CerebrumIn this article, we describe the structure and concepts of the cerebrum, cerebral cortex, and homunculus. Have you ever wondered how humans take in sensations from their external environment, decipher their meaning and respond to them in a conscious way? In other words, how do we recognize that an object is a book and manage to move it across a desk? A structure buried inside your skull, called the cerebrum, plays a key role in all of these and many more similar capabilities.
While neuroscientists have been trying to crack the enigma of the cerebrum for decades with incomplete success, we’ll hop on a simpler journey. In this article, we’ll take a look at all its lobes, structures, grooves, folds, and the two imaginary people constantly living there.
The cerebrum, also known as the forebrain, is the largest part of the brain. It is derived embryologically from the telencephalon. The cerebrum consists of two cerebral hemispheres (right and left) separated by a deep longitudinal fissure which contains the corpus callosum. It is enveloped inside thin but protective membranes called meninges, one of which houses the subarachnoid space filled with cerebrospinal fluid.
If you’ve ever seen a picture or plastic model of a human brain, you will have noticed that it is not perfectly smooth. Instead, the cerebrum is full of grooves and ridges running in every direction. These are called sulci and gyri, respectively. Their role is to increase surface area, and hence the number of neurons, within the cerebrum. This permits larger processing and cognitive abilities within the cerebral hemispheres.
Each cerebral hemisphere is composed of five lobes:
The frontal lobe is the most anterior part of the cerebrum. It is involved in activities like muscle control, higher intellect, personality, mood, social behaviour, and language. Posteriorly, the frontal lobe is separated from the parietal lobe by the central sulcus (of Rolando) and inferiorly from the temporal lobe by the lateral sulcus (of Sylvius). The most significant convolutions of the frontal lobe are the precentral, superior, middle, inferior and orbital. The entire frontal lobe is supplied by the anterior and middle cerebral arteries, which are branches of the internal carotid artery.
The parietal lobe is situated between the frontal and occipital lobes, and separated from them by the central and parieto-occipital sulci respectively. It is involved in language and calculation, as well as the perception of various sensations such as touch, pain, and pressure. The lobe consists of two parts called lobules (superior and inferior) separated by an intraparietal sulcus. Other important landmarks include the postcentral sulcus together with the postcentral, angular, and supramarginal gyri. The parietal lobe is supplied by branches of the anterior, middle, and posterior cerebral arteries. The latter originates from the basilar artery.
Continuing down the list, we have another lobe of the cerebrum called the temporal lobe. It is responsible for memory, language and hearing. It sits underneath the previous two lobes, from which it is separated by the lateral sulcus. The temporal lobe consists of the superior, middle, and inferior temporal gyri that are delimited by the superior and inferior sulci. It is supplied by the middle and posterior cerebral arteries.
There are quite a lot of head spinning terms regarding the cerebrum, right? Let’s simplify the matter slightly by showing you a diagram of the brain and help you get your bearings:
The occipital lobe is the most posterior portion of the cerebrum and it is involved in processing visual stimuli. It rests on the tentorium cerebelli, a fold of dura mater that separates it from the cerebellum. The occipital lobe is separated from the parietal and temporal lobes by the parieto-occipital and calcarine sulci respectively. Additional important features and landmarks include the superior and inferior occipital gyri (divided by the lateral occipital sulcus), lingual gyrus and the cuneus. The vascular supply of the occipital lobe stems from the posterior cerebral artery.
Last but not least, we have the insula or insular lobe which is buried beneath the frontal, parietal, and temporal lobes. This lobe is involved in processing various sensations such as taste, visceral, pain and vestibular function. The central sulcus of the insula divides its surface into short and long gyri. This lobe is supplied by branches of the middle cerebral artery.
One final anatomical structure evident on a sagittal view of the brain is the limbic lobe which is involved in the modulation of visceral, hormonal, and autonomic functions, as well as emotions, learning, and memory. The limbic lobe is actually a misnomer because it is considered a region of the cerebrum rather than a lobe. Located superior to the corpus callosum, it consists of subcallosal, cingulate, and parahippocampal gyri, together with the hippocampal formation (a subcortical structure).
Learning all of the above information is definitely not easy, but we’re here to make it as easy and enjoyable as possible. Take a look at the following videos and tackle the quizzes to master lobes of the brain.
Now that we’ve covered the cerebrum, let’s take a look at the cerebral cortex. These two terms are often used interchangeably but they are actually quite distinct. The cerebrum describes the whole main part of the brain. It consists of two types of tissues called grey and white matter. Grey matter is composed of neural cell bodies and forms the outer, surface layer of the cerebral hemispheres. It is involved in processing and cognition. White matter, on the other hand, is made up of myelinated axons and forms the bulk of the deeper structures of the cerebrum. Its role is to join various areas of the cerebrum together. Strictly speaking, only the outer grey matter layer can be called the cerebral cortex.
The cerebral cortex is divided into smaller areas structurally by sulci and histologically by its cellular organization. The latter results in Brodmann areas, of which there are 52 in total. Together this information can help us start to form an understanding of the functional areas of the brain. The most important are the following:
|Frontal lobe||Primary motor cortex (Area 4), premotor cortex and supplementary motor cortex (Area 6), frontal eye field (Area 8), prefrontal cortex (Areas 9, 10)|
|Parietal lobe||Primary somatosensory cortex (Areas 1, 2, 3), somatosensory association cortex (Areas 5, 7), angular and supramarginal gyri (Areas 39, 40)|
|Temporal lobe||Primary auditory cortex (Areas 41, 42), secondary auditory cortex (Area 22)|
|Occipital lobe||Primary visual cortex (Area 17), secondary visual cortex (Area 18), associative visual cortex (Area 19)|
|Special Brodmann areas||
Wernicke’s area (22, 39, 40) - speech fluency
Broca’s area (44, 45) - motor speech
As you can see, cerebral cortex function is quite diverse and complex. More information about the brain's function, including Brodmann areas and the histology of the cerebral cortex are provided below:
Let’s focus even closer on two cortices of the cerebrum, the sensory and motor cortices. The primary somatosensory cortex is found within the postcentral gyrus of the parietal lobe. It is responsible for recognizing sensations, like when your socks are too tight. But how does it do this? The somatosensory cortex recognizes which part of the body is the source of sensation through the sensory homunculus, which is a representation in the shape of a little man. However, its proportions are sized according to the amount of sensation carried out by each body part. In other words, the sensory homunculus has huge hands and lips, but very small knees and elbows.
The primary motor cortex is found within the precentral gyrus of the frontal lobe and follows a similar logic. It is responsible for executing your motor actions and also contains a homunculus sized according to the amount of movement performed by each body part. For instance, he has a huge tongue and thumb compared to his chin or elbow.
Who would have thought that we actually have imaginary friends inside our brains? Joking aside, the following two resources can help you learn more about the two homunculi:
This quiz is specially designed to focus on the anatomy of the cerebrum and cerebral cortex. We encourage you to take it and solidify your knowledge about the structure and function of the cerebrum!