Nervous System Lecture 1 (with notes) PDF

1 This is Lecture 1 of the Nervous System section of the module. The topic is the cells of the nervous system. 2 Here’s a picture of the human brain. We’re going to be talking a lot about the human brain, but let me let you in on a...

1 This is Lecture 1 of the Nervous System section of the module. The topic is the cells of the nervous system. 2 Here’s a picture of the human brain. We’re going to be talking a lot about the human brain, but let me let you in on a secret right from the start. Nobody really knows how it works. 3 Here’s a picture of the human heart. In Trimester B we’ll be looking at how the heart works. But I’m showing you this picture now because, when it comes to the heart – and in fact, when it comes to most organs in the body – you can get a fair idea of what it does just by looking at it. Its structure relates very closely to its function. It’s got chambers, it’s got valves, it’s made from muscle tissue. There are tubes going in and out of it. The structure of the heart lets it do what it does, which is pump blood around the body. 4 Now, if you look at the structure of the brain, it’s not at all obvious how its structure relates to its function. How does this lump of nervous tissue allow you to remember things, and imagine things, and get angry and sad and happy and frightened? How does it allow you to experience the world and interact with the world? And if you’re expecting to learn the answers to these questions, you’re going to be disappointed, because nobody really knows. The brain is, in many ways, mysterious. 5 Let me give you some examples of how mysterious it is. Here’s a photo of a brain scan. The brain has been sliced – not literally, of course – across the way, and we’re looking down at it. The white border is the skull, the grey stuff filling the inside of the skull is the brain tissue, and there are a few dark areas that are spaces within the brain. 6 Now, the brain scan on the right is a scan of the brain of a 44 year old Frenchman. He was healthy, he was married, he had children, he worked as a civil servant. So, perfectly normal life. But as you can see, he had hardly any brain. He had about 10% of the normal amount of brain tissue. 7 Those dark areas contain only fluid. The brain tissue, what there is of it, is the thin layer of grey stuff around the inside of the skull. How can this be possible? How can you have hardly any brain tissue, yet live a fairly normal life? The short answer is: nobody knows. 8 Here’s another mysterious aspect of nervous system function. You may have heard of what’s called phantom limb sensation. A high proportion of people who have a limb amputated – an arm, a leg – report that they can still feel it, even though it no longer exists. In fact, they often report feeling pain in the phantom limb. 9 But even stranger, I think, is something called supernumerary phantom limbs. This is a rare condition that can happen after certain types of stroke. And the person feels that they have extra limbs. You can see here the positions that these extra limbs tend to be experienced. So, they feel theiractual own arms but they also feel as if they have these extra arms. How do you explain that? Well, there are various suggestions, but no-one knows for sure. 10 Here’s a final example of a weird condition that involves the nervous system, but we’re going to try to explain this one later on. It’s called crocodile tears syndrome. Apparently, crocodiles weep when they’re having their dinner. When they’re chomping down on some poor antelope, tears pour out of their eyes. 11 And people, human beings, with crocodile tears syndrome also weep when they’re eating. 12 In fact, they’ll even weep when food is placed in front of them, or even simply at the thought of food. At the end of the lecture we’ll have a go at explaining why this should happen. 13 Anyway, let’s start getting down to business by looking at the general structure of the nervous system. 14 Inside the skull is the brain. We’ll look at the brain in detail later in the module. 15 Travelling down from the brain, running down your back, is the spinal cord. Again, we’ll come back and look at this in a later lecture. 16 And together, the brain and spinal cord make up what’s called the central nervous system, the CNS. Now, you can see lots of blue strands leaving the brain and the spinal cord and running down the arms and legs. They’re also part of the nervous system. And they’re what’s called nerves. We’ll be looking at nerves in more detail shortly, but for the moment let’s just name some of them. 17 The nerves that travel in and out of the brain are called the cranial nerves. And they kind of control what’s going on in the face and the head: things like facial expression, for example. 18 Further down, travelling in and out of the spinal cord, are the spinal nerves, and they control what’s going on in the body from the neck down. 19 Now, the spinal nerves – and also the cranial nerves – branch and split and go on to form what we can call the peripheral nerves that travel to pretty much every part of the body. 20 And all those nerves – cranial, spinal, peripheral – form what’s called the peripheral nervous system. So the central nervous system is the brain and spinal cord; the peripheral nervous system is all the nerves outside the brain and spinal cord. 21 So, that’s the basic structure of the nervous system. What about the the functions of the nervous system? What are these functions? Well, this is a question that’s quite difficult to give a simple answer to. 22 But if you had to do that, if you had to provide a simple statement of nervous system function, the answer would be communication and control. 23 Now the control might be relatively simple and straightforward, like controlling the size of your pupil, making it bigger or smaller depending on light levels. It’s the nervous system that does that. 24 Or it could be something more complicated. To do what this guy’s doing, the nervous system has to process balance messages, messages from the eyes, messages to and from the muscles, it has to coordinate complex movements. It has to do all that from moment to moment. 25 So let’s take a look at the cells of the nervous system, because it’s those cells that allow this communication and control to take place. And although there are billions of cells in the nervous system, there are two main types. 26 The first type is what are called simply nerve cells, or neurones. 27 And it’s nerve cells that actually carry out the function of communication and control. We’ll look at how they do that in a moment. 28 The second type of cell in the nervous system are called glial cells, also known as neuroglia. That word “neuroglia” – the literal translation of that word is “nerve glue”, and that kind of gives you a clue as to what these glial cells do. 29 Glial cells are the supporting cells of the nervous system. There are various types – we’ll be mentioning a few of them as we go – and they all do different things, but what they all do, in various ways, is they help the nerve cells to function. 30 But let’s concentrate on the nerve cell for the moment. Here’s a diagram of a nerve cell. Let’s label its key structures. 31 Over here is what’s called the cell body of the nerve cell. Picture it as a kind of ball- shaped structure. That circle at the centre of it represents the nucleus. 32 And sticking out from the cell body is a long thin extension, called the axon. Now, axons can be incredibly long. Axons that travel from your spinal cord in your back down your leg to your foot: that’s, what, a metre in length? In cellular terms, an incredible distance. So the axon you see here is much too short. It should really travel off the right of your screen and out your front door and half way down the street. 33 The first part of the axon – the bit where it leaves the cell body – is called the axon hillock. That’s an important part of the axon that we’ll return to in a later lecture. 34 The axon is the structure that carries the electrical impulse. The impulse starts at axon hillock, and travels along the length of the axon. We’ll be looking at that process of electrical communication in more detail in the next couple of lectures. 35 So the axon is a long thin extension, and when it gets close to its destination – here the destination is the purple-coloured nerve cell over on the right of the screen – the axon splits into branches called telodendria. 36 And at the end of each of those branches is a tiny swollen structure called a terminal button. Terminal buttons contain chemicals called neurotransmitters, and what the electrical impulse does is it causes the terminal buttons to release this neurotransmitter. And that neurotransmitter chemical is going to pass on a message to the next cell in the pathway. In this case, the purple nerve cell. Again, we’ll be looking at this process of chemical communication in more detail in a later lecture. 37 The final important structure – or structures – are these short, very thin, highly branched extensions sticking out from the cell body. These extensions are called the dendrites. What the dendrites do is collect the chemical messages passed on from other nerve cells. So when a nerve cell releases its neurotransmitter chemical, it’s mainly the dendrites that pick up this neurotransmitter. Again, like pretty much everything I’ve been saying so far, we’ll return to this later. 38 Now, some nerve cells have an axon that is what’s called myelinated. Some axons are myelinated axons. Here’s a diagram of a myelinated axon. So you can see the cell body of the nerve cell over on the left, with dendrites sticking out from it, and you can see the long axon, and the axon is surrounded by sections of a kind of yellowish substance. 39 This yellowish substance is called myelin. Myelin is a lipid material, and what myelin does is it insulates the axon. 40 Myelin is made by a type of glial cell – one of the supporting cellsI mentioned earlier – called a Schwann cell. Each section of myelin is made by a separate Schwann cell. 41 And one very important point is that there are gaps between the sections of myelin – the insulation isn’t continuous – and the gaps, the small areas of exposed axon, are called the nodes of Ranvier. 42 Here’s how the myelination process works. Each Schwann cell grabs hold of a section of axon 43 And then begins to wrap around it. 44 And what you end up with is a section of axon surrounded by lots of layers of myelin insulation that’s been made by that Schwann cell. 45 Now, Schwann cells are found only in the peripheral nervous system. Schwann cells myelinate the axons in the nerves that run to and from different parts of the body. There are no Schwann cells in the central nervous system. What you do have in the central nervous system, though, is a different type of glial cell called an oligodendroglial cell, and those oligodendroglial cells myelinate the axons within the central nervous system. In other words, within the brain and spinal cord. 46 It’s a very similar process, but as you can see here, oligodendroglial cells can provide the myelin insulation for more than one axon. 47 So, while we’re talking about oligodendroglial cells, let’s just take the opportunity to mention some of the other glial cells of the central nervous system. This rather complicated picture shows some of the cells within the central nervous system. 48 Down in the left corner here is the cell body of a neurone. You can see another one just above it. 49 And here’s one of the oligodendroglial cells I just mentioned, providing some of the myelin insulation for the axons of these two nerve cells. 50 What you also have in the diagram is another type of glial cell, a type called an astrocyte. What astrocytes do is they form part of what’s called the blood-brain barrier. We’ll return to the blood-brain barrier in a few weeks’ time. 51 The ependymal cells that you can see in the top left of the picture are cells that help to make cerebrospinal fluid, the fluid that surrounds the brain and fills the spaces inside the brain. 52 Finally, microglial cells are the defensive cells of the central nervous system. They patrol the brain and spinal cord and attack and destroy any foreign cells or any damaged cells. 53 Now, let’s return to a term I mentioned earlier. The word “Nerve”. What exactly is a nerve? 54 Here’s a definition. A nerve is a bundle of axons running together outside the CNS; in other words, outside the brain and spinal cord. You’ll remember that the axon is the long thin extension of the nerve cell that carries the electrical signal. Well, those axons, when they travel out to the body – or when they travel in from the body – are grouped together into nerves. 55 Each of the long yellow strands in this picture, passing out from the spinal cord up at the top, then running down the arm, is a nerve. 56 A reasonable analogy is to think of a nerve as being like a cable. The individual wires in the cable are the axons. And as you can see in this picture, the wires are actually packaged together into bundles within the cable. And the same is true for axons. Axons are grouped into bundles within the entire nerve. 57 Here’s a diagram that shows the structure of a nerve. Down at the right corner is a single myelinated axon – you can see a couple of sections of the myelin insulation. That axon – and all the axons – is surrounded by a protective wrapping of connective tissue called the endoneurium. Axons are grouped together into bundles called fascicles that are surrounded by another protective layer of connective tissue called the perineurium. Finally, the protective wrapping around the entire nerve is called the epineurium. 58 I’ll be talking about the CNS – the central nervous system – in later lectures, but while I’m talking about bundles of axons I might as well give you another definition. Bundles of axons within the CNS – within the brain and the spinal cord – are called tracts. So, nerves lie outside the central nervous system, tracts lie within the central nervous system. 59 Let’s finish off by saying a bit about nerve damage. Although the module concentrates on normal physiology, we’ll also occasionally talk about pathophysiology, what happens when damage or dysfunction takes place. So what happens when a nerve is damaged? 60 Let’s imagine that your ulnar nerve has been completely severed, it’s been completely sliced across the way: so all the axons are cut. The ulnar nerve runs down your arm, it runs behind your elbow. When you bash your funny bone it’s your ulnar nerve that you’ve bashed. 61 So let’s imagine that this is an axon that lies within the ulnar nerve. 62 This axon – like all the other axons in the nerve – has been sliced in two. 63 Here’s a close-up view of the axon. The dashed red line represents the point where it was sliced. Bear in mind that the diagram shows only one axon, but that all the processes I’m going to talk about will involve all the axons that have been damaged in this way. At the top you can see two anatomical terms: proximal and distal. Proximal means “closer to the point of attachment” (in this case, the attachment is to the cell body, which will be up in the spinal cord); distal means further away from the point of attachment, so in this case beyond the point of damage. 64 So you can see that distal to the point of damage – further down the arm in this case – the axon begins to degenerate, it begins to break down. So does the myelin that was wrapped around the axon: the myelin also degenerates. But the Schwann cells that made that myelin do not degenerate, they remain alive. And what you can also see is that proximal to the damage point, the axon and myelin stay intact. 65 What then happens is that the axon starts to regenerate, it starts to regrow down the arm. It does this at a rate of about 1mm every day. Meanwhile, macrophage cells – defensive cells of the body – are swallowing and destroying the broken-down axon and broken-down myelin. In other words, they’re clearing the path. And the Schwann cells start to form a line. 66 And as the axon regrows, it’s guided to its destination – which might be much further down the arm, might be in the hand and fingers – by that line of Schwann cells. 67 And as the regrowth continues, as the new axon makes its way down the arm, the Schwann cells start to remyelinate it, they start to wrap around it and form the myelin insulation. Now, in an ideal world the new axons will travel to exactly the same place the old axons did, but this doesn’t always happen. The success rate is dependent on a number of factors, including how far the new axons have to go. 68 One condition that can sometimes result in misdirected axons is a condition called Bell’s Palsy. 69 Bell’s Palsy involves viral infection of one of the cranial nerves that controls various things to do with the face. For example, as the infection damages axons in this nerve, the face can become paralysed on one side as the facial muscles are no longer being stimulated. 70 The cranial nerve in question is called the facial nerve. Here you can see it passing out from the skull and then branching and travelling to various parts of the face. 71 One branch of the facial nerve goes to the lachrymal gland, the gland around the eyeball that produces tears. That branch is shown in blue on this diagram. 72 Another branch of the facial nerve goes to the salivary glands to trigger the production and release of saliva into the mouth. That branch is shown in red in this diagram. 73 What can sometimes happen though is that, as the person recovers from Bell’s Palsy – and the majority of patients recover without any long term effects – but what can happen in a small proportion of patients is that some of the new axons can end up travelling to the wrong destination. Some of the red axons, that normally trigger salivation, end up travelling to the lachrymal glands, the tear glands. 74 And that’s how crocodile tears syndrome occurs. Normally, when a person has a meal – or thinks about having a meal – particular axons in their facial nerve tell their salivary glands to release saliva. In these patients – patients in whom those axons are now going to the tear glands –secrete tears in response to stimulation by these axons. 75 So, we’ve introduced some aspects of nerve cell structure. In the next lecture we’re going to start to look in more detail at how nerve cells work. And we’re going to begin by looking at how nerve cells send an electrical impulse along their axon. That’ll be the topic for lecture 2. 76