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Okay, this is 914, and the people in the back may sometimes have trouble hearing me, so... You should always get, maybe we should actually change tables in the future, so you can all sit closer. I don't have a real loud voice.
I can't ask them for an amplifier, but in a small class that's sort of awkward. So, yeah, well if you stop hearing me just come. Yeah, there's one there. A few there that are closer. Okay, so this is the plan.
I want you to acquire, and by acquire I mean in here, in your mind, I want you to have an outline of vertebrate neuroanatomy, especially for mammals, but we'll learn enough about non-mammals that you'll have some understanding of them too. And neuroanatomy is not exactly an exciting topic. It's usually boring. But it doesn't stay that way if you know why it's like that and what it does. So that's why when I wrote the book, that's the text for the class, I try to explain where this came from, how it develops, how it evolved.
For evolution, yes, I can use molecular data, but that is only in conjunction with comparative anatomy. Look at a broad range of species, some of which are very primitive. That is, fossil record can trace them way, way back, like the sea lamprey, for example. And of course, if you're going to talk about evolution, you've got to talk about function, because... Things don't evolve.
They evolve mainly to serve adaptive functions by natural selection. I'm assuming that. I'm a Darwinian in that sense. And I recognize there are other mechanisms of evolution that result in change, but I believe the major things, especially for things like the CNS, which controls function, is due to... Yeah.
Selective survival of the species. So today we'll start out with a little terminology and then we're going to talk about cells and the next time we'll be talking about the way some of the cellular mechanisms are used to study pathways and connections. You'll hear a little bit about that today.
You should be getting familiar with the stellar site. You go to the, you log in, you go to the materials section, and you'll see at the top the general section and the resources section. You'll find there a book list. Now, this year I'm really only requiring as essential reading of my book. Some very interesting other books, and I post some of those readings as supplementary readings, and some of them are very helpful.
If you get through with the reading and you haven't put your nine hours a week in yet, do some of the supplementary readings. The Allman book is a particularly interesting one. It's out of print. It's a problem with Freeman Press and Scientific American books.
They didn't keep books in print. Other publishers could pick them up, but so far Allman hasn't been republished, and that's true also for the Nauta and Firetalk book. Nauta was my teacher in neuroanatomy, and he was a great neuroanatomist and wrote a very interesting book that's still a good research, so I will be posting those too.
I'm not posting Larry Swanson's book, Brain Architecture. It's a book with similar goals. to my book, but it's a very different book.
It doesn't have the general kind of coverage that I have, but it's very unique in many things, and I did make use of that book in writing mine. So some people might want to pick that up, and I always list where the relevant things are in that book. You have a copy, and the reserve room, I think...
Do I have a personal copy? Okay, well we should check the reserve room and find out to make sure that they put the library's copy on reserve. So in case you have time and want to read some of that, that's fine. Now glossaries are important because you will soon find out there's a lot of terms in there on there. And they come from the Greek, they come from the Latin, there are various combinations.
There's a lot of structures, you know, a lot more than we have time to mention. I doubt if I will mention the bundle of Victazir. I might mention the Habanulo-Interpeduncular Tract. These are just, they're synonyms.
They mean the very same thing. The second way, that's easy. If you know what the Habanulo is, you know what the Interpeduncular Nucleus is, because it's the Habanulo-Interpeduncular Tract.
It goes from one to the other. And a lot of tracts, fortunately, are named that way. But not all.
And there's a lot of synonyms. We'll see some of that right away. So you just get used to it. A little suffering, especially earlier in the class.
And it might make you feel a little lost once in a while. Okay. But be patient with yourself.
And just put the time in. And I will go over things. The more important things I will return to a number of times to make it easier for you. I use that method in the book and will use it in the class because I'm basically... The book originated from this class, from teaching here at MIT for a number of years.
Alright, I'm posting not only the readings for each class, but I'm posting questions on the readings. Okay, I want you to read before the class. Don't just come here and expect me to feed it all to you. We will answer questions in this class, and you will have a chance to ask me questions. But to make sure you're reading, I am sometimes just going to say, what's the answer to that question?
You know, you'll all be embarrassed from time to time, but... It gets you to read. I can also use quizzes for that, but that takes up class time, takes up discussion time, so we're not going to have time to do too many quizzes. I will let you know in advance when we're going to have one. Alright, let's go through the questions.
And I know this is the first class, we had a little trouble getting the book online, MIT Press was going to do it, as you probably know, and they ended up... not being satisfied with their setup for textbooks for a specific class. They have a deal with the libraries to make their books, their e-books, but mine isn't actually even out yet, so that will come.
March 28th is when we can actually get the book, the print book. After a few classes, I'll get all the names of you who expect to stay in the class or for other reasons want the book, and I will give that list. to the guy at the MIT Press Bookstore to make sure he holds that, those for you guys.
And then anybody else that comes in, he'll have to have extra copies for them. All right. Should brain structures in your organization make sense to you?
Because, you know, it seems a little arbitrary a lot of times when you're studying brain structure. What kind of sense should it make? What kind of sense do you want it to make?
How would you answer that? I want it to make sense in terms of evolution, and in terms of development, and in terms of function. I want the connections we talk about, most of them, to make some kind of functional sense.
It will help you remember it, and you will build gradually this outline in your mind. And if it doesn't seem to make any sense, bring it up. Just tell me, it doesn't make any sense. Okay, put me on the spot.
Okay, so could somebody define central nervous system for me? Yes. Okay, so brain and spinal cord.
The brain is in the skull. The word encephalon means in the skull. So we talk about the different main parts of the brain as cross encephalon, mesencephalon, raman encephalon, the different three major brain vesicles in the skull.
And then the marrow, the center of the spinal column, enclosed by the vertebrae. There's the spinal cord or the medulla spinalis. That's why the caudal end of the hindbrain is actually the medulla oblongata, the elongated center medulla of the spinal cord. Okay, so here are pictures of it.
This is one from Herrick. This is from... A dissection at a medical school museum in Lausanne, Switzerland that I visited.
They have a wonderful museum there. And this is a dissection of a child who died. And they've exposed the whole spinal cord and the roots and the brain where they've sectioned part of it there. This is one where...
You know why that looks so different from this one? What's different there? It's encased by the dura mater.
Right. The dura is the tough mother, the canvas-like covering of the brain, the outer meningeal there. Okay. And we found this...
Picture after my book, I would have liked to use this in my book. I've actually used these, but this one will probably be in the next edition because it's such a beautiful dissection of the adult human spinal cord and attached to the brain with the nerve roots, of course, all cut off. This would lead to the whole network of the peripheral nervous system.
Okay, so now, beginning at the caudal end of the CNS, what are the names of all the major subdivisions? Let's name them. The most caudal, spinal cord. So what's just above it?
I've already mentioned it in the class. I've actually mentioned all the main answers here, but I want to see now. You've had 901. You've had brain things before.
You should know all these things. But don't be too embarrassed if you forget. A neuroanatomy doesn't stick with some people very well, but that's why you're taking the class, right?
So what's above the spinal cord? Sorry? The brainstem. Okay, the brainstem is a general name for everything in the encephalon, inside the skull, that's not cerebral hemispheres. These are cerebral hemispheres.
There's cerebellum those are the cortical areas it's everything else that's not those two things okay so let's start at the bottom and name the most call one simplest in English hindbrain midbrain forebrain okay the hindbrain the lower part is the medulla oblongata the rostral part is often called the pons just because of the structure called the pons that's located there and we'll be talking about that in these connections okay and then above the hindbrain the midbrain okay and we'll be studying that in a special unit and it will come back in various chapters of the book and then the forebrain but what are the major parts of the forebrain Hemispheres We've already named cerebral hemispheres. That's the end brain. It contains a little more than the hemispheres. It contains the olfactory bulbs and what we call the basal forebrain.
And what's in between the end brain and the midbrain? The tween brain of course, the between brain. Sure it's between the midbrain and the end brain, but it's also between the hemispheres.
The hemispheres kind of blossom out of the tween brain. All right, so here's a picture of the embryonic neural tube where I've taken the developing hemispheres and I've sort of pushed them apart. So you can see the thin, what we call the roof plate here in the rhombencephalon. There you see the hindbrain right there. And you see how it's got that...
Sort of one cell thick membrane across the top. It's a tube. The whole nervous system is a tube. But the walls get very thick with development. This is early in development.
They're not that thick yet. But that roof plate never gets thick in much of the hindbrain. Only in the rostral part where the cerebellum developed. Then it gets huge. Okay?
Sorry? The tween brain. No, no, that's what die and self-blown means.
I'm just giving you here the equivalent English and classical names. These are basically Greek. And the Romans imported a lot of the terms from the Greek language and added their own. So the other question is here, why is the hindbrain called the rhombencephalon? Hindbrain, rhombencephalon.
Well, rhombencephalon doesn't mean hindbrain. Sorry? For that rhombic shape there, it's the shape of the roof plate seen from the top, and that stretches out like that when the development of the tube develops flexures, bends.
It's like a pea pod that you've bent and it stretches out part of the top there. Okay, so what are the coordinates we use now? We want directions when we're looking at brain sections, and we want to know what the common planes of section are.
Of course, we talk about anterior and posterior, rostral and caudal. Do those always mean the same thing? No, not for humans.
They do for most animals. Here, anterior and posterior, same as rostral and caudal. You have trouble remembering those names? Rostrum, you know the term rostrum. He's at the rostrum.
He's at the front. Caudal actually means tail. Dorsal, ventral.
Dorsal towards the back. Ventral towards the belly. Also...
But look at the human here. The only way the terms are really equivalent for the human is when he's in that position. Because if he's standing up, now ventral is also anterior.
You see? Not so here. And it's similar for the bird. And so that's... the reason we normally, and by the way, you can use the term oral too, instead of rostral, but that's why I prefer just the dorsal ventral, rostral caudal terms, because we can use them right across all the vertebrates, okay, and even for invertebrates, and for the planes of section, very simple, you know, but just notice the synonymous terms.
And I will sometimes, without even thinking, switch from one of these to the other. Transverse, frontal, corona, they all mean the same. Horizontal always means horizontal. You have mid-sagittal and parasagittal, but parasagittal, people don't bother with that.
They still just call them all sagittal, whether they're mid-sagittal or parasagittal, at the midline or off the midline. And then oblique sections are just used for special purposes in order to get certain axons all in the plane of sections. And here I'm just showing these are not these are what you're seeing here are the drawings that I based the textbook figures on.
We did redraw a lot of them for the book. But here I just sketched the brains used in the lab the most mouse, rat or hamster from the side and showing. If you make a series of frontal sections, they're cut like that. This would be the horizontal plane. And then I turn the brain around.
You're looking at it from the front. The olfactory bulb's in front. Sagittal sections would look like that.
Okay, so what kind of tissue makes up the CNS? It's not really a lump of porridge. It just looks like that if it's not fixed.
So what kind of tissue is it? You can say it's ectodermal tissue because it arises from the embryonic ectoderm. And part of that ectoderm forms the central nervous system and peripheral nervous system.
And we will see that because one of our early topics is spinal cord development. Let's see. So when you do histology of nervous system tissue, it's very much like histology of skin, because the skin is ectodermal. So how do we define and recognize cell groups?
Anybody? What do we do? How are they initially named? Well, of course, dissection.
But you can't see. A lot of detail with dissection that you can see with anatomical methods. What kind of anatomical? Yeah, yeah, you. You were indicating you were going to tell me.
Speak louder. Okay, anybody. Give me some methods.
Sorry? I can't hear very well. You know, I can't hear well with any background noise.
I have more trouble than you do, I think, in hearing me. Stains. One word, yeah.
Histological stains. Give me an example of a nistle stain. What is a stain for? Well, a nistle substance. Where is the nistle?
In the cell body. Right. Not very much of it gets into the dendrites.
Maybe the large proximal dendrites get a little of it. And it doesn't enter much of the axon either. So when we stain for nissil substance, we're seeing the cell bodies. So you'll see whether the neurons are big or small, whether they have a lot of nissil substance or less.
So there'll be dark staining or lighter staining. And just those properties help us define different cell groups. They help us define layers. Okay, so we will see more pictures of that. There's many pictures in the book of these things.
We'll see more pictures next time in Chapter 2. Some of you have read it already. What are the kinds of stains? Well, Golgi, but if you just take a more general stain, other than the nistle stain, we could stain for fibers. And the fiber stains might just stain for myelin, so that's only the thicker myelinated fibers, but they might stain for all the fibers, like silver stains for hexane. That gives you a different picture.
Like, for example, when I wanted to map the whole neocortex of the hamster, I found the nistle substance not to be all that clear. I could see boundaries, but they weren't. It was pretty hard to make out. So I used a silver stain for axons and found suddenly I could really see clear boundaries.
And I was able to map using a lot of quantitative care and the histology, I was able to map the cortical areas. And you'll see results of both of those kinds of methods applied to mapping the parts of the brain. Okay, if we use the term primitive primitive cellular mechanisms the way I do in the book. What does it mean when we're talking about nervous system?
What is a primitive cellular mechanism? Basically I'm talking about mechanisms that we see in single-celled animals that we still see in neurons, okay? And here I list them as the way I list them and discuss them in chapter one. They're all present in one-celled organisms. They're retained in the evolution of neurons.
Irritability and conduction. Irritability means it responds in some way to stimulation, like even just simple mechanical stimulation, but also other kinds of stimulation, chemical stimulation, electrical stimulation. Something changes in the membrane.
And it conducts those changes to other parts of the cell. It happens in the amoeba. It happens in other protozoa.
It happens in neurons. And then we get specializations. That happens in single-celled organisms, too. Parts of the membrane respond better to some stimuli.
And we see that in neurons, of course. Specializations at the synapse, of course. There are specializations for responding to stimulation from the outside world. Movement. Some cells specialize in movement.
You say, well, that applies only to muscle cells. No, it applies to neurons too. They have to move a lot when they develop.
And they still use contractile proteins just like muscle cells. And then secretion. Single cell organisms secrete. They use that in catching prey, for example.
secretion many neurons specialize and even central nervous system neurons some of them don't just secrete chemicals at the synapses that they secrete into the bloodstream okay they are neuro secretory cells and then parallel channels and information flows some way to integrate different information coming in different parts single-celled organisms it's easier for them to throw in one cell Well, when you get a multicellular organism, especially if it's big, then it becomes a real problem. How do you integrate different things? Different stimuli can be contradictory. Your left hand might be touching one thing, and the right hand something that doesn't make any sense in terms of what's in your left hand. How do you solve the problem?
How do you integrate? Well, you need connections. So, what we'll be dealing with.
And then the last property, endogenous activity. We'll come back to that one. Can someone answer question 11 for me?
Contrast the meaning of synapse and bouton in descriptions of neuronal structures. You find both of them. You find a bouton near the axon ending or at the axon ending.
Often many boutons associate with one axon because it branches and has many endings. And we talk about synapses. What is the difference in the way we use those two terms?
Yeah, let's make an even simpler answer. Sorry? You know, I can hardly hear any of you.
Yeah, but you're still not getting the major point. You're looking at details. I want the main picture.
What's the difference between a butan and a synapse? A butan can have a lot of synapses. The synapse is just one little area of the membrane that's specialized for its contact and communication with another cell.
The butan is the enlargement of part of the axon where most synapses occur. So if it's a, let's say it's an axon going along like this, and along its way it has an enlargement, and then it just keeps going. And at that enlargement, that would be the place to look for synapses. They're called boutons en passant, boutons in passage.
Often we use the French because it sounds so nice. And that's where the word, of course, bouton is a French word, bouton terminal. Terminal Bhutan. Now we talked about a Bhutan Pasar.
So it's where synapses are formed, usually. Do all axons end in boutons and synapses? No.
You will see, we'll talk about this, different types of endings. Okay, but we're talking about even peripheral nervous system, an axon going to a muscle cell, it ends in a type of enlargement. But there it's more specialized. It's the end plate, the muscle end plate.
It's a flat structure, but it has all the synapses on the muscle cell. Okay, next question there. What membrane structure had to evolve in order for action potentials in axons to evolve? You know that dendrites...
Most of them don't conduct action potentials. A few of them actually do, but not very many. They conduct differently from axons. Axons conduct by action potentials.
I want to know what membrane structure had to evolve for this to happen. Sorry? No, no, no, no.
Myelin didn't exist when axons first evolved. And when action potentials evolved, I'm talking about a molecular structure in the membrane. Exactly.
A particular type of ion champ. A voltage-gated ion champ. It appeared in jellyfish, which have been around longer than any chordate. Okay?
That just means that when the membrane potential changes, remember that the cell is irritable, it responds to input. And what's the usual response of a neuron to stimulation? Or an axon.
Take an axon in my arm and I pinch it, especially if I pinch right here. I get an effect. I'm causing depolarizations.
Simple word, think in terms of the main thing here. Depolarization, the main response, the neuronal membrane to stimulation. Okay, so now, what about, let's deal with these, and you'll see examples of that in a minute here.
I want you to be able to contrast excited terrain. Inhibitory post-match potential. You should all be able to do that by now with the studies you've done, unless you're from another department and you've just wondered what the brain is all about.
Well, we'll teach you, but I don't expect you to be able to answer yet. Okay, contrast the nature of conduction in a dendrite and an axon. Just what we were talking about. And what's the functional purpose of an active pumping mechanism in the axonal membrane?
Usually people say, oh, action potentials. And the answer is no, it's not. So first of all, excitatory, inhibitory, postsynaptic potentials. This is from an introductory biological psychology textbook. It shows intracellular recordings of the microelectrode where they record from this axon.
Okay, in here, yeah. So the presynaptic recording shows the action potential. Big potential that goes from, you know, minus 60, minus 70, becomes momentarily positive and then the membrane potential recovers.
And if you record on the other side of the synapse, you get a little bit of depolarization. If you're getting depolarization, it's excitatory. Why are those two things equivalent?
Because it moves. There's one point here where the axon begins, where if the depolarization reaches a critical level, that triggers the action potential. All these little EPSPs, some 8, at the...
beginning of the axon, the axon hillock we call it. Okay, so the more of them there, and the conduction in the cell body is decremental. So if it's happening way over here, and there's a depolarization, it has less effect on the axon hillock here than something happening right there.
Okay, so it makes a big difference where the terminal is on the axon. But the EPSDs look the same everywhere. They might be a little bigger, a little smaller, but they're conducted decrementally by the dendrites and the cell body membrane. All right.
Inhibitory postsynaptic potential is opposite. It's when there's a hyperpolarization, as you see here. The membrane, if it was... polarized at minus 70 might go to minus 80. And it's inhibitory because it takes the membrane further from the point where an action potential will be triggered.
So let's talk a little bit more about that difference in conduction of dendrites in an axon. Looking at this picture, I drew it without myelin for a reason. Axons don't need myelin to conduct.
So here I draw, I'm drawing... Functionally equivalent parts of two neurons. A dorsal root ganglion cell that conducts from the body surface, where there are endings here, and then a long axon that goes right by the cell body into the central nervous system where it ends in terminals with synapses on other cells. Secondary sensory neuron. So it's a primary sensory neuron.
And here I have a motor neuron. So here you're inside the CNS and there is the cell body. There's the axon going to a muscle cell. OK, so this part of both of those cells is the receptive part.
OK, this part where I show the arrow is the conductive part. And here transmission is occurring. Transmissive part.
Okay? And that's... So now let's take a little piece of that. First of all, the conduction here... Now, in some cases, the axon might begin further out, but here I have it beginning right here.
You get decremental conduction there. What are the characteristics of decremental conduction? other than the fact that it gets less and less, further away from the site of stimulation you get.
The other characteristic you should know is it's very, very fast. Okay? Almost instantaneous. Not like light, but it's close.
Okay? Very fast. And then, the point where the action potential begins. What is an action potential?
Here I pictured it. I've taken a little snapshot of it at one little point. I've enlarged the tube with the axon and I'm just showing how ion distributions are polarizing the membrane.
Positive on the outside mainly because of an accumulation of sodium ions. Negative on the inside. Okay. There are potassium ions in there that are positive also, but a lot of negatively charged.
anions, large ones, and you see here. What happens when at the beginning of the action potential there's an implosion of sodium ions that momentarily reverses the potential? This curve matches that piece of axon. So there's the beginning of the action potential.
Sodium ions rush in, an implosion of sodium ions, the potential to remember reverses, okay, momentarily, and then it rapidly recovers. And the first reason it starts recovering is potassium ion channels are also voltage gated, and so potassium, which is in high concentration inside, rushes out. The channels open up. You see it's a semi-permeable membrane. It's not, all these ions can't get through very rapidly.
unless the channels open up. So that's why the voltage-gated ion channels are so important. Okay, and then so the membrane recovers. And there's another way to look at that membrane where I show the polarization indicated a lot of sodium ions on the outside there's also chloride ions and then on the inside the negatively charged anions, the big ones that don't pass through the membrane at all and the positively charged potassium ions.
But there are molecules in the membrane I'm just showing a couple of sites here We call the sodium potassium pump. It's always moving sodium ions out and potassium is in. Because with a lot of action potentials, you basically lose that concentration of sodium on the outside and potassium on the inside if you get a lot of action potentials. So eventually it'll just stop, unless you use energy.
to redistribute those ions. And that's why we need an active pumping mechanism. Okay? Now we've answered those questions. We've tried to...
What's a dorsal root ganglion? I've mentioned it here. Actually, I said a dorsal root ganglion cell. Let's answer that a little better. Talk about the oligodendrocytes and Schwann cells.
Somebody already mentioned myelin. These are the cells that make myelin. And then I want to talk about the main function of the myelin sheath.
First of all, what's the dorsal root ganglion? This is a picture from Cajal that I put in the book. Here you see in an earthworm and mollusk, primary sensory neurons, the neurons responding to the outside world. These are... neurons in the surface layer of the body, so in the skin of the animal.
Here the cell primary sensory neurons are right in the epithelium. Here the cell body is below the epithelium but it still got it extends right out into the epithelium. Okay, but as soon as you get to the vertebrates, oh this is the central ganglion here.
We don't talk really about a CNS but Sometimes we do just because of similarity to the vertebrates. Here's a fish, an amphibian reptile bird or mammal. The fish has these bipolar cells that contain the primary sensory neuron, and they are collected in a ganglion. You talk about a collection of cells outside the central nervous system as a ganglion. Okay, so here in us and...
Other mammals and also amphibians and reptiles, the primary sensory neurons that carry input from the skin are in a dorsal root ganglion. And the dorsal roots, because the roots of the spinal nerves that enter the CNS, they always divide. The more dorsal one contains the sensory axons. The more ventral ones, the motor axons going to the muscles. Okay, so that's the dorsal root ganglion.
So now what about oligodendrocytes and Schwann cells? When axons acquire myelin, it is the Schwann cell that myelinates these axons of the periphery in the purple nervous system. Soon as you enter the central nervous system, It's a really a very different type of tissue and you get a different glial cell making the myelin. It's now an oligodendrocyte. There are other differences too.
One Schwann cell will form the myelin in one little stretch. And then there will be a little place with no myelin. And then another Schwann cell will myelinate the next thing. In the CNS, one oligodendrocyte can myelinate or form a segment of myelin in a whole bunch of axons. Nearby X. So quite different.
But the function is quite similar. What is the function? It's a kind of insulator.
It prevents ion flow. The ions can't flow through the membrane where the myelin is. Okay, it's a tight fit. So if you get depolarization, just before myelin begins, okay, we're talking here about space or one end, okay, you get depolarization. What happens?
It triggers the action potential right there in that little piece of bare axon. And then, the only way it can conduct down the axon is by flow of ion, by decremental conduction, until it reaches the next node where there's no myelin. And if the decrement of the depolarization isn't so great... It will depolarize that membrane, trigger an action potential there. And then the same thing will happen.
And the conduction will go flip, flip, flip, like jumps. Okay? That's what saltatory conduction is.
Saltation means jumps. Okay? And that's why it speeds up.
Because the decremental conduction is very fast. I said before, the action potential is not so fast. But it's self-regenerating.
So in these, in a picture like this, you just get this continuous movement of the action potential all the way down the axon. But it's limited in how fast it can go. It goes faster if the axon's bigger.
So in animals without myelin, the axons get huge, that you need rapid conduction, right, for an escape response. But we don't have, mammals generally don't have such huge axons. It would be very, it would cost too much energy and space. The myelin was the invention to solve that problem.
All right, receptor cells, I said many receptor cells are not actually neurons. So how are they different from neurons? How do they interact with neurons? They depolarize, just like neurons. But they don't have the membrane of an axon, okay?
They don't have any action potentials, but their depolarization affects neurons that contact them and can trigger an action potential in those neurons. So that's a receptor cell. Some receptors, like in the nasal epithelium, olfactory receptors, the primary sensory cell is a receptor. Okay, you have the same thing in the retina with the specialization. All right, and here is just a list of various specializations you already should know about.
Several of them here are mechanical, and each here also responds to light, to chemicals, to heat or cold. They're all specialized receptor cells, except in the case of chemicals, it's the neuron itself, but it's still a specialized receptor. What kind of molecules are actin and myosin? What kind of molecule is that we're talking about?
Tractal proteins. When is actin found most abundantly in neurons? During development. When the axon is growing.
The axon has to move a lot. The growth cone is very active. And we will see that. study development.
You should know about Otto Loewy's discovery. Don't have time to describe his dream. I like to describe it. It's a lot of fun. But you should read.
I have a little bit in the book about it. And you can find things online very easily. He discovered, he didn't know he was discovering acetylcholine and norepinephrine. But that's what it was. That's what the molecules turned out to be.
He was the one who settled a big argument in neuroscience, okay, in the early part of the 20th century about whether conduction at synapses was electrical or chemical. And there were a lot of arguments on both sides. He proved that now we know that both exist, but for the most part, most synapses, the conduction is the transmission from one cell to the other is chemical.
And he discovered that. in his experiment on frogs. By just stimulating the axon of a heart, the accelerator nerve, one heart in a Petri dish, taking a little fluid from that Petri dish and putting it in another Petri dish where you haven't stimulated the nerve, but the heart speeded up anyway.
That means... The reason I said it is I and maybe a few of you want to go to the talk it for discover didn't talk about. OK, I want to read this and you ask me a question about it. Is there anything about the way I portrayed synapses and their various types in the central nervous system?
The hardest thing is probably the concept of presynaptic facilitation and inhibition. So see if you can. Get some understanding of that. The rest of this, I think, is quite clear from the book, Endogenous Activity.
Also, just read it in the book. You can look at the slides. You know what I'm stressing here.
And these concepts will recur later in the class. Okay?