The Science of Everything Podcast - Episode 63: The Nervous System

Episode Date: June 29, 2014

An overview of the structure and function of the nervous system, including a discussion of the autonomic and somatic divisions of the peripheral nervous system, the spinal cord, and the brain. I discuss the major regions and structures of the brain, including the brainstem, the cerebellum, the cerebral cortex, and various subcortical structures. I conclude with some brief remarks about lateralization and the relationship between brain size and IQ.

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, the cerebral spinal fluid, and there will be a 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 hind brain stem.

We'll talk about the different regions of the cortex in 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 defined by the nervous system or defines nervous tissue is called a neuron or a nerve cell that's also called a loaves. 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, also abbreviated C&S 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 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, the energy. 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 myelin, swan cells, and then surrounded again by layers of connective tissue called enderutriam. 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 brain, the base of the skull, basically. Now, these nerves connect up either the spinal column or the brain, depending on whether they're 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 enervated by

nerves. These nerves carry signals to and from the parts of the body. So particularly skeletal muscles are innovated by nerves. That means we can send signals through the nerves to the muscles in order to move, so to walk or 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 appropriate perception, appropriate perception, 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, not just one, afferent and efferent. So afferent means it's carrying information from the periphery, say sensory receptors in the

or mucus membranes or internal organs or from the eye or ear or wherever into the brain. So that's aphorant. It's bringing in, essentially. Efferant, with an E, is the opposite. It carries signals out from the brain or through the spinal column out to our muscles or to innovate the visceral organs. So that's our digestive system and things like that. Right. So the peripheral nervous system includes both aphorant and efferent neurons. It's sort of like a 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 aphorant neurons.

And particular nerves will be specialized for particular purposes. So, for example, there's one nerve that's responsible for innovating the heart and causing it to speed up. And there's another one that innovates the heart to cause it to slow down. And so that you might think that there would only be one, but in fact there are two, one is sort of the accelerate, and one is like the brake in a sense. And there are nerves for all different parts of the body and different organs and different muscles and so on.

Now, as I said before, the cranial nerves leave the skull, and so they project directly from the base of the brain. Spinal cord nerves leave the vertebra through openings in the bone. So, you know how we have a spinal column, and then there are, with the different vertebrae. There are openings in the bone, and one of the reasons those are there is to allow the spinal nerves to project outwards, and therefore project to the different parts of the body that they need to. Now, you remember that I said that the nervous system is divided up into two components. There's the peripheral nervous system and the central nervous system. Well, the peripheral nervous system is further subdivided into what it are called the autonomic nervous system and the somatic nervous system.

So I'll talk about each of those in turn. The somatic nervous system is associated with voluntary control of body movements, particularly skeletal muscles. So in particular, the somatic nervous system includes afferent nerves, traveling, bringing sensory information to the brain, particularly about the location and and stretching and so on of muscles and tendons, while efferent nerves from the brain, particularly from the motor cortex, stimulate the skeletal muscles to initiate contraction and therefore motion. So whenever we engage in voluntary motion moving that we can sort of choose to do voluntarily, that's mediated by the somatic nervous system. The second branch, if you like,

of the peripheral nervous system is called the autonomic nervous system. And you can remember that because it's kind of like automatic. It happens automatically. You don't have conscious control, for the most part, over the autonomic nervous system. So it controls bodily functions below the conscious level, including things like heart rate, digestion, the rate of respiration, respiration, urination, sexual arousal, breathing, swallowing, all other things like that. So whenever you do any of those activities, muscles need to contract, either smooth muscles for digestion or skeletal muscles for motion, or changing the rate of the beat of the heart or of the contraction of the diaphragm, say, for respiration. All of that requires innovation by nerves from the brain.

Basically, the rate of action potentials will tell the organs how rapidly they will, how rapidly the muscles will contract or how strongly they will contract. So that's how the peripheral nervous system works, essentially. It just sends action potentials along the axons through the, nerves, and the rate of those at which those action potentials arrive at their destination essentially determines how frequently and intensely the muscles contract, thereby controlling the contraction of the muscles, and hence the relevant motion of that part of the body or of the digestive system or whatever it be.

So, as I said, most autonomic nervous system functions are involuntary, however, they can sometimes work in conjunction with somatic nervous system to provide voluntary control over certain components. So respiration would be an example of that. There's an involuntary component, but you can also change the rate of respiration if you want to. So this leads us to a final point about the peripheral nervous system, which concerns the two separate sub-components, if you like, of the autonomic nervous system, the sympathetic and parasympathetic nervous systems. And this relates to what's commonly called the fight or flight response,

which you probably have heard of before. So let me just set the stage a bit here. So remember, there's a number of levels of categorization here, which can get a little bit confusing. So let me go over them. The nervous system describes all nervous tissue and everything we're talking about in this podcast, basically. Within the nervous system, there are two most basic components. There's the peripheral nervous system, which are all of the nerves and also sensory organs outside of, basically all throughout the body, except for the central nervous system.

The central nervous system consists of the brain and the spinal column alone. That's it. That's all the central nervous system is. Peripheral nervous system is everything else. So there's that distinction between peripheral and central nervous system. Within that, within the peripheral nervous system, so now we're just zoom into that part. There are two subcomponents to that. There's the somatic nervous system and the autonomic nervous system. Somatic is responsible for. Basically, voluntary muscle control. Autonomic is for automatic non-voluntary functions. Now, let's zoom in further to the autonomic nervous system.

Within that, within the autonomic nervous system, of the peripheral nervous system, there are two further subcomponents, the sympathetic and the parasympathetic nervous system. Now, let me explain the difference. Effectively, you can think of them as just being opposites of each other. They do sort of exactly the opposite thing. Basically, the sympathetic component of the autonomic nervous system is responsible for what we might think of as the... initiating the

fight or flight response, so-called, although I don't like that name very much because it's not very specific. So the sympathetic nervous system effectively initiates the fight or flight response by such actions as dilating the pupil, inhibiting the flow of saliva, accelerating heartbeat, dilating the bronch eye in the lungs, inhibiting digestion, various activities of digestion,

inhibiting bladder contraction, and also causing the stimuli, sorry, this secretion of adrenaline, the hormone. It's important to note that the nervous system and the endocrine system, which is responsible for producing and secreting hormones, these two systems work closely together in controlling the behavior of the body and the activity of the different organs and tissues. So one particularly close way that they work is through the sympathetic nervous system

because the nervous activity activation of the sympathetic nervous system leads to secretion of adrenaline, which then further is partly self-responsible for initiating the sympathetic nervous response. So they work closely together there. The flip side to the sympathetic nervous system is the parasympathetic nervous system, which essentially, as I said before, just does the opposite of everything that the sympathetic nervous system does. It constricts the bronchia in the lungs. It slows the heartbeat, stimulates the flow of saliva and stimulates parasolstores and secretion of bile and enzymes for essentially stimulating digestion. So effectively, This is parasympathetic is associated with what's called rest and digest.

Sympathetic is associated with fight or flight. So usually sympathetic nervous system is activated when there's some threat. The animal feels threatened in some way or there's a need for vigorous action. In contrast, the autonomic nervous system is particularly activated when there's a need for feeding or for rest or for secretion or other sort of maintenance activities like that. That's the basic sort of really rough idea of what the two subsystems. do. It's important to understand that it's not an either-or situation. It's not like the sympathetic nervous system is on or the parasympathic or the parasympathetic nervous system is on. It's one or the

other. That's not how it is. Both are essentially always active to one degree or another. So it's a situation, rather than sort of having two buttons where one other, the parasympathetics on or the sympathetics on, it's more like two dials where they can be turned up to various degrees and down to various degrees depending on the circumstance, and that is regulated by hormones, for example, like adrenaline, for instance, regulates how much each of these subsystems was activated in a given situation. Right, so that's enough on the peripheral nervous system. Let's move on now to talk about the central nervous system. The central nervous system is so named because it integrates information and coordinates the different components of the body. So it takes information from all of the

different sensory apparatuses of the body, brought through the peripheral nervous system. It integrates that processes it, synthesizes it, and then uses that information to coordinate and manage, essentially, the activities of all of the different parts of the body. So it sends out those efferent signals to then control the rest of the body. So the central nervous system consists of the brain and the spinal cord. Let's start by talking about the spinal cord. The spinal cord is just a long, thin, tubular bundle of nervous tissue, sort of like axons, although they're not, sorry, sort of like nerves, so the nerves we talked about in the peripheral nervous system, although they're not quite nerves because they're part of the central nervous system,

and nerves are sort of technically part of their peripheral nervous system, but it's kind of a similar idea, bundles of axons surrounded by connective tissue, and, of course, the spinal cord protrudes out of the bottom part of the brain. An interesting thing, which I didn't know until I was just doing the research for this podcast, actually, that in an adult, the spinal cord only occupies the upper two-thirds of the vertebral colon, So if you think of your spinal, of your spine, the vertebra, the bones that comprise the spine, only the top two-thirds of those roughly are actually occupied by the spinal cord. The lower third does have nervous tissue in it, however, it's not part of the spinal cord.

The spinal cord itself is specifically refers to a sort of a long, straight, tubular, particularly sort of rigid bundle of nervous tissue and axons and support cells, down from that, it protrudes a number of nerves which run down through the bottom third of the vertebra and then out through the rest of the body and joining up to the peripheral nervous system, or becoming the peripheral nervous system essentially. But correctly speaking, that bottom third of the vertebra is not part of the spinal cord per se, so that's not part of the central nervous system. Now, the spinal cord is protected by three layers of tissue called spinal meninges. They're similar to the meninges we'll talk about in the surrounding the brain in a moment. And the names of these are the Dura, the

arachnoid, and the Pia layers are surrounding the, these are just layers of connective tissue which surround the spinal cord to protect it. Between two of these layers, the sort of the second and the third layer, so between the bottom two layers is sort of like a pocket, if you like, or a a region which contains cerebrospinal fluid. And this is, well, it's just a fluid. It's mostly water, but it contains electrolytes and other things. It has a number of purposes, which I'll discuss more in a moment. But an interesting thing about this is that you may have heard of a medical procedure

called a spinal tap, or more correctly, it's called a lumbar puncture. This involves the use of a needle to withdraw cerebral spinal fluid from the subarachnoid space, so that's the space between these two layers where the cerebro spinal fluid sits. usually from the lower region of the back, which is called the lumbar, so hence the lumbar puncture. And one reason you might do that, for example, is to diagnose various conditions, which lead to a change in the composition of cerebrospinal fluid. It's very difficult to get access to cerebrospinal fluid, or much more difficult to say than getting access to blood,

because it's only found in certain parts of the body, and you don't really want to inject a needle into your head, which would be a bad idea. So the lumbar puncture is essentially the easiest way to get access to cerebrospinal fluid. So that's what that refers to if you've ever heard of a spinal tap before. Now, cerebrospinal fluid isn't just found in the layer surrounding the spinal cord. It also surrounds the brain. Well, I should say it surrounds the brain and also is found inside the brain. So let me explain how that works.

There is a system of sacks, essentially, you could call them, or ventricles, more correctly, located in the middle of the brain called the ventricular system. Well, I said in the middle of the brain. Some of the ventricles are in the middle. Others are sort of in other regions. But there are several ventricles which are all connected to each other, and they contain cerebrospinal fluid. These ventricles are located, as I said, in the brain.

But the cerebrospinal fluid is free to circulate between the ventricles and also through the subarachnoid space and surrounding the... Throughout the meninges that surround the brain as a whole. This cerebro spinal fluid serves a number of purposes. One is to serve as a means of... of transporting nutrients to the cells of the brain and the spinal cord, and then transporting wastes away from them,

so it serves a nutritive function. It also serves, the cerebrospinal fluid also serves to protect the central nervous system by oftening cushioning support. In particular, what's actually quite interesting is the brain sits, as I said, it sits essentially in a pool of cerebro-spinal fluid. it's surrounded by a membrane, well, it's surrounded by membranes in between which is found a certain quantity of surre spinal fluid. So these are the meninges I was talking about before, and the subarachnoid space where you have that surreeper spinal fluid. But the point is it completely surrounds the brain, except for the part where, of course, the spinal cord connects up to it.

And effectively, because of the consistency of the brain itself, you know, the brain itself is made of nervous tissue, which is mostly water, and the cerebral spinal fluid also has some various enzymes and other things in it, so it's not completely water. The point is that the brain has basically neutral buoyancy in the cerebrospinal fluid. What does that mean? Well, it means it neither floats nor sinks. So if you imagine putting the brain in a bath of cerebrospinal fluid, it wouldn't bob to the surface or sink to the bottom. It would just have neutral buoyancy. It would just sort of sit wherever you put it.

Essentially what that means is that a crude way of putting it is that the brain floats, in a bath of cerebrospinal fluid. Which is kind of interesting if you think about it. One of the main purposes of this, as I mentioned before, is to protect the brain, because if it gets knocked a bit to either side, as long as the knock isn't too hard, a large portion of that force or the energy

can be cushioned by the brain moving with respect to this fluid and just pushing the fluid out of the way. So it's a way of cushioning the brain and protecting it from harm. Another important thing to know about the central nervous system is the existence of what's called the blood-brain barrier. Now, the blood-brain barrier

refers to essentially this, well, barrier that prevents large molecules from, or other substances, from traveling from the blood system into the brain. The main mechanism behind this is essentially a high density of cells connected by tight junctions, well, that's literally what they're called tight junctions, that essentially like glue the cells together might be a good way of thinking about it,

right next to each other so that the cells are squeezed right tight next to each other so that you can't fit any big molecules in between them. So normally what happens is that surrounding the capillaries, the small blood vessels elsewhere in the body, are what are called endothelial cells. They're just cells that are responsible for forming the boundary between two different organs or whatever. These endothelial cells surround the capillaries, but they're not tightly welded to each other in a sense.

There are gaps between them, and so big molecules or other substances, for example, like bacteria or antibodies, can pass between them. However, in the brain, this can't happen, because these endothelia cells are stuck together by these tight junctions, and there are some other mechanisms as well that do that. But effectively, these gaps between the endothelial cells are closed, and so only small molecules or particular substances, like, for example, glucose, which have special,

mechanisms to pass across the, are able to pass across. Everything else is prevented from from traveling between the blood system or the cardiovascular system through to the, into the brain or the spinal column. There are a couple of areas of the brain that are not on the brain side of the blood barrier, so they're not protected by this, but most of the brain in the spinal cord is. The main reason for this is, as I sort of indicated before, to protect the brain from bacterial infections, because bacteria are too large to pass

across the blood-brain barrier, which means that infections of the brain are quite rare, and that's very fortunate because bacterial infections of the brain would be not a very nice thing to have. The downside, of course, is that antibodies, the components of the immune system that are largely responsible for dealing with infections when they do occur, are also too large to pass across the blood-brain barrier, and most antibiotics are also unable to pass across it, so it's difficult to administer drugs that target the brain, because many of them can't cross the blood-brain barrier. Okay, so that's enough about the peripheral and central nervous systems in general terms.

I'm now going to talk about the brain specifically, again, focusing on the human brain. The brain is, of course, why we all care about the nervous system. It's not the peripheral nervous system that we're really interested in. It's the brain. That's where the interesting stuff really happens. The adult human brain weighs, on average, about 1.5 kilograms. So it's not actually very heavy. It's not very large either.

It has a volume of about 1.1 to 1.3,000. cubic centimeters, which, again, is not particularly large. You can easily fit it in the, in two hands. The human brain only comprises about 2% of the body weight of an adult human, but it consumes 20% of the total body's oxygen consumption and 25% of total body glucose utilization, which is just ridiculous. So it consumes dramatically disproportionate amount of resources for this such a small component of the brain. And that's because the nervous issue is ridiculously active and requires an enormous amount of energy to support that metabolic function. The human brain contains something on the order of 100 billion neurons. We don't really know exactly

how many, but it's something like that, and perhaps as many as 100 trillion or maybe even one quadrillion, which is 1,000 trillion, synapses, that is, connections between the neurons, which is just a mind-boggling number. So obviously we can't specify all of the different structures in the brain since there are just so many. Here I'll just attempt to talk about some of main ones to give an idea of the different structures and functions within the brain. One way of understanding the complexity of the human brain is to think of it as comprised of three regions. The brain stem, the cerebellum, and the forebrain. And I'll talk about each of those. The brain stem is the most posterior part of the brain, which effectively means it's

closest to our feet, for humans anyway. And it's directly joined onto and continuous with the spinal cord. So there's no sort of clean break between the brain and the spinal cord, actually. It's sort of merges into it. They're directly connected to each other and continuous with each other. We usually describe the brain stem as comprising of the main components being the medulla, or also called the medulla oblongata, the ponds, and the midbrain. These three different components are all responsible for slightly different functions, but they're also all kind of similar. They don't look like anything very particularly interesting. They're all just sort of extensions of the spinal cord.

The brain stem is responsible for functions including cardiovascular control and respiratory control, control of pain sensitivity, alertness, and consciousness. So it's basically responsible for sort of low-level regulatory functions, monitoring heart rate and respiration and other things like that. So again, just to recap that, the spinal cord is, again, sort of a thin oblong cord that runs up and is directly continuous with them. dueler, which is the posterior most part of the brainstem, that then connects up into the ponds, which is sort of like a bit of a bulb. You'll see it as a sort of a bit of a sticking out bulb

on the brain stem if you see your diagram of the brain. And just superior to that, so just above that is the midbrain. And these three together form what we call the brain stem. And it's responsible for many of the sort of really basic, evolutionarily, very old regulatory functions to do with alertness and breathing and sleep and heart rate and things like that. Okay, so that was one of the three parts of the brain that I mentioned. The second of the three that I mentioned before was the cerebellum. Cerebellum is Latin for little brain, because that's exactly what it looks like. It looks like it's kind of a little, small version of the brain,

which sits to the back of and near the bottom of the brain. So if you sort of touch the top back of your neck just above the spinal, just above the top of your spine, the cerebellum is kind of in that region. there. It sits just behind or it sits just behind the brain stem and below the rest of the brain. It is responsible mostly for motor control, fine motor control. So it's not responsible for initiating movement. It doesn't send that, you know, those main signals that move or that innovate the skeletal muscles, but it does contribute to coordination, precision and

accurate timing of motor control. So you think of it as being responsible for fine motor control. It may also have some other roles in other cognitive functions, but the most well-established function it serves is in fine motor control. So that's the second of the three brain regions. So we've discussed the brain stem and the cerebellum. Now the third brain region is called the four brain. And it's the superior most region of the brain, so it sits on the top. And it's what most people think about when you think of the brain. You think of that sort of wrinkled walnut-looking thing.

Most of that is the fore-brain. The forebrain doesn't just include those wrinkles on the outside. That's actually the cortex. It also includes a number of what are called subcortical structures, which are underneath inside, sort of in the middle of all that, which are very important as well. So some of these are called the hippocampus and the basal ganglia and other things like that, which we'll talk about it a bit later.

The forebrain is responsible for most of what we might call the interesting stuff. So sensory processing, volitional movement, sensation, perception, memory, learning, language, all of these sorts of things are done in the forebrain for the most part. So let's now talk in a bit more detail about the different regions and sub-components of the forebrain because it's quite complicated and intricately structured. It should be noted that these basic regions of the brain, the brain stemmed, the cerebellum and the forebrain are found in most mammals, but the relative sizes of these different regions

of the brain differ dramatically. So particularly the forebrain or the cerebellum, which is part of the forebrain, gets much, much larger as you move to more human-like animals. So, say, primates. So it's much larger in primates than it is in animals like cats, for example, and it's much larger in a cat than it is in a mouse and so on. And indeed, mammals generally have much larger brains than sort of evolutionarily older types of animals like reptiles, for example.

In fact, I believe reptiles don't even have a forebrain at all. They just have the hind brain. Sorry, the brainstem. I may have said hindbrain and brainstem interchangeably. They're not exactly the same thing. but they're kind of similar ideas, so we didn't get into the distinctions here. So let's talk about the different regions of the forebrain, which in the human is by far the most prominent and important part of the brain,

important in terms of doing interesting things anyway. Of course, you can survive without your forebrain, but you can't survive without your brain stem. If your brainstem is destroyed, you die because your heart and lungs and things won't work. You can survive without your forebrain, although it wouldn't be a very interesting life. This is someone who's brain dead, for example, but still alive, may well have their brain stem functioning correctly, but their forebrain or their higher cognitive functions are not working, and so they're effectively, you know, they're alive, but no one is home,

so to speak. There's nothing going on there. So there are two main regions to the forebrain, not exactly regions, because they're not spatially distinct so much. The main reason that they're distinguished this way, as far as I understand, is because of the way that they develop in terms of the different types of tissue differentiating from each other, but don't think of these as being sort of spatially separate as being region one or region two. two of the brain, because they're sort of mixed up together. But the two parts of the forebrain that I'm talking about will be the diencephalon and the Tileancephalon. The latter is also called the cerebrum, so I'll probably call it that, call it that because it's a bit easier.

So let's talk about the dion cephalon first. The dyncephalon consists of a number of components. The three main ones that I'm going to discuss are the thalamus, the hypothalamus, and the pituitary gland. And you may have heard of some of these before. Thalamus is Latin for chamber, and it kind of looks like a chamber. It's basically just a sort of an roughly spherical, sort of oval shape that's located near the center of the brain. It's sort of just above the brain stem. It's like a ball sitting on top of the brain stem in a sense. Again, I'm simplifying here, but just to get the idea.

It's a collection of nuclei. Nuclei are the, that is, the nuclei of a bunch of different neurons, all bunched together. That's a nuclei. So when we say nuclei here, we're not talking about the nucleus of an atom, nor are we talking about the nucleus of a single cell. we're actually talking about a bunch of those cell nuclei sitting around together. So it's a collection of nuclei that has a very diverse range of functions. The thalamus is very, very important. It does a whole lot of different things.

Probably the simplest way of thinking about its function is that it's a relay center. It acts to send information between different subcortical regions and also between those regions and the cerebral cortex, which we'll get to in a moment. In particular, every sensory system, with the exception of the olfactory system, includes a thalamic nucleus that receives sensory signals and then sends them to the corresponding primary cortical area. So apart from olfaction, all of the other senses go through the thalamus first. So it's sort of a relay station in a sense that is responsible for mediating the exchange of information between brain regions. It's also thought to play a role in regulating

sleep and weightfulness. The hypothalamus, which just means under the thalamus, because that's where it is, is a much smaller region at the base of the forebrain, which is very important, even though it's quite small. It's, again, comprised of a number of nuclei, each with its own distinct connections and particular purposes. But the main purpose of the hypothalamus is it produces a number of hormones. So the hypothalamus can be thought of as an important connection between the endocrine system and the nervous system.

So it's sometimes described as like a neuroendocrine tissue, because it sort of does a bit of both. It's involved particularly in regulating sleep and weightfulness, eating and drinking behaviors, hormone releases, and other things. The hypothalamus is also particularly important for controlling the activity of the pituitary gland, which is the third component of the diencephalon that I'm discussing here. The pituitary gland is an endocrine gland, which sort of sticks out the bottom of the hypothalamus. It's almost like it's stuck to it by a string.

It's not quite like that, but it's sort of a strange design there. The pituitary gland secretes nine different hormones which regulate homeostasis. So homeostasis refers to maintaining the stability of internal conditions within the body, like pH and temperature and metabolism and things like that. So the pituitary gland has two main parts to it, the anterior and the posterior parturatory gland, and they each have slightly different functions and they control different hormones. We didn't get into the details of that. I'll talk more about that when I do an episode on the endocrine system.

But the important thing to remember is that the hypothalamus and pituitary gland form a system, which sort of links the nervous and endocrine systems together. So they're quite important. There are other parts to the diencephalon as well, apart from the thalamus, hypothalamus, and pituitary gland. For example, there's something called the epithalamus and the subthalamus, and there are others as well. But we won't go into those in too much detail

because they're not as well studied or as prominent. Let's now move on from the dion cephalon to talk about the Tilean cephalon or the cerebrum. Now, the cerebrum comprises pretty much all of the other components of the forebrain, which remember is one of the three regions of the brain that we're discussing, along with the brain stem and the cerebellum. So the cerebrum includes the, includes white matter, which I'll discuss in a moment, the hippocampus, the amygdala, the basal ganglia, rhineencephalon, and the cerebral cortex. And I'm going to discuss each of those, each of those in turn.

Let's start by talking about the white matter, which is quite interesting. So white matter is so-called, it is literally white or sort of a pinky white. And the reason it is that color is because it's basically just fat or mostly lipids. The white matter is a very important component of the central nervous system because it consists mostly of glial cells and myelinated axons. So these are the support cells and sort of insulation that surrounds the axons of neurons and that helps conduct the information from one body of neurons to another. You can think of white matter as being sort of the central nervous system version of the nerves in the peripheral nervous system. I mean, it's a bit different because white matter is a lot more condensed.

It's sort of pushed up and squashed together rather than distributed throughout the rest of the body. But it serves a sort of a similar function of transmitting information and insulating to ensure that the signals can be propagated in the most efficient way. If you sort of cut open the brain and look into it, what you'll actually find is that there's a small outer surface, which is only, I think, a few millimeters thick, which is called the cerebral cortex, and that's where most of the cell bodies are.

This is called grey matter, because that's the actual bodies of the cells, of the neurons. Most of the interior of the brain is comprised of what we call white matter, so it's these axons and support cells. Of course, the interior of the brain also contains the ventricles, containing cerebrospinal fluid that I talked about before, and there are also some other subcortical structures containing sort of bunches of nuclei, so bunches of grey matter interspersed amongst the white matter.

So these are the subnuclear structures, sorry, the subcortical nuclear structures. But most of the internal volume of the inside of the brain is actually white matter. And one particularly important white matter structure is called the corpus callosum, which connects, which is a very sort of dense bunch of fibre tracts, which connects the two hemispheres of the cerebrum, so the left half of the brain and the right half of the brain, essentially, because the brain is sort of bilaterally symmetrical, or sort of the same on both sides, on either side.

Right, so that's the white matter. The hippocampus, which means seahorse, are two structures which are located on either side of the thalamus. And they're called the seahorse, because they kind of look like a seahorse. They're sort of a weird bent shape. They're subcordical structures, so they're bunches of nuclei.

They're part of the limbic system, and they're particularly important for consolidating memory. So processing information taken in, from stored in short-term memory and storing it in long-term memory. They're also used for spatial navigation. But particularly people who have memory difficulties may have some damage to the hippocampus, particular types of memory deficits, I should say. Particularly antrograde amnesia can be caused by bilateral damage to the hippocampi.

Bilateral, meaning on both sides of the brain. So that's the hippocampus. The amygdala, which means almond, again it sort of just refers to its shape, is a small nuclei. It's quite a bit smaller than the hippocampus, located sort of at the base of the hippocampus, it's sort of near the thalamus and the end of the hippocampus. Again, there are two of them, because there's one on either side of the brain. And they are responsible in particular for some roles in memory in decision-making, but particularly in emotional reactions, especially negative reactions like fear.

Amygdala activity has been very strongly associated with, say, fear responses and other things like that. And because the amygdala is also considered to be part of the limbic system. The limbic system, I should say, is essentially this subcortical structure, sorry, this set of subcortical structures, which has particularly important roles in emotion and other things like that. But it's not especially well-defined the notion of a limbic system. It's not even clear if it's really a system as such, or it's just a lot of connected components.

But anyway, it's a term you might hear occasionally, so I just thought I'd mention it. So that's the amygdala. The basal ganglia is another part of the cerebrum. It is a group of interconnected substructures in the forebrain, the primary function of which appears to be related to the selection of action. So it sends inhibitory signals to all the different parts of the brain that generate motor behaviors, and in the right circumstances, it will release that inhibition, so letting an action happen.

It's also thought to be related to sort of reward and punishment, so motivation as well. Action selection is sort of the sort of takeaway you should remember about the basal ganglia. Another important part of the cerebrum is called the rhine encephalon, which means nose brain, It's, again, a collection of structures involved with olfaction. It includes the olfactory bulb and the olfactory tract and some other things as well. It's very important for many animals because olfaction is extremely important for a lot of animals, a lot of mammals, but it's relatively underdeveloped in primates, particularly humans, who've evolved to rely more on vision than olfaction. When I say, by the way, that's something like the basal ganglia or the rhine encephalon is a collection of structures,

It really means that it's a complicated structure which is sort of dispersed throughout various regions of the subcortical internal region of the cerebrum. So it's hard to sort of describe exactly what it looks like or where it is. It's easy to try and use diagrams if you're trying to visualize these things. Some structures like, for example, the phallumis or the amygdala are more relatively contained in single structures, whereas, say, the basal ganglia or the rhine encephalon are a bit more dispersed and a bit harder to describe or localize, at least for our purposes. Okay, so that's the Ryan Encephalon. The final part of the forebrain that I want to discuss is, in my view, most interesting, the cerebral cortex.

Cortex means bark, I think that's Latin as well. So it basically refers to the thin layer at the very outside of the brain. It's the outermost layered structure of neural tissue in humans and other mammals. It's divided, sorry, before I talk about divisions, the cerebral cortex plays a vital role in memory, attention, perceptual awareness, thought, language, consciousness, all of those sort of higher cognitive functions is occurring mostly in the cerebral cortex and also the associated white matter because you can't have any of this happening without the white matter, transporting the signals around.

The cerebral cortex traditionally divided up into four different regions. These are literal regions, so you can distinctly map them out on different regions of the brain. If you've seen sort of covered maps of the, or coloured maps of the external surface of the brain, this is probably what they're referring to. It's the four different lobes of the brain or regions of the brain. The occipital lobe is at the back, and that's responsible largely for vision. So V1, for example, the primary visual cortex is located there at the very back of the head. I talked about that when I talked about vision in the episodes on that.

Temporal lobes are located at the temple, so sort of on the side of the head. That's responsible for some language function, thought to play a role in audio and visual memories and perhaps the formation of new memories and some various other things as well. The parietal lobe is sort of on the top of the head. It is responsible for somatosensory perception, so the perception, receiving and processing information about perception from different parts of the body. It's also thought to play a role in processing visual information and visual memory. And finally, the largest lobe of the frontal lobe, also called the frontal cortex, is responsible for executive

function. So that's making decisions and sort of higher thought. And it's also thought to play some role in language. The frontal lobe also includes the motor and the primary motor and premotor cortex, so it's responsible for primary control over the semantic nervous system that we talked about earlier, so control of the

skeletal muscles. Damage to regions of the frontal lobe, particularly the regions of the prefrontal cortex at the very front of the frontal lobe, is associated with behavioral abnormalities and changes in personality and things like that. So if you've heard of Phineas Gage, for example, is a famous example of an individual who had frontal lobe damage and led to changes in his personality and behaviors, and also difficulty in regulating his behavior and self-control and things like that.

So that's the sort of things that the frontal lobe is thought to be responsible for. There's just a couple of other points that I should make about the cerebral cortex. again, if you've seen a picture of the brain or seen a diagram of it, which I'm sure everyone has, you would remember that it looks sort of lumpy. It's got these sort of lines going through. It looks sort of wrinkled like a walnut. These wrinkles serve a very important purpose, and they're actually referred to as gyri and sulkases. In particular, the protruding parts that the protruding parts are called the gyri,

and the sort of the valleys, or the holes, if you like, in between them are referred to as the sulcuses. So the gyrae and the sulkyte comprised the brain. The reason that it's folded up and wrinkled in this way is so that the total surface area of the brain can be packed. The total surface area of the brain can be larger, but packed within a smaller volume. Because there's a trade-off, essentially. The way that the cerebral cortex is structured is that it's layered. There's like six layers actually, and there are different types of neurons on each layer. And so it seems to require that it's structured in these layers, and below that, as we know, is the white matter and then the subcordical structures.

In order for the brain to grow larger and to do more and interesting things, we need to have a larger surface area. However, there's a limit to that, because, evolutionarily speaking, human brains got larger and larger over time as we evolved. but there was a limit to that because the brain has to come out of the birth canal. It has to fit through the mother's pelvis, and that means it can't get beyond a certain size. So one way we got around this is by having babies earlier and earlier. So if you look at the gestation period for a lot of other mammals our size, it's much, much longer than for humans. Humans are born relatively underdeveloped. Like, humans can't, human babies can't walk for many months until after they're born, whereas many animals can walk as soon as they're born.

Well, that's one product of the fact that human babies are born very immature. And the reason that's happened is because there's been evolutionary selection in favor of premature births. Again, evolutionarily speaking, they're not premature for humans, but they're premature compared to other animals. Because that's a way of producing a larger brain overall, but keeping it small enough to fit it out through the birth canal. But another way around this is instead of increasing the volume of the brain, just increase the surface area of the brain and then fold it up more. So that's where you get the gyri and the sulkyers being a way of fitting more surface area into a smaller volume. You actually see if you look over different mammals that things like rodents, for example, and rat brains, they're pretty much completely smooth because there hasn't been as much pressure on them to involve larger brains.

If you look at, say, cat brains, they're a bit more, they have some ridges, and if you look at primate and human brains, they have many, many ridges, and they're very, very folded. And dolphin brains are even more folded up than the human brain is, because again, they have a similar issue of trying to fit more surface area into a smaller volume. So that's the reason that the brain is sort of wrinkled and folded up like that. It's because of the way the nervous tissue is structured. Just a couple of final points. I want to mention lateralization. This is the notion that each hemisphere of the brain interacts primarily with one half of the body.

It's actually crossed over so that the right half of the body is controlled by the left half of the brain and vice versa. Also, there's a specialization of each hemisphere. So most people, particularly right-handed people, have language lateralized, so sort of focused on the left hemisphere of the brain. That means that damage to certain regions of the less hemisphere produces severe linguistic deficits in terms of understanding and production of language, whereas comparative damage to only the right side of the brain doesn't produce. are nearly so much of a deficit. This is not the case for all people. Some people are almost completely bilateral in language, and some people are actually right lateralized, but most people are left lateralized. I don't think it's really understood why that's the case, but it seem to be

the case. However, you might hear sometimes people talking about left brain and right brain thinking about the left side of the brain being analytical and sort of deductive and things like that, and the right side of the brain being more creative or holistic or things like that. That's total nonsense. There's just, there's no evidence for that. There is evidence for degrees of lateralization of some functions, like language, for instance, and maybe some other things. Certain reasons, sometimes one hemisphere of the brain, one half of it can be a bit more active in some tasks than another. That's true, but this differs a great deal between different people and between different tasks, and depends on exactly how you test it. It's not a particularly large effect from any phenomena.

It's reasonably large for language, but for other things, it's very small if you can detect it at all. and there's no evidence that it's important in any sense, like, that there's sort of one way of thinking, which is more left-brain thinking, and another way of thinking that's more right-brain thinking. That's just rubbish. There's no evidence to that at all. One final thing that I want to mention

is the relationship between brain size and intelligence, or IQ. This has been something that's been studied for quite some time. In the late 19th and early 20th century, it was thought that there was a very sort of clear relationship between brain size and intelligence, and it was thought that white people had larger brains and therefore were more intelligent or were more intelligent because they had larger brains. It was also thought that men were more intelligent because they had larger brains or larger

cranial, higher cranial capacity than women. It is true that men do have a slightly higher cranial capacity on average than women, although there's a great degree of variability there. I don't think that there is any overall differences in terms of cranial capacity between races. I'm not sure what the latest research on that is. However, the more interesting question of how important is that in terms of intelligence is much clearer. There's really no evidence that there's any significant effect of brain size on intelligence. From what I could gather, there have been a number of studies which have shown a moderate correlation between brain volume and IQ,

but it's not a very large correlation. So it's like 0.3 or 0.4, which means that, so for example, if the correlation was point 3, 0.36, just to pick a sort of intermediate figure, that would mean that about 13% of the variation in IQ is explained by variation in brain size, which is really a very small amount. And I don't think that that carries over between the sexes either. There's certainly no average intelligence differences between the sexes. The difference in cranial capacity is almost certainly just a product of, again, small average differences in overall body size between the two sexes. So anyway, the bottom line is there does seem to be something to the idea that brain size and intelligence are related, but there's not that much to it.

And certainly historically and even arguably today, it's very much overblown that there's no very clear or concise, or precise relationship between the two. And it certainly doesn't mean that if you have a big head, you're intelligent or vice versa. It's, if there's any relationship, it's a subtle and not that important one. All right, that brings us to the end of what I wanted to discuss today. I hope you found this episode interesting and not too difficult to follow. It's sort of a bit silly of me to try and explain the different regions of the brain without any visual material, but I will be posting some up on the Facebook page for the podcast,

so go and check that out. Just type in The Science of Everything podcast on Facebook, and you should be able to find the page and give us a like. Also, feel free to email me. Fods12 at gmail.com. That's F-O-D-S-1-2 at gmail.com. I'd love to hear your feedback about this episode or any other episode, you've listened to or any feedback you might have about the podcast in general, suggestions for

future episode topics or questions you might have that you'd like to be addressed. I'd love to hear from you. So thanks again for listening and I'll talk to you next time.