or directly towards the suprarenal gland. The fourth way is that they may synapse directly in a paravertebral ganglion at the same level. Now what happens is, after synapsing inside the ganglion, postganglionic fibers leave through the gray ramus communicans and they re-enters the same anterior ramus, which it initially travelled through, to continue with the spinal nerve, innervating all structures related to the branches of the anterior and posterior rami of the same spinal nerve. These fibers can also combine with fibers from other levels to form splanchnic nerves, which then pass onto the thoracic viscera. Same goes to the pre-vertebral ganglia. The postsynaptic fibers pass onto the abdomen and pelvic viscera via a visceral motor nerve plexus. Alright, so that was the 4 major things that I wanted you to keep in mind. The first thing being the difference between the word nuclei and ganglion. The second thing being where the parasympathetic and sympathetic outflow is located, and the difference between a pre-ganglionic and post-ganglionic fiber is. Third thing being the anatomical relation between the sympathetic ganglia. They can either be paravertebral as in the sympathetic chain, or prevertebral ganglia as in the celiac ganglia, superior mesenteric ganglia, and so on. The fourth thing I want you to know is that prevertebral fibers can do 4 things once they enter the sympathetic chain. They can go down, up, leave towards the prevertebral ganglia, or they can synapse with cell bodies within the ganglia at the same level. Alright. Finally, let us now make a scheme for the sympathetic nervous system. Just remember that this is just a scheme, nothing that is topographically correct. This is just to make it easier for you to remember. And keep in mind there are a lot of variations to these things, two different sources may show you to different schemes. I’ll just try to show you the general one. Alright. Now. Let’s cover the sympathetic trunk. Here again we see Pons, Medulla and the spinal cord. Laterally to the spinal cord we see the paravertebral ganglia, also called the sympathetic chain, or the sympathetic trunk. It receives it’s sympathetic fibers from the thoracolumbar area, that’s T1-L2 but again take it with a grain of salt. Sources also say C8 to L2, L3 region. When fibers reach the sympathetic trunk, they may ascend and synapse with a nuclei located some segments above. They may decend. They may synapse with nuclei at the same level, or they may leave the sympathetic trunk as preganglionic fibers. The sympathetic trunk is divided into cervical ganglia, which are three pairs usually. Thoracic gangli, which are 10-11-12 pairs approximately, Lumbar ganglia, which are 4-5 pairs, sacral ganglia, usually 4 pairs, and last ones are the coccygeal ganglia, they’re often fused together to form gnalgion impar, which is the unpaired ganglion at the bottom. Now, let’s talk about the sympathetic innervation of the head, neck and thorax. Most of which are innervated by the cervical ganglion. Now the cervical ganglion received fibers from approximately T1-T4, and since it’s not adjacent to the thoracolumbar outflow, they don’t have white rami communicans, only gray rami communicans. Kinda makes sense doesn’t it? Now, the cervical ganglia are divided into the superior cervical, which is the largest one of them. Middle cervical ganglion, which is extremely small. It may be divided into 2-3 smaller parts, or sometimes it may be even absent. And we got the inferior cervical ganglia, which is often fused with the 1st thoracic ganglion, to which then it’s called cervicothoracic ganglion, or stellate ganglion. It’s a headache sometimes studying the sympathetic nervous system, there are a whole lot of variations. But let’s start with the superior cervical first. The gray rami communicans of the superior cervical ganglion is going to continue into the rami of the first 4 cervical nerves, innervating structures associated with those nerves, but what’s special here is that the superior ganglion is going to give off the postganglionic internal carotid nerve, which forms the internal carotid plexus around the internal carotid artery. From here, these postganglionic fibers will go through the otic ganglion without synapsing with it, then further on towards the eye and release norepinephrine. Now think about it logically, in a fight or flight response, what is the eye gonna do in order to help in this situation? Well you wanna be able to see all options that you have in a fight or flight situation right? So the pupils are gonna dilate. So norepinephrine si gonna act on the dilator pupillae muscle, causing pupillary dilation, allowed more light to come in and give you possibility to see far. The norepinephrine is also gonna act on the ciliaris muscle, to flatten the lens, allowing to focus on far vision aswell, this is called accommodation, so norepinephrine here also helps with accommodation. Alright what else do we have from the internal carotid plexus? We got a deep petrosal nerve, which carries sympathetic fibers into the pterygopalatine ganglion and remember those are postganglionic fibers, they’re not gonna synapse in these ganglia. These postganglionic fibers come from the superior cervical ganglia. The deep petrosal nerve is gonna act on two places. It’s gonna act on the blood vessels supplying the lacrimal glands, releasing norepinephrine, decreasing the blood flow to decrease the lacrimation. Or it can act directly on the lacrimal gland decreasing the lacrimation, decreasing the tear production basically. Other places is it can act also on glands located in the oral cavity, nasal cavity, and the sinuses, specifically the maxillary and sphenoid sinus. It’s gonna do the same here. Norepinephrine cause vasoconstriction, decrease blood flow to these areas. It’s also gonna act on the salivary glands within the oral cavity, to make a thick mucus production. So that is the branches associated with the internal carotid plexus. Since we have an internal carotid nerve, we gotta have an external carotid nerve as well, right? This one is gonna form a plexus around the external carotid artery. From here, we gonna have the sympathetic root going towards the submandibular ganglion, and a Sympathetic root towards the otic ganglion, both providing sympathetic innervation to primarily the major salivary glands, which are the parotid, submandibular and the sublingual glands. Decreasing the mucus production there too. The superior cervical nerve aso give off a laryngopharyngeal nerve which join the pharyngeal branches from the glossopharyngeal and vagus nerves and form the pharyngeal plexus to basically provide motor, sensory and sympathetic innervation to the pharyngeal area. Alright let’s do the middle cervical one. Remember the middle cervical ganglion is extremely small and varies between the population. After receiving its preganglionic fibers from the upper thoracic segments, it’ll give off gray rami communicans towards 5th and 6th cervical nerves and supply structures associated with them. There are gonna be fibers from the middle cervical, as well as from the inferior cervical ganglion that’s going towards the thyroid and the parathyroid glands. Remember these glands are under hormonal control from the pituitary gland. So sympathetic nerves act as vasomotor nerves, not secretomotor, basically increasing blood flow to help produce more thyroid hormones for energy during the fight or flight response. From the inferior cervical ganglion, we got gray rami communcans that join the 7th cervical nerve and the 1st thoracic nerve. Again there are variations, but the inferior cervical ganglion also give off a vertebral nerve that travels towards the vertebral artery and forms a plexus around it. Other nerves we got here is a Superior cervical cardiac nerve, a middle cervical cardiac nerve, and an inferior cervical cardiac nerve, which all go together towards the heart, to help form the a cardiac plexus. So that was everything for the cervical ganglia. Now the thoracic ganglia all have gray rami communicans that join the intercostal nerves. But what they also have are fibers that contribute to the sympathetic innervation of the thoracic viscera. From approximately T2, T3, T4 and T5, we got thoracic cardiac nerves, which join in with the cardiac plexus to help with the sympathetic innervation. Now, if you remember from the anatomy of the heart, the heart has mainly two systems. It has a myocardial system, and a conducting system. The sympathetic nerves are gonna act on both of them, what do you think is gonna happen now? In a fight or flight response? The conducting system is composed of nodule cells, right? The SA node primarily is the functional one. They function to regulate the heart rate, so what’s gonna happen is that the sympathetic nervous system is gonna try to increase the heartrate, so it has a positive chronotropic action. And we said the heart has two main systems, the other one being the myocardium. You’re in a fight or flight response, you need more blood to the body, and so you need the cardiac muscles to contract harder to increase its cardiac output and again increase the blood pressure. So that’s the other thing that’s gonna happen, it’s gonna increase the contractility, we call that positive inotropic effect. Alright. Now let’s add some more organs here. From around the same region, there’re gonna be postganglionic fibers that’s’ gonna give sympathetic supply to the esophagus and the bronchi, basically to help form the oesophageal plexus and the pulmonary plexus. You don’t eat while you’re fighting or running, usually. So you’d wanna decrease the peristalsis in the esophagus. That’s basically what the sympathetic innervation here does. The respioratory system is opposite, you need oxygen during a fight or flight response, so it causes bronchodilation, and vasoconstriction of the bronchial arteries to decrease the blood flow and decrease the secretion of glands to help open up the airways as much as possible. So that was mainly what I wanted to talk about when it comes to the innervation of the head neck and thorax. Let’s do the sympathetic innervation of the abdominal and pelvic viscera. There are two main things here that you need to remember. The first thing is that all nerves that exit and supply the organs are called splanchnic nerves. Remember those are pre-ganglionic fibers. They don’t synapse with the paravertebral ganglia at all. What they do, is that they exit and travel towards the pre-vertebral ganglia. This is so important that you understand this concept. Here you see a section of the abdominal aorta along with some plexuses aswell. The pre-vertebral ganglions we have are the celiac ganglia, superior mesenteric ganglia, aorticorenal ganglia, and the inferior mesenteric ganglia. These ganglia have postganglionic fibers that travel through their plexuses to innervate specific organs. Again before we continue, there are a whole lot of variations when it comes to the sympathetic outflow. Different sources may give you different information, and to be fair they are all true. The reason is many of these ganglia are interconnected, but let’s go through a very basic scheme. From approximately T5 to T9, we got preganglionic fibers called the greater splanchnic nerves, that carry their fibers to the celiac ganglia. After synapsing, the celiac ganglia give off postganglionic fibres that travel along the plexus on the arterial branches from the celiac trunk, and they mainly provide sympathetic innervation to the upper organs here. So it goes to the liver to promote glycogenolysis, we’re in a flight or flight situation so we need sugar. We’re breaking down glycogen into glucose. Fibers will also go to the stomach and upper part of the duodenum to inhibit peristalsis, and also increase the contraction of the pyloric sphincter. We really don’t need our GI tract to be active at this moment, so the bile pathway is also inhibited. The pancreas is also innervated here to basically help keep our blood sugar high, it’s going to increase the glucagon and decrease the insulin production. Fibers can also go to the spleen. Now the spleen is ideally positioned within the circulatory system as a blood filter to detect, respond to, and protect against blood-borne antigens. So it’s packed with blood and lympocytes. In the spleen we got a non-lymphoid red pulp and a lymphoid white pulp, and between them is a marginal zone, a transitional site from circulation to peripheral lymphoid tissues. One important function of the marginal zone is to trap particulates and initiate innate responses against them, or to initiate immune tolerance. The sympathetic nerves here theoretically help regulate the immune activities within this system through norepinephrine. I’m not sure to what extent it’s completely relevant to the overall sympathetic response, but I’ll put my sources down in the description if you wanna read a bit more about it. Now. The greater splanchnic nerve can have a branch that comes off here and goes to the adrenal medulla. What does that mean? That means that the adrenal medulla receives pre-ganglionic fibers directly, and so these fibers go directly towards the chromaffin cells in the adrenal medulla, we call these an ‘’intramural ganglia’’. And when the chromaffin cells are stimulated, it’s gonna release norepinephrine and epinephrine directly into the bloodstream to have a widespread strong effect on the whole body. Alright. From T10 and T11, we got the lesser splanchnic nerve that primarily go to the superior mesenteric ganglion, which supplies things like the ascending colon, cecum, the proximal 2/3 of the transverse colon. The small intestine and the distal duodenum. All to inhibit the GI system, again. Decrease peristalsis, decrease absorption by constricting the blood vessels, and decreasing the secretion process. Then from T11, we got the least splanchnic nerve which may go to the aorticorenal ganglion. Again there are lots of variations. Fibers from the lesser splanchnic nerve may go to the aorticorenal ganglion. The least splanchnic nerve may go directly to the renal plexus. Different sources might tell to different things, just keep those things in mind. But essentially what’s important is that postganglionic fibers are gonna innervate the kidneys. Now what do you want the kidneys to do in a fight or flight response? You don’t really wanna pee. So you cause vasoconstriction of the blood flow to decrease the urine production within the kidneys. And, remember we got a RAAS system, a renin angiotensin aldosterone system. So sympathetic response help produce more renin from the kidneys to help increase the blood pressure. Fibers also go to the ureter to decrease the peristalsis of the ureters. From the lumbar region, we got the lubar splanchnic nerves going towards the inferior mesenteric ganglion. The inferior mesenteric ganglion is the most inferior of the prevertebral sympathetic ganglia in the abdomen. It’s a small ganglion and sometimes it may just be a loose collection of cells rather than a defined ganglion. So some sources don’t even consider it as a ganglion, you might find some sources write that preganglionic fibers synapse with cells within the plexuses of this region, wether that may be the inferior mesenteric plexus or the intermesenteric plexus. But the overall postganglionic supply is the same. And that is highlighted here. Basically supplying the rest of the GI tract which is the descending colon, sigmoid, the distal transverse colon and the rectum. Postganglionic fibers also go towards the smooth muscles within the walls of the urinary bladder. Do you wanna urinate when you’re in a flight or fight response? No. So it’s gonna decrease the contraction of the urinary bladder. And it’s going to increase the constriction of the internal urethral sphincter to help keep the urine in. Then we got the genitals. So ovaries, uterus, and so on for the female. For male it’s the penis, scrotum and so on. In male, the sympathetic response is gonna primarily help with ejaculation. For female it will act on the uterus to primarily cause uterine contractions. Not to go in too much detail. Lastly we have the sacral splanchnic nerve, which may contribute to the innervation of organs around the inferior hypogastric plexus, mainly the gonads. Remember I keep telling you this that there are a lot of variations when it comes to this system, and different ganglia and fibers are interconnected between each other. The last thing I wanna mention here is, something I haven’t mention here. But you see all of those nerves in between the ganglia? Those are all the plexuses in the abdominal region that contain autonomic nerves in general, so both sympathetic nerves and also parasympathetic fibers through the vagus nerve. So for example, the celiac ganglia, how does it reach its target organs? Through the celiac plexus. Which may be connected to the superior mesenteric ganglia as well. There is the superior mesenteric plexus, the inferior mesenteric plexus. In the pelvis we have the superior hypogastric plexus and the inferior hypogastric plexus. We got the intermesenteric plexus aswell. They contain cell bodies that act as postganglionic fibers. And when these cell bodies are clustered, that is when we get those prevertebral ganglia. And remember I told you the inferior mesenteric ganglion is sometimes scattered so much it’s not even considered a ganglion. So some sources might write that fibers synapse with cells within the inferior mesenteric plexus for example. But overall this is how the plexuses are arranged in the abdominal and pelvic cavity. In the thorax you know you may have the pulmonary plexus, cardiac plexuses, and also the esophageal plexuses. There are some other plexuses here aswell. But overall that was mainly all I had for the sympathetic nervous system. I hope it made sense! Now, comes the fun part. Remember I told you that we got some important preganglionic nerves coming from the thoracic lumbar and sacral region going towards the prevertebral ganglia? What’s the name of those preganglionic fibers? 10 points if you got that one correct. What about here? What’s the 4 main branches that is given off from the superior cervical ganglion? And I’ll reveal the answer now. Got those correct aswell? In that case you’ve grasped the most important things that you need to know when it comes to the anatomy of the sympathetic outflow The next video is going to be about the parasympathetic nervous system. Thank you so much for watching another one of my videos. If you enjoyed, learned something from it, please remember to like, comment your favourite moment, subscribe. Turn on those notifications. If you looking for other ways to support, go ahead and check out the link in the description box. Have fun ya’ll. Peace.