Primary+Somesthetic+Receiving+Area

Structure
Consists of three narrow strips of tissue (areas 3ab, 1, 2) which differ histologically, in architectural composition, and in sensory input. Each of these areas maintains a complete and independent representation of the body **Area 3a:** Receives input from the muscles spindles (group IA muscle afferents) and can also signal muscle length (e.g. flexion or extension)
 * Area 3b:** Receives cutaneous stimuli.

Area 3 appears to maintain cutaneous and muscular maps of the body. However, almost all of the cells in area 3ab receive input only from the contralateral half of the body. Hence, only half the body is represented.

These are semi-independent, and in some respects they do not directly correspond to the location of body parts along the body surface, but instead are organized in regard to those parts which most frequently interact. That is, certain body parts more greatly represented in accordance with their sensory importance.

Information received and processed in area 3 is relayed to the immediately adjacent areas 1, and 2; each of which also contain a specialized spatial map of the body.
 * Area 1:** Maintains an overlapping cutaneous-joint body map
 * Area 2:** Maintains a map of the joint receptors and can signal the position and posture of the limbs based on input from the muscle spindles.

Degree of activation is also dependent on the attentional state and degree of arousal. With minimal attention to the source of input, there is minimal activation, which is why sensations from the clothing, shoes, or while sitting, lying down, and so on, can rapidly fade from consciousness. Together these four strips of tissue comprise an interactional functional unit and are responsive to touch, texture, shape, motion, and the direction of stimulus movement, including temporal-sequential patterning, and can directly monitor the position and movement of the extremities. Many cells are also responsive to changes in temperature as well as the presence of noxious stimuli applied to the skin.

Because the majority of these neurons receive input concerning pressure, light touch, vibration, the movement of joints, and muscular activity they can signal and determine whatever posture or position the body is in as well as the amount of force or pressure being exerted by the limbs, i.e. if carrying or lifting some object. Conversely, via the reception and analysis of this input an individual can detect an insect crawling up or down their leg, the direction it is moving, as well as determine the position of their arms and legs without looking at them. Nevertheless, predominantly elementary and simple contralateral somesthetic information is processed in this region.

Electrical Stimulation
Simple, albeit well localized sensations on the opposite half of the body such as numbness, pressure, tingling, itching, tickling and warmth.

Body Image Representation
The primary receiving areas for somesthesis continues up and over the top of the hemisphere and along the medial wall where the lower half of the body is represented. **Medial wall:** rectum, genitals, foot and calf
 * Superior surface:** Leg
 * Lateral Convexity:** Shoulder, arm, hand and then face

Body parts are also represented in terms of their sensory importance, i.e. how richly the skin is innervated. Example: more cortical space is devoted to the representation of the mouth, fingers and the hand than to the elbow or trunk.

In fact, the area devoted to representation of the fingers is 100 times larger than the area devoted to the trunk. Because of this the cortical body map is very disorted. However, some areas are also juxtaposed, such as the hand and mouth area.

Functional Laterality
There is clear evidence that the //right parietal area// is dominant in regard to many aspects of somesthetic information processing. Neurons in this half of the brain appear to be more sensitive and more responsive and to more greatly monitor events occuring on either half of the body, but particularly the left. The left half of the body exceeds the right in regard to most forms of tactual sensitivity. The left hand and the soles of the left foot, as well as the left shoulder are more accurate in judging weight, have a more delicate sense of touch and temperature. The left hand judges warm substances to be hotter, and cold material to be colder as compared to the right hand, even when both hands are simultaneously stimulated. The right parietal lobe appears to have more neocortical space devoted to maintaining images of the body.

**Somesthetic Agnosias**
Surgical or other forms of destruction involving the primary somesthetic receiving areas results in a complete, albeit, temporary loss of sensation from the entire half of the body. If the lesion involves just the hand, or leg area, then sensation only from the hand or leg will no longer be perceived.

-Elevation of sensory detection thresholds -Loss of position and pressure sense: two-point discrimination and reduced ability to detect movement of the fingers. -Capacity to determine texture, shape, temporal-sequential patterning -Capacity to recognize objects by touch or to discriminate among different forms or their properties, e.g. size, texture, length, shape -Stereognosis is significantly attenuated -Passive, (non-movement) sensation is less impaired. -Motor disturbances such as paresis with hypotnonia, and/or produce inaccuracy and reduced speed of movement -Ability (or will) to initiate movement may also be reduced (d/t connections with primary motor area)
 * Depending on the extent of the lesion, the effects may include:**

In some instances, over time a remarkable recovery of somesthetic discrimination sense may be observed. Nevertheless, even with complete removal of the post-central gyrus, stimuli applied to the face are much better perceived as compared to the same stimuli applied to the hand. This is because more area is devoted to the face than the hand, whereas more area is devoted to the hand than to the legs.

Lesions to the parietal lobe which spare the hand area of the post-central gyrus, but which destroy the remaining tissue, will result in mild or no permanent sensory deficits when the hands are tested (with the exception of stereognosis). The patient will continue to experience sensations from their hand, but not from the rest of the body. Sensations from across other body parts will not be perceived. Hence, when testing for parietal lobe dysfunction, not only the face and hands, but other body parts should be examined.