Hello we're going to finish our consideration of the nervous system by now considering an efferent pathway, the somatic nervous system. We talked about the afferent pathways coming into the central nervous system. We will combine the two pathways after we consider just the efferent pathway, the semantic nervous system. This pathway activates skeletal muscle. Then in the next session, we will discuss reflexes, which include afferent sensory information, the central nervous system integraton of that information, decides what to do, and then we will consider the efferent pathway, the somatic nervous system, and how that controls motor reflexes and locomotion. So in this session, we will consider the general principles of the Somatic Nervous System, some aspects of spinal cord structure and briefly the neuromuscular junction. A fair amount of this will be a review of just the nervous system in general. Then in the next session we're gonna talk more about how the afferent pathways and efferent pathways come together to control movement. So you've seen this figure before. This is an overview of of the nervous system, where we have sensory information coming in through afferent pathways. That's what we've talked about most recently. Then the CNS is going to integrate the information. Today we will 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 nervous system which is controls other organs. This again is review to remind you that in the autonomic nervous system, which has the sympathetic portion and the parasympathetic portion, we have two neurons in series. The first neuron is in the central nervous system. The second one is sitting out in the periphery. But with the somatic nervous system, we have a single neuron which starts in the central nervous system and sends it's axon all the way out to the muscle to stimulate it. So, it's a single neuron system in terms of the efferent portion. Also keep in mind that the skeletal muscle performs a variety of actions, not just voluntary actions. It is also involved in reflexes such as breathing, which are going to be somewhat involuntary actions. Let's review our spinal cord structure where we said that there is an afferent neuron coming in. It's cell body is sitting usually in the dorsal root ganglion. This neuron transmits the body's sensory information. Then there will be neurons sitting in the spinal cord called interneurons that are a part of the system that is going to interpret the stimuli coming in. We'll see in the next session concrete examples of this circuitry. So we have interneurons that are collecting information. Then based on what they decide, we will have firing of the efferent neuron for the somatic nervous system. This neuron is sitting in the ventral region called the ventral horn of the spinal cord. This is the ventral portion of the spinal cord. Here we have all of our motor, somatic motor neurons. Each of the cell bodies sends its axon all the way out to the skeletal muscle. We need to now consider that somatic motor neuron that we're just looking at in the spinal cord. What is it synapsing with? That brings us to the definition of a motor unit. A motor unit is a single motor neuron and all of the muscle fibers, which is the same thing as a muscle cell, that it controls. So that's what a motor unit is. We'll be talking about this more when we talk about skeletal muscle as well. Usually a motor neuron is going to control anywhere from 3 to around 1,000 different muscle cells, skeletal muscle cells, 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. That's what's shown here in this muscle. These blue muscle cells are the ones that are enervated and receiving signals from one neuron. This is one motor neuron's motor unit. Keep in mind that one, a single muscle cell is only going to be intervated by one neuron. Which means that it is only being controlled or regulated by one neuron. So the decision about whether or not a muscle cell contracts is made at the level of the somatic motor neuron. If it sends an action potential down it's axon to the muscle, then the muscle will contract. We'll talk about that in just a minute. Remember that the muscle cell is only getting inputs from that one neuron. So the muscle cell does not decide whether or not it's going to contract because it's only being controlled by one neuron. Whenever that neuron fires, that muscle is going to contract. So the decision is made by 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. This is a certain type of synapse which we've already talked about. There's nothing so unusual about it. We have the axon from the somatic motor neuron coming to contact the plasma membrane of the skeletal muscle cell. The 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. This will to vesicles of neurotransmitter, specifically acetylcholine, to fuse and then release the acetylcholine into the synaptic cleft. Acetylcholine is 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, contains nicotinic acetylcholine receptors. So these are nicotinic synapses. We know that nicotinic acetylcholine receptors are going to be acetylcholine gated sodium channels. That means that if this neuron fires, it's going to open sodium channels. That's going to cause 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. This is somewhat unique compared to what we saw with neurons. That means that basically we are having a synapse that is equivalent to the axon initial segment. That means whenever we have firing of the somatic motor neuron, for all intents and purposes, we will 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. That's going to cause a contraction because we're having a graded potential right next to voltage gated channels. It's pretty much a given that 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. For instance, the diaphragm that controls our breathing is skeletal muscle. Obviously the muscles that are important for posture, and locomotion are under somatic nervous motor neurons control. These motor neurons start in the spinal cord and act on skeletal muscle. A motor unit is going to be a somatic motor neuron and the fibers that it innervates or controls. These motor neurons are located in the grey matter of the ventral horn of the spinal cord. Then we mentioned just briefly, but we'll talk more about it. That there are interneurons in the spinal cord. These interneurons are going to be important in coordinating responses and integrating information. This is a subject that we're going to really focus on in the next lecture when we talk about how we make reflexive movements.
