Nervous system anatomy introduction.

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Transcript
00:00Hello, a little bit frazzled today because I'm trying to squeeze this in amongst lots
00:07of other teaching stuff I've got to do, but what I thought we'd do is we'd do an introduction
00:13to the nervous system as a whole, the anatomy of the nervous system. So we'll talk about
00:18a neuron and then from there, well I guess we'll start at the top and we'll work our
00:21way down and out and the main aim will be to cover the major terms, the major structures.
00:30So when you come across them elsewhere, you know what people mean when they say spinal
00:33nerve, medulla oblongata and things like that, okay? And I'm going to be really careful to
00:39not go into too much detail. So this isn't going to be detailed, this is going to be
00:43introductory, there's other detailed stuff to go and look at, all right? Right.
00:50A pipe cleaner, that's an idea. We use pipe cleaners for making things and poking skulls
01:05and that sort of thing, but if we're to consider a neuron, a neuron then is the basic cell
01:11of the nervous system and a neuron has a, you know, there are various types of essentially,
01:17if you think about a neuron as having a cell body at one end, all right, so the nucleus
01:25and it's got a little bit of a fat bit there, the cell body is at one end and then it sends
01:29off a long axon to somewhere else in the body and that may be very, very long, it might
01:35be more than a metre in you, it might be many metres long in a blue whale, so these are
01:38incredibly long cells, but a neuron sends an axon down to meet another neuron or to
01:46become a receptor for something or motor to something like trigger a muscle contraction
01:51or something like that and that is what the nervous system is largely made up of and that
01:56is the cell and the shape you should be thinking about when we're thinking about the wiring
02:01of the central nervous system, the neuron. There are other cells in the nervous system,
02:06there are lots of supportive cells and connected tissue cells and fat producing cells and all
02:10sorts of other cells, but functionally, that's our guy, all right? So when we talk about
02:16grey matter and white matter, the grey matter, those are collections of nerve cell bodies,
02:23so the cell body, the neuron cell body there and they, so for example in the brain, the
02:28grey matter is around the outside, it's more superficial and the white matter, when we're
02:35talking about white matter, we're talking about axons covered in myelin, fat, which
02:39is why they look white, so nerves again, when we look at, when we think about nerves, they
02:45are bundles of neuron axons generally covered in myelin, so grey matter and white matter.
02:56The brain, or the brian as many students seem to spell it in some anatomy exams, the brain
03:02has a number of lobes and we can see these sulci and gyri, the gyri being the fold and
03:09the sulci being the depression, which give a lot more surface area to the brain, so a
03:14lot more room for grey matter, so a lot more room for more neurons, more neurons, more
03:20connections, more complexity. And it has a number of lobes, we have the frontal lobe,
03:25parietal lobe, temporal lobe and occipital lobe and these are all parts of the cerebrum
03:31and there are two cerebral hemispheres, left and right, split down the middle but joined
03:36by big tracts of white matter, so they do communicate, so those are the cerebral
03:41hemispheres, this is the cerebrum and then coming out of the cerebrum, descending down
03:50here we have, well in the centre there we have the midbrain, which we can't really see,
03:54it's surrounded by the rest of the cerebral hemispheres up here and then we have the pons,
04:01it's curved like a bridge, pons, pont, so we have the pons and the medulla or the medulla
04:07oblongata and then the medulla oblongata continues as the spinal cord descending down through
04:13the back. So that's the brainstem, the midbrain, pons and medulla comprise what we call the
04:21brainstem and we can see a number of nerves coming out of the brainstem and indeed coming
04:27out of the cerebrum and these nerves we call cranial nerves because they come out of these
04:35parts. If we spin around posteriorly we see these chunks of tissue here, these are the,
04:43well this is the cerebellum and the cerebellum, I'm keeping it simple, let's not talk about
04:53functions, we're just talking about names. So the cerebellum is attached to the brainstem,
05:03boom, okay, so then spinal cord, so then the spinal cord is a continuation from the medulla
05:14oblongata through the vertebrae all the way down the back, I say all the way down the back,
05:19in adults the spinal cord is not the same length as the vertebral column, so the spinal cord
05:25actually ends here and what we can see here are spinal nerves running down which we'll talk about
05:29in a moment. But that's the central nervous system, so the cerebrum, the brainstem, the cerebellum
05:42and the spinal cord are the central nervous system which is commonly abbreviated to CNS.
05:50The other division of the nervous system is the peripheral nervous system,
05:54the PNS. Now the peripheral nervous system is everything that comes out of the central nervous
06:00system, so here we see spinal nerves coming off regularly and other nerves, those are all parts
06:06of the peripheral nervous system, and those cranial nerves that we see here are also parts
06:12of the peripheral nervous system, all right. So what's a spinal nerve then? Well, a spinal nerve
06:21is a nerve that leaves the spinal cord and passes off to somewhere else in the body, and we've
06:28talked about all of these things in more detail elsewhere, so I'll leave that to those videos,
06:33but remember our basic building block of the neuron. A spinal nerve then is a collection of
06:39lots and lots of neurons, some of them are sensory and some of them are motor, and they're all
06:45bundled together and the reason they're running together as a spinal nerve really is because
06:51they're all going to a similar location. We're a segmented animal and we can see that these spinal
06:57nerves are showing us those segments, those segments are most obvious in the ribcage,
07:02but they continue elsewhere throughout the body, and sometimes those spinal nerves come together,
07:07so this is the brachial plexus forming up here, these are the nerves that are going to supply
07:12the upper limb. We can see these spinal nerve roots coming together very soon after they leave
07:19to form new nerves, because by coming together they're forming, you know, new structures,
07:24anatomists like to name everything, so we'll give those new names, and then they change and link and
07:28change and blend and join and eventually form the major nerves of the upper limb. And the plexus
07:34then, a plexus is, it's like an organization of wires, it's like wiring, the nerves are all
07:41crossing over and joining and forming new nerves, but there aren't any connections in there, it's
07:46just cabling, it's just cable management, that's a plexus. Whereas a ganglion, which is what we're
07:55seeing here, a ganglion is a collection of neuron cell bodies outside the central nervous system.
08:05We would call, I said that gray matter is collections of nerve cell bodies, but a ganglion,
08:15this mass that we can see here, if we see a ganglion inside the central nervous system,
08:19it tends to be called a nucleus, but a nucleus inside the central nervous system is, again,
08:24just a collection of nerve cell bodies. See, terminology is one of the first things that can
08:29trip you up when trying to learn neuroanatomy, and neuroanatomy is hard enough as it is.
08:34So, the spinal cord descends and gives off a number of spinal nerves, and those run off to
08:39their target organs and tissues and what have you, bam, job done. They might be
08:45innervating muscles and causing them to contract, they might be carrying all sorts of sensory
08:48information back, and there are maybe one or two connections between that nerve
08:53and the central nervous system before they get to the higher centers, but as I said, that's all
08:57covered elsewhere. So, we were talking about the spinal cord, there is another video about the
09:01spinal cord, but the spinal cord only reaches this far, and then all these spinal nerves that you see
09:08leaving the vertebral column at regular intervals, when we get down here into the, so we're into the
09:15lumbar region, the sacral region, those spinal nerves just carry on running down inside the
09:21vertebrae and then leave a little bit later. So, if you were just to look at the spinal nerves
09:26leaving the vertebrae, it remains this nice neat segmented pattern that you would expect to see,
09:32it's only when you look inside the vertebrae that you see that in fact the spinal cord has ended
09:36here and the spinal nerves are forming like a horse's tail, the cord required down here.
09:42That's almost it, isn't it? Because not only do we divide the nervous system into central nervous
09:48system and peripheral nervous system, but we also divide it into autonomic and somatic.
09:55Now, somatic, soma, soma meaning the body, so the somatic nervous system means of the body,
10:01and essentially it's, the somatic nervous system is involved with things you can,
10:09things you can consciously control or things that you are consciously aware of.
10:14The best examples are of, are this, so somatic motor control is I'm deciding to make these
10:21movements and I'm making them, so that's the somatic nervous system that's controlling my
10:27skeletal muscles. And when I poke my skin, when I get stung by a bee, when a wasp, when a fly
10:38flies into my eye, those are things I'm very consciously aware of, so that would be the sensory
10:45side of the somatic nervous system, so of the body, do you see what I mean? Whereas the autonomic
10:51nervous system is involved with things that we're not consciously aware of. If we think of a motor
10:58example, the autonomic nervous system would be involved in controlling the muscles in the GI
11:05tract that pass your food along the tube, you really don't want to be aware of that, you don't
11:10want to have to think about where something is and then move it along, no, no, no, no, that's what the
11:14autonomic nervous system does, it's automatically looking after your GI tract for you, you're not
11:21even aware of it, you can't be aware of it unless you have some pain or something. So then the
11:27sensory side of that, of the autonomic nervous system, again would be carrying sensory things
11:32back from the gut, but another good example would be blood pressure and blood gas, blood gas levels.
11:42So in some of the major blood vessels in the body and deep inside the brain, there are baroreceptors
11:50and chemoreceptors, and those baroreceptors are looking at the stretch in artery muscle walls, so
11:57they can look at the blood pressure in that blood vessel, and chemoreceptors are looking at
12:03the chemical composition of the blood and sending that information back to the brain, so the brain
12:08is aware of how it needs to modulate the blood vessels in the body and the heart to manage blood
12:15pressure and to make you breathe in faster or slower to make sure the chemical composition
12:23of the blood in terms of oxygen and carbon dioxide and what have you is appropriate. Do you
12:27see what I mean? So the autonomic nervous system has a sensory function in monitoring your body
12:33and making sure it is as it should be, and then autonomically changing things to make sure we
12:40have this level of homeostasis to make sure things are as they should be. So that's the
12:44autonomic nervous system, but what about physically? Well somatic, okay, so I was talking about the
12:50brachial plexus. Most of the nerves of the brachial plexus are innervating skeletal muscles
12:55of the upper limb and carrying sensory innervation back from the skin of the upper limb. So these
13:00nerves are very big because they have a large number of neurons. It's a physical thing. If you
13:07have greater sensitivity in your fingertips, you need more sensory neurons. More sensory neurons
13:13means that nerve gets bigger. So the brachial plexus might be a really good example of a section
13:19of the somatic nervous system. Now most of these spinal nerves will in fact be mixed. They'll have
13:27autonomic fibres and somatic fibres in there because in your skin you are autonomically
13:35controlling blood flow to your skin. When you're hot, more blood flows to your skin. You lose heat
13:40and when you're cold, less blood flows to your skin. That's all under autonomic control.
13:45That's the smooth muscle in the arterioles being controlled there. Also the hair sticks up and so
13:51on, right? So throughout your body, particularly the skin and elsewhere, but most of these nerves,
13:58most of the nerves in the body are mixed and are carrying autonomic nerves and somatic nerves.
14:05So it's not an easy physical distinction to make. It's more of a thoughtful distinction.
14:10But we can see some autonomic structures. So this here, these ganglia, this is the
14:19sympathetic trunk or the sympathetic chain or sympathetic ganglia. And what these are, are,
14:26okay, so the sympathetic and parasympathetic divisions of the nervous system are both
14:34parts of the autonomic nervous system. So the autonomic nervous system gets divided into
14:38sympathetic and parasympathetic. And all sympathetic and parasympathetic neurons are
14:44motor. They're all driving something. And then you'll have visceral sensory fibers coming back,
14:51visceral afferents, which would be part of the autonomic system, really. They're the sensory part.
14:56So the sympathetic nervous system, and there's a whole video on this, I'm sure, it originates in a
15:02section of the spinal cord. It sends a neuron out to a sympathetic ganglion. It meets another neuron
15:09there because, of course, a ganglion is a cell body sending out another axon. So we have a
15:15collection of sympathetic nerve cell bodies here. And then the sympathetic neuron from the ganglion
15:19runs out with another nerve, usually, to get to wherever it's got to go. So the sympathetic
15:27nervous system comes out of the spinal cord. The parasympathetic nervous system
15:34is described as craniosacral. Craniosacral. So this is the sacrum down here. And that's because
15:44parasympathetic neurons that are driving parasympathetic things come out of the brain.
15:50The vagus nerve is probably the biggest and most famous one. And the vagus nerve carries
15:54parasympathetic fibers from the brain down through the body to your GI tract, your abdomen,
16:00and what have you. But it doesn't reach all the way down to your pelvis. So there are also some
16:05sacral parasympathetic neurons in the spinal cord that come out of the sacral vertebrae and carry
16:12parasympathetic innervation to the pelvis and what have you. So sympathetic and parasympathetic
16:17innervation is largely to organs. And you may well know this already, but they get described
16:24in two different ways. So the sympathetic nervous system is described as your fight or flight
16:30response because it's involved with adrenaline. And it can also result in a release of adrenaline
16:37into the blood, which triggers all of your sympathetic stuff. So the fight or flight
16:41response gets you ready to flee. It fires up all your muscles, releases a whole bunch of energy,
16:47gets you like superpowered, ready to run or to fight. So it's a survival thing.
16:53Whereas the parasympathetic division of the nervous system is kind of the opposite. It's
16:58described as rest and digest. So the parasympathetic nervous system is largely driving, for example,
17:05your gastrointestinal tract. So you have a meal and it drives all of those functions.
17:10And in many parts of the body, they kind of work in opposition.
17:14You know, like they'll both, both sympathetic and parasympathetic nerve fibers will run to the heart
17:20and will affect the rate at which the heart beats and the force of contraction of the heart.
17:25And that's quite a good example of them working in opposition. One will speed your heart up,
17:30one will slow it down. You can probably work out which is which. But in other areas of the body,
17:34they don't always work in opposition. It's a little bit more complicated than that.
17:40How's that? I think I've achieved what I set out to do, which was to give an overview
17:46of the anatomy of the nervous system. So what we wanted to do was be able to identify
17:51the parts of the brain and the parts of the central nervous system and know what those
17:55terms meant. So we can recognize the cerebral hemispheres, the cerebellum, the midbrain,
18:00the pons and the medulla, and then the spinal cord. We've talked about cranial nerves and spinal
18:06nerves, and we've split the nervous system into somatic and autonomic. And we've talked about
18:13some of those structures. And then we've talked about how we divide the autonomic nervous system
18:18into sympathetic and parasympathetic. That's a really good starting point.
18:25I mean, I don't find neuroanatomy easy. And what I try to do when we're teaching students here,
18:30because some are very good, and some are not very good, and some, you know, as they find it
18:34difficult, what I'm trying to do is just try and get everybody up to the same level. And then when
18:38we're all on that same level, we can add detail and add detail and add detail slowly. But if you
18:43don't get up to that initial level, the rest of it just gets really patchy. All right.
18:51Right, I better go do get ready for my next thing. See you guys next week.