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.
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