Parietal Lobe

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.