Term
| T/F Movement of the basilar membrane is frequency dependent, i.e., tonotopic. |
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| Three distinct chambers of the cochlea are: |
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Definition
scala tympani
scala media
scala vestibuli |
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T/F Four important membranes of the cochlea are:
Basilar
Tectorial
Reissner’s
Reticular |
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| T/F For hearing and balance, transduction is based on mechanical gating of CI channels. |
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Definition
Gyrencephalic vs. Lissencephalic
What you have in this depiction is to scale, actually, on one side-- one hemisphere of the human brain-- and just to have an idea as to the sizes of other species. You have, on the right-hand side on the top, for comparison, a complete brain from a macaque. All of these human and macaque from a dorsal view-- from the top. And then below the macaque, you have ferret, rat, and mouse. And you have a scale by there for comparison.
So of course, the human brain is much larger than these other species, but the human brain is not the largest brain as sometimes people think. A whale's brain, for that matter, is much larger. Dolphin brains are not only bigger sometimes, but more gyrencephalic.
This brings me to the concept of gyrencephaly and lissencephaly. Important for you to notice, also, all of these foldings in the cortex of the brain, which are believed to [? help you ?] both to provide a larger surface area, what we call gyri. So the bulges are the gyri, and then going deep into the wrinkle is the sulcus. So gyri-- or gyrus in singular-- and sulci, or sulcus in singular.
What you notice here is rats-- in mice, at least, [? encephalic-- ?] they do not have these wrinkles. Macaques, ferrets, and humans are gyrencephalic. |
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Definition
different types of cortex
This table, that you will see several times, I used to depict differences between neocortex, mesocortex, allocortex, and other types of cortical areas.
When we talk about the neocortex, we are talking about the newer cortex that has evolved in evolution. So that constitutes, remember, our telencephalon. Remember, in the introductory lecture, I talked about the prosencephalon, mesencephalon, and the [? rhinencephalon ?] of the back. And we said that the front had differentiated [INAUDIBLE] to diencephalon and telencephalon, and the telencephalon is what gives rise [INAUDIBLE] development and human development to the cerebral hemispheres.
Evolutionarily speaking, the neocortex is the new type of cortex, and is believed to give us, again, very human characteristics. It's what we use for our high cognitive functions. It's a six-layered structure, and we will see this in a second in terms of the organization. |
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Definition
In terms of different cortical areas, here very, very shortly, what you have is a frontal part, OK? The frontal comes up to the center sulcus, then you have parietal. In the back you have occipital. Here you have the temporal.
Now, it is important to recognize that according to location, there are different functions. So while premotor, higher cognitive, premotor areas are here, then you're moving into motor, then primary somatosensory, association cortical areas. Primary visual is back in here into the occipital.
These things are important for many reasons, including clinical relation. So if a person comes with a lesion that you can easily identify in one particular location in the cortex, there will be very, very particular symptoms associated with that lesion. And that, again, is because the functions are in different areas of the cortex. |
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Definition
In terms of the organization for the different types of cortex, here you have examples between archicortex and neocortex, in the motor cortex, and then in the visual cortex. What I want you to pay particular attention to-- and here is the hippocampus-- is that only the neocortex is a six-layer structure. But even within the six layers, they are not always the same.
In the motor cortex, the most prominent layers are layers 3 and 5. That's where you have the motor neurons. In here, [INAUDIBLE] cells are the largest ones. These cells are among the largest in the animal kingdom.
Imagine, in a giraffe, again, for instance, a [INAUDIBLE] cell, motor cell in the primary motor cortex. When it has to have an action that has to travel many, many meters, it has to have a soma which is very large to be able to provide nutrients to that very long axon-- very long cable. In the visual cortex, the most prominent layers are 2 and 4. And in particular, 4 in primates is very developed and split into 4a, b, and c, or alpha, beta, gamma, and so on-- again, according to function.
In the archicortex, you have one through about three or four layers, depending on where you're looking and how you look at them, as well as in the hippocampus. These are not neocortical areas. These are much older in development. Only the neocortex is what we refer to as isocortex, or six-layer type of cortex. |
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Definition
When Brodmann did the cytoarchitecture of the brain, he was just looking at the cellular components. He was able to come up with about 52 different regions. Now, keep in mind, these very early studies, he was just looking at the shape of the cells. That's why we call it cytoarchitecture.
And he noticed not only the different shapes, but the different distributions and the different layers. At that point, he did not know yet that these differences in a structure and differences in the architecture were going to be for different functional components. Some later studies determined, for instance, that the cells in what Brodmann called area 9 turned out to be prefrontal cortex, higher cognitive functions. Area 4 turned out to be primary motor cortex. Areas 1, 2, and 3 turned out the primary somatosensory, and so on and so forth.
Here, you also have what then later on was demonstrated to be Broca's area-- and we'll talk a little bit about Broca's aphasia-- while in the back here, you have Wernicke's areas in the side, and in this area, you have primary auditory. And as I mentioned before, this is primary visual. Area 17 is the primary visual cortex. 18 and 19 are also visual, but not primary-- but secondary visual cortex. |
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Definition
When we look in the circuitry, when we look at layers in the circuit and we go put layers 1, 2, 3, and 6, what we realize is that there are different types of organization, but some very unique principles. So again, motor cortex-- very prominent pyramidal cells in layers 3 and 5-- very big pyramidal cells. Granule cells in layers 2 and 4 are very prominent for receiving sensory information. And these are just different types of cells.
Again, remember, when we talk about the different types of neurons according to shape or according to the dendritic arbor or the axonal arbor, so this is what is making reference to. Pyramidal, because of the shape of the soma, is like a pyramid. Granule, because it's little, round granule cells. Looks like granules.
But the dendritic arbors and the axonal arbors might be very, very different. These cells are usually the receiving cells, mostly receiving inputs from other cortical areas or from subcortical structures, as well as for intracortical connections. And the pyramidal cells may have communication which is local, but as well an action that will go faraway and communicate with faraway cortical and subcortical areas, including, also, connections to the spinal cord. |
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Definition
When we look in more detail, here you have, again, the different layers. And now we are emphasizing, for instance, that if you go in terms of the inputs, this is where inputs from brain stem modulatory systems have come in. It's no surprise that the modulatory systems are coming heavily into the different layers, because they need to modulate excitation and innervation all over the cortex.
On the other hand, more selective information-- for instance, sensory information-- remember, when we did the sensory information being collected from specialized receptors, and it was going to the ventral posterior-lateral nucleus of the thalamus. Now, the thalamus is now projecting very heavily to layer 4.
So granule cells in layer 4 are heavily receiving sensory information and processing that information. This organization also comes from development-- the inside-out type of development, meaning deeper layers are born first-- cells in the deeper layers are born first-- and then the cells migrate, bypass layer 6, for instance, and then locate themselves in layer 5. Then later on, other cells are born, bypass layer 5, and go form layer 4, and so on. That's the development of the cortex.
The cortex is organized. It's complex, but the fact that it's complex doesn't make it a random structure. Now, when we look not only in layers but also in columns, we realize that there is organization-- columnar organization. |
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Term
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Definition
So there is horizontal organization in layers-- or lamina-- and there is also vertical organization in columns. Many people look at the columns in different ways. Columns might be studied as dynamic columns according to how information is being processed, and dynamic columns are moving from area to area. They are dynamic because what is activated-- the columnar cells; the cells organize in a column-- might seem to be moving around.
But the wiring is all static in the sense-- not static all the time, of course, because you can grow new connections and so on, especially as you develop-- but static in the sense that once they need to be stable-- this is believed to be the case for the human brain, to make it different from other animals and so on-- is that they become stable. You cannot keep changing a circuit and expect it to be stable. So stability seems to be the key for us to be able to have memory, and memory is absolutely essential for learning. |
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Definition
| When we look at-- one example of looking at columns that I did myself was trying to understand columns in the ferret cortex. And so using tracers of different kinds, when we inject different places such as the talons, we are able to understand the columnar organization in different parts of the cortex. Many different ways to look at columns, but one of them is by doing the histology, of course. |
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Definition
Now, one type of column-- or types of columns-- that became very famous are ocular dominance columns that have to do with the visual system. And it's absolutely amazing that information coming in through our eyes is separate-- separate by eye, and then separate by channels and so on-- and all of this information is still separate all the way to the cerebral cortex-- to the primary visual cortex.
And what you have here are examples of those types of columns. Here, appear more like stripes because of the way in which the cortex has been cut. Also, there are columns or stripes in the somatosensory cortex-- for instance, for the mystacial pad, the whiskers in some animals that go to very particular areas in what we call the [? barrel ?] cortex. |
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Definition
Orientation columns in primate cortex: orientation changes around a radial axis but remains constant along an angle.
Now, going back to the visual cortex, more modern techniques have allowed to use, for instance, calcium imaging to depict the dynamics and the way in which the columns are organized, and this is what you see in these figures-- very colorful figure-- looking on the surface of the cortex and the activity as it is imaged using, in this case, calcium fluctuations. And that concludes our very, very brief introduction to part 1 to the cerebral cortex. |
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Term
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Definition
(transverse fibers) connect the two hemispheres:
• Corpus callosum
• Anterior commissure
• Posterior commissure
• Hippocampal commissure |
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Term
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Definition
connect regions within the
same hemisphere; short and long
- Long association fibers:
• Fornix (hippocampal formation—mammillary bodies) • Mammillothalamic tract
• Uncinate fasciculus (frontal—temporal)
• Cingulum (cingulate gyrus—entorhinal cortex)
• Superior longitudinal fasciculus (frontal—occipital)
• Inferior longitudinal fasciculus (occipital—temporal) • Occipitofrontal fasciculus |
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Term
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Definition
connect cortex to subcortical structures (brainstem and spinal cord) e.g., internal capsule and ascending and descending pathways including: corticobulbar, corticospinal pathways, thalamocortical radiations, etc.
-go far away |
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Term
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Definition
In here, you have some depiction of what we are just talking about. This diagram is making the point of some of the short and long association fibers that you have-- the corpus callosum in here, as well. Remember, that is not in color because this will be between the two hemispheres. So those would be commissural. In here, you have, again, corpus callosum. We can always split this into components. In the corpus callosum, just to give you an example, you have the rostrum right here. That's because it's the place that is closest to the face.
Then you have the genu, which is the bending-- this part-- the body of the corpus callosum. And the splenium, which is at the back. This, of course, is a sagittal view of the brain-- mid-sagittal view, to be precise. In here, you also have the fornix, and there, you have the anterior commissure, as well. This is a horizontal cut in which myelin appears in-- it's very dark. It's almost black. And what you have is components. So all of this is myelin fibers-- myelinated fibers in the cortex, and then you can distinguish in here the internal capsule. In anterior and posterior limbs, they make a letter X on both sides. And then here, you have another component of the corpus callosum.
Now pay attention to this beautiful depiction in here. There you have part of the optic radiations, so you have all of these fibers. Now, this is a brain from which a lot of gray matter has been removed so that you can actually see these fibers, and you can see these fibers coming into the occipital pole in here, going around the calcaneal sulcus, which is where we have area 17-- remember area 17-- Brodmann area 17 is the primary visual cortex. |
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Term
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Definition
Imaging the brain’s structural connections using diffusion MRI
Now more modern techniques can show us something very colorful and beautiful. This is done-- at least some of it is done using diffusion tensor imaging or diffusion MRI. And what is usually being detected is water molecules, and the water molecules are being polarized because of the magnet and the magnetic resonance. And then through computer programs, they can be color coded. And so you can see in different colors, the different pathways. This is a much, much more modern way of looking into the different types of fibers. This has helped us, to a great extent, understand some of the connections that have been much more difficult to study, using, for instance, immunohistochemistry for basic myelin proteins or doing just general histology. |
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Term
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Definition
Now we move into something different from the pathways, and it's the rhythms of the cortex-- in particular, the EEG-- the electroencephalogram, which is-- we can put electrodes on the scalp and collect information. And this has been done for a long, long, long time, and what was noticed is that different states of activity in the cortex, which are related to how active you are or your brain is-- like active thought or being alert and working-- relaxed, reflective, drowsy, or sleeping. It's mostly related to one particular type of activity. These Greek letters depict the different types of waves. So here, for instance, you have gamma-- what we call gamma. This is the Greek letter gamma from 30 to 100 Hertz. Beta is from 15 to 30 Hertz and so on.
Now be aware that in the literature, you might find small differences on how different authors refer to this. So if you read in a book that they were doing gamma between 25 or between 35 and 125 Hertz, it's not that different. It's still within the gamma range. If they start it much higher than 100 Hertz, they might say that they are in the higher spectrum of gamma.
But that is all understandable. The main point here is look at the activity in here. This is low voltage, fast activity. And this low voltage, fast activity is typical of being awake. Now very interestingly, as we are falling asleep in the different stages of falling asleep, you become drowsy and so on. Less of this activity is prominent, and what becomes more prominent is this low wave activity. These are low waves depicted in here. You have, for instance, delta waves-- very slow waves. And this is very typical of being asleep.
However, you reach a point when you are dreaming in which the activity becomes REM-- rapid eye movement because your eyes are moving and it's usually associated with dreaming. And at that point, the EEG goes back to low voltage, fast activity. And it's paradoxical to some extent that it might become very, very difficult to determine if the person is actually dreaming, deeply sleeping, or if it is awake. |
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Term
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Definition
OK. Now in here, you have more things that have to do with the rhythms of the cortex and I want to show you what we call up and down states. The point here is that some of the membrane-- this VM is the membrane voltage, so voltage across the membrane. MU is the multi-unit activity, and LFP is the local field activity. These are recordings in the cortex of an animal in the brain. And what we want to show you is that there is oscillatory behavior in the cortex.
So oscillatory behaviors are not unique to humans-- animals-- the cortex of animals also show oscillatory behavior. Some of the cells and some of the oscillatory behaviors show superimposed action potentials on the slow waves. Some of the slow waves-- at least some of them are calcium mediated. Well, you remember action potentials are mostly sodium, potassium mediated. Those are the main ions for action potentials.
So the ability to have this type of behavior, in terms of the electric properties of the cortex-- the cells of the cortex-- means that you can have different states of the cortex. You can have bi-instability in cells. The cell could be stable at a lower membrane potential, and then something may happen, and the cell can become also stable at a higher membrane potential. That complicates, among other things, our capacity to understand what is happening in the cortex, in terms of the electrical activity, but also conveys a much larger repertoire of a capacity to the cortex, in terms of what it can do, electrophysiologically. All of that, remember-- the result of all those different types of ion channels, which are embedded in the membranes of neurons.
Here is more of the same. Just to show you again what we call down here-- the down state, and then a depolarized up state. So here, we can see that the cells-- to be hyper polarized. And then here, we can see that the cells-- to be depolarized. Of course, the cell inside continues to be negative, and the action potentials that you see in here continue to be your strengths in reversals. So that very, very, short period of time-- the inside of the cell might turn out to be positive.
Up = Depolarization Down = Hyperpolarization |
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Term
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Definition
| That, if we invert the local field, potential-- we can show that this oscillatory behavior in the cortex is not only rhythmic, but that it's a mechanism, for instance, to communicate-- mechanism of communication between different cortical areas. There are hypotheses that indicate, for instance, that our capacity to move information among different cortical areas is so observed by these synchronized activities. So one reason why one hemisphere is able to communicate with your other hemisphere is that at some point, some mechanism might be able to put them in sync, and the synchronization happens via oscillatory behavior. |
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Term
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Definition
Now, as a very quick example, I bring you back to this that I showed you before just to make the point that in spontaneous barrages of synaptic activity that propagate down the axon-- and I mentioned that before-- might have a pinch or have an effect on the post-synaptic cell. This turns out to be important, as I mentioned before, for many reasons. Not only-- the pointer I made before was that communication may be digital and analog locally, when it's only digital far away. But what it does besides that is that it increases the capacity of the cortex to process information and it increases it tremendously.
So being aware of this, it's necessary to take these things into account when doing, for instance, neural models. So things just-- the way we learn more details, but things get much more complicated. Capacity of the brain is incredibly high. |
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Term
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Definition
• Increase circuit complexity.
• What happens if recurrent excitation is removed?
• What happens if there is too much excitation?
• What happens if there is too much inhibition?
• Think of consequences of changes in the flow of Information.
This again, you've seen before, is the embedding of these lots of different cells within the circuit, and I told you this before for the concept of a neural network. And then here, you have again the same thing that I showed you before, in terms of when communication is digital and when it is both digital and analog |
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Definition
A lot of study is done in vitro to understand how cells communicate with each other, and what we many times learned is that the oscillatory behavior of one cell impinges on the oscillatory behavior of the other cell. In neuroscience, I want to end on this part of the lecture by mentioning something very interesting-- something that we call the binding problem. And the binding problem in neuroscience is basically that how come we see with our eyes, we hear with our ears, and so on, and we collect all these different types of sensory information via different channels. Transduction-- different transduction mechanisms. Eventually, everything makes it into the brain via the common language of the brain, which is action potentials.
But it is not the case that we are aware of it. We sense the world as a complete entity-- as a whole. It's not the case that I first see and then I hear and then I somehow have to think about it. In fact, I could, right now, disappear. You don't have to see me, and if you continue to hear me, you will already know that it is me who is talking to you, even if you don't see me. And these are complex processes. The brain somehow binds everything together and makes our understanding of the world a single, very compact, very succinct experience. We have evolved to be this way, and those processes and how these things happen are, again, very hot topics of research in neuroscience. And this concludes this part of our talk. |
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Term
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Definition
Now we come to the third part of our very short lecture on the cerebral cortex. What I want to discuss here-- there are several points. Unfortunately, we don't have time to expand extensively for this course the real details of any one of them. But it is good for you to just be aware of these. And then you can read more on your own. One of the things that I want to talk about is how memory processes are studied. How can you study memory? What is it that you do, actually, to study memory?
Now let me just give an example of memory at the cellular level. This is a diagram of a monkey, macaque monkey, brain. And what we have here-- the principle sulcus, this gray area, is the area of the prefrontal cortex. |
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Definition
Now, the animal is trained to react to a target that is shown to them on a computer screen. The computer screen is divided into sectors. What you have here is from 0 to-- going this way, actually, to 360 degrees. And then a cue might appear in one of those sectors. And the animal has to make a saccade. A saccade is an eye movement, showing where they saw the-- for instance, a light.
And there is also a delay which is introduced. The delay is important because it allows us to determine for how long cells were able to retain this information. At the same time, recordings, electrophysiological recordings, are taking place in cells of the prefrontal cortex.
And so first, the animal fixates on a-- let's say here at the center. Then the-- a cue appears-- let's say this light-- at 135 degrees. Then there is a delay-- in this case, 2 and 1/2 seconds, 2,500 milliseconds. And then the animal is allowed to give the saccade. And if the saccade is given to the right place, where the cue had appear, then the animal receives a reward. |
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Term
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Definition
1. Someprimateprefrontalneuronsshowelevatedactivityinresponse to sensory stimulation (C1–C2)
2. Insome,theactivitythenbecomespersistent(self-sustainedneuronal firing) in the absence of continued sensory stimulation (C2)
3. Inthoseneurons,firingfinallysubsidesaftermotorresponse
Cells also have a preferred direction, i.e., they respond best to a stimulus in a particular place in space/direction.
Now, on the next slide, what you have is a depiction of what happens in terms of electrical activity from one single cell. And upon doing the experiment, you may have different types of cells. There are cells that, while being recorded, are mostly responsive to the cue-- so the firing increases tremendously at the cue-- and some others that then are firing much more during the delay.
The cells which are firing heavily during this delay are thought to be firing and containing information, containing the memory, of the event. So this is what we call online memory. And then there are those that will fire during the response period. Some of them may have to do with the actual motor command to do the response. So we are very interested in these types of cells. |
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Definition
Now, on the next slide, you have, actually, plotted activity. And what you can see-- all of these little tiny lines represent action potentials. And they go line to the next line to the next line, and so on. And what you can see here is an increase, a tremendous increase, in the activity, followed by a decrease, and then going back to a baseline. And here, you have non-preferred direction.
So this has allowed the discovery, years ago, of cells that are selected for a particular direction. So they not only contain information about the event that has occurred, but where in this space it has occurred. So a cell might be very responsive to the 135 degrees that we had. But it may not be responsive to the 180 degrees opposite direction. |
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Term
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Definition
LTP: Long-Term Potentiation (The Basis for Long-Term Memory LTM)
Now, this is how electophysiologically, a memory is studied. This brings us to the concept of long-term potentiation. Long-term potentiation, as a cascade of events, as many things that happen, is believed to be the basis for long-term memory or how memories can be maintained.
Now, that is-- this is different from what I just talked about a second ago in terms of memory, online memory. Online-- it seems to be mediated by some cells able to maintain or sustain a very high frequency of firing for some time while you retain certain information in memory while you are able to then do a particular task. Now it's different. Now we are going to talk about perhaps what gives rise to long-term memories, how do you store memories.
And so experiments, early experiments, using tetanus-- tetanus is a very high-frequency input that is given to a single cell, for instance, or to a structure-- show these experiments in the hippocampus-- show that after tetanus, the cell was much more responsive. So in other words, at the beginning, it's harder to make it go. And then later on, it's much easier to make it go.
So in a way, I think of these things as being-- having sensitization of the cell. These processes might be mediated by much longer processes that might require the incorporation of, for instance, more receptors into the membrane of the particular cell, even perhaps at the presynaptic level, a larger number of vesicles, so that on-- given enough time, a response, an electophysiological response that is small might become a much larger one.
These mechanisms of-- that might take-- because it takes time also to manufacture the receptors and to embed them into the membranes, and so on-- might be responsible for long-term potentiation. To some extent, this might explain why repetition is good for memory. The more you repeat and repeat and repeat the same thing, the more you give time, you provide the same stimulus, basically, and you provide more time until these changes might take place. And then your memories become much more permanent.
Now, one thing that we usually don't talk much about is that as important as it is to have memory, it is important to forget. We do not want to remember every single little detail of every single thing we do during the day. In fact, when you go home at night, if somebody asks you how your day went, you can recollect the whole day, the events of the whole day, in maybe one or two minutes. And the reason for that is that only the most salient stuff is what you remember. You remember things that became very important to you. You don't remember every single detail unless those details happened to be important. |
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Definition
| Now, what you have here next to the long-term potentiation is the long-term depression and different mechanisms for long-term depression without-- because we don't have enough time, so I'm not going to go into the details-- but you can look in here-- is that I already told you long-term potentiation might be mediated by the increasing in the number of receptors into the membrane, and so on. And so if you do the opposite, you may achieve the opposite effect. This is how you forget things. Maybe things are removed. And so one mechanism is long-term potentiation. The other mechanism is long-term depression. |
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Term
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Definition
| (or working memory): online memory that allows you to remember just enough so that you can function—lasts seconds to hours |
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Term
| Long-term memory (LTM) and types |
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Definition
lasts days to a lifetime
• Declarative memory: consciously available, encoded by hippocampus, entorhinal cortex, and adjacent areas, but consolidated in distributed systems, i.e., their precise storage location is not known
• Episodic: memory of episodes—specific events in time
• Semantic: knowledge about the external world, what things are and what they are for
• Implicit memory (or procedural): how to use things, e.g., how to ride a bicycle or use a pen, etc. If the series of steps necessary to do something involves movements, then at least some of the memory is most likely stored in areas related to movement, e.g., cerebellum, etc. |
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Term
| There are other various classifications of memory with implicit names: e.g |
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Definition
| emotional memory (would involve the amygdala), prospective vs. retrospective, etc. |
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Term
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Definition
inferior frontal gyrus (44, 45)
• Broca’s aphasia: difficulty to produce speech |
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Definition
superior temporal gyrus (22)
• Wernicke’s aphasia: difficulty in speech comprehension |
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Definition
| (hippocampus, mammillary bodies, fornix): loss or impaired ability to form new memories |
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Definition
| loss of pre-existing memories to conscious recollection |
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