The purpose of the peripheral nervous system is to gather information and relay commands from the central nervous system. So it's going to bring information to the CNS and bring commands from the CNS out to the body. Now, the peripheral nervous system is often divided into a bunch of different categories. One of the major divisions is between what's called the somatic nervous system and the autonomic nervous system. Now the somatic nervous system is usually frequently referred to as the motor system because it controls voluntary movements. It has 2 divisions, the afferent division and the efferent division. F as in 2, ef as in away. So afferent nerves carry sensory information from sensory receptors to the central nervous system. Whereas, efferent nerves carry signals from the central nervous system out to the body. And here, you can see the skin has these nerves connected to it, those are part of the afferent division. They'll go to the spine, which is part of the CNS, and there they might connect with an efferent nerve that's going to go and form a neuromuscular junction on muscle. So, it doesn't always work out this way. The point is there are 2 divisions, some afferent nerves are going to have their signals go all the way to the brain, others will simply stop in the spinal cord. Here, I just want to point out the, this is the primary somatosensory cortex. Basically, that's where all the sensory information from the body is going to come in. And just want to show that this is connected to a particular pathway in the spine, and that will go radiate down and, of course, nerves will exit through the body. So just want to show you that, these efferent and afferent nerves, they're all over the body. It's not just a small little loop like this image implies.
Now, the autonomic nervous system regulates unconscious functions. So the motor system is voluntary muscle movements. Right? The autonomic nervous system can result in muscle movements. However, these are not conscious functions, they are involuntary and unconscious functions. And the autonomic nervous system is really going to control organs of the endocrine system, the cardiovascular system, digestive system, and excretory system. All the systems that we don't really have conscious control over. Right? Now, in and of itself, the autonomic nervous system can be broken between 2 divisions. The sympathetic division and the parasympathetic division. Actually, there's another division too, the enteric division. You don't need to worry about memorizing this, but it is becoming a hotter subject of research now that we are understanding the importance of our guts, and this enteric division is going to control the organs of the digestive tract, as well as the pancreas and gallbladder. I only bring it up again because, you know, you know me, I love gut bacteria, I think the whole enteric system is super cool, and cutting edge research. Right? So, let's get to the stuff that you need to make sure you know, and that is the sympathetic and parasympathetic division. These are often simplified as the fight or flight division and the rest and digest division. Now, this simplification is pretty much good enough for our purposes, however, when you try to delve, or if you I should say delve into greater detail in these matters you'll see why it's not a perfect definition. For our purposes, it's good enough. So the sympathetic division is going to have neurons that release norepinephrine, and remember that's going to trigger that fight or flight response. That's why this is the fight or flight division. And you can see what happens in the sympathetic nervous system down here, you know, dilation of pupils, dry mouth inhibiting saliva production, increasing heart rate, and all that stuff that gets your body ready to either run away or fight, you know, a tiger or whatever it is. Now, the parasympathetic division releases acetylcholine. And this is going to be the rest and digest division, and it's going to have some effects that are opposite, but not always. I don't want you to try to think of these two things as, sympathetic does this and parasympathetic does the opposite because they don't really line up quite so perfectly. So there are some contrasts, for example, parasympathetic division constricts the pupils, as opposed to the sympathetic dilating them, but and, you know, likewise, it reduces heart rate as opposed to raising heart rate. Of course, it has other kind of unrelated things, and, you know, you can see all of its activities here. And, the reason, you know, just to give a little preview, the reason we sort of say it's not a great, you know, the rest digest, fight or flight division isn't perfect. You know, for example, when you look at reproductive functions, rest and digest, well, stimulating the genitals just doesn't really sound like rest. But that's a parasympathetic function. So, you know, these aren't perfect definitions, but for our purposes it's a good enough generalization.
So with that let's go ahead and flip the page.