Welcome back, this is our last session on the nervous system proper. We're going to talk about the organization. of this system. Previously we've talked about how cells communicate along their lengths, so how neurons communicate along their lengths as well as between neurons. Now we're going to talk about how they're put together to form the nervous system. Then we will talk about how the nervous system is organized. And you can think of it as having kind of three parts, where we will have to get information into the nervous system and in a minute we will talk about what kinds of information that is. That is going to be called, that's going to be part of the peripheral nervous system. It is going to be mostly outside of the brain and spinal cord. That's going to be called the afferent portion of the nervous system. Remember that afferent means coming into and it starts with A. So that's kind of the first part of the nervous system. That's what's shown in red here. What is shown ia a touch receptor. It senses this pin stuck in the skin. The receptor sends a signal into the spinal cord. So the afferent part of the nervous system is going to feed into the central nervous system. The central nervous system, spinal cord or brain, is where the integration of the information occurs. It is where the decision is going to be made about what to do about this stimulus. Then we're going to have the efferent portion of the nervous system which is going to sit mostly in the peripheral nervous system. This causes the effect. So we have sensory information about what's happening. The decision is made in the central nervous system about what to do with it. Now we need to create or cause the response, that's going to be the efferent part, it starts with e. Later in the alphabet, it's the second part. It comes out to cause the effect and move away from the central nervous system. We're going to have several lectures or sessions on the senses. But we're just going to briefly mention what kind of sensory input we're talking about. The first thing we're going to talk about are not a surprise. It's going to be vision, hearing, taste, your sense of balance, your sense of smell. So those are certainly types of sensory information that are coming in through these afferent systems. Another one is somatosensation, which is based in the body. That's why it's somato-. These recetors are going to sense touch and pain and temperature that are in the skin as well as in other organs. Also part of somatosensation is proprioception, which is how you know where your body is in space without using your eyes. It is sensing stretch in your muscles or in your tendons or in your joints that can tell you where your body is in space. So, proprioception is part of somatosensation, it's part of the sensation that occurs in your body. Apart from your special senses that are in your head. Then the other stimuli that we also should consider, in that we'll be talking about throughout the rest of the course, are our visceral stimuli, things that are necessary to maintain proper homeostasis. So knowing the pH of the blood or the oxygen content of the blood, things like that the nervous system is also going to monitor and respond to. So we’ve talked about the afferent systems coming in. Then we'll talk a little bit more in the future, about the integration of the central nervous system. But now we're going to move to talking about the efferent branches of the nervous system. We have the somatic Nervous system which controls skeletal muscle. We also have the autonomic nervous system that controls smooth muscle and many other organs of the body. The autonomic nervous system, which you probably are familiar with, can be divided into the sympathetic and parasympathetic portions. It works in tandem with the enteric nervous system, which is very important in the GI tract. We're not going to talk too much about the enteric nervous system. We will briefly talk about the actual circuitry of these efferent parts of a nervous system. Let's start at the bottom of the slide, with the somatic nervous system. This controls the skeletal muscle. It has a single neuron that originates in the central nervous system and send its axon out to the muscle. That axon will release as a neurotransmitter, acetylcholine. It's going to bind nicotinic acetylcholine receptors on the skeletal muscle. We'll be talking much more about that When we consier the somatic nervous system and skeletal muscle. The autonomic nervous system uses two neurons in series. That is what is shown in this diagram. The first neuron is starts in the central nervous system. It then synapses with a second neuron then leads to the tissue or the organ. For the parasympathetic nervous system, the first neuron release acetylcholine. The second neuron binds that acetylcholine with a nicotinic acetylcholine receptor. The second neuron is also going to release acetylcholine. The organ that the second neuron synapses with is going to bind that acetylcholine using muscarinic acetylcholine receptors. In the sympathetic portion of the autonomic nervous system, again, we're going to have acetylcholine and nicotinic acetylcholine receptors. So I remember this as follows: both the autonomic neurons and the somatic neurons, leave the spinal cord. The first neuron for the autonomic and the somatic neurons, they all three release acetylcholine and then bind nicotinic acetylcholine receptors. It's just in the autonomic system, the second neuron of the autonomic system where things change. We said already in the parasympathetic system, it's acetylcholine and muscarinic acetylcholine receptors. In the sympathetic nervous system we're releasing mostly norepinephrine or noradrenaline and then binding andrenergic receptors on the organs. Also in the sympathetic nervous system we have a special case, the adrenal gland. It is the first sympathetic neuron in the series that synapses with cells in the adrenal gland medulla causing them to release epinephrine, as an endocrine hormone into the blood. Then epinephrine binds to adrenergic receptors in the organ. It acts very similarly to just a normal synapse, except that the epinephrine is released into the blood. It is still going to bind adrenergic receptors in a similar way to norepinephrine. So really, the only reason why I'm concerned about this is just for you understanding because we're going to come back to these systems over and over again when we talk about the nervous system's control of the organs. We're going to finish by reminding you of what the sympathetic and parasympathetic nervous systems do in terms of controlling organs. This figure is meant to show you that the red lines are the Sympathetic Nervous System or the Parasympathetic Nervous System. And the blue lines are the opposite system. The point is, just that many organs are innervated by both systems. The reason behind that is so that we can have exclusive control over the activation of these organs. You probably know that the sympathetic nervous system is more active when you're doing flight or fright. When you are running from a bear and you're super afraid. So you increase your heart rate and do all sorts of things that are going to help you get away from that bear. And you will shutdown systems or suppress systems like the digestive tract that are not helping you get away from the bear. That you're trying to conserve your resources so that you can use all of them to try to get away from the situation. Then you have the parasympathetic system which is going to be more active during times of rest and digestion. Your heart rate would be slower, and your digestive system will be active. Very often you're not going to have complete situation where one system is completely going full out, and one shut down. Is always going to be a balance where both systems are somewhat active. Is just by matter of which one is more active than the other. The whole point of it is fine control. You can think of driving a car or maybe riding a bicycle. If you drive a car you could drive it with only the gas pedal. You'd have to let off the gas for a long time and coast to say you wanted to stop. But you could get around somewhat well. However, having a gas pedal and a brake, having the sympathetic nervous system and the parasympathetic nervous system, gives you exquisite control of where you put that car. That's the same thing with having these two nervous systems that are both innervating many organs. THey give you exquisite control about how active they are at any one given time. We'll be referring to these many times when we talk about the control of other organs. So we've talked about just the organization of the nervous system where we're going to have the central nervous system that's going to integrate. The peripheral nervous system that's going to send in the sensory information and is going to send out the directive of what needs to happen in response to the sensory system. I didn't mention this explicitly, but what this means is it's a reflex arc with the sensory information coming in. The integration of what should happen and then the control and the signal of what to do in giving output to the effectors via the efferent pathways of the nervous system. Then we talked about how we've got many different efferent portions of the peripheral nervous system. Somatic, skeletal muscle, and autonomic. For the organs, where we have sympathetic and parasympathetic, they act reciprocally and in opposition. The Enteric division governs the gut. It can be modulated by the autonomic nervous system, usually the parasympathetic nervous system..
