Hello. We're going to finish our consideration of the nervous system by now considering an efferent pathway, the somatic nervous system. So we talked about the afferent pathways coming into the central nervous system, and now we're going to kind of combine the two when we, after we'd considered just the efferent pathway, the somatic nervous system that activates skeletal muscle. Then in the next session, we're going to talk about reflexes, which is where we bring in sensory information. The central nervous system integrates that information, decides what to do. And then we're going to consider the efferent pathway through the somatic nervous system and how that controls things like reflexes and locomotion. So in this session, we're going to talk about general principles of the somatic nervous system, some aspects of spinal cord structure, and just touch again on the neuromuscular junction. But a fair amount of this will be some review from just the nervous system in general. And then in the next section, we're going to talk more about how the afferent pathways and efferent pathways come together to control movement. So you've seen this figure before. Just to place this in the context of the larger nervous system where we're going to have sensory information coming in through afferent pathways. That's what we've talked about. Most recently the CNS is going to integrate and then we are going to be talking about the somatic nervous system which is the part of the efferent nervous system that controls skeletal muscle. That's going to be in contrast to the autonomic's neurvous system which is going to be what controls the organs. And this again is reviewed to remind you that with the autonomic nervous system, such as the sympathetic portion and the parasympathetic portion; we have two neurons in series, where the first neurons in the central nervous system. And the second one is sitting out in the periphery, but with the somatic nervous system, we have the neuron, the single neuron sitting in the central nervous system and sending its axon all the way out to the muscle to stimulate it. So it's a single neuron system in terms of the efferent portion. And also keep in mind that this skeletal muscle is going to perform a variety of actions, not just voluntary actions. So, it's going to be involved in reflexes and breathing, which are going to be somewhat involuntary actions. Let's review our spinal cord structure, where we've already talked about how we're, going to have a afferent neuron coming in. Itself cell body is sitting usually in the Dorsal root ganglion for the body sensory information. And then there will be neurons sitting in the spinal cord called interneurons That are going to be part of the system that is going to interpret the stimuli coming in and we'll see in the next session, concrete examples of this circuitry. So we're going to have inner neurons that are Collecting information. And then based on what they decide we will have firing of the ether neuron for the somatic neverous system that's going to be sitting in the ventral. What we call the ventral horn of the spinal chord. So this is the ventral portion of the spinal chord and here we're going to have all of our motor, somatic motor neurons with their cell bodies sending their axon all the way out to the skeletal muscle. We need to now consider that somatic motor neuron that were just looking at in the spinal chord, what is it synapsing with? And so, that comes to the definition of a motor unit, where a motor unit is a single motor neuron, and all the muscle fibers, which is the same thing as a muscle cell, that it controls. So, that's what a motor unit is and we'll be talking about this more when we talk about skeletal muscle as well. Where a, usually a neuron is going to control anywhere from three to around a thousand different, muscle cells, skeletal muscle cells or, which is the same thing as skeletal muscle fibers. All of those fibers are going to be in the same muscle. But they're usually going to be spread out. So that's what's shown here in this muscle. Where we're saying that these blue muscle cells are the ones that are innervated and are receiving signals from one neuron. This is this ones motor neuron motor unit. Keep in mind, that one, a single muscle cell is only going to be innervated by one neuron which means it's only being controlled or regulated by one neuron. So our, our decision making about whether or not a muscle itself should contract is coming at the level of this somatic motor neuron. If it sends an action potential down its axon to the muscle, the muscle is going to contract, and we'll talk about that in just a minute. And it's that muscle cell is only getting inputs from that one neuron. So the muscle cell is not having to decide whether or not its going to contract, it's because it's only being controlled by one neuron. And whenever that neuron fires, that muscle cell is going to contract. So the, decision making is happening at, with the somatic motor neuron, not with the muscle cell receiving multiple inputs from multiple neurons and deciding whether or not to contract. So let's talk about the neuromuscular junction which is just going to be a certain type of synapse which we've already talked about. There's nothing so unusual with it. We're going to have the axon from the somatic motor neuron coming to contact the plasma membrane of the skeletal muscle cell. So that action potential will travel down that axon, and when it does just like in a normal synapse it's going to open voltage gated calcium channels, that will let calcium into the terminus of the axon. Which is going to be the signal to because vesicles of neuro transmitter, and more specifically, acetylcholine, to fuse and to be released into the synaptic cleft. Cleft. So, acetylcholine is going to be what's released by all somatic motor neurons, the muscle motor end plate, which is just the portion of the muscle plasma membrane, that is at the synapse. It's going to connect, contain nicotinic acetylcholine receptors. So these are nicotinic synapses. And we know that nicotinic acetylcholine receptors are going to be acetylcholine Gated sodium channels. So that means that if this neuron fires, it's going to open sodium channels, that's going to because a graded potential. However, keep in mind that right next to this synapse on the skeletal muscle plasma membrane we're going to have voltage gated, ion channels. Which is going to be somewhat unique compared to what we saw with Neurons, so that means that basically we are having a synapse what to, what would be equivalent to the axon initial segment. And so that means that whenever we have firing of the somatic motor neuron, for all intents and purposes we're also going to have an action potential that occurs in the skeletal muscle membrane as well, and we'll be talking more about that once we cover muscle. But again, it's this idea that if that neuron fires we're going to have an action potential in the muscle membrane and that's going to because contraction, just because we're having a graded potential right next to voltage gated channels. And so it's a pretty much given that they're going, one's going to follow the other. So we've just had an introduction to the somatic nervous system, where we're going to control many different types of movement. One, because for instance, the diaphragm that controls our breathing is skeletal muscle. So, things like that as well as obviously the muscles that are important for posture and locomotion. And the somatic motor neurons are going to act. They're sitting in the spinal cord and they're going to act on skeletal muscle. A motor unit is going to be a somatic motor neuron and the fibers that it innervates or controls. And we're going to have these motor neurons in the grey matter of the ventral horn of the spinal cord. And yet we mentioned just briefly but we'll talk more about it, this interneurons in the spinal cord and how they're going to be important in coordinating responses and integrating information. And this is a subject that we're going to really focus on in the next section in terms of talking about how we make reflexive movements.
