Human Anatomy - Kotsan I. Y. 2009

Brain
Localization of Functions in the Cerebral Cortex

Experimental research data indicate that impairment or removal of specific areas of the Cerebral Cortex in animals leads to the disruption of certain Functions. These facts are further supported by clinical observations of human patients with tumors or traumatic injuries affecting specific Regions of the cerebral cortex. All of this has led to the Conclusion that centers regulating various functions are located within the cerebral cortex. I. P. Pavlov viewed the cerebral cortex as a continuous receptive surface and a totality of cortical ends of analyzers. The term "analyzer" refers to a complex system of anatomical structures consisting of a receptor-perceptive apparatus, Nerve Impulse pathways, and a cerebral center where all stimuli originating from the environment or The Human Body are analyzed.

Due to the close interconnections among various analyzers, the cerebral cortex carries out analysis, synthesis, and the generation of appropriate responses that regulate diverse bodily activities.

The cortical end of an analyzer is not a strictly demarcated zone; rather, the cerebral cortex distinguishes between a core (Nucleus) and scattered elements. The core is the concentration site of cortical Nerve Cells that form a precise projection of all elements of a given peripheral receptor, where higher-level analysis, synthesis, and functional integration take place. Scattered elements may be located both at the periphery of the core and at a considerable distance from it, mediating simpler forms of analysis and synthesis. The presence of scattered elements partially compensates for impaired functions when the core is damaged. The zones of scattered elements belonging to different analyzers lack clear boundaries and may overlap one another.

The most important centers (cores) of the cortical ends of analyzers have the following localization.

The core of the cortical analyzer for proprioceptive and general sensitivity (Temperature, pain, tactile, and Muscle-joint Senses) is located in the cortex of the postcentral gyrus and the superior parietal lobule. Because sensory pathways cross over in the Spinal Cord and partially in the Medulla Oblongata, each postcentral gyrus is connected with the opposite half of the body. Within the postcentral gyrus, the receptor fields of various body parts are projected such that the highest-located cortical ends of the sensitivity analyzers correspond to the lower regions of the trunk and lower limbs, whereas the lowest projections (closer to the lateral sulcus) correspond to the upper regions of the trunk, HEAD, and upper limbs.

Because in animals the general sensitivity receptors are particularly well-developed on the head, specifically around the Mouth—which played a major role in food acquisition—humans have likewise retained a high level of development in oral receptors. Consequently, the area corresponding to these receptors occupies a very large zone in the postcentral gyrus. At the same time, with the Evolution of the human hand as an organ of labor, the number of tactile receptors in the Skin of the hand has dramatically increased, turning it into a primary organ of tactile sensitivity. Accordingly, the cortical areas corresponding to the receptors of the upper limb are much larger than those corresponding to the lower limb receptors. Therefore, if one were to map a human figure onto the postcentral gyrus upside down (with the head toward the base of The Skull and the feet toward the upper edge of the hemisphere), one would need to draw a large face with a disproportionately large mouth, a large hand—especially the thumb, which is vastly enlarged compared to the other digits—a small trunk, and a tiny leg.

In the cortex of the superior parietal lobule, near the general sensory center, lies the center for stereognosis (a specialized form of cutaneous sensitivity that enables the recognition of objects by Touch). For the right upper limb, the core of this analyzer is located in the left hemisphere, and for the left upper limb, in the right hemisphere.

The lower PARTS OF THE precentral and postcentral gyri contain the center for interoceptive sensitivity (the analysis of impulses originating from Internal Organs and Blood Vessels). Afferent impulses from internal organs, blood vessels, involuntary musculature, and Skin glands arrive at this cortical region, which in turn gives rise to efferent pathways directed toward subcortical autonomic centers.

The core of the motor analyzer is located primarily within the so-called "motor area of the cortex," which encompasses the precentral gyrus and the paracentral lobule. This region of the cerebral cortex controls all human motor activity and The formation of conscious motor responses.

In the motor cortex, the human body is also projected upside down. That is, the upper regions of the precentral gyrus and the paracentral lobule contain cells whose impulses travel to the Muscles of the lower trunk and lower limbs, whereas the lower part of the precentral gyrus houses the motor centers that regulate the muscles of the head and face.

Because the Pyramidal Tracts—which originate from large pyramidal cells (layers V and partially VI of the cortex)—cross over either at the Brainstem level (corticonuclear fibers) or within the spinal cord segments (corticospinal tract), the motor areas of each hemisphere are connected with the skeletal muscles of the opposite side of the body. The limb muscles are unilaterally connected to one hemisphere, whereas the Muscles of the Trunk, Larynx, and Pharynx are connected to the motor areas of both hemispheres.

The core of the motor analyzer responsible for conjugated (synchronous) Rotation of the head and eyes in the opposite direction is located in the posterior regions of the middle frontal gyrus, within the so-called premotor zone.

The core of the motor analyzer that coordinates human movements associated with professional skills or complex acquired habits is situated in the inferior parietal lobule within the supramarginal gyrus. In right-handed individuals, this analyzer is located in the left cerebral hemisphere; in left-handed individuals, it is found in the right hemisphere. Damage to this area preserves basic movement capabilities, but abolishes The ability to perform complex coordinated movements, resulting in a condition known as apraxia (from praxis, meaning action).

The core of the static analyzer—responsible for head position and movement—has not yet been precisely mapped in the cerebral cortex. It is believed that the vestibular apparatus is projected within the temporal lobe, specifically in the region of the middle and inferior temporal gyri. Lesions of the static analyzer core lead to ataxia, manifested as balance disturbances and body sway during standing. This analyzer plays a decisive role in human bipedalism and is of particular importance for jet pilots, as the sensitivity of the vestibular apparatus is significantly reduced under flight conditions.

The core of the Auditory Analyzer is located in the middle section of the superior gyrus of the temporal lobe, On the surface facing the insula. This area houses Neurons that perceive and analyze impulses originating from the cochlea of the Inner ear. Nerve cells comprising the auditory analyzer core of each hemisphere receive pathways from both the left and right sides. Consequently, unilateral damage to this core does not result in total Hearing loss, whereas bilateral damage leads to cortical deafness.

The core of the visual analyzer is located in the occipital lobe. The optic tract terminates on the inner surface of the occipital lobe on both sides of the calcarine sulcus, where the retina is projected. Specifically, the visual analyzer core of the right hemisphere is connected via pathways to the lateral half of the right eye's retina and the medial half of the left eye's retina, while the core of the left hemisphere is connected to the lateral half of the left eye's retina and the medial half of the right eye's retina. Just as with the auditory analyzer, only bilateral destruction of the visual analyzer cores results in complete cortical blindness.

The core of the olfactory analyzer is localized on the Inferior surface of the temporal lobe, in the region of the uncus and partially the hippocampus. From a phylogenetic standpoint, these areas belong to the most ancient parts of the cerebral cortex.

The core of the gustatory (taste) analyzer is believed by some researchers to reside in the lower part of the postcentral gyrus, close to the centers for mouth and Tongue muscles, while others locate it in the region of the uncus, adjacent to the cortical end of the olfactory analyzer—a proximity that accounts for the close association between olfactory and gustatory sensations. The gustatory and olfactory analyzer cores are connected with receptors from both the left and right sides.

The Anatomical Features of the cortical speech centers are characterized, firstly, by the fact that in over 95% of right-handed individuals, speech is controlled by the left hemisphere, and in the remaining 5% by the right hemisphere. The majority of left-handed individuals (about 70%) also have their speech zones in the left hemisphere. Secondly, the cortical speech centers lack direct connections with peripheral receptors. Impulses reach the cortical ends of the speech analyzers exclusively from the cortical ends of the visual and auditory analyzers. Spoken language is based upon the auditory analyzer, whereas written language is based upon the visual analyzer.

Spoken language manifests as the ability to understand the meaning of words and conversational phrases (sensory speech) and to speak (motor speech).

The core of the auditory speech analyzer is located in the posterior part of the superior temporal gyrus, adjacent to and closely connected with the general auditory analyzer. Impulses enter the speech-auditory cortex exclusively from the general auditory cortex. The function of this core is not only to enable a person to hear and comprehend another's speech, but also to monitor their own. Damage to this core does not impair general auditory sound perception, but abolishes the ability to comprehend words and spoken language (sensorimotor aphasia).

The core of the motor speech analyzer (speech articulation) is localized in the posterior part of the inferior frontal gyrus (Broca's area). Damage to this area abolishes the capacity for articulate speech, although voice production remains intact; it also results in agrammatism—the inability to construct sentences from words or to grammatically coordinate words within a sentence. The speech of such patients consists of a disjointed, meaningless collection of words.

Written language manifests as the ability to write (motor speech) and to read (sensory speech).

The core of the motor written-language analyzer (the analyzer of movements associated with writing letters and other symbols) is situated in the posterior section of the middle frontal gyrus and is closely linked with the general motor analyzer. Damage to this core does not cause a generalized motor deficit, but specifically results in the loss of ability to perform the precise, fine movements required for writing letters, symbols, and words (graphical agraphia).

The core of the visual written-language analyzer is located in the angular gyrus of the inferior parietal lobule and is strongly connected with the cortical end of the general visual analyzer (the calcarine sulcus of the occipital lobe). Lesions of this core result in the loss of the ability to recognize letters, rendering the individual unable to read (alexia).