Lecture 19 - Autonomic Nervous System PDF

NOTE: Transcripts are made from the auto-generated Lecture Captions, so are not edited for grammar/spelling. Lecture 19 - Autonomic Nervous System Video 1 Introduction Welcome to the next online lecture. In this lecture we're going to be looking at the autonomic nervous system. So we've talked a...

NOTE: Transcripts are made from the auto-generated Lecture Captions, so are not edited for grammar/spelling. Lecture 19 - Autonomic Nervous System Video 1 Introduction Welcome to the next online lecture. In this lecture we're going to be looking at the autonomic nervous system. So we've talked already about the organization of the autonomic nervous system. So we'll do a quick review first, and then we'll look at some of the specifics of the sympathetic nervous system and the parasympathetic nervous system. We're also going to be talking about some of the neurotransmitters that are part of the autonomic nervous system, as well as the specialized receptors that will change what happens on our postsynaptic membranes. And then we're going to finish up by talking a little bit more the enteric nervous system. So let's get started. Slide 1 The material for this online lecture can be found in chapter 16 of your textbook. This image is showing you the different ways that the autonomic nervous system works on target tissues in the body. Recall the divisions of the autonomic nervous system that we talked about earlier in the course. The sympathetic nervous system and the parasympathetic nervous system. The sympathetic nervous system, which is highlighted here in blue. And the parasympathetic nervous system, which is highlighted in red, have an extremely diverse range of connections to organs in the body. And often these connections play an important role in maintaining homeostasis. Notice that in many cases, the divisions of each of these reached the same organs. That enables us to have tight regulation of these tissues. And it's called dual innervation. Some of these tissues include the smooth muscle of specific tissues, cardiac muscle and glands. The autonomic nervous system is important for creating reflex loops which sense organ function and coordinate an appropriate neural response to maintain homeostasis. We will be looking at an example of this on an upcoming slide. Slide 2 So if we want to look at what's different about the autonomic nervous system... Let's first take a look back at the organization of the peripheral nervous system. The peripheral nervous system is divided into the sensory or afferent division, which transmits action potentials to the central nervous system from the periphery. And we also have the motor or efferent division, which transmits action potentials from the central nervous system to our effector organs, which are basically our target tissues. We'll be focusing mostly on the motor division. The motor division is further divided into the somatic nervous system and the autonomic nervous system. The somatic nervous system innervates skeletal muscle. It's a one neuron system, meaning the cell body portion is located in the central nervous system, in either the motor nuclei of the brain for the cranial nerves, or in the anterior horn of the spinal cord for the spinal nerves. The axon then travels directly to its target tissue, the skeletal muscle. Somatic motor neurons are always myelinated and they can only ever cause an excitatory response. So you can either send an action potential and get a muscle contraction or you don't send an action potential and the muscle doesn't contract. You can't send an inhibitory action potential with a somatic motor neuron. So you can't send a signal to relax skeletal muscle. Instead, you just don't send any signal. In contrast, the autonomic nervous system innervates cardiac muscles, smooth muscles, and glands, and it's a two neuron system. The first neuron is the preganglionic neuron, and that's the one that's closest to the central nervous system. It has its cell bodies within the central nervous system. And they're located in neither the autonomic nuclei of the cranial nerves, which are located within the brain stem, or in the lateral horn of the spinal cord. The preganglionic neuron is always myelinated and it synapses with the postganglionic neuron, which is the second neuron in the two neuron system. It extends from the synapse, which is located in the ganglion outside of the central nervous system, to the effector or target tissue. The postganglionic neuron is always unmyelinated. And in contrast to the somatic system, it releases neurotransmitters that can cause either an excitatory response or an inhibitory response. While we focus on the motor side of things, there is a somatic and autonomic nervous system division on the sensory side as well. The somatic division responds to consciously perceived sensations like those that come from our skin or our special senses. Whereas the autonomic division responds to unconsciously perceived visceral sensations within the body. You can see a number of these comparisons made between these two systems in your textbook in table 16.1, if you're interested, but you're not required to memorize all of that extra information. Slide 3 So this just illustrates these two systems. In our somatic system, we have the cell bodies in the anterior horn of the spinal cord. They exit through the ventral root and then they join into the spinal nerve. It then travels out to our effector or target tissue, which is skeletal muscle. Here we release the neurotransmitter acetylcholine, and this will cause muscle contraction. However, in our autonomic nervous system are preganglionic neuron has cell bodies in the lateral gray horns or within the brainstem. It also leaves through the ventral root and joins into the spinal nerve however, it will then synapse with the postganglionic neuron in a region called the autonomic ganglion. The preganglionic neuron, as I mentioned, is always myelinated and the postganglionic neuron is always un-myelinated. The preganglionic neuron always releases acetylcholine as its neurotransmitter. Whereas the postganglionic neuron can then synapse with our effector organs, either smooth muscle, cardiac muscle, or glands. The postganglionic neurons can release neurotransmitters that are either acetylcholine or norepinephrine. And we'll look at the examples of where we would release one over the other in a moment. Slide 4 Within the autonomic nervous system, there are two main divisions, and one division that isn't really part of the autonomic nervous system, but it's closely associated with it. The two major divisions are the sympathetic nervous system and the parasympathetic nervous system. Most organs in the body have dual innervation from both of these divisions. So one tends to speed things up and the other tends to slow things down. For example, heart rate,the parasympathetic nervous system will decrease your heart rate and the sympathetic nervous system will increase your heart rate. The third division is the enteric division, which provides innervation to your digestive tract. Slide 5 This slide is showing the anatomy of the sympathetic nervous system, which is often referred to as the thoracolumbar division because of the location of the cell bodies of these neurons within the central nervous system. They originate in the lateral horns of the spinal cord at levels T1 through L2, which are represented by these blue lines. The neurons will then exit the spinal cord through the ventral route. They'll join in the spinal nerve for a short period of time. And then they'll enter one of two types of sympathetic ganglia. The first type of sympathetic ganglia is called the sympathetic chain ganglia or the sympathetic trunk ganglia. Its a collection of postganglionic cell bodies that are close to the spinal cord, and they actually connect to each other to form two chains on either side of the spinal cord. The other type is the collateral ganglion or pre-vertebral ganglion, which is a collection of postganglionic cell bodies that are closer to the effectors or the target tissues. There are four main routes for our sympathetic axons after leaving the chain ganglion. These are highlighted on the next two slides, but you can see some of them here. One is showing the axons of the preganglionic neuron can exit the ventral root and form the spinal nerve. They then exit the spinal nerve and enter into the sympathetic chain ganglion. They can either synapse at the same level or travel up and down the chain ganglion and synapse with the post ganglionic axon on a different level. The post ganglionic axon will then leave the sympathetic chain ganglion and travel to its effector. Another option is for the preganglionic nerve to move through the chain ganglion without synapsing and travel to the collateral ganglion. It would then synapse there with the postganglionic neuron, which would then leave and travel to its target or effector tissue. Slide 6 This shows in more detail the four main routes of sympathetic neurons as they leave the spinal cord, and it's also summarized on the next slide. As we just saw on the last slide, the first one is showing the axons of the preganglionic neuron leaving through the ventral root and forming the spinal nerve. They then can exit the spinal nerve and enter into the sympathetic chain ganglion. They can then either synapse at the same level or they can travel up or down the sympathetic chain ganglion and synapse with the post ganglionic axon at a different level. The post ganglionic axon will then leave the sympathetic chain ganglion and re-enter the spinal nerve in the anterior ramus. Because they re-enter the spinal nerve, they're present at each spinal level. It can then travel to, it's effector, which in this case includes the skin from the neck down, sweat glands, and smooth muscle for blood vessels, or smooth muscle for the arrector pili muscles of the skin, which are those that make your hair stand up on end or give you goosebumps. In the second pathway, everything's pretty much the same up to the postganglionic neuron. So these preganglionic axon still entered the sympathetic chain ganglion and can either synapse there or travel up or down. The differences that the post ganglionic axon does not re-enter into the spinal nerve. Instead, it leaves the chain ganglion and goes directly to its target tissue. The post ganglionic axon with this configuration is called the sympathetic nerve. The target tissues of this nerve are those organs of the thoracic cavity, like in the example here the heart, but also the lungs. Slide 7 In the third route, the preganglionic axon enters into the sympathetic chain ganglion, but instead of synapsing here, it actually moves through the chain ganglion without synapsing. It then travels to the collateral ganglion. The nerve after it leaves the sympathetic chain ganglion is referred to as the splanchnic nerve. Once it reaches our collateral ganglion, it will synapse with our postganglionic neuron. The effectors for these types of pathways are usually found within the abdominopelvic cavity. In this example we have a portion of the GI tract. And finally, the fourth root is quite unique because it travels directly to the adrenal gland. This time the preganglionic axon bypasses both the sympathetic chain ganglion and the collateral ganglion. And it travels directly to the center of the adrenal gland, called the adrenal medulla. Here it synapses with specialized cells that are really a modified cluster of postganglionic cell bodies, which are considered to be the second neuron in this sort of two neuron system, which doesn't really have a second neuron, it's just the cell bodies of these particular cells. They don't have dendrites are axons, but when you synapse with these cells, it stimulates them to release epinephrine and norepinephrine into the blood. These can travel to various parts of the body as hormones. Slide 8 So again, this slide is just summarizing all of those points that I just made on the last slide. So in our first route we had the synapse in the chain ganglion, but then that postganglionic neuron was going to rejoin with the spinal nerve,to basically catch a ride to the skin. So there it's going to have its target, the skin of the neck, the trunk, and the limbs. We also had another situation where there is a synapse in the chain ganglion, but now the postganglionic neuron exited right out of that chain ganglion and went to the target tissues, which were in the thoracic cavity ie. the heart and the lungs, and that was then called the sympathetic nerve. And the next situation, the preganglionic neuron actually passed through the chain ganglion and then it synapsed in the collateral ganglion, which is much closer to the target tissue. The nerve that extends out of the chain ganglion is referred to as the splanchnic nerve. And the target tissue for these nerves are the abdominalpelvic organs. And then finally we had the unique situation where the preganglionic neuron went through the chain ganglion and the collateral ganglion, and it went straight to the adrenal medulla. And there, there's some specialized cells that act as our postsynaptic neurons, and their job is actually to release epinephrine and norepinephrine into the blood, and then these will act as hormones. So let's pause here and do some practice questions before we move onto the parasympathetic nervous system. Video 2 Slide 9 The parasympathetic nervous system is sometimes referred to as the craniosacral division, because the cell bodies of the parasympathetic neurons are associated with the nuclei of four different cranial nerves. Cranial nerve three, the oculomotor nerve, cranial nerve seven, the facial nerve, cranial nerve nine, the glossopharyngeal nerve, and cranial nerve ten, the vagus nerve. They also have cell bodies in the lateral gray horn of the sacral region of the spinal cord, from S2 to S4. These are what we referred to as the pelvic splanchnic nerves. In the parasympathetic nervous system, the preganglionic axons synapse with the postganglionic axons at something called the terminal ganglia, which are usually somewhat closer to the effector tissues. For cranial nerve three, the parasympathetic nervous system stimulates the ciliary muscles of the eye, as well as the sphincter pupillae. For cranial nerve seven, it stimulates the glands for tears, the salivary glands, and the glands for nasal secretions. For cranial nerve nine, it supplies the parotid salivary gland. Parotid is spelt p, a, r, o, t, i, d, parotid. And for the vagus nerve, cranial nerve ten, which is the most important and significant and has many branches which supply the heart, the pulmonary system, and the GI tract as far as the midpoint for the colon. The pelvic splanchnic nerves supply the smooth muscle and glands of the colon from the midpoint on, the ureters, the bladder, and the reproductive organs. Slide 10 We'll begin looking at the physiology of the autonomic nervous system by first examining the types of neurotransmitters that each neuron releases. Since both the sympathetic nervous system and the parasympathetic nervous system innervate the same structures. There needs to be a way for the structure to know which one is sending the signal. So we need to be able to create two different signals. We do this by the type of neurotransmitter that's released at the target tissue. The two neurotransmitters in the autonomic nervous system are acetylcholine and norepinephrine. Acetylcholine is considered to be cholinergic and norepinephrine is adrenergic. In the sympathetic nervous system, the preganglionic neurons release acetylcholine or are cholinergic. And most of the postganglionic neurons release norepinephrine or are adrenergic. One exception is that some sweat glands of the sympathetic nervous system actually release acetylcholine. In the parasympathetic nervous system, the preganglionic neurons also release acetylcholine. But here, all postganglionic neurons also release acetylcholine. So both will be cholinergic in this case. Slide 11 There are also different receptors for these two neurotransmitters. All preganglionic neurons and the postganglionic neurons of the parasympathetic nervous system release acetylcholine. But the release of acetylcholine can be either excitatory or inhibitory, depending on which receptors they bind too. There are two types of acetylcholine or cholinergic receptors there called the nicotinic or the muscarinic receptors. Nicotinic receptors are excitatory and open sodium channels. They're located on the cell bodies of all postganglionic neurons and on the plasma membrane of skeletal muscle, which is commonly referred to as the neuromuscular junction. Therefore, if an action potential reaches an autonomic ganglion, the only option is to excite the postganglionic neuron. Same goes for skeletal muscle. Muscarinic receptors are found on the plasma membrane of all parasympathetic nervous system effectors or target tissues. These include smooth muscle, cardiac muscle, or glands. Binding to these receptors causes a G protein signalling pathway to begin, and the result will either be an excitatory or an inhibitory response. Slide 12 Adrenergic receptors are those that bind norepinephrine, or frankly, epinephrine as well, and are found in most of our sympathetic nervous system effectors, except for those sweat glands that I've already mentioned. That's because the sympathetic nervous system, postganglionic neurons are the ones that release norepinephrine. There are two classes of adrenergic receptors, alpha and beta, and both can excite or inhibit based on the receptor themselves. In most cases, A1 and B1 cause excitation, and A2 and B2 cause inhibition. There's a detailed table of a variety of these receptors and their target tissues. If you're interested, you can take a look in your textbook, it's table 16.3. Slide 13 So on this slide we're looking at the location of these various receptors. So in the first image we're looking at a preganglionic neuron releasing acetylcholine, which is all of the preganglionic neurons of the sympathetic and parasympathetic. And then we have nicotinic receptors on the postganglionic neuron cell body and dendrites. So we're always going to get an excitatory response in the postganglionic neurons in both the sympathetic and parasympathetic situations. So in the next image we're looking at the muscarinic receptors. So you can still see the preganglionic is exactly the same, we've got nicotinic receptors and acetylcholine being released at the ganglion. But then at our target tissue we now have muscarinic receptors. So these muscarinic receptors are going to have either an excitatory response or an inhibitory response. And we're going to find these receptors in the parasympathetic system and some exceptions in the sympathetic nervous system like the sweat glands. And finally, we have the adrenergic receptors that are part of the sympathetic nervous system. So we had the same preganglionic and nicotinic receptors with acetylcholine in the ganglion region. Except now our postganglionic neuron is going to release the neurotransmitter norepinephrine. So on our target tissues here of the sympathetic nervous system, we have adrenergic receptors. Again, we're going to have alpha and beta receptors, depending on which type we have, will have either an inhibitory or excitatory response. Also notice here that all of our preganglionic neurons are myelinated and all of our postganglionic neurons are unmyelinated. So this is a good image that's depicting that as well. So let's take a pause again here and we'll do some more practice questions. Video 3 Slide 14 Our ability to maintain homeostasis is dependent on the regulation of the autonomic nervous system and the use of autonomic reflexes. These are loops created from a direct link between the visceral sensory input and the autonomic nervous system motor responses. We're going to look at an example of this by looking at baroreceptors, which are receptors in the walls of larger arteries that detect stretch and changes from blood pressure. Slide 15 So in these examples of autonomic reflexes, we're looking at regulating blood pressure. We have those baroreceptors in the walls of the carotid artery. It's important to have these here because these are the arteries that lead to the brain and we always want to maintain blood flow to the brain. So when there's an increase in blood pressure, this is detected as more stretch on the baroreceptors. And the sensory signal is sent to the medulla oblongata through a sensory fiber of the glossopharyngeal nerve. At the medulla oblongata, there's an integration of the action potentials which cause a reflex action and an action potential that sent through the vagus nerve. The vagus nerve here acts as the parasympathetic preganglionic neuron, which goes to the wall of the heart in synapses with a very short postganglionic neuron, which is also located within the wall of the heart. It releases acetylcholine and this will decrease heart rate, which in turn will decrease blood pressure. Slide 16 In this situation, blood pressure is now decreased, so there's less stretch on the baroreceptors. This will result in decreased firing rate on the sensory portion of the glossopharyngeal nerve. This reaches the medulla oblongata is interpreted differently. The decreased rate of action potentials causes a different response. Now the signal moves down the spinal cord to the thoracolumbar region, or the sympathetic nervous system division. The preganglionic neuron leaves the spinal cord and synapses in the sympathetic chain ganglion. And then the postganglionic neuron extends as a sympathetic nerve to the walls of the heart. It releases norepinephrine, which will then act to increase heart rate. It also increases the force of contraction of the heart, which will ultimately increase blood pressure. Both of these examples show how the two divisions can control the same tissue but have opposite effects. Slide 17 There are some generalizations that we can make about the functions of our autonomic nervous system. One, both divisions can produce stimulatory and inhibitory effects, and this will depend on the target tissue. Two, most of the organs have dual innervation. So both the parasympathetic nervous system and the sympathetic nervous system will have some control over the tissue. There are some organs that only have one or the other, and the effects on each of the target organs may vary because some may have more influence from one division versus another. But more often than not, a lot of tissues will have connections from both of these systems. (Three) In tissues with dual innervation, the parasympathetic nervous system, and the sympathetic nervous system usually have opposite effects. So one will increase something and the other one will decrease it. So in the example that we've already been talking about, the changing heart rate with signals from our baroreceptors, the parasympathetic nervous system decreases heart rate and the sympathetic nervous system increases heart rate. Four, we can also produce cooperative effects. So within one system we can stimulate several organs to work together to have an effect. An example of this is in the parasympathetic nervous system. It can stimulate the pancreas to release digestive enzymes, but at the same time it also stimulates the smooth muscle of the small intestine to contract and mix these enzymes with our food. The sympathetic nervous system usually has more of a general effect or a whole body effect compared to the parasympathetic nervous system, which is more localized to the effector. The general response is mostly due to the innervation of the adrenal gland by the sympathetic nervous system, releases the adrenal hormones, epinephrine and norepinephrine into the blood. This also makes the response last longer than the parasympathetic nervous system, because these circulating hormones would need to clear from the blood in order to stop the response. And finally, we have rest versus activity. So the parasympathetic nervous system is considered to be our rest and digest system. It acts to help conserve and restore body energy at rest. The actions of the parasympathetic nervous system are often referred to as the sludd responses, S, L, U, D, D. Salivation, lacrimation, urination, digestion, and defecation. It also has the three decreases, decreased heart rate, decreased diameter of the airway, and decreased diameter of the pupil, which is basically constriction. The sympathetic nervous system is dominant in times of physical or emotional stress. So these are the E situations, emergency, embarrassment, excitement, or exercise. The reaction is also sometimes referred to as the flight or fight response. And it causes a variety of responses including increased heart rate, increased blood pressure, increased force of heart contraction, pupil dilation, decrease blood flow to nonessential organs like the gut, and increased blood flow to skeletal muscle and the heart. The airways will dilate and you'll get an increase in respiratory rate, and you'll also see an increased blood glucose concentration. There's one exception to these responses, and that's paradoxical fear. This is when you feel that there's no escape route or no way to win. In this case, you get a mass action effect of the parasympathetic nervous system. And this is why people will lose control over things like urination and defecation when you would normally expect to have a sympathetic nervous system response dominating. Slide 18 The final division of the autonomic nervous system is the enteric nervous system. This system is comprised of nerve complexes in the walls of the digestive tract. Something different about these plexuses is that they contain nerve cell bodies, not just axons, compared to the ones that we've already looked at in the spinal nerves. There are three points of nervous input that can contribute to these plexuses. There are sensory neurons which connect the digestive system to the central nervous system; and these sends sensory information about the homeostasis of the digestive system to the central nervous system. There are also autonomic nervous system motor neurons that connect the central nervous system to the digestive tract to control smooth muscles and gland. secretions. There are also enteric neurons, and these are found only within the plexus itself. They provide control through autonomic reflexes. They're actually capable of controlling digestive homeostasis without input from the central nervous system, and they work through local reflex loops. Slide 19 There are three major types of enteric neurons. The enteric sensory neurons, which maintain homeostasis by detecting stretch on the walls of the digestive tract or the chemical composition of the contents of the digestive tract. There are also enteric motor neurons which stimulate or inhibit smooth muscle of the digestive tract or control secretions from glands, which would help break down and move food through the digestive tract. Finally, there are enteric interneurons which connect the sensory and motor enteric neurons. These help create reflex loops which allow for rapid responses to sensory information that has been detected. So let's pause again here and we're going to follow with first a nice summary animation of many of the things that I've talked about in the lecture today. And then we'll try out some more practice questions. Animation No caption file Video 4 Conclusion So that completes today's online lecture. Today we talked about the autonomic nervous system and its organization, and compared it to the somatic motor nervous system. And then we looked in more detail at the sympathetic nervous system as well as the parasympathetic nervous system. We continued to look a little more deeply by looking at the neurotransmitters that are part of the autonomic nervous system, as well as the receptors that are involved in the sympathetic and parasympathetic responses. And then we finish by talking about the enteric nervous system. And our next online lecture module, we're going to begin to look at the special senses by looking at taste and smell. So until then, take care.