Cerebrum: Anatomy & Function

Cerebrum. Cerebral cortexAnatomy and function of the cerebrum. Cerebral cortex. Brodmann areas
~ 17 min

In this note, we are going to discuss the anatomy and function of the cerebrum.

External features

The cerebrum consists of two cerebral hemispheres: the left and the right,

they are separated by the median longitudinal fissure.

The surface of the hemispheres is uneven, and on it, we can see numerous depressions called sulci.

Large sulci divide the hemispheres into lobes, while small ones divide the lobes into gyri.

There are four lobes:

Frontal lobe

Temporal lobe

Parietal lobe

Occipital lobe

Additionally, there’s the insular lobe (or simply insula),

which is kind of hidden deep into the brain and is covered with the so-called operculum.

Sulci

Let’s consider the three largest and deepest sulci. They are the most significant landmarks on the surface of the brain. The central (or Rolandic) sulcus runs between the frontal and parietal lobes,

while the lateral (or Sylvian) sulcus runs between the frontal and temporal lobes.

The last sulcus is called the transverse fissure of the cerebrum, which separates the cerebral hemispheres from the cerebellum.

Other noticeable sulci include the parieto-occipital sulcus and the collateral sulcus.

Functions

Let’s move on to the functions of each lobe. For now, we’ll only discuss them in general terms, as you will gradually learn the more detailed principles of why a particular part of the brain performs a specific function as you study the brain and the nervous system as a whole.

The frontal lobe is responsible for controlling skeletal muscles, i.e., our movements, as well as for complex functions such as decision-making, motor planning, and determining what is good and what is bad.

Simply put, it defines us as humans in general and as individuals in particular.

The temporal lobe contains auditory, gustatory, speech, and memory centers.

The parietal lobe receives and processes sensory information, such as the sense of touch, pain, vibration, etc.

The occipital lobe is predominantly responsible for vision.

And the insular lobe participates in a bit of everything: from speech formation to the control of the heartbeat and other vital functions.

Internal features

Now let’s look at the section of the cerebral hemispheres. They consist of gray and white matter. Gray matter is represented by the neuron cell bodies, which generate the nerve impulse,

while white matter consists of neuron processes that conduct the nerve impulse, thereby providing communication between different parts of the nervous system.

So, here in the cerebral hemispheres, the gray matter is represented by the cerebral cortex,

which is located on the surface and the subcortical nuclei, or so-called basal ganglia, located just a bit deeper.

And the white matter is grouped into thick bundles of nerve fibers that ascend and descend to various parts of the nervous system, providing a connecting function.

Brodmann Areas

Let’s move on to a detailed examination of the cerebral cortex. It is divided into 52 so-called Brodmann areas, each of which has its own histological structure and functions.

Precentral gyrus

As a reminder, each of the cerebral hemispheres has the central sulcus (also known as fissure of Rolando) between the frontal and parietal lobes. Anteriorly to this sulcus, we have the precentral gyrus.

And the cortex covering the precentral gyrus is called the primary motor cortex (Brodmann area #4).

It contains giant pyramidal neurons, actually, the largest neurons in the central nervous system (also called Betz cells) that are responsible for voluntary movements of the opposite or contralateral side of the body.

So, the precentral gyrus sends a signal to the skeletal muscles, causing their contraction. And the term «voluntary» means that we can control this process: if we want to flex the arm – we flex the arm, if we intend to extend the leg – we extend the leg, and so on.

The key point here is that the primary motor cortex is responsible for our movements.

The primary motor cortex is organized in a particular way of somatotopic arrangement. Its upper and medial parts are responsible for the movement of the lower limbs, and the lower and lateral parts, respectively, of the head and neck.

This particular pattern, this concept, is called Penfield’s motor homunculus.

Postcentral Gyrus

Posterior to the central sulcus, we have the postcentral gyrus. The cortex of this gyrus is called the primary somatosensory cortex (Brodmann areas #1, 2, and 3)..

It receives and processes somatic sensory information from the contralateral side of the body.

These are sensations like touch, pain, temperature, pressure, vibration, and proprioception (body position feeling).

And just like the primary motor cortex, it has a particular pattern of somatotopic arrangement. Its upper and medial parts are responsible for the sensory input from the lower limbs, and the lower and lateral parts, respectively, from the head and neck. This concept is called Penfield’s sensory homunculus.

Motor association cortex

Anterior to the primary motor cortex, we have the motor association cortex. This is Brodmann area #6.

It consists of two regions – the premotor area and the supplementary motor area. They are responsible not only for simple muscle contraction and voluntary movement but also for the planning and sequencing of movement. So, the function gets more complicated, compared to the primary motor cortex.

Keep in mind this principle of complication, we will get back to it later.

Frontal eye field

Let’s move a little bit anterior. Area #8 is the frontal eye field involved in voluntary rapid (saccadic) eye movements.

The term «saccadic» means rapid, simultaneous eye movement in the direction that we need. For example, when we track across the page from one line to another while reading.

Broca’s Area

The so-called Broca’s area is located a little lower, within the inferior frontal gyrus. It is predominantly, but not exclusively, responsible for speech production – or, to be more specific, for stimulating the muscles that are involved in speech production.

It is important to note that Broca’s area can be found particularly in the dominant hemisphere. Most people, right-handed and left-handed, have it in the left hemisphere, but this is not an absolute rule, it may be different, some people have it in the right hemisphere.

The lesion of Broca’s area is called Broca’s aphasia, or expressive aphasia, and it’s commonly associated with non-fluent, grammatically incorrect speech accompanied by intact comprehension.

And note that Broca’s area is located exactly next to that area of the motor cortex, which is responsible for the movement of the lips and tongue. So there are some connections between them, and this is quite logical.

Wernicke’s Area

And now let’s look at the Wernicke’s area, located within the posterior part of the superior temporal gyrus and is predominantly, but not exclusively, responsible for language comprehension (both written and spoken language). Like the Broca’s area, it’s located only in the dominant hemisphere – left or right.

The lesion of Wernicke’s area is called Wernicke’s aphasia (or receptive / sensory aphasia).

Its key feature is impaired comprehension, commonly associated with fluent but meaningless speech, called “word salad”.

Prefrontal cortex

Anteriorly to all these areas, we have the prefrontal cortex. It is responsible for such complex higher functions as cognition, learning, working memory, decision-making, various aspects of personality, social behavior, and so on.

Also, it is partially involved in motor planning. In evolutionary terms, this is the newest and the most complex cortical area.

Association cortex

Remember, we said that we have the primary motor cortex that performs a relatively simple function, and next to it, we can find the motor association cortex that performs a more complex function.

This is also true for sensory modalities.

For example, posterior to the primary somatosensory cortex, we can find the somatosensory association cortex and posterior association cortex (or area) that not only receive sensory input but also perform integration or advanced processing, and analysis of it.

For example, identify the position of an object, its various characteristics, color, shape, size, all this stuff. In short, these areas collect diverse sensory input together and create a more comprehensive picture of the world around us.

It is the same with auditory, visual, and other sensory modalities. There are primary cortical areas: auditory, visual, and others.

Primary and part of the secondary auditory cortex

And next to them, we can find association areas, that provide analysis and recognition of stimuli.

Wernicke’s area and the adjacent regions<br />
also act as the associative auditory cortex — Wernicke’s area and the adjacent regions
also act as the associative auditory cortex

Medial surface

Well, now let’s briefly look at the medial surface of the cerebral hemispheres.

The most significant structure here is the cingulate gyrus and the cingulate cortex that covers it.

It is a component of the limbic system that plays an important role in memory, emotional responses, motivation, and performs many other functions.

Here we can also find the olfactory cortex (primary and association) that provides processing, analysis, and recognition of the olfactory (smell) sensory input.

Cerebral white matter. Types of fibers

The white matter is represented by the neuronal processes (predominantly axons) that conduct nerve impulses and provide connections between the different parts of the nervous system.

The white matter fibers are classified into three main types, based on their origin and destination:

Association fibers connect various parts of the cerebral cortex within the same hemisphere. They run, for example, from the frontal lobe to the temporal lobe, from the temporal lobe to the occipital lobe, and so on. Short association fibers connect adjacent regions, and long association fibers connect distant parts of the cortex.
Projection fibers, also called nerve tracts, connect the cerebral cortex to the lower parts of the nervous system – the brainstem and spinal cord. There are two types of projection fibers: ascending or afferent and descending or efferent.
Commissural fibers connect corresponding regions of the left and right hemispheres. They run from right to left or from left to right.

White matter structures

Well, all these fibers form a large number of different structures in the brain. Let’s start with those that we can find between the basal ganglia. They look like three bundles, each of which is called a capsule:

The internal capsule is located between the caudate nucleus, lentiform nucleus, and the thalamus

The external capsule – between the lentiform nucleus and the claustrum

The extreme capsule – between the claustrum and the insular cortex

Internal capsule

The most important is the internal capsule because of the large number of nerve tracts running within it.

It has three parts:

the posterior limb,

the genu,

and the anterior limb.

In addition, the fibers in the posterior limb are organized in a particular way of somatotopic arrangement, where the descending motor fibers for the face, arm, and leg run in front of ascending sensory fibers, also for the face, arm, and leg.

The fibers that run between the internal capsule and the cerebral cortex have a fan-like arrangement, and this structure is called the corona radiata.