PARIETAL LOBE
Parietal lobe extends from
central sulcus and merges with occipital lobe behind and temporal lobe below.
This lobe is separated from occipital lobe by parieto-occipital sulcus and from
temporal lobe by Sylvian sulcus.
Parietal lobe is divided
into three functional areas:
A. Somesthetic area I
B. Somesthetic area II
C. Somesthetic association
area.
In addition to these three
areas, a part of sensory motor area is also situated in parietal lobe.
SOMESTHETIC AREA I
Somesthetic area I is also
called somatosensory area I or primary somesthetic or
primary sensory area. It
is present in the posterior
lip of central sulcus, in the postcentral gyrus and in the paracentral lobule.
Areas of Somesthetic
Area I
Somesthetic area I has
three areas, which are called areas 3, 1 and 2. Anterior part of this forms
area 3 and posterior part forms areas 1 and 2.
Connections of
Somesthetic Area I
Somesthetic
area I receives sensory fibers from thalamus via parietal part of thalamic
radiation.
Localization –
Homunculus
Different sensory areas of
the body are represented in postcentral gyrus (primary sensory area) in an inverted manner as in the
motor area. Toes are represented in lowest part of medial surface, legs at the
upper border of hemispheres, then from above downwards knee, thigh, hip, trunk,
upper limb, neck and face. Representation of face is not inverted.
Representation of parts of face from above downwards is eyelids, nose, cheek, upper
lip and lower lip.
Functions of
Somesthetic Area I
1. Somesthetic area I is
responsible for perception and integration of cutaneous and kinesthetic
sensations. It receives sensory impulses from cutaneous receptors (touch, pressure,
pain, temperature) and proprioceptors of opposite side through thalamic radiation.
Area 1 is concerned with sensory perception. Areas 3 and 2 are involved in the
integration of these sensations.
2. This area sends sensory
feedback to the premotor area
3. This area is also
concerned with the movements of head and eyeballs
4. Discriminative functions:
In addition to perception of cutaneous and kinesthetic sensation, this area is
also responsible for
recognizing the discriminative features of sensations.
Discriminative functions
are:
i. Spatial recognition:
Tactile localization, two point discrimination and recognition of position
and passive movements of
limbs
ii. Recognition of
intensity of different stimuli
iii. Recognition of similarities
and differences between the stimuli.
Effect of Stimulation
of Somesthetic Area I
Electrical stimulation of
somesthetic area I produces vague sensations like numbness and tingling.
Effects of Lesion of
Somesthetic Area I
If lesion occurs only in
the sensory area without involvement of thalamus, the sensations are still
perceived.
But, the discriminative functions
are lost. If thalamus also is affected by lesion, there is loss of sensations
in the opposite side of the body.
SOMESTHETIC AREA II
Somesthetic area II is
situated in postcentral gyrus below the area of face of somesthetic area I. A part
of this is buried in Sylvian
sulcus. It is also known as secondary somesthetic area or somatosensory area
II.
Functions of
Somesthetic Area II
Somesthetic area II
receives sensory impulses from somesthetic area I and from thalamus directly.
Though the exact role of this area is not clear, it is concerned with
perception
But, the discriminative functions
are lost. If thalamus also is affected by lesion, there is loss of sensations
in the opposite side of the body.
SOMESTHETIC AREA II
Somesthetic area II is
situated in postcentral gyrus below the area of face of somesthetic area I. A
part
of this is buried in
Sylvian sulcus. It is also known as secondary somesthetic area or somatosensory
area II.
Functions of
Somesthetic Area II
Somesthetic area II receives
sensory impulses from somesthetic area I and from thalamus directly. Though the
exact role of this area is not clear, it is concerned with perception of sensation.
Thus, the sensory parts of body have two representations, in somesthetic area I
and area II.
SOMESTHETIC
ASSOCIATION AREA
Somesthetic association
area is situated posterior to postcentral gyrus, above the auditory cortex and
in front of visual cortex. It has two areas, 5 and 7.
Functions of
Somesthetic Association Area
Somesthetic association area
is concerned with synthesis of various sensations perceived by somesthetic
area I. Thus, the somesthetic
association area forms the center for combined sensations like stereognosis.
Lesion of this area causes astereognosis.
Sensory Motor Area
Sensory area of cortex is
not limited to postcentral gyrus in parietal lobe. It extends anteriorly into
motor area in precentral gyrus of frontal lobe. Similarly, the motor area is
extended from precentral gyrus posteriorly into postcentral gyrus.
Thus, the precentral and
postcentral gyri are knit together by association neurons and are functionally
inter-related. So, this
area is called sensory motor area. Function of sensory motor area is to store
the timing and programming of various sequential movements of complicated
skilled movements, which are planned by neocerebellum.
APPLIED PHYSIOLOGY
Lesion or ablation of
parietal lobe (sensory cortex)
results in the following
disturbances:
1. Contralateral
disturbance of cutaneous sensations
2. Disturbances in
kinesthetic sensations
3. Loss of tactile localization and
discrimination.
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