Hi everyone, Dr. Mike here. In this video, we're going to take a look at the cerebral hemispheres. So, when we take a look at the brain itself, we know the brain is made up of the cerebrum, which is everything colored here, the cerebellum, which is that little brain that sits underneath and towards the back, and the brain stem underneath, which sits just above the spinal cord. Today, we're talking about the largest part of the brain, which is the cerebrum, which has a fissure or segmentation down the middle.
that separates it into two hemispheres, a left hemisphere and a right hemisphere. So it's the largest part of the brain, but it's also the most developed part of the brain. Now, what do I mean by developed? Well, let's take a look at some of the functions of the cerebrum.
First of which is that the cerebral hemispheres contain the primary motor and sensory cortices. Now, when I say cortices, I'm referring to the cortex. And the cortex is simply the outer one to five millimeters of the cerebrum. It contains grey matter. Remember, when we look at a neuron, you're going to have the cell body of a neuron, you're going to have the axon of the neuron, and then you're going to have the axon terminals.
That axon terminal is likely going to talk to another axon, or neuron I should say, for example. So importantly, the axon is surrounded by fat, myelin, and fat in this case looks white. So when we look at grey matter, grey matter...
are the cell bodies and the areas of synapsing. So that's grey matter. And the white matter is actually simply the axons.
So that means that when you see grey matter, this is the site of integration, where we make sense of information. The white matter is simply just highways. This is simply where one signal is sent to the next area.
Alright, keeping that in mind, when we look at the primary motor, and sensory cortices that sit within the cerebral hemispheres. This is the highest level of activity. For example, when we look at the motor cortex, or the primary motor cortex, which actually sits in this part of the frontal lobe, this is the area where we want to initiate some sort of voluntary or conscious motor movement. If I want to begin walking, this is where it begins. So that's why it's the highest level in which motor activity will function.
When we look at the sensory cortex, this is where information coming from the external environment or the internal environment is coming up for us to be consciously aware of it. Again, it's the highest level of integration of sensory information. This is where we become aware. So think about if you had your hand in your pocket.
When you feel something in your pocket, this is going to the sensory cortex. Alright, now in addition to that, I'm going to skip this part here. We've got association areas.
That's part of the cerebral hemispheres. Now, association areas, they help the primary motor and sensory cortices. By associating previous information and current information, it's comparing and contrasting what it already knows so that you now have more information.
It's basically a way to analyze complex information. So first of all, the association areas for the motor cortex. So I said, you're going to have the motor cortex here. I want to initiate walking for example, that's happening there at the primary motor cortex, but the association areas allow for you to sequence and plan more complex tasks.
So if I wanted to go play the piano for example, or juggle some balls, this is where we have the association areas come into play for the motor cortex. It allows for us to plan and sequence complex movements. When we look at the sensory cortex, which I didn't say, sits within that parietal lobe here, the association cortices help you understand what piece of sensory information you are receiving.
So I said before that primary sensory cortex there, you put your hand in your pocket and you feel something, what the association cortex does is it analyzes whether it's smooth or rough, whether it may be hot or cold. Have I felt it before? What might this be from previous experience?
So it's an association cortex because it's bringing associations from previous experience and throwing it to that of the primary somatosensory cortex. So these are these association areas, all part of the cerebral cortex. The cerebral hemisphere is the substrate for conscious experience.
When we become consciously aware of anything, it's because it's gone to the cortex. of the cerebral hemispheres. So if you want to be consciously aware, it needs to get to those one to five centimeter thick, millimeter I should say, layer of the cerebral hemispheres. If you want to be consciously aware, this is where consciousness is going to arise, where you become aware of experience.
More deeper areas of the brain may receive sensory information, but you will not become aware of it occurring. Or it may make a decision about conscious, may make a decision about motor movement, but you're not going to be consciously aware of that. For conscious awareness, it must get to that one to five centimeter external layer that we call the cortex of our cerebral hemispheres.
The cerebral hemispheres also contain the limbic system. Now the limbic system is important for memory and emotional aspects of behavior. So think of emotional aspects of things that you do.
So the emotional aspects of feeding, the emotional aspects of relationships, the emotional aspects of reproduction, the emotional aspects of fight or flight. This is the limbic system and you can see part of the limbic lobe. sitting here, but there's deeper limbic structures as well, right near the thalamus that you can see here. So we've got the limbic system.
It's also the highest aspect of cognitive function. The cerebral hemisphere is the highest aspect of cognitive function. So when we look at cognitive function, we're thinking about planning, judgment, emotion, making sense, being reasonable, making sure that your behavior is appropriate for the scenario in which you're in. Most of this sits within the frontal lobe, specifically an area called the prefrontal cortex.
If that area is damaged, we have issues when it comes to behavior and planning and things like that. So, highest aspect of cognitive function. Now, like I said, when we take the cerebrum, it's going to have the 1 to 5 millimeter layer, and then there's going to be some deeper structures.
The 1 to 5 millimeter layer, like I said, is the gray matter. That's the site of integration and making sense and consciousness. But we've got deeper layers of the cerebrum where we project information. These are the highways.
These are the axons. This is the white matter. All right.
Now, you can send information from one side of one, from this one cerebral hemisphere to the other. And if you do this, you're sending it through white matter tracks or fibers. They're called commisural fibers.
They are predominantly going to be through what we call the corpus callosum. That's the connection between those two hemispheres. Some people had a corpus callos otomy.
Otomy means to cut. It separates those hemispheres. These individuals had very severe cases of epilepsy and epilepsy is this misfiring of neurons. If you cut it, it stops it from spreading across the two hemispheres and help mitigate the severity of the epilepsy. But it also separated the hemispheres anatomically.
And if you have a look at the split brain experiment studies, you'll find some very interesting things when it comes to splitting the consciousness of the brain. So we've got the commisural fibers from one hemisphere to the other. We've also got projection fibers. These fibers take it from more superficial areas of the brain to deeper aspects of the brain, brainstem and spinal cord. These are going to be projection fibers.
You can see that they're going to cross over to the other side because the right hemisphere controls the left-hand side of the body, left hemisphere controls the right, or the right side of the body sends information to the left hemisphere. and vice versa, which means these projection fibers must cross at some point. Usually they're going to cross at around about the medullary region.
Association fibers. I spoke about association areas. Association fibers allow for one part of the cortex to speak to another part of the cortex so that we can make more sense of what's happening in the situation. If I want to sequence a motor task, I need these association fibers. If I want to make sense of what I'm feeling in my pocket, again, We need these association fibers.
So when we look at the cerebral hemispheres, it's made up of cerebral cortex, one to five millimeter layer. This is the site of, what is it? Consciousness, experience, integration.
It's made up of lobes. And these lobes, which we should probably look at before we finish, I'm gonna focus more attention on those lobes in future videos. These lobes are made up of the frontal lobe, the parietal lobe, the occipital lobe at the back. the temporal lobe near the temple and we've also got an insular lobe if i were to sort of pull that temporal lobe away from the frontal you could see the insular underneath i'm going to do videos on each of these lobes more specifically but final point is that when we look at the superficial or surface and anatomy of the brain you're going to find that there's areas which have a bump up and a dip down. Now the bump up is known as a gyrus, that's singular.
If it's plural it's gyri. The dip down is known as a sulcus, again that's singular, a sulci is plural. And this is what gives the brain its wrinkled appearance.
It just increases the surface area of the brain which means there's more room to fit more neurons, more sites of integration, all that type of fun stuff. Couple of points. Because I said you've got the frontal lobe, the parietal lobe, the occipital lobe, the temporal lobe, how do we separate them out?
How do we anatomically know where their demarcation points are? Well, we use gyri and sulci. For example, we separate out the frontal lobe from the parietal lobe with a sulcus called the central sulcus.
Central sulcus. Now, wherever there's a sulcus, In front and behind is going to be a gyrus, right? So this central sulcus, we've got a gyrus in front called the precentral gyrus. And that's that primary mode cortex. We've got a gyrus behind it.
And that's going to be called the postcentral gyrus. That's going to be the somatosensory cortex that we spoke about as well. When we look at the parietal lobe separated out by the occipital lobe, we're going to have the parieto-occipital sulcus. parietal or occipital sulcus. And again separating out the occipital to the parietal lobe.
We've also got the temporal lobe here. How is that separated out from the frontal and parietal lobe above it? We have a lateral sulcus here also known as a lateral fissure or sylvanian fissure situated there. All right in a future video I'm going to focus on each of those lobes but to summarize The cerebral hemispheres is the largest and most complex and most developed part of the brain.
It allows for us to have conscious awareness, it has lobes and these lobes have particular functions, it has cognitive, higher order cognitive function, it has limbic areas for emotional and behavioral understanding, it has association areas, primary motor, primary sensory areas, but more deeper to this it has fibers, axons that can project to other parts of the cerebral hemispheres. but also deeper parts of the brain as well.