Transcription
Hello everyone and welcome back to my playlist of physiology. Chapter number 61 from Guyton, Unit 11. And you will see the name and realize how important the chapter is going to be. It's about the ANS, autonomic nervous system, as well as a few bits about the adrenal medulla because the adrenal medulla technically is, you know, very much connected with the sympathetic nervous system. And we'll, uh, discuss how this happens.
But, uh, from the very outset, I would like to tell you that if you really have to understand this chapter, you first need to watch my anatomy videos. I would strongly recommend a video on the strong organization of the autonomic nervous system, and then you will very easily understand this chapter.
You know that the autonomic nervous system is the portion of the nervous system that controls most visceral functions of the body. So, body internal organs, stomach, lungs, heart, visceral functions, functions of internal organs, they are largely controlled by the autonomic nervous system. We also call it the involuntary nervous system because this is the nervous system which is actually not directly under the control of your will. So, for example, if you have to pick up a ball from here, you know that you have to pick up the ball, and you contract your muscles, you bend your bones, and you move towards the ball and pick up the ball. So that is known as a voluntary action. But see, your heart is beating, you know, and your heart is beating, um, even if you want to stop it, it will not stop. It will continue beating because this is an involuntary control under the autonomic nervous system. So, uh, stomach, for example, if there are stomach, um, glands which are secreting something, uh, if I ask you to stop that secretion on your own will, can you, can you do that? No. This is not possible because the stomach glands are secreting their secretions involuntarily because they are under the control of the autonomic nervous system.
So, most of the, you know, functions of the viscera of your body are controlled by this, such as control of the arterial pressure, GI motility, GI secretion, urinary bladder contractions, emptying, and sweating and body temperature regulation. These are all called, um, if you like, visceral functions of the body. Some of these activities are controlled almost entirely, and some partly by the autonomic nervous system. One of the most striking characteristics of the autonomic nervous system is the rapidity and intensity with which it can actually change the visceral function. So, very quickly, it can change the blood pressure, very quickly, it can change the rate of the motility of your GI tract. For example, in three to five seconds, it can increase the heart rate to twice the normal. That is the effectiveness of your autonomic nervous system. Within 10 to 15 seconds, the arterial pressure can be doubled. So, it's, it's, it's, I mean, super duper fast, actually. At, uh, the other extreme, the arterial pressure can be decreased low enough within 10 to 15 seconds. So, remember this point that, um, the autonomic nervous system is capable of changing the visceral functions of your body actually very, very rapidly and very, very quickly.
Okay, now, general organization of the autonomic nervous system is something very important to understand. And as I told you in the very beginning, please watch my anatomy videos. Please watch my neuroanatomy videos which explain the general organization in much more great detail. But anyways, we are going to recap it here. The autonomic nervous system is activated mainly by the centers which are located in the spinal cord, brain stem, and hypothalamus. So, these are the areas which actually are the control powers for the autonomic nervous system. Also, portions of the cerebral cortex, which is what we call the higher centers of cognition, especially the limbic cortex, can transmit signals to control the, uh, the word used to influence the autonomic control. So, remember that your autonomic nervous system, which is the sympathetic and the parasympathetic nervous system, these are not controlled by anything and they are actually automatically, you know, regulating different body functions. They are well controlled and well influenced by other parts of your brain. Okay, of not only the brain but also the other parts of the nervous system, I would say, because of the spinal cord control.
Now, the autonomic nervous system also often operates through visceral reflexes. That is, subconscious sensory signals which go to the system, and then the system fires back. So, stomach, for example, GI motility sends signals, and then the signals come back, and the motility either increases or decreases by autonomic signals. Now, this word "efferent" signals, basically what does it basically mean? Suppose this is your stomach, and then this is the sympathetic system, for example. So, the fibers going from the stomach to the system, the nervous system, these are called afferent fibers. And then the fibers which emerge from the nervous system and come towards the organ, these are called efferent fibers. So, whenever the word "efferent" means these are the fibers which are coming towards any system, basically nervous system component.
Now, the efferent autonomic signals are transmitted to various organs of the body by two major pathways. One is known as the sympathetic system, and the other one is known as the parasympathetic system. So, these two comprise together the autonomic nervous system. Okay, the characteristics and functions of which are described in the following sections. We'll discuss what is the parasympathetic system, what is the sympathetic system. So, that will be dealt with in, uh, great detail in the next few headings. But remember, these are, um, you know, the two major key distributions of the autonomic nervous system.
Before I move forward in this particular chapter, it is important to understand the basic anatomy and the concepts which are actually described in these purple boxes, which I usually leave when I'm studying Guyton for you. The reason being, sometimes it's about the pharmacology, or sometimes it's about the clinical stuff which is actually dealt with in more detail during the medicine lectures or the pharmacology lectures. But sometimes these purple boxes are actually an integral component of the chapter that we are going to understand. Such as in this particular instance. So, uh, if you don't understand this anatomical distribution of the sympathetic and parasympathetic nervous system, we will not be able to understand the chapter with clarity. Let's go through this. I'll take you first through the organization of the sympathetic system, and then we'll talk about the parasympathetic system. So, let's have a look at this diagram. This clearly tells the organization of the sympathetic system.
So, what basically happens? Okay, obviously, you know this is your brain, and this is your spinal cord. Alongside the spinal cord, there is a chain of sympathetic ganglia, and this is known as the paravertebral chain. It's present on both sides of the spinal cord. And then we have a series of prevertebral ganglia, including the celiac ganglion, the superior mesenteric ganglion, the aortico-renal ganglion, inferior mesenteric ganglion. Now, ganglia is a place where the neurons meet. So, say, for example, if I'm seeing this is a ganglion, it would mean that there will be a synapse between one neuron and the other neuron. The neuron which is coming from somewhere to the ganglion is known as the preganglionic fiber. So, that is what is known as preganglionic, and the name indicates, uh, this with great clarity, preganglionic. So, ganglion, say, pre, pre to the ganglion, before the ganglion. So, that's called a preganglionic fiber. And preganglionic fiber will obviously be coming somewhere from the central nervous system towards the ganglia. And then there will be a synapse, and then there will be another neuronal fiber which will get away from the ganglia, and it will go to some target organ. And this fiber is then known as the postganglionic fiber. So, there are two sets of neurons which are called preganglionic neuron and the postganglionic neuron. And the pre and the post words are with reference to the ganglion where the synapse is actually happening. The preganglionic neuron is coming from the central nervous system, going towards the ganglion. And the postganglionic neuron is coming from, after the synapse, obviously coming from the ganglion and moving towards the organ, the target organ.
Okay, now, if we look at the ganglion chain here, which is the paravertebral ganglion chain, we will appreciate that the preganglionic fibers are actually coming from the spinal cord and they come towards this chain. Now, there are three things which are possible. Number one, uh, the preganglionic fibers actually, they are present in red here. So, if I enlarge the image, you will appreciate that these red fibers, these are the preganglionic fibers coming from the spinal cord and going towards the ganglionic, sympathetic chain. So, they come to the sympathetic chain. Now, they have three options. Either they synapse here at the level where they are entering into the, the ganglion. They synapse with the same ganglion. Or the preganglionic fiber can arise from the spinal cord and travel up or travel down and synapse with some other ganglion, not with a ganglion at the same level. And the third option is that the preganglionic fiber comes into the ganglion and then does not synapse here with the ganglion, and it travels away. It synapses somewhere else. For example, it synapses in the celiac ganglion. Or it is possible that this preganglionic fiber, uh, comes from the spinal cord, goes into the ganglion, does not synapse here, and goes straight to the adrenal medulla and synapses there. So, these are different possibilities, different routes that the preganglionic fiber can actually take once it is out from the spinal cord. So, that's the basic, basic idea.
Now, once the preganglionic fiber decides what it has, it has really, you know, what is going to be the fate of this preganglionic fiber in terms of synapse. If it is synapsing within the same level ganglion present in the paravertebral sympathetic chain, then the postganglionic neuron will exit from here and go somewhere towards any particular organ. Um, but if the preganglionic fiber is not synapsing at the same level, it will go up, synapse there, and then the postganglionic fiber will come from there. Or if the preganglionic fiber is coming to the ganglion chain, not synapsing here, and then going to the superior mesenteric ganglion, then it will synapse here, and the postganglionic neuron will emerge from the superior mesenteric ganglion. Or if the preganglionic fibers come from the central nervous system, go to the paravertebral sympathetic chain, does not synapse here, does not synapse with the prevertebral ganglia, and then goes straight to the adrenal medulla and synapses there. So, all these possibilities you have to bear in mind with great clarity. Okay, otherwise, you'll be struggling actually to understand the organization of the sympathetic chain.
Now, let's read the text which is then, uh, easily understandable once we have gone through the diagram. So, what they're talking about is Figure 61.1, which we have just seen, shows the general organization of the peripheral portion of the sympathetic nervous system. It contains one of the two, uh, paravertebral sympathetic chain ganglia. So, there are two, on one on each side of the spinal cord. We also had a look at the prevertebral ganglia, which include celiac, superior mesenteric, inferior mesenteric, and hypogastric. And number three, we had a look at the nerves extending from the ganglia to different internal organs. So, these nerve fibers are going to, for example, to the liver, for the pancreas, to the GI tract, to different parts of the body.
Now, the organization of the sympathetic system is, uh, in the way that the preganglionic fibers are originating from, you know, thoracic and the lumbar vertebrae. Therefore, it is also known as thoracolumbar outflow. Okay, usually from thoracic vertebrae 1 to lumbar vertebrae 2. And the preganglionic and the postganglionic fibers are arranged in a way that the preganglionic fibers emerge from the central nervous system. And to be very specific, in the anterior medial lateral horn of the spinal cord. From this is from where the preganglionic fibers actually emerge. Now, they can actually take one of the three routes, as I already told you. They can either synapse with the postganglionic sympathetic neurons in the ganglia when they enter. They can move up or down. And they can pass through the parasympathetic, through the paravertebral sympathetic chain without synapsing, and they may go to the, you know, prevertebral ganglia. All right.
Also, let's discuss the concept of the gray and the white rami communicantes. So, this diagram nicely explains this concept. So, this is, for example, the spinal cord, and then the spinal cord central root and the dorsal root. And this red bit is the preganglionic fiber. It emerges from the spinal cord, goes into the spinal nerve, and from the spinal nerve, white rami, it gets connected with the sympathetic chain. And in the ganglia, synapses happen. And then the postganglionic fiber goes via the gray rami and again enters the spinal nerve and goes to different parts of the body, such as muscles. So, you have white ramus communicans and gray ramus. White ramus communicans because it is communicating between the spinal nerve and the sympathetic chain. White, so that basically means it contains preganglionic fiber. And gray ramus usually contains the postganglionic fiber. Okay, so these are the terminologies that you have to understand.
And, um, then if we talk about, um, uh, more than this, okay, what organs are affected and what the functions, it's more sort of details in my anatomy videos. Okay. Now, let's talk about the parasympathetic nervous system arrangement. The parasympathetic nervous system arrangement is well depicted here. It's a craniosacral outflow. So, from the sacral vertebrae and from the cranial portion, which means the cranial nerves, the preganglionic fibers are emerging from the central nervous system, from the cranial nerves and from the sacral nerves. So, these are the preganglionic fibers. Unlike the sympathetic system, they do not synapse with the paravertebral system of ganglia. They are long preganglionic fibers. They go to some organ, and the ganglia which are present near the organ. So, for example, cranial nerve 3, the preganglionic fibers go to the ciliary ganglion, synapse there, and then the short postganglionic fibers go to the eye and supply to the eye. Similarly, if you talk about cranial nerve 7, the preganglionic fibers travel to the pterygopalatine, to the submandibular, and then the postganglionic fibers supply the lacrimal gland and the submandibular glands respectively. Vagus nerve contains preganglionic fibers, and then they go to the heart. The postganglionic fiber, this is where the synapse is happening.
So, if you consider, uh, this particular sympathetic system, most of the preganglionic fibers are short, and the postganglionic fibers are long. And this is, I mean, apart from the adrenal medulla. I would say in the adrenal medulla, the preganglionic fibers are going all the way to the adrenal medulla. So, they are long preganglionic fibers. But most of those preganglionic fibers which are actually synapsing with the paravertebral sympathetic chain, they are very short preganglionic fibers. But in case of the parasympathetic system, these are very, very long preganglionic fibers and short postganglionic fibers because the ganglia are present near to the target organs. So, that is something very, very important which you have to understand. So, that's a good diagram to remember, and that's a good diagram to remember the anatomical organization of sympathetic and parasympathetic nervous systems.
Now, some basic principles about sympathetic and parasympathetic function. Although I repeat from the very beginning of this chapter that you have to watch my autonomic nervous system anatomy videos because I've discussed it, but again, since it is part of this chapter, we are not going to skip it. We will talk about it. So, first of all, cholinergic and adrenergic fibers, secretion of acetylcholine or norepinephrine. So, the sympathetic and parasympathetic nerve fibers secrete mainly one or the other two types of synaptic neurotransmitters, either acetylcholine or norepinephrine. The fibers which are secreting, so if there is a neuron which is secreting acetylcholine, it is known as a cholinergic fiber. Or if a fiber which is secreting norepinephrine is called an adrenergic fiber. What is a cholinergic fiber and what is an adrenergic fiber? A cholinergic fiber is the one which is releasing acetylcholine at its nerve ending. And the, uh, other one, the adrenergic fiber, is the one which is secreting norepinephrine. Important point: all preganglionic neurons are cholinergic. So, that's a rule of thumb. All those neurons which are emerging from the central nervous system, either in the parasympathetic system or in the sympathetic system, all of them release acetylcholine. So, they are all cholinergic. Acetylcholine or acetylcholine-like substances when applied to the ganglia will excite both the sympathetic and the parasympathetic system. That's another important point. So, point number one: all the preganglionic fibers are releasing acetylcholine. Point number two: they all excite the ganglia. Now, either all or almost all the postganglionic neurons of the parasympathetic system are cholinergic. Now, here is an exception. In contrast, most of the postganglionic sympathetic neurons are adrenergic. So, um, so you see the divide here. All preganglionic, whether it is sympathetic or parasympathetic, cholinergic. All postganglionic from the parasympathetic system, cholinergic. All postganglionic from the sympathetic system, adrenergic. So, now you get a feel that which part of your autonomic system is secreting what type of neurotransmitter. Therefore, the terminal nerve ending of the parasympathetic system, all or virtually all, secrete acetylcholine. Because in preganglionic fiber, acetylcholine is released, and postganglionic, releases acetylcholine. So, I can, with a lot of confidence, I can say that the neurotransmitter of the parasympathetic nervous system is basically acetylcholine. So, if I go back to this diagram, for example, that's a diagram which is showing you the parasympathetic system. So, this is a preganglionic fiber of the parasympathetic system. So, what will be released here? Acetylcholine. What will be released here? Acetylcholine. Because this is the preganglionic fiber, and this is the postganglionic fiber. So, uh, if I change the scenario to the sympathetic system, we need to understand that the, so, so if I ask you, what will be released here? Acetylcholine. Because this is where the preganglionic fibers are coming. But what will be released here? Norepinephrine. Because this is the postganglionic fiber of the sympathetic nervous system. Okay, that is super important information. You must understand this right now.
The molecular structures, they are not important for acetylcholine and not for epinephrine, not at all. They don't test you, um, on these topics in examinations. The mechanism of transmitter secretion and removal at the postganglionic endings, again, not so important, not very high. But let's go through it very quickly. Secretion of acetylcholine and norepinephrine by the postganglionic nerve ending. So, we're talking about those nerve endings which are postganglionic. A few of the postganglionic autonomic nerve endings, especially those of the parasympathetic nervous system, are similar too, but much smaller than those of the skeletal neuromuscular junction. Getting this point? However, many of the parasympathetic nerve fibers are almost, and almost all the sympathetic nerve fibers, merely touch the effector cells of the organs that they innervate as they pass, or in some cases, they terminate in the connective tissue. So, they are exactly not touching the cells, but they are some distance away. So, that's fine. Where these filaments touch or pass over or near the cells to be stimulated, they usually have bulbous enlargements, and those are known as varicosities. That's that's okay. I mean, varicosity is something where they have a terminal bouton type of thing. Okay. When an action potential is spread over the terminal fiber, the depolarization process increases the permeability of the fiber membrane to calcium ions, and because of the calcium which enters, it helps expulsion of the neurotransmitter from the nerve ending. It's, it's almost similar to what happens in the neuromuscular junction.
Now, synthesis of acetylcholine, its destruction after secretion, and its duration of action. Acetylcholine is synthesized in the terminal nerve endings and varicosities of the cholinergic nerve fibers, both preganglionic fiber and postganglionic fiber of the parasympathetic system. It is synthesized or degraded. But the basic chemical reaction for the synthesis is acetyl-CoA plus choline. This is the enzyme used, and you get acetylcholine, abbreviated as ACh. Once acetylcholine is secreted into a tissue by a cholinergic nerve ending, once it is released, it resists in the tissue for a few seconds, then it splits into acetate and choline, catalyzed by acetylcholinesterase, which you have done this enzyme in neuromuscular junction physiology, which is bound with collagen and glycosaminoglycans in the local connective tissue. The mechanism is the same as that of acetylcholine signal transmission and subsequent destruction that happens in the NMJ, the neuromuscular junction.
If you talk about synthesis of norepinephrine, this is where we are talking about the postganglionic fibers of the sympathetic system. Uh, it begins in the axoplasm of the terminal nerve ending, obviously, the postganglionic fiber's nerve ending. Okay. The basic steps include tyrosine, key hydroxylation to dopa, dopa decarboxylation to dopamine, and then transport of dopamine into the vesicles. Then dopamine to norepinephrine by hydroxylase. Norepinephrine can also be converted into epinephrine by methylation. Okay. After secretion of norepinephrine by the terminal endings, it is removed from the secretory site by number one, reuptake into the adrenergic nerve ending. So, this is getting again into the postganglionic fibers. Number two, diffusion away from the nerve ending. And number three is the destruction or destruction by enzymes. Obviously, the enzyme which is very active is the monoamine oxidase, monoamine oxidase, the MAO enzyme, which is found in the nerve ending. And another which is called catechol-O-methyltransferase, catechol-O-methyltransferase, the COMT, which is present diffusely in the tissue. So, these are the two important enzymes which are involved for, you know, degradation of norepinephrine. Ordinarily, the norepinephrine secreted directly into the tissue remains active only for a few seconds and then is acted upon by MAO and the COMT. Okay. So, this is how they are synthesized, and they are secreted, and they are degraded.
Now, obviously, we, um, are talking about, see, this is the whole scheme. This is the nervous system. These are the preganglionic fibers. Here is the ganglion. Then this is the postganglionic fiber, and this is the organ. Now, the postganglionic fiber, depending upon the sympathetic system, postganglionic fiber, parasympathetic system, depends. And then with this, it also depends on the receptor because whatever is being released, the neurotransmitter is obviously going to act on some sort of receptor. Receptors, what are the different types of receptors on the effector organ? Effector organ, it can be stomach, it can be heart, just an organ. Per, postganglionic fibers for their action, that is known as an effector organ or effector organ receptor. That is obviously called the receptor.
So, now the heading, this is very actually interesting heading: Receptors on the effector organs. Before acetylcholine, norepinephrine, or epinephrine, which are secreted at the autonomic nerve endings, before they can stimulate the effector organ, they must first bind to the receptors. H, the receptor is on the outside of the cell membrane of the effector organ. Obviously, binding of the transmitter substance with the receptor causes conformational changes of the protein molecules. In turn, the altered protein molecule excites or inhibits the cell, causing a change in the cell membrane permeability or activating or inactivating the enzymes which are attached to the other side. So, that's how basically any receptor system works. If this is the cell, this is the end organ, there will be a receptor, and the neurotransmitter will bind to them. Simple. Excitation or inhibition of effector cell by changing its membrane permeability. So, either the membrane permeability is changed, usually by either opening or closing of any ion channel, so that the cell is either excited or, you know, inhibited. Receptor action by altering the intracellular protein, which is what we call the second messenger system, the cyclic AMP system, for example. There is also the GMP protein. So, if the intracellular proteins are the cyclic or the second messenger system is affected, that is also a mechanism by which the neurotransmitters work. Important. Yeah, you should know what are the different types of receptors.
One category is what we call the muscarinic receptors, and the other category is what we call the nicotinic receptors. Fibers and acetylcholine activate mainly two types of receptors, which are called muscarinic and nicotinic receptors. So, these are both activated by acetylcholine towards the, you know, end organs in the sympathetic or the parasympathetic nervous system. So, what type of receptors would it work on? It will work on two types of receptors: muscarinic and nicotinic. The reason for these names, uh, is that muscarine, a poison from Amanita muscaria, activates only muscarinic receptors. And so, it has, like, kind of historical perspective of why they are called so. Muscarinic receptors, which use G protein as their signaling mechanism, are found on all the effector cells that are stimulated by the postganglionic cholinergic neurons of either the parasympathetic nervous system or the sympathetic nervous system. So, everywhere in this distribution. Nicotinic receptors are ligand-gated ion channels found in autonomic ganglia at the synapse between the preganglionic and the postganglionic fiber. So, ganglia, there are nicotinic receptors, and postganglionic effector intervention, here, there, we have got muscarinic receptors.
Okay. Alpha and beta adrenergic receptors. Two major classes of adrenergic receptors also exist. They are called alpha receptors and beta receptors. These are adrenergic receptors. So, not acetylcholine, but norepinephrine and epinephrine receptors. Now, there are two major types of alpha, alpha subcategories: alpha-1 and alpha-2. And similarly, beta is beta-1, beta-2, and beta-3. And the beta receptors also use G protein signaling. Again, receptor physiology, I've already, so please watch my physiology videos to understand, uh, these receptors in more great detail. Now, norepinephrine and epinephrine, both of which are secreted into the blood by the adrenal medulla, have slightly different effects in exciting alpha and beta receptors. Norepinephrine excites mainly alpha receptors, but excites the beta receptors to a lesser extent as well. So, that's how the norepinephrine works. Epinephrine excites both types of receptors approximately equally. Memorize Table 61.1. It lists the distribution of alpha and beta receptors in some organs and systems controlled by the sympathetic system. So, we'll have a look at these, um, you know, this particular table. So, alpha receptors' job, beta receptors' job. So, alpha receptors, for example, do vasoconstriction. Iris dilates. Medulla, intestine, relaxation. Intestinal sphincter, contraction. See, intestine itself, relaxation. Like in constriction of the, uh, sphincters. Pylorus, contraction. Bladder, contraction. Sphincter, urination, inhibit neurotransmitter release. So, these are all alpha receptors. And the beta receptors are all these: glycogenolysis, lipolysis, bladder wall relaxation, thermogenesis. So, just read this table. Important one for you to remember.
Okay. Excitatory and inhibitory reactions of sympathetic and parasympathetic stimulation. So, we'll now go through this particular table in a minute, which lists the effects of different visceral functions of the body caused by stimulating either the parasympathetic system or the sympathetic system. Note again, or receptor types. It's very, super important to understand what we are going to discuss now. Note again that sympathetic stimulation causes excitatory effects in some organs, but inhibitory effects in others. Likewise, the parasympathetic system causes excitation in some organs, but also inhibition in others. So, depending on the context, it can do both of the stuff. There is no generalization one can use to explain whether sympathetic or parasympathetic stimulation will cause excitation or inhibition. It depends upon the organ. It depends upon the tissue. So, for example, if you look at this table here, if we simply take the eye and pupil, it is dilated by sympathetic stimulation. It is constricted by parasympathetic action. So, this is a way to read this table. Pick up an organ. So, sweat gland, for example, see what is the effect of the sympathetic system. It is sweating. See what is the effect of the parasympathetic system. It is sweating on palms and hands. See, pick anything. Heart, for example, increased heart rate by sympathetic system, decreased heart rate by parasympathetic system. Okay, so go through this. Try to memorize. I don't have to get exams, you have to. So, you must remember this. Okay.
Right, right, right, guys. Next, move on to the next heading, which is the function of the adrenal medulla. I told you, adrenal medulla is actually sympathetic because spinal cord preganglionic fibers. So, long preganglionic fiber, and this is just like a ganglion of the sympathetic system. So, stimulation of the sympathetic nerve to the adrenal medulla causes large quantities of epinephrine and norepinephrine, which is released by the chromaffin cells. On average, about 80% of the secretion is epinephrine and 20% is norepinephrine. So, the preganglionic fiber, when excited, chromaffin cells, they release epinephrine and norepinephrine. The circulating epinephrine and norepinephrine have almost the same effect on different organs as effects caused by direct sympathetic stimulation. So, adrenal medulla is actually considered a part and parcel of the sympathetic system. The circulating norepinephrine causes constriction of most blood vessels in the body. Of the autonomic nervous system, the stimulation of the adrenal medulla causes release of hormones epinephrine and norepinephrine, which have sympathetic activity. The adrenal medulla supports sympathetic nervous system function because it works just like a sympathetic system.
Relation of the stimulus to rate of sympathetic and parasympathetic effect. A special difference between the autonomic nervous system and the skeletal nervous system is that only a low frequency of stimulation is required for full activation of the autonomic effectors. So, property because of what? Fast action, quick action from the sympathetic and parasympathetic system. And the word sympathetic and parasympathetic tone, basically means what? Normally, sympathetic and parasympathetic systems are continually active, and the basal rate of activity are known respectively as the tone. The basal tone of the autonomic nervous system. The value of tone is that it allows a single nervous system to both increase and decrease the activity. Theoretical concept. I would not worry about this too much. Okay, so just don't worry about this basal, uh, tone of your autonomic nervous system. It's not something that would, you know, be a big trouble in your examination. They won't actually touch upon it. Right.
So, uh, we now have to start another very important concept, which is how basically the sympathetic and the parasympathetic system fire. Do they fire individually? Do they fire to particular organs? Or do they have mass discharge property? So, let's go through this concept. Sympathetic system, it sometimes responds by mass discharges. Mass discharges, as the name indicates, a portion of the sympathetic nervous system actually discharges simultaneously. So, it is going to affect, uh, the blood pressure, for example. It will affect the rate of metabolism. It will affect, for example, blood glucose concentration. So, there will be mass firing or mass discharge of the sympathetic nervous system. So, mass discharge is usually in a stress situation, body response, fight and flight situation. At other times, the activation occurs in isolated portions. So, for example, isolated activation. If the body requires heat regulation, then the sympathetic control towards sweating and blood flow of the skin, that is modulated. So, that is called a local reflex. Yeah, isolated response. Many local reflexes involving sensory afferent fibers travel centrally in the peripheral nerves to the sympathetic ganglia and spinal cord and cause highly localized reflex responses. So, that's response phenomena which include localized reflexes. It is also a local response. Many of the sympathetic reflexes that control GI functions operate by the way of nerve pathways that do not even enter the spinal cord, merely passing from the gut, mainly to the vertebral ganglia and then back through the sympathetic nerves to the GI parts to perform their secretory function. So, the point is, okay, if we talk about the sympathetic system, there are two modes of, you know, operations. One, there can be a mass discharge, and this is something which the body requires during fight and flight situation, yeah, alarm situation, yeah, a stress situation. And the other phenomena is the local or, you know, individual kind of response. Heat regulation, sympathetic system will be activated. If GI tract response, then the sympathetic system going towards the GI will be activated, that sort of thing.
Parasympathetic system. Parasympathetic system usually, mass response does not exist. So, there's no mass response in the parasympathetic system. It's usually localized response, and it's actually very precise control. So, the control functions by the parasympathetic system are often highly specific. For example, parasympathetic cardiovascular reflexes usually act on the heart only to increase or decrease its rate. So, if heart rate is the target, only the parasympathetics towards the heart are stimulated. Likewise, other parasympathetic reflexes causing secretion mainly by the mouth glands, or in other cases, secretion, um, is mainly in the stomach glands. So, specific target organ, specific response. Yet, there is often association between closely allied parasympathetic functions. For example, although salivary secretion can occur independent of the gastric secretion, the two usually occur together. But that is still not known as the mass response. It is still known as the individual organ response. Okay.
All right. So, uh, with this concept, let us, uh, spend a few minutes on the alarm or the stress or the fight and flight situation, which is what we call the mass discharge of the sympathetic nervous system. Sympathetic system, different organs' effects, multiple. So, for example, there will be increased arterial pressure, so increase in the blood pressure. Increase in the blood flow to active muscles, so that you can run appropriately. Increase in cellular metabolism. Increase in blood glucose concentration, increase glycolysis, blood glucose. Increase muscle strength. Increase mental activity. Increase rate of blood flow. All these things. So, if you are standing like this, and there's a dog behind you, and now is the situation where you have to either run or get bitten by the dog. So, you would rather prefer to run. And in order to run, the body may, there is a rush of sympathetic nervous system. That rush is known as the stress response or the mass response by the sympathetic system. The sympathetic system is especially strongly activated in many emotional states as well. For example, the stage of rage, sympathetic discharge is there. So, that's the sympathetic alarm reaction, making you ready for fight or flight situation. So, either you fight with the dog, or you fly away from here.
Okay. Now, medullary, pontine, and cortical control of the autonomic nervous system. Now, autonomic nervous system anatomy, already, already. So, you know how it is controlled, how it is regulated. But a few words here. Many neuronal areas in the brain stem, reticular substance, and along the course of the tractus solitarius of the medulla and pons, and also in the, you know, special nuclei, they all together control different autonomic functions such as the blood pressure, the heart rate, the glandular secretions of the GI tract, as well as the contraction of the bladder. Control of each of these is discussed appropriately, obviously, when, uh, individually they are discussed in neuroanatomy. But the point is that you should know, obviously, and you know this, that the sympathetic system is very well controlled and regulated by the higher centers. Okay.
Now, the control of the brain stem autonomic centers by higher areas. It's the same concept. Your, I mean, neurology, lectures, always my point is the fact that you have to understand that no system in the brain is like individual or respectively working of the other system. They're all very well connected. They're all very well concerted, and they perform their functions in a very coordinated manner. So, the sympathetic system is not controlled by the higher centers. Brain stem, there's very good control, or the brain stem is left without any control. It is controlled by the higher centers in the cerebral cortex. So, uh, that's the bottom line, basically, that the whole system is very well regulated.
Okay. So, basically, with these concepts, uh, the blue boxes, you know, I'm leaving blue boxes in physiology for now. These particular blue boxes deal with pharmacology of the autonomic nervous system, which we have actually done with from Guyton in detail. So, if interested, so please go watch my Guyton videos. Already available, autonomic nervous system Guyton pharmacology. Please go review this there. Okay, so I'm not going to go into the details of, you know, how each muscarinic receptor is blocked, or how do you block the postganglionic neurons. So, this has all been dealt with already. Okay.
So, with this, this chapter ends here. I wish you all the very best. Please subscribe to the channel if you like the video. Share the video with your colleagues. Hit the bell icon, and I'll see you in another video very soon. This is Professor Asif Qureshi, and you are watching Dr. Asif Lectures.