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Physiology of the cerebral cortex of the cerebral hemisphere

The large, or terminal, brain is one of the complex organs of man. The functions of this part of the central nervous system are significantly different from the functions of the stem and spinal cord. They form the basis of the physiology of higher nervous activity. Under the higher nervous activity I.P. Pavlov meant behavior, activity aimed at the adaptability of the body to changing environmental conditions, at equilibrium with the environment. I.P. Pavlov with his studies proved not only the reflex activity of the cerebral cortex, but also discovered a qualitatively new higher type of reflexes - conditioned reflexes. It was further found that conditioned reflexes are elementary acts that form the behavior of humans and animals. At the same time, it was found that damage to the cortex of the hemispheres in the experiment leads to an irrevocable loss of the acquired reactions developed in the process of individual life - conditioned reflexes. The morphological confirmation of the physiology and clinic data was the doctrine of the different-quality structure of the cerebral cortex of the cerebral hemispheres in different parts of it - cyto and myeloarchitectonics of the cortex. As a result of a detailed study, special maps of the cerebral cortex were created, reflecting the totality of the cortical ends and analyzers.

The analyzer is a nerve mechanism consisting of a receptor receptor apparatus, nerve impulse conductors and a brain center, where all those irritations that come from the environment and the human body are analyzed. Various analyzers are closely interconnected, in connection with this, analysis and synthesis take place in the cortex, development of responsible reactions that regulate all types of human activity. It is known that in the cerebral cortex there is a core and scattered elements occupying a certain area. The cerebral cortex is a collection of cores of various analyzers, between which are scattered elements of different adjacent analyzers.

So, in accordance with the cytoarchitectonic maps of the cerebral hemispheres in humans, it is possible to determine the cortical ends of various analyzers (nuclei) relative to the convolutions and lobes of the hemispheres (Fig. 141).

Fig. 141.

Cytoarchitectonic fields of the cerebral cortex

(according to Broadman, 1925):

A is the lateral surface; B - medial surface; numbers - cortical fields

In the cortex of the postcentral gyrus and the superior parietal lobe there are nerve cells that form the core of the cortical analyzer of general sensitivity (temperature, pain, sensory) and proprioceptive. Sensitive pathways leading from the cerebral cortex have a cross at the level of the spinal cord and medulla oblongata. As a result, the postcentral gyrus of each hemisphere is associated with the opposite half of the field. In the postcentral gyrus, all receptor fields of different parts of the body have their own projection onto the region of the cortical end of the analyzer of general sensitivity. The core of the motor analyzer is located mainly in the motor region of the cortex and is localized in the precentral gyrus, which lies in front of the central (Roland) groove. It is known that in the upper parts of the precentral gyrus and in the paracentral lobe there is a nucleus, the impulses from which go to the muscles of the lower parts of the trunk and limbs.

In the lower part of the precentral gyrus is the core of the motor analyzer, which regulates the activity of the facial muscles.

In the posterior sections of the middle frontal gyrus there is the analyzer core, which provides the function of the conjugate rotation of the head and eyes in the opposite direction.

In the region of the inferior parietal lobe, there is the core of the motor analyzer, the function of which is the implementation of all targeted complex movements.

In the cortex of the superior parietal lobule is the core of the skin analyzer, one of the types of sensitivity that has the function of knowing the subject by touch.
The cortical ends of this analyzer are located in the right and left hemispheres. Damage to these areas of the cortex leads to loss of recognition of objects when feeling.

The core of the visual analyzer is located on the medial surface of the occipital lobe. Receptors of the lateral half of the retina of the left eye and the medial half of the retina of the right eye are projected respectively in the cortex of the occipital lobe of the left hemisphere. The defeat of the visual analyzer leads to complete loss of vision or visual memory with impaired ability to navigate in an unfamiliar space.

The core of the auditory analyzer is located in the upper edge of the temporal lobe. The pathways from the receptors of both the left and right sides go to it. In this case, a unilateral damage to the nucleus does not cause a complete loss of the ability to perceive sounds. In case of bilateral lesion, “cortical deafness” is observed. On the lower surface of the temporal lobe of the cerebral hemispheres, in the area of ​​the hook (end of the parahippocampal gyrus) is the core of the odor analyzer.

Senses of smell and taste are closely interconnected, which is explained by the close location of their analyzers. The nuclei of the hemisphere analyzers are associated with receptors on both the left and right sides of the body.

In the back section of the middle frontal gyrus is the core of the motor writing analyzer. Damage to this area leads to the loss of precise movements when writing letters and numbers.

The core of the motor analyzer of articulation of speech is located in the rear sections of the lower frontal gyrus (Brock's center). The defeat of this zone leads to a loss of motor ability of the muscles that are involved in speech formation. In the lower frontal gyrus is the core of the language analyzer, which is associated with singing; its damage causes the loss of memorization of musical phrases. In the upper part of the temporal gyrus lies the core of the cortical analyzer, the defeat of which leads to musical deafness.

The projection zones of the cortex occupy a small portion of the surface of the cortex of the cerebral hemispheres of the human brain in comparison with associative zones, which have no close connection with either the sensory organs or muscles, they communicate between different regions of the cortex; integrate, unite all impulses entering the cortex into integral acts of learning (reading, language, writing), logical thinking, memory and provide the possibility of a targeted reaction of behavior.

In case of violations of associative zones, agnosia appears - the inability to cognize and apraxia - the inability to perform learned movements. For example, if the outer surface of the occipital lobe - the associative zone of vision - is damaged, visual agnosia is observed, the patient is not able to read the text, to recognize a familiar person. In case of violation of the associative zones of speech of the cerebral cortex, aphasia is possible - loss of speech. Aphasia can be sensory and motor.

Sensory aphasia (Wernicke aphasia) is characterized by a violation of the understanding of oral speech while maintaining the possibility of repeating what was said by another person; observed in lesions of the pathways between the back and middle of the superior temporal gyrus (Wernicke area) and other areas of the cerebral cortex. Motor aphasia occurs when the posterior third of the lower frontal gyrus on the left (Brock's center) is affected: the patient understands the speaker’s speech, but cannot speak.
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Physiology of the cerebral cortex of the cerebral hemisphere

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