Human Anatomy, Part 2 - K. A. Diubenko, A. K. Kolomiitsev, Yu. B. Chaikovskyi 2008

Special Section
Nervous system, systema nervosum - Special Section
Pallium - Functional areas of the cerebral cortex - Sensory areas of the cortex

The primary somatosensory cortex, or general sensory area, is located immediately posterior to the central sulcus of the Brain in the postcentral gyrus of the parietal lobe. It extends from the longitudinal fissure at the apex of the brain to the lateral sulcus. In Fig. 155, the general sensory area is designated as areas 1, 2, and 3. The primary somatosensory cortex receives sensory inputs from cutaneous, muscular, and visceral receptors from various PARTS OF THE body. Each point in this area receives sensory input from a specific part of the body; thus, a map of the entire body is represented in the postcentral gyrus (Fig. 155 A). This Localization of Functions in the postcentral gyrus of the parietal lobe is known as the sensory "homunculus". The receptive fields of human body parts are projected such that the representations of the lower trunk and lower limbs are located superiorly, followed by the face, fingers, hand, trunk, and thigh, while the leg and FOOT are represented on the Medial surface of the hemisphere, in the posterior part of the paracentral lobule. The anal and urogenital regions are also located here, whereas the receptive fields of the upper body and HEAD (Pharynx, Tongue, jaws) project to the lowermost Divisions of the postcentral gyrus. The size of the sensory area receiving stimuli from a body part does not depend on the physical size of that part, but rather on the density of receptors located within it. For example, the sensory cortex receives more impulses from the Lips and fewer from the chest. The primary function of the primary somatosensory cortex is the precise localization of where sensory perceptions arise. Although the thalamus can detect general localization because it receives sensory inputs from large areas of the body, it cannot distinguish precisely between specific fields of excitation. This precise discrimination is a characteristic function of the primary somatosensory cortex.

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Fig. 155. Functional areas of the brain (diagram). Dorsolateral surface of the right hemisphere

I - frontal lobe, lobus frontalis; II - parietal lobe, lobus parietalis; III - occipital lobe, lobus occipitalis; IV - temporal lobe, lobus temporalis; 1, 2, 3 - cortical areas of the primary sensory cortex; 5, 7, 39, 40 - cortical areas of the sensory association cortex; 4 - cortical areas of the primary motor cortex; 6 - premotor area; 8 - frontal eye field; 9, 10, 45, 11 - cortical areas of the frontal lobe; 44 - Broca's primary motor speech area; 43 - primary gustatory area; 41, 42 - Wernicke's primary auditory (association) area; 22 - auditory association area; 38, 21, 37 - cortical areas of the temporal lobe; 17 - primary visual area; 18, 19 - visual association area; 20 - visual association area

The secondary somatosensory area is a small area on the posterior wall of the lateral sulcus, aligned with the postcentral gyrus. For the most part, it is involved in the finer, discriminative aspects of sensation.

Posterior to the primary somatosensory area lies the somatosensory association area. It corresponds to areas 5 and 7 (Fig. 155). The somatosensory association area receives information from the thalamus and other brain regions located below the primary somatosensory cortex. Its role is to integrate and interpret sensory perceptions. This area makes it possible to determine the exact shape and texture of an object without looking at it; to estimate or compare THE POSITION OF one object relative to another; and to perceive The Relationship of one body part to another. Another role of the somatosensory association area is to store memories of past sensory experiences. Thus, you can compare current sensory perceptions with those previously acquired.

Other sensory areas of the Cerebral Cortex include:

1. Primary visual cortex (area 17). Located on the medial surface of the calcarine sulcus of the occipital lobe, occasionally extending near the lateral surface. It receives sensory inputs as impulses from the retina and decodes color, shape, and movement.

2. Visual association area (areas 18 and 19). Located in the occipital lobe. It receives sensory inputs as impulses from the primary visual cortex and the thalamus. The visual association area relates current visual experiences to past ones and evaluates them (the "evaluative" visual area) (area 18), which processes the signals received by area 17 into recognizable images. Patients with lesions in area 18 can see but cannot recognize objects.

3. Primary auditory cortex (areas 41 and 42). Located in the posterior part of the temporal lobe, inferior to the lateral sulcus. It consists of two or three transverse temporal gyri, gyri temporales transversi (Heschl's gyri)*—the cortical center of Hearing, situated on the superior temporal gyrus. It decodes basic sound characteristics such as pitch and rhythm. While the anterolateral portion of the auditory area is responsible for low pitches, the posterolateral portion is responsible for high pitches.

4. Auditory association area (area 22) (Wernicke's area)**. Located inferior to the primary auditory cortex within the auditory cortex. It becomes active when sound is received in the form of speech, music, or noise. It also decodes speech signals, translating words into sounds. Wernicke's area is believed to be responsible for the integration of sounds and the comprehension of language.

5. Primary gustatory cortex (area 43). Located at the Base of the postcentral gyrus, superior to the lateral sulcus in the parietal cortex. It decodes sensory perceptions related to taste.

6. Primary olfactory cortex. Located in the uncus. This area processes sensory perceptions related to smell.

7. The gnostic* area, represented by the association Zones of the parietal cortex (areas 5, 7, 39, and 40). This common integrative area is situated between the somatosensory, visual, auditory, and association areas. The gnostic area receives nerve impulses from these areas, as well as from the gustatory and olfactory areas, the thalamus, and the lower Brainstem. The gnostic area integrates sensory perceptions from the association areas and impulses from other areas so that a single, cohesive experience can be formed from various sensory signals. The area then transmits signals to other parts of the brain to generate an appropriate response to the sensory input.

* Richard L. Heschl (1824–1881) was an Austrian neurologist.

** Carl Wernicke (1848–1905) was a German psychiatrist. He described sensory aphasia. In 1874, he discovered the sensory speech center (Wernicke's area).

*** Gnosis (from Greek gnosis, meaning knowledge or recognition) refers to the cognitive function of perceiving and understanding various aspects: the shape, size, and meaning of objects, language comprehension, and the recognition of processes.