Lobes of Cerebral Cortex

LOBES OF CEREBRAL CORTEX

In each hemisphere, there are three surfaces lateral, medial and inferior surfaces. Neocortex of each cerebral hemisphere consists of four lobes :

1. Frontal lobe

2. Parietal lobe

3. Occipital lobe

4. Temporal lobe.

Lobes of each hemisphere are demarcated by four main fissures and sulci:

1. Central sulcus or Rolandic fissure between frontal and parietal lobes2. Parieto-occipital sulcus between parietal and occipital lobe

3. Sylvian fissure or lateral sulcus between parietal and temporal lobes

4. Callosomarginal fissure between temporal lobe and limbic area.

CEREBRAL DOMINANCE

Cerebral dominance is defined as the dominance of one cerebral hemisphere over the other in the control

of cerebral functions. Both the cerebral hemispheres are not functionally equivalent. Some functional

asymmetries are well known.

CEREBRAL DOMINANCE AND HANDEDNESS

Cerebral dominance is related to handedness, i.e. preference of the individual to use right or left hand.

More than 90% of people are right handed. In these individuals, the left hemisphere is dominant and it

controls the analytical process and language related functions such as speech, reading and writing. Hence, left hemisphere of these persons is called dominant or categorical hemisphere. Right hemisphere is called representational hemisphere since it is associated with artistic and visuospatial

functions like judging the distance, determining the direction, recognizing the tones, etc. Lesion in dominant hemisphere leads to language disorders. Lesion in representational hemisphere causes

only mild effects like astereognosis. Left hemisphere is the dominant hemisphere in about 75% of the right-handed persons. In the remaining left-handed persons, right hemisphere controls the language function. Some of these persons do not have dominant hemisphere.

BRODMANN AREAS

Brodmann area is a region of cerebral cortex defined on the basis of its cytoarchitecture. Cytoarchitecture means organization of cells. Brodmann areas were originally defined and numbered in 1909 by Korbinian Brodmann depending upon the laminar organization of neurons in the cortex. Some of these areas were given specific names based on their functions. During the period of a century Brodmann areas had been extensively discussed and renamed.

FRONTAL LOBE OF CEREBRAL CORTEX

Frontal lobe forms one third of the cortical surface. It extends from frontal pole to the central sulcus and

limited below by the lateral sulcus. Frontal lobe of cerebral cortex is divided into two parts:

A. Precentral cortex, which is situated posteriorly

B. Prefrontal cortex, which is situated anteriorly.

PRECENTRAL CORTEX

Precentral cortex forms the posterior part of frontal lobe. It includes the lip of central sulcus, whole of

precentral gyrus and posterior portions of superior, middle and inferior frontal gyri. It also extends to the

medial surface. This part of cerebral cortex is also called excitomotor cortex or area, since the stimulation of different points in this area causes activity of discrete skeletal muscle. Precentral cortex is further divided into three functional areas :

1. Primary motor area

2. Premotor area

3. Supplementary motor area

Primary Motor Area

Primary motor area extends throughout the precentral gyrus and the adjoining lip of central sulcus. Areas 4 and 4S are present here.

Structure of primary motor area

Though this area has all the six layers, the granular layer is thin. Special structural feature of this layer is the presence of giant pyramidal cells called Betz cells in ganglionic layer.

Premotor Area

Premotor area includes areas 6, 8, 44 and 45. The premotor area is anterior to primary motor area in the

precentral cortex. The premotor area is concerned with control of postural movements by sending motor signals to axial muscles (muscles near the midline of the body).

Structure of premotor area

Premotor area is similar to primary motor area in structure except for the absence of giant pyramidal cells in ganglionic layer.

Supplementary Motor Area

Supplementary motor area is situated in medial surface of frontal lobe rostral to primary motor area.

Various motor movements are elicited by electrical stimulation of this area like raising the contralateral

arm, turning the head and eye and movements of synergistic muscles of trunk and legs.