Cerebral cortex

Lobes of the cerebral cortex

The cerebral cortex is divided into six lobes based on the organization of major sulci. Each lobe has gyri that contain neuronal cell bodies involved in specific functions. Four of these lobes, the frontal, parietal, temporal and occipital lobes take their names from the overlying cranial bones. The insular lobe is located deep to the lateral sulcus, while the limbic lobe is located on the medial aspect of the hemisphere. The boundaries of these lobes are defined by major sulci that separate one region of the cerebral cortex from the other.

The major sulci on the lateral surface of each cerebral hemisphere are the lateral sulcus (Sylvian fissure) which separates the temporal lobe from the frontal and parietal lobes, and the central sulcus which divides the frontal and parietal lobes. Notable sulci on the medial surface of the cerebral hemisphere include the parietooccipital sulcus which separates the parietal and occipital lobes, the cingulate sulcus which separates the limbic lobe from the frontal and parietal lobes, and the collateral sulcus which separates the limbic lobe from the temporal lobe. The insular lobe, which lies at the base of the lateral sulcus, is separated from the surrounding frontal, parietal, and temporal lobes by the circular sulcus of the insula.

Histology of the cerebral cortex

Microscopically, the cerebral cortex is composed of cell bodies of billions of neurons, their dendrites, myelinated and unmyelinated axons which altogether form a unique, multilayered arrangement. In addition, it contains a dense population of supporting glial cells which include oligodendrocytes, astrocytes, microglia, and ependymal cells, and blood vessels. The neuronal cells of the cortex consist of six main cell types. These are the pyramidal cells (the main output neurons of the cerebral cortex), fusiform cells, stellate (granular) cells, basket cells, horizontal cells of Cajal-Retzius and cells of Martinotti. Histologically, the cerebral cortex is organized into six layers or horizontal laminae based on the size and shape of the neuronal bodies. These layers are designated by Roman numerals, and from superficial (at the pial surface) to deep, they are the:

• Molecular (plexiform) layer (I): Contains mainly nerve axons and a few scattered horizontal cells of Cajal-Retzius
• External granular layer (II): Composed of a varying density stellate (granular) cells and pyramidal cells
• External pyramidal layer (III): Contains predominantly pyramidal cells of varying sizes
• Internal granular layer (IV): Consists mostly of the stellate cells and a smaller portion of the pyramidal cells. It is usually the narrowest layer
• Internal pyramidal layer (V): Contains mainly medium-sized to large pyramidal cells
• Multiform (fusiform) layer (VI): Composed of different kinds of neuron types, mostly fusiform cells with less dominant pyramidal cells and interneurons

Columnar organization of the cerebral cortex

The cerebral cortex can also be functionally divided into vertical formations that are called columns. These represent the functional units of the cortex and are capable of memorizing relations and performing more complex operations than a single neuron. Each column is oriented perpendicular to the cortical surface and it consists of all of the 6 cellular layers. Neurons are tightly connected inside one column, but also share connections with the adjacent and distant columns, and with the subcortical structures as well, particularly the thalamus.

Functional areas of the cerebral cortex

Additionally, the cerebral cortex can be divided into three functional areas: primary, secondary and associative. The cortical areas responsible for the elementary functions of either motor or sensory are primary areas. Secondary areas are located around every primary area and receive afferent projections from the corresponding primary areas and the thalamus. They are responsible for integrating the raw signal from the primary areas with the information received from the thalamus, to refine the primary area stimuli. Association areas, on the other hand, are cortical areas that integrate, process and analyze different kinds of stimuli that reach the brain and are involved in mediating higher mental functions.

Phylogenetic types of cortex

Different regions of the cerebral cortex can be categorized based on evolutionary history and relationships into three groups, the allocortex, mesocortex, and neocortex (isocortex).

• The allocortex is the most ancient type of cortex and is composed of the archicortex and the paleocortex. It consists of three cellular layers: polymorphic, pyramidal and the molecular layer, and is associated with the limbic system. The paleocortex contains three to five layers of cells and mediates the sense of smell.
• The mesocortex is a transitional form between the allocortex and isocortex. It contains three to six layers and is found in the insula, cingulate and parahippocampal gyri.
• The neocortex, as its name suggests, is the most recent cortical region and makes up to 90% of the human cortex. It includes all of the lobes of the cortex except the limbic lobe and consists of six layers of cells or laminae.

Check out our videos, articles, illustrations and quizzes in this study unit to learn more about the histology of the cerebral cortex:

Brodmann areas

The cerebral cortex is divided into smaller areas structurally by sulci based on its histological structure and cellular organization. The resulting smaller regions are known as Brodmann areas. There are a total of 52 areas which have been shown to have distinct neuronal organization and have also been correlated to various cortical functions.

The Brodmann areas can sometimes be challenging to understand! Take advantage of the videos, articles, illustrations and quizzes included in this study unit to learn the material from different perspectives.

Frontal lobe

The frontal lobe is the largest lobe of the cortex and contains four main gyri located on its lateral surface: the precentral gyrus, superior, middle and the inferior frontal gyri. The precentral gyrus lies between the central and precentral sulci. It contains the primary motor cortex (Brodmann area 4), and is involved in integrating signals from different brain regions to modulate voluntary movement to the body and limbs on the contralateral side.

The superior frontal gyrus lies anterior to the precentral sulcus while the middle frontal gyrus lies between the superior and the inferior frontal sulci. The inferior frontal gyrus lies in the inferolateral surface of the frontal lobe below the inferior frontal sulcus, which separates it from the middle frontal gyrus. It is divided into three parts, a pars opercularis, pars triangularis, and pars orbitalis. In the dominant hemisphere, this region contains the Broca’s speech area and is important in the motor component of speech production.

Parietal lobe

The parietal lobe contains the postcentral gyrus, superior and inferior parietal lobules. The postcentral gyrus is known as the somatosensory cortex (Brodmann area 3,1,2). It lies between the central and postcentral sulci and is the region of the cerebral cortex that receives and integrates sensory information related to touch and the sensing of body position and movement. The inferior parietal lobule (Brodmann area 39,40) consists of two gyri, the supramarginal gyrus and the angular gyrus. The supramarginal gyrus is located superior to the posterior aspect of the lateral fissure. The angular gyrus lies just posterior to the supramarginal gyrus. Both gyri are involved in auditory and visual perception and form part of Wernicke’s area, which is a functional unit of the cerebral cortex that plays an important role in the comprehension of spoken language.

The superior parietal lobule (Brodmann area 5,7) is separated from the inferior parietal lobule by the intraparietal sulcus. This region is involved in integrating sensory and motor functions and provides input to the premotor cortex.

Temporal lobe

The temporal lobe consists of the superior, middle and inferior temporal gyri, separated by superior and inferior temporal sulci. The superior temporal gyrus contains a specialized region called the transverse temporal gyri of Heschl, which is also known as the primary auditory area (Brodmann area 41, 42). This region of the cerebral cortex is primarily responsible for the reception of auditory information. Posterior to the primary auditory area in the superior temporal gyrus, lies the auditory association area (Brodmann area 22), which is responsible for the interpretation of sounds and for the association of the auditory input with other sensory information. The posterior part of this region, in the dominant hemisphere (usually the left), contributes to Wernicke’s area, which is important in the comprehension of language. The middle temporal gyrus is associated with the perception of movement within the visual field, while the inferior temporal gyrus plays a role in facial recognition.

Occipital lobe

The occipital lobe forms the caudal aspect of the cerebral hemisphere. Its lateral surface often exhibits significant variability in the sulci resulting in either two or three occipital gyri (superior, middle and inferior). On the medial surface of the occipital lobe, the calcarine sulcus separates the cuneus which lies above the sulcus from the lingual gyrus below. The regions of both gyri adjacent to the calcarine sulcus form the primary visual cortex (Brodmann area 17), which is responsible for the integration and perception of visual information.

Insular lobe

The insular lobe is buried deep to the lateral fissure. It contains a group of short gyri in its rostral region called short gyri (gyri breves) and another group of long gyri in its caudal region called long gyri (gyri longi). These sets of gyri are separated from each other by the central sulcus of the insula. The insular cortex is involved in receiving and processing and integration of various types of information, including taste sensation, visceral sensation, pain sensation, and vestibular function.

Limbic lobe

The limbic lobe is a ring of cortex on the medial aspect of each hemisphere that surrounds the corpus callosum. It forms part of the larger limbic system which is involved in emotional and behavioral expression. The limbic lobe is composed of the subcallosal, cingulate, and parahippocampal gyri.

• The subcallosal gyrus lies below the rostrum of the corpus callosum. This area also corresponds with sections of Brodmann areas 24, 25, and 32, and is believed to be involved in depression.
• The cingulate gyrus lies directly above the corpus callosum. It continues anteriorly around the genu of the corpus callosum, blending with the subcallosal gyrus. Posteriorly, the cingulate gyrus wraps around the splenium of the corpus callosum. It is separated from the corpus callosum by callosal sulcus, while the cingulate sulcus separates it from the frontal and parietal lobes. The cingulate gyrus is involved in modulating emotion, as well as in visceral motor processes.
• The parahippocampal gyrus lies between the parahippocampal sulcus and the collateral sulcus which continues anteriorly as the rhinal sulcus. It contains the hippocampal formation and extends anteromedially as the uncus which contains the amygdala. The parahippocampal gyrus provides a path of communication between the hippocampus and all cortical association areas through which afferent impulses enter the hippocampus.

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