What Is the Primary Somatosensory Cortex?

The sensory cortex is a representative area of the cerebral cortex. The sensory projection of this area is characterized by upside down and left and right crossing. That is, the sensations of the head and face are projected to the lower part of the central back, the sensations of the lower limbs are projected to the upper part of this area, and the somatic sensations on one side are shot to the corresponding representative areas of the contralateral cortex. In addition. The size of the sensory representative area is related to the sensory sensitivity of different parts of the body. The sensory representative areas of the hands, lips, and mouth are larger than the sensory representative areas of the body. The proprioceptive sensation represents the central back. The visual representation area is on both sides of the occipital fissure. The right occipital lobe is related to the left visual field of the two eyes, the left occipital lobe is related to the right visual field of the two eyes, and the lateral occipital cortex is injured causing the contralateral blindness. The auditory representative area is in the lateral and superior temporal gyrus of the temporal lobe. The projection of auditory impulses is bilateral. Damage to the representative area of one cortex will not affect hearing. The representative area of smell is in the hippocampus. The representative area of the sense of taste is at the lower side of the central back looking facial projection area. Visceral sensation when the representative area is on the edge. Feeling cold, warm, painful, and so on, the body surface feels like the center is back. The deterministic force method of the cortical sensory representative area is the subjective report of each part of the brain that is electrically stimulated by the subject, and the brain area that generates the evoked potential is determined by the observation of a certain part of the body. The functional positioning of sensations is not absolute, and some sensations can also be caused in other cortical areas. ^[1]

The formation of sensory cortical functional structures depends on the congenital inheritance and the shaping of external stimuli at certain stages of the day after tomorrow. For example, in the first week after the kitten is born, one eyelid is sutured until 1 to 3 months later, and then it is found that only a few cells in the visual cortex respond to the eye's stimulus, and the feeling of these neurons Wild anomalies. Some kittens wear glasses that can vertically shift the field of vision of their eyes when they are reared. After the critical period of development, these kittens have been able to determine their orientation with glasses, but they will stop moving and remove their movements. Unstable. If the above two tests are after the critical period of cat development, such as four weeks later, the above two results will not appear. It can be seen that the development and maturity of animal sensory systems have certain stages, and they have strong stability after maturity. Of course, there will still be a certain degree of plasticity after maturity, but it is mainly further improved in acquired activities to better adapt to the requirements of the external environment. ^[2]

The sensory cortex of humans and other mammals belongs to the neocortex of the brain, and its six-layer structure can be clearly distinguished by Neissl staining. The fourth layer receives sensory information from the sensory nerve nucleus of the thalamus, including vision. , Auditory and somatosensory. All kinds of sensory information pass through the final sensory relay station of the thalamus to reach the cerebral cortex and enter the field of consciousness. The areas of the cerebral cortex that directly receive thalamic visual, auditory, and somatosensory uploads are called the primary visual cortex, primary auditory cortex, and primary somatosensory cortex, respectively, and are located in the central posterior gyrus of the occipital lobe, upper temporal lobe, and parietal lobe, respectively. Other areas of the cerebral cortex are often referred to as associate cortex. The contact cortex regions adjacent to the primary sensory cortex regions also process sensory information and are more integrated. It is worth noting that the study found that the contact cortex not only receives information from other parts of the cortex but also the sensory afferents of the thalamus, so some researchers have called these cortical areas "additional sensory areas".

Sensory neurons in the cerebral cortex are organized into clusters in structure and function, which researchers call cortical columns. Cortical column is a basic functional unit for encoding and processing sensory information. Its axial direction is perpendicular to the surface of the cortex. Its height is equivalent to the thickness of cortical gray matter, about 2 to 5 rnlTl, and its diameter is about 0.1 to o. 5 rnm.

Each cortical column contains about 105 neurons. The neurons in the same cortical column form a close neural connection with each other and have the same functional characteristics. They respond similarly to a specific type and specific sensory area. For example, in the somatosensory cortex, some cortical column neurons respond similarly to skin tactile stimuli on the palm of the left hand, while other cortical column neurons respond similarly to joint activity in the right leg. Neurons in a cortical column in the auditory cortex respond to sound stimuli of a certain frequency. Neurons in a cortical column in the visual cortex only respond to bright rods (gratings) in a particular orientation; neurons in another adjacent cortical column only respond to gratings in another orientation, so they are It is called the visual cortical azimuth column. ^[3]

In the ventral (outer, inner) nucleus of the thalamus, the afferent fibers are arranged in an orderly manner according to the body surface source, and there is a topological projection relationship with each part of the body surface, that is, the body positioning relationship, and it continues to project to the somatosensory cortex. on. In the somatosensory cortex, the size of the representative area of the body surface is allocated according to the sensitivity of the skin to the physical sensation, such as the face and hands, especially the lips and fingers. Figure 14-5 is the body surface projection intention of the human primary somatosensory cortex. The central posterior gyrus (cortex 3, 1, and 2) is the primary somatosensory cortex. According to its coronal section, the order of the projection regions on the body surface is an inverted malformed human hand. And the face is extra large. ^[4]

Electrical stimulation of the central gyrus in humans can produce a sense of Qi on the opposite side of the body (Penfield and: Rasmussen, 1950). The sensations experienced are relatively thick, described as numbness, tingling, tactile sensation, or feeling like a part of the body removed, with very little pain. The reaction zone also has the kind of body positioning seen in the cortical motor zone, which causes facial sensation when stimulating the lower back of the center and leg sensation when stimulating the upper part. However, observations in humans failed to confirm the cortical nodularity demonstrated by Woolsey, Marshall, and Bard (1942) in monkeys. There is some evidence that humans accept three main divisions into faces, hands, and feet, as Liddell and Phillip (1950) found in the cat's cortical motion zone. Dusser de Barenne (1924) placed strychnine in a small area of the sensory cortex of monkeys, recorded the range of electrical changes, and found that the electrical changes spread to all corresponding communities, but not to other large areas. . For example, it does not spread from the face area to the hand area. Post-central gyre resection does not bring us any new knowledge, but confirms the conclusions made in studies of evoked potentials and the effects of direct stimulation of the central gyrus. ^[5]