Episode 63: The Nervous System

James Fodor (0:33)

Hello, you're listening to The Science of Everything Podcast, episode 63, The Nervous System. I'm your host, James Fodor. In this episode, we're going to look at the different components of the nervous system, including the peripheral nervous system and the central nervous system. We'll talk about the spinal column, cerebral spinal fluid, and there'll be particular focus on the brain. So we'll look at the anatomy of the brain, the different regions of the brain, including the forebrain and the hindbrain, the brainstem. We'll talk about the different regions of the cortex and their structure and functions. Recommended pre-listing for this episode is episode 38, Neurons and Synapses.

Okay, so let's make a start. The nervous system is the part of an animal's body that coordinates voluntary and involuntary motions, activities, and transmit signals between different parts of the body to essentially coordinate actions and manage the different organs of the body. The special type of cell that is sort of defined by the nervous system or defines nervous tissue is called a neuron, or a nerve cell. It's also called a nerve, as we'll see a bit later. That's a bit of a misnomer.

Most animal species have two parts to the nervous system, a central nervous system and a peripheral nervous system, abbreviated CNS and PNS respectively.

Basically, the central nervous system consists of the brain and the spinal cord, and everything else is the peripheral nervous system.

So let's start by talking about the peripheral nervous system, and then we'll move on to talk about the central nervous system in some more detail. And the focus here will be on the human peripheral and central nervous systems and later the human brain, although many of the basic principles that I'm going to discuss apply to other animals as well, particularly mammals and primates in particular. Okay, so the peripheral nervous system. The peripheral nervous system includes 31 pairs of spinal nerves, 12 pairs of cranial nerves, and also the various other nerves and ganglia and associated sensory receptor organs located throughout the body. So let's explain what we mean by those terms. First of all, let's explain what a nerve is. You've probably heard about nerves before. I don't mean when you're anxious. I mean nerves as in in the body. This is what I meant before when I said that the phrase nerve cell is a bit of a misnomer because nerves aren't cells. Nerves are actually enclosed bundles of axons in the peripheral nervous system. They're sort of like cables. Remember that an axon is a protrusion of the cytoplasm of a neuron, a nerve cell, so-called, which is used to transmit information from one neuron to another. Now, each individual axon is very small and narrow usually, and in order to transmit signals from, say, the brain to the end of the toe or to one's hands or wherever else, it's necessary to send more than a single axon. So you have a bunch of axons all sort of bundled together. They're all next to each other and surrounded by or insulation cells called, for myelin, Schwann cells, and then surrounded again by layers of connective tissue called endonutrium. And so this bundle of axons surrounded by connective tissue is called a nerve.

Nerves can either protrude from the spinal column or from the brain directly. The nerves that protrude from the spinal column are called spinal nerves, and there are 31 pairs of those in humans. And there are 12 cranial nerves which protrude directly from the base of the brain, the base of the skull, basically.

Now these nerves connect up either the spinal column or the brain, depending on whether they are spinal nerves or cranial nerves. They connect those to the various organs throughout the body. So our hands and our feet and our digestion system and our heart and skin and all of the other different parts of the body that need to be innervated by nerves.

These nerves carry signals to and from the parts of the body. So particularly skeletal muscles are innervated by nerves. That means we can send signals through the nerves to the muscles in order to move, so to walk and to run and to whatever else. And also we get signals back from muscles and skin telling us when we feel pain or hot or cold, or telling us the relative positions of the different parts of our body. That's proprioception and things like that. So those different types of nerves are called afferent neurons or afferent fibers in this case, because we're talking about bundles of neurons, afferent and efferent. So afferent means it's carrying information from the periphery, say sensory receptors in the skin or mucous membranes or internal organs, or from the eye or ear or wherever, into the brain. So that's afferent. It's bringing in, essentially. Efferent, yes, with an E, is the opposite. It carries signals out from the brain or through the spinal column out to our muscles or to innervate the visceral organs. So that's our digestive system and things like that. Right, so the peripheral nervous system includes both afferent and efferent neurons. It's sort of like one-way roads in a sense. You've got two lots of one-way roads. Some go out, that's the efferent, and some go in, that's the afferent neurons. And particular nerves will be specialised for particular purposes. So, for example, there's one nerve that's responsible for innervating the heart and causing it to speed up, and there's another one that innervates the heart to cause it to slow down. And so you might think that there will only be one, but in fact, there are two. One is sort of the accelerated one, and one is like the brake in a sense. Nerves are all different parts of the body, and different organs, and different muscles, and so on.