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Lateral views of the brain

Structures seen on the lateral views of the brain.

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Transcript

In today’s tutorial, we’re going to go way back to the 1930s and meet this guy, Dr. Wilder Graves Penfield, a neurosurgeon, once hailed as the greatest living Canadian. Penfield is most well-known for his techniques for treatment of epilepsy as well as his role in the functional mapping of the brain. Penfield was aware that in many cases of epilepsy, seizures originated from a scarred or damaged region of brain tissue and also that it was quite common to patients suffering with epilepsy to get what’s known as an aura – a sensation such as a smell, taste or thought before the onset of a seizure.

For example, one of Penfield’s patient was famously described as smelling burnt toast before her seizures would come on. So, Penfield helps develop what you could call a somewhat out-of-the-box technique known as the Montreal Procedure, in which he would remove a portion of a patient’s skull while they were awake. He’d then zap different areas of the patient’s brain with a small electrode, taking note of what the patient experienced each time. And he would keep proving until eventually he would find the area of the brain producing the aura, thus identifying the troublesome brain tissue causing the seizures and removing it in hopes of reducing the risk of future seizures. Science fiction, this is not.

So, over his career and after probing many, many brains, Penfield learned much about the functions of different regions of the cerebral cortex and eventually developed a functional map of brain function. So, in today’s tutorial, we’re going to relieve Penfield’s exploration of the brain and, unfortunately, we couldn’t find anyone who was willing to let me saw open their brain. So, we’re going to take a more somewhat conservative approach and stick with this illustration of the brain to be our guide instead.

So, stick with me now as we explore a lateral view of the brain.

So, to begin, let’s get a lay of the land of what we’re looking at, remembering first that this is the anterior, posterior, superior, and inferior aspects of the brain. And looking at the general anatomy first, let’s begin with the structure here which is the cerebellum.

And the cerebellum is mainly responsible for functions such as the coordination of muscle activity and movement, and helping us to stand upright and to maintain posture. Anterior to that is the brainstem, which mainly functions to relay information to and from the higher regions of the brain and the rest of your body. And the brainstem, of course, can be divided into three parts, which are the midbrain, the pons, and the medulla oblongata, and you can see in our illustration that these are largely covered by the cerebrum which is the center for higher processing in our brain. And it is specifically the surface anatomy of the cerebrum that we’ll be focusing on today.

So, in this coronal section of the brain, we can see that the cerebrum is divided into two cerebral hemispheres – a left and a right one – which are separated by the medial longitudinal fissure. Each hemisphere has three surfaces – the superolateral surface, the medial surface, and the inferior surface. When looking from a lateral perspective, we are therefore specifically looking at the superolateral surface of the brain.

Now, would you believe me if I said that when we look at the surface of the brain, we’re really only able to see approximately one-third of the actual surface area of the cerebral cortex? This is due to the presence of these elevations or folds, which are known as gyri, and depressions or grooves, which are known as sulci. Of course, there’s a reason for this folding. It’s actually to allow more surface area to accommodate the millions and millions of neurons and synapses which are needed to facilitate higher processing. If our brains didn’t have these sulci and gyri, it would be around the same size as a basketball which, unless you’re going for the crazed-villain look, is neither becoming nor very practical.

The sulci of the cerebral cortex simply serve to define each gyrus from the next while some larger grooves known as fissures define and demarcate the cerebral cortex into subdivisions, which are known as lobes. And these are the frontal lobe, the parietal lobe, the temporal lobe, the occipital lobe, and the insular lobe or insula for short which is hidden in here, but more on that in just a short while.

The superficial four lobes are named according to their overlying cranial bone. For example, the frontal lobe lies deep to the frontal bone, the occipital lobe lies deep to the occipital bone, and so on. And when looking at the cerebral cortex from a lateral perspective as we’re doing right now, we can identify three somewhat pointed ends, which are known as poles. And these include the frontal pole located at the anterior tip of the frontal lobe, the occipital pole located at the posterior tip of the occipital lobe, and finally, we have the temporal pole, which is located at the anteroinferior tip of the temporal lobe.

So, let’s take a closer look at the major gyri and sulci of each lobe right now beginning anteriorly with the frontal lobe. And the frontal lobe is responsible for controlling a diverse range of functions such as muscle movements or muscle control as well as cognitive functions such as planning, concentration, language as well as rationale and executive control – meaning that it helps prevent us doing dumb or stupid things when we’re angry or upset that we’ll later regret. Interestingly enough, the frontal lobe doesn’t fully finish developing until after adolescence which in itself says a lot about adolescence.

The frontal lobe is limited posteriorly by a fissure called the central sulcus, or the fissure of Rolando, which runs anterolaterally towards a second fissure called the lateral sulcus, or the fissure of Sylvius, which defines the inferior border of this lobe.

Let’s take a look now at some of the subdivisions of the frontal lobe. So, highlighted in green now is a sulcus called the precentral sulcus which runs parallel and a little bit anterior to the central sulcus, or the central fissure, and the area between it and the central sulcus is the precentral gyrus. The precentral gyrus is home to the primary motor cortex which, true to its name, controls motor function.

So, our friend, Penfield, also discovered that motor function is topologically arranged, meaning that each part of the body is represented in a specific area of this cortex as you can see in this illustration. In the region anterior to the precentral gyrus, there are two sulci that run in an anteroposterior direction, and these are the superior and inferior sulci, and these sulci divide this region into three gyri – the superior frontal gyrus which is associated with laughter, working memory and self-awareness; the middle frontal gyrus, and the inferior frontal gyrus.

The inferior frontal gyrus is further divided by two somewhat vertical sulci which are the anterior ramus of the lateral cerebral sulcus and the ascending ramus of the lateral cerebral sulcus. And this divides the inferior frontal gyrus into three parts – the pars orbitalis or the orbital part, the pars triangularis or the triangular part, and the pars opercularis or opercular part.

And yes, I know, there’s a whole lot of terminology going on here, but really if you can just remember this major landmarks, then you’re doing really well with the anatomy of the frontal lobe.

I want you to pay particular attention to this pars triangularis for a moment, though, because it’s around this area, usually in the left hemisphere that we’ll find Brodmann area 44 and 45, which is more commonly known as Broca’s area. And this is the area which is responsible for speech production and articulation. We will find out more about this area a little bit later in this tutorial, so stay tuned.

We’re going to continue on to our next cerebral lobe of interest though right now, and this which is the temporal lobe.

So, the temporal lobe which is defined as this area just here highlighted in green which is largely inferior to the lateral sulcus. It has two major sulci of note which are the superior temporal sulcus seen here and the inferior temporal sulcus running roughly parallel beneath it. And these two sulci divides the temporal lobe into three primary gyri which are the superior temporal gyrus, the middle temporal gyrus, and the inferior temporal gyrus.

One particular region of interest which we must mention is this one here located at the posterior end of the superior temporal gyrus, and this is Wernicke’s area or Brodmann area 22. And this is the control center of your brain for language development and comprehension of speech. Most often, Wernicke’s area is generally only found on the left-hand side of the brain, however, that’s not always the case.

Alright, it’s time for our next cerebral lobe now, and this time, we’re going to be talking about the parietal lobe.

So the parietal lobe is primarily responsible for the processing of the sensations of touch, pain and pressure, which are also known as somatosensory sensations. So, if you somehow managed to drive a nail through your finger one day, it will be your parietal lobe telling you all about it.

Looking at the anatomy of the parietal lobe, we can see that it is limited anteriorly by the central sulcus and posteriorly by this sulcus here known as the parietooccipital sulcus, which is more visible on the medial aspect of the cerebral hemisphere. Inferiorly, the parietal lobe is defined by the posterior ramus of the lateral sulcus continuing across via an imaginary line inferior to the parietooccipital sulcus.

Within the parietal lobe, the postcentral sulcus runs posterior to the central fissure defining the postcentral gyrus in between which houses the primary somatosensory cortex. And just like we saw with the primary motor cortex, the primary sensory cortex is also topologically arranged with different areas of cortex dedicated to certain regions of the body.

The remainder of the parietal lobe is divided into two primary parts divided by the intraparietal sulcus which runs anteroposteriorly and superior to the intraparietal sulcus is the superior parietal lobule. And, unsurprisingly, on the opposite side, is the inferior parietal lobule.

So, some notable gyri of the inferior parietal lobule include the supramarginal gyrus which is this part arching around the end of the posterior ramus of the lateral sulcus, and the angular gyrus, which is this section that arches over the superior temporal sulcus.

Alright, time for our next cerebral lobe which is the occipital lobe.

So, next time you’re staring at your beautiful loved one’s eyes, be sure to stop and take a moment to thank your occipital lobe for the fact that you can actually see them. Yes, this is the part of our brain which houses our primary visual cortex and processes visual stimuli received by the retina to produce the image of the world as we know it.

The occipital lobe is the smallest of the four paired lobes of the brain and, topologically, it sits posterior to the parietal and temporal lobes separated from them by an imaginary line reaching from the parietooccipital sulcus down to the preoccipital notch.

From a lateral perspective, one of the sulci which we can identify on the occipital lobe is the lateral occipital sulcus, seen here dividing the occipital lobe into superior and inferior occipital gyri.

So, we’ve spoken about the four superficial lobes of the brain so far. Let’s explore a fifth area now which is known as the insular lobe.

So, if there’s one type of food that I love, it’s gotta be Thai food. It’s really, really good and I love the combinations of sour and sweet and salty and spicy food. It’s an explosion of flavor that sets my taste buds on fire, and it also sets my gustatory or taste cortex equally alight, which happens to be partially located within the insular lobe.

The name “insula” in itself means hidden, and as you can see in the illustration, it can only be visualized by means of retraction of the lateral sulcus, and it is sometimes described as the fifth lobe of the brain. However, other people might simply describe it as a distinct area, rather than an actual lobe. But we’re not going to get into that debate today.

So, looking at its anatomy, we can see the insula or the insular cortex is overlapped by the surrounding frontal, parietal and temporal lobes which occlude it from view. And the surrounding areas are known as the opercular and there are three of these in total – the frontal operculum, the parietal operculum, and the temporal operculum. As with the other lobes, the surface of the insula can be divided into smaller gyri and in this case, the dividing sulcus is this one here, which is the central sulcus of the insula. Anterior to the sulcus are the short gyri of the insula which posterior to it will find the long gyrus of the insula.

And with that, we’ve explored the major landmarks and divisions of the cerebrum from a lateral perspective.

So, before we finish up, let’s take a moment to take a clinical look at one of the areas that we studied today.

So, you may remember earlier, we briefly mentioned this part of the brain which is known as Broca’s area, and you might also remember that we said this area is responsible for the production of speech, and is more often located on the left hand side of the cerebrum in most people. And this part of the brain receives its arterial supply via the superior division of the middle cerebral artery, the one which you can now see highlighted on the illustration.

Now, what would happen if someone experienced an occlusion of this artery – say, for example, due to a clot? Well, as you might have guessed, we would commonly refer to this as a stroke or a cerebrovascular accident. A stroke affecting Broca’s area can result in a condition called Broca’s aphasia, also known as non-fluent or expressive aphasia. And since Broca’s area is primarily responsible for putting words together to form sentences, Broca’s aphasia decreases our ability to do the same.

For example, if a sufferer of a stroke within Broca’s area wanted to say “I worked in an office doing accounting”, they’d be more likely to say something like “worked… office… accounting…” So, as you can see, a person with Broca’s aphasia relies on keywords to communicate their message, but often might omit functional words such as prepositions and articles. For example, in and an. Their speech, nevertheless, appears effortful and often pretty frustrated as the sufferers usually pretty aware of the difficulties that they’re having in finding the words.

Although comprehension remains generally intact with Broca’s aphasia, sufferers may have difficulty in understanding order and sequencing sentences. For example, as “I was bitten by a dog”, could be understood as “I bit the dog.” Interestingly though, although sufferers of Broca’s aphasia lose their ability to speak, their ability to sing usually remains intact. And this presents a theory that speech and singing may have different pathways in our brain.

In fact, functional studies of the brain have shown that the brain shows increased right hemispheric activity when we’re singing. And many sufferers who have received therapeutic singing or what’s known as melodic intonation therapy have shown an improvement in speech production suggesting that these activities help activate language processing centers in the right hemisphere.

It’s all very interesting. Our brains are truly very impressive supercomputers, to say the very least.

Alright so, on that note, we’re going to wrap things up on this video tutorial, but before we do, let’s quickly summarize the anatomy of the cerebrum that we learned today.

So, we started off by first identifying the four major superficial lobes of the brain which were the frontal, temporal, parietal, and occipital lobes. Beginning with the frontal lobe, we saw that it was limited posteriorly by the central sulcus and inferiorly by the lateral sulcus with its anterior most tip being referred to as the frontal pole. We learned about some of its major sulci and gyri including the precentral sulcus which defines the precentral gyrus which is the home of our primary motor cortex. We also identified the superior and inferior frontal sulci which separates the superior, middle, and inferior frontal gyri. And when we then focused specifically on the inferior frontal gyrus, we saw that it could be further subdivided into three main parts which were the orbital, triangular, and opercular parts.

We next looked at the temporal lobe which we learned was bounded superiorly by the lateral sulcus and anteriorly at the temporal pole, and looking at its posterior limit, we learned the temporal lobe extended back as far as this landmark here which is the preoccipital notch. While looking at its subdivisions, we first identified the two primary sulci which were the superior and inferior temporal sulci which defined three main gyri – the superior, middle, and inferior temporal lobes.

Moving on to the parietal lobe next, which is located posterior to the central sulcus, there’s the postcentral gyrus which houses the primary somatosensory cortex and is bounded by the sulcus here which is the postcentral sulcus. Other notable landmarks of the parietal lobe included the intraparietal sulcus which separated the superior parietal lobule from the inferior parietal lobule. And when looking at the inferior parietal lobule, we also noted two major gyri which were the supramarginal gyrus and the angular gyrus.

The posterior end of each cerebral hemisphere, we took a quick look at the occipital lobe which is roughly bounded anteriorly by an imaginary line reaching from the parietooccipital sulcus down to the preoccipital notch. As most of the optical lobe is located along the medial aspect of each cerebral hemisphere, we mentioned just one sulcus here, which was the lateral occipital sulcus which separates the superior and inferior occipital gyri.

Finally, we prised apart the lateral sulcus here to reveal the insular lobe, and we identified the surrounding cortex to the insula as the opercular namely the frontal operculum and the parietal operculum, and the temporal operculum. And focusing on the insula itself, we finally identified the central sulcus of the insula which separates the short insular gyri from the long gyrus of the insula.

And that’s it! Take a breath of relief and a moment to relax because this tutorial is done and tested. I hope you enjoyed it and please be sure to check out our website at kenhub.com for more quizzes, articles and atlas sections on all that we’ve studied today.

And until next time, happy studying!

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