Video: Overview of the cerebrum

The brain is the most complex organ in the human body. It's crown jewel, if you will. It produces our every thought, action, memory, feeling, and experience of the world. This jelly-like, walnut-shaped mass of tissue, weighing in at around 1.4 kilograms, contains a staggering 100 billion neurons. That's about the same as the number of stars in the Milky Way.

You probably already know that the brain can be divided into three main parts – the cerebrum, brainstem, and cerebellum. In this tutorial, we're going to focus on and get an overview of its largest part, the cerebrum.

When we see an image of the brain, it is usually the cerebrum that we first notice. It's this part here situated at the most superior part of the brain and is also known as the telencephalon, or endbrain. Structurally, the cerebrum is organized into two hemispheres – left and right – separated from one another by the longitudinal cerebral fissure.

Communication between the right and left sides of the brain is achieved via what are known as cerebral commissures, such as the corpus callosum, which is the largest bundle of white matter tissue connecting the two cerebral hemispheres.

While the two hemispheres of the cerebrum appear to be mirror images, they often have specialized functions. For example, the left hemisphere is typically more involved in language-related tasks, such as word formation and comprehension, whereas the right hemisphere tends to play a greater role in spatial abilities and processing abstract reasoning. However, for most processes, both hemispheres work together and many cognitive functions require integrated activity from both sides of the brain.

The cerebral hemispheres are each composed of superficial gray matter known as the cerebral cortex, underlying cerebral white matter, and finally, deeper masses of gray matter known as the basal nuclei, also commonly referred to as the basal ganglia.

Let's begin our exploration of the cerebral hemispheres now and take a look at the superficial gray matter.

The outer surface of the cerebrum – the cerebral cortex – exhibits many elevated ridges of tissue called gyri that is separated by grooves called sulci. The morphology of the gyri and sulci is primarily driven by the need to maximize the surface area of the cerebrum within the confines of the skull. When folded, each cerebral hemisphere measures just over 1/10 of 1 square meter or 1 square foot. However, if it were unfolded, laid out flat, it would cover about 1 square meter or 9 square feet. There are three main sulci in each hemisphere – the central sulcus, the parietooccipital sulcus, and the lateral sulcus.

When discussing the cerebrum, we can anatomically divide each hemisphere into six lobes, as you can see here. We'll take a look at each of these in a moment.

From a functional perspective, we can alternatively look at the cerebral cortex according to three generalized areas, which are the primary motor area, primary sensory areas, as well as several association areas which are areas where motor or sensory information is integrated to allow for more complex functions.

Unimodal association areas serve to integrate one specific type of information and generally relates to perception. For example, your primary visual area might see a vehicle, but your visual association area recognizes its shape to identify it as a car and its color to more specifically identify it as a taxi.

Higher order, or multimodal association areas, function by combining input from different sources. For example, the visual stimulus of seeing the taxi can be combined with the auditory input of a honking horn, telling you that the taxi is coming in your direction. This information is passed on to motor association areas to initiate a course of action appropriate for this danger.

As we explore each of the lobes of the cerebrum, we'll take a look at some of these functional areas in more detail.

As I mentioned, each hemisphere of the cerebrum is subdivided into six lobes based on location. They are mostly named after the overlying bones and include the frontal, temporal, parietal, occipital, insular, and limbic lobes. Let's explore each of the lobes of the brain while discussing their gross and functional anatomy.

The frontal lobe is the largest of the six lobes and accounts for more than a third of overall cortical volume. It lies largely in the anterior cranial fossa with its superolateral aspects conforming to the internal surface of the frontal bone. It extends from its most anterior part, the frontal pole, and travels posteriorly to the central sulcus which separates it from the parietal lobe. Posterior and inferior to the frontal lobe is the temporal lobe, separated from it by a lateral sulcus.

Various parts of the frontal lobe are associated with different functions and responsibilities. Perhaps best known is the frontal lobe's role in motor control and there are several areas of the frontal lobe involved in this. First is the primary motor area, or cortex, which is localized around this gyrus here, known as the precentral gyrus. It sends the majority of electrical impulses from the motor cortex to the body and requires the least amount of electrical stimulation to elicit movements.

Anterior to this are two additional or non-primary motor regions which are the premotor and supplementary motor areas. These are unimodal association areas involved in the planning of coordination of movements with the premotor area thought to be important for movements which are guided by external input while the supplementary motor area is more involved with learned movements performed from memory.

Anterior to the motor areas, we have the prefrontal association area, often simply referred to as the prefrontal cortex, which is located around the regions of the superior, middle, and inferior frontal gyri. The prefrontal association area is often described as the personality center of the brain which makes us uniquely human.

Through its extensive connections to other cortical areas, the prefrontal cortex is involved in a long list of higher order cognitive processes related to executive function. It's where your brain is able to compare the different thoughts, determine good from bad, and make decisions based on expected outcomes – all of that sort of thing. It's also a center for expression of personality and moderation of social behavior. It's well known that damage to this area can cause changes in these.

Before we move on, two other areas of note for the frontal lobe are Broca's area which is essential for producing the motor component of speech and articulation, and the frontal eye field, that controls eye movements related to visual attention, meaning it guides our eyes towards relevant visual stimuli.

I think that's enough on the frontal lobe. Next, let's explore the temporal lobe.

After the frontal lobe, the temporal lobe is the second largest lobe of the brain. It largely occupies the middle cranial fossa and is so-called due to its proximity to the temporal bone of the skull. The temporal bone is limited superiorly to the lateral sulcus, separating it from the frontal lobe and part of the parietal lobe, and posteriorly, by the preoccipital notch, which marks the beginning of the occipital lobe.

Functionally, the temporal lobe contains the cortical areas that process hearing as well as sensory aspects of speech and memory. The primary auditory area is located along the superior temporal gyrus and represents the specialized part of the cerebral cortex responsible for the reception of auditory information.

The auditory association area can also be found on the superior temporal gyrus of the temporal lobe. It serves as an intermediate stage for auditory processing which is responsible for analyzing more sophisticated features of sound such as distinguishing between different types of auditory stimuli like speech, music, or environmental sounds.

The information processed in the auditory association area is then sent to the other higher association areas for further integration and interpretation. An example of such an area would be Wernicke's area, which is located in the posterior portion of the superior temporal gyrus, usually in the left cerebral hemisphere. It is responsible for the comprehension of written and spoken language which means it functions to build upon information processed by the auditory association cortex by interpreting the meaning of words and sentences allowing for comprehension of spoken language.

The rest of the temporal lobe is given away to non-auditory higher association areas such as the limbic association area which is located along the anteroventral portion of the temporal lobe as well as parts of the frontal and limbic lobes. It serves to give emotional context to sensory input which is related to memory and learning. For example, when you're hungry, it's a relief to see food in your refrigerator and it's this emotional response that helps you recognize and associate that this is a place to find food next time that you're hungry.

Time now to explore the parietal lobe. It extends from the central sulcus anteriorly which separates it from the frontal lobe to the parieto-occipital sulcus posteriorly which separates it from the occipital lobe. The lateral sulcus corresponds to its inferolateral boundary separating it from the anterior portion of the temporal lobe. An intraparietal sulcus runs from the postcentral sulcus towards the occipital pole, dividing the lateral parietal lobe into the superior and inferior parietal lobules.

Functionally, the parietal lobe is best known for its involvement on the perception of somatosensation, or body sensation, which relates to sensation of the skin, movement, and spatial awareness which is known as proprioception and sensation related to internal organs. So this is where your brain processes things like touch, pressure, temperature, pain, and position of the body.

Initial processing of this type of sensation occurs in the primary somatosensory area which can be localized to this gyrus here known as the postcentral gyrus. Information from the primary somatosensory area is then integrated and interpreted in the somatosensory association area which is largely located in the region of the superior parietal lobule. It plays an important role in things like tactile object recognition as well as spatial awareness which relates to the perception of position and movement of different parts of the body.

The inferior parietal lobule, on the other hand, contributes to a large and complex higher order association area known as the parietotemporal-occipital or posterior association area. It's involved in a long list of complex neurological functions and plays a key role in integrating and combining auditory, visual, and somatosensory information. This is where we gain deeper understanding and context to language, where we pick up on social cues and interpret body language, and where numerical processing and operations occur – and that's only to name a few of this area's functions.

Moving on to the smallest of the brain's six lobes now – the occipital lobe – which is located posterior to the temporal and parietal lobes, superior to the cerebellum. Functionally, the occipital lobe is the visual processing area of the brain. Two functional areas are found in this lobe. These are the primary visual area and the visual association area, also known as the secondary visual cortex.

The primary visual area is located on either side of the calcarine sulcus on the medial surface of the occipital lobe and extends to the occipital pole. It functions to receive visual input from the eyes, and therefore, like the other primary sensory areas, is the first area to receive visual input from photosensitive receptor cells in the retina.

The visual association area constitutes the remaining regions of the occipital lobe and small parts of the adjacent lobes. This region functions to interpret visual attributes like color, shape, movement of objects, and depth.

Second to our last in our list of cerebral lobes is the insula. The lobe is hidden away in the floor of the lateral sulcus and is covered by parts of the frontal, parietal, and temporal lobes, known as the opercula. When the lateral sulcus is opened and the insula is exposed, we can see a central sulcus that divides the lobe into an anterior part composed of short gyri and a posterior part formed by a single long gyrus.

Functionally, the insula is not as well explored as other regions of the brain; however, it is thought to house the primary gustatory area as well as serve as a center for interoception, which relates to the process of bringing information about the physiological condition of the body to consciousness.

For example, it contributes to the conscious experience of things like hunger, thirst, and a desperate need to use the loo. It also integrates emotional states with physical feelings. For instance, your insula plays a role in your gut feeling when making decisions or physical feelings of empathy or compassion.

The last lobe in our list is the limbic lobe. The limbic lobe is arc-shaped and refers to a specific group of anatomical structures largely found along the medial aspect of the cerebral hemisphere forming a rim around the corpus callosum.

Functionally, the limbic lobe serves a diverse set of functions which are broadly related to emotion, memory, and behavioral actions. It's one of the regions which contributes to your fight or flight responses or that gets your heart racing when in the midst of a deep moment of passion. It's also the part of our brain which houses our olfactory cortices which explains why certain smells can trigger vivid memories or emotions.

And that completes our exploration of the cerebral cortex which we said represents the superficial gray matter of the cerebrum. Let's not forget, though, that the cerebrum also contains a number of deep gray matter structures. Let's now have a quick look.

So the deep gray matter structures of the cerebrum are generally referred to as the basal nuclei or basal ganglia. These are found deep within the white matter of each cerebral hemisphere and comprise three main components – the caudate nucleus which is the C-shaped ring of gray matter here; the putamen, and the globus pallidus.

The basal ganglia play a crucial role in the modulation and regulation of voluntary movements. So they're the ones that make your dance move smooth and rhythm sweet. They are also involved in motor learning and skill, helping difficult and complex movements become second nature with practice, and also cognitive processes which help determine appropriate motor responses based on goals and sensory output.

So now that we've explored most of the cerebrum, let's finish up by taking a quick look at cerebral white matter.

Deep to the gray matter of the cerebrum is the cerebral white matter which is composed of axons of neurons reaching between different areas of the brain. You may or may not be aware of this but all neurons in the cerebrum, regardless of their function, are interneurons. This means that they only communicate with other neurons.

There are two types of interneurons – short axon, or local interneurons, which are mainly found in gray matter and form circuits with nearby neurons in the same region, and long axon, or relay interneurons, which are those which extend from one region of gray matter to another and it's the axons of these neurons which form the white matter.

Most of these axons are surrounded by a type of fatty sheath called myelin which gives the white matter its color. While gray matter facilitates information processing, white matter serves the important function of enabling information transfer and integration.

There are three types of fibers which make up the white matter of the cerebrum. First, we have association fibers which interconnect different cortical regions within the same cerebral hemisphere like those which we said form connections between primary motor or sensory areas with association areas. Next, we have commissural fibers which are those which form connections between the cerebral hemispheres. The largest of these is the corpus callosum, a dense plate located in the depths of the longitudinal cerebral fissure.

And finally, we have projection fibers which connect different cortical regions within lower regions of the brain or spinal cord. An example of such white matter tract is the internal capsule which is a major pathway for ascending and descending axons traveling between the cerebrum and the brainstem or spinal cord.

And with that, we've covered the main ins and outs of the cerebrum. There's lots more to learn but we've made a great start at becoming more knowledgeable about this part of the brain.

Be sure to solidify your knowledge now with our quizzes and other learning materials in this study unit. And when you're ready, continue on our more detailed topics on different parts of the cerebrum.

See you next time.