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,

Cerebral hemispheres
Cerebral hemispheres

they are separated by the median longitudinal fissure.

Median longitudinal fissure
Median longitudinal fissure
Median longitudinal fissure
Median longitudinal fissure
Cerebrum
Cerebrum

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

Sulcus
Sulcus

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

Gyrus
Gyrus

There are four lobes:

  • Frontal lobe
Frontal lobe
Frontal lobe
  • Temporal lobe
Temporal lobe
Temporal lobe
  • Parietal lobe
Parietal lobe
Parietal lobe
  • Occipital lobe
Occipital lobe
Occipital lobe

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

Insular lobe / insula
Insular lobe / insula

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

Operculum - temporal part
Operculum - temporal part
Operculum - fronto-parietal part
Operculum - fronto-parietal part

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,

Central (or Rolandic) sulcus
Central (or Rolandic) sulcus

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

Lateral (or Sylvian) sulcus
Lateral (or Sylvian) sulcus

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

Transverse fissure
Transverse fissure

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

Parieto-occipital sulcus
Parieto-occipital sulcus
Cingulate sulcus
Cingulate sulcus
Collateral sulcus
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,

Gray matter
Gray matter

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

White matter
White matter

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

Cerebral cortex
Cerebral cortex

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

Basal ganglia
Basal ganglia

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.

Neural tracts
Neural tracts

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.

Precentral gyrus
Precentral gyrus

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

Primary motor cortex
Primary motor cortex

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)..

Postcentral gyrus
Postcentral gyrus

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.

Supplementary motor area
Supplementary motor area
Premotor area
Premotor area

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.

Frontal eye field
Frontal eye field

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.

Broca’s Area
Broca’s Area

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.

Wernicke’s area
Wernicke’s area

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.

Prefrontal cortex
Prefrontal cortex

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
Primary and part of the secondary auditory cortex
Primary visual cortex
Primary visual cortex

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

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

Neural tracts
Neural tracts

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
Internal capsule
Internal capsule
  • The external capsule – between the lentiform nucleus and the claustrum
External capsule
External capsule
  • The extreme capsule – between the claustrum and the insular cortex
Extreme capsule
Extreme capsule

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,
Posterior limb
Posterior limb
  • the genu,
Genu
Genu
  • and the anterior limb.
Anterior limb
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.

Corona radiata
Corona radiata

Commissural fibers

The left and right hemispheres are connected by three structures made up of commissural fibers.

  • These are the anterior commissure,
Anterior commissure
Anterior commissure
  • the posterior commissure,
Posterior commissure
Posterior commissure
  • and the most significant, the corpus callosum,
Corpus callosum
Corpus callosum

which has four parts:

  • Splenium
Splenium
Splenium
  • Trunk
Trunk
Trunk
  • Genu
Genu
Genu
  • Rostrum
Rostrum
Rostrum

Fornix

Let’s look below the corpus callosum. There is another cluster of white matter, called the fornix.

Fornix
Fornix

It is a component of the limbic system, and it plays an important role in memory processing.

The fornix looks like two bundles, which are connected in the midline, forming the body of the fornix.

Body of the fornix
Body of the fornix

The front part of each bundle is called the column,

Columns
Columns

and the back part is called the crus.

Сrura
Сrura

The left crus and the right crus (pl. crura) are connected by the commissure of the fornix (also called the hippocampal commissure).

Сommissure of the fornix
Сommissure of the fornix

And the last structure – the thin translucent partition, called the septum pellucidum is stretched between the fornix and the corpus callosum.

Septum pellucidum
Septum pellucidum
Golosary

Cerebrum: Anatomy & Function

cerebral hemispheres
postcentral gyrus
longitudinal fissure of the cerebrum
cingulate gyrus
telencephalon / cerebrum
internal capsule
sulcus
external capsule
gyrus
extreme capsule
frontal lobe
posterior limb
temporal lobe
genu
parietal lobe
anterior limb
occipital lobe
corona radiata
insular lobe / insula
anterior commissure
operculum
posterior commissure
central (Rolandic) sulcus
corpus callosum
lateral (Sylvian) fissure
splenium
transverse cerebral fissure
trunk / body
parieto-occipital sulcus
genu
cingulate sulcus
rostrum
collateral sulcus
fornix
gray matter
body of fornix
white matter
column of fornix
cerebral cortex
crus of fornix
basal nuclei
commissure of fornix
precentral gyrus
septum pellucidum
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