Evolution of the Mammalian Brain

Evolution of the Mammalian Brain. From the perspective of the evolution of the brain, a progressive increase in its mass, volume, and complexity of neocortical organization is characteristic. In the course of evolution, the archicortex and paleocortex are displaced towards the cerebral commissure. In animals of different orders, a gradual formation of folding can be traced. Mammals are characterized by several types of brain organization. The lissencephalic type of brain organization is characteristic of monotremes (platypus). With this type of organization, the central sulcus, which divides the cortical structures into frontal and parietal parts, is constantly present. The paleocortex quantitatively dominates, and the neocortex occupies a relatively smaller area of cortical structures. A distinctive feature of this type of organization is the absence of the corpus callosum. In the lissencephalic brain, there are overlapping projections of different sensory modalities between cortical fields. Neurons overlap and receive information from different channels. Motor and sensory areas overlap in the cortex. In the diencephalon, the differentiation of thalamic nuclei deepens: a group is observed that in insectivores forms an associative group of nuclei. In the cortex, the area of α-motoneurons, which regulate the α-motoneurons of the spinal cord, is identified, leading to the formation of the neocerebellum in the cerebellum, which is involved in the regulation of motor activity. In marsupials, the same brain organization is preserved. The gyrencephalic type of brain organization is characteristic of insectivores and rodents. With this type of organization, the corpus callosum appears, the area of the neocortex increases, and sulci and gyri are evident. In insectivores, two polysensory zones are formed – analogues of associative areas. In rodents, frontal and parietal areas are morphologically distinguished, but they do not yet function as associative zones. This function is performed by several neocortical fields of different localization. Also, in this group of animals, there is a clear specialization of sensory zones for specific sensory modalities, i.e., they no longer overlap. In rodents, another variant of visual information projection appears (originally, the main switching was retino-tectal (monotremes, marsupials, insectivores)) – retino-thalamic. It is characterized by the fact that the main selection and processing of information is carried out by the geniculate bodies of the thalamus. In rodents, formed thalamo-cortical connections are observed, but their functional differentiation is relatively weak. The deuto-gyrencephalic type of brain organization is characteristic of predatory mammals. In predators, in addition to constant sulci and gyri, significant folding of the neocortex develops. Differentiated zones are clearly distinguished: special projection zones (process information from the sensory system), motor zones, and well-formed associative zones, represented in the parietal and frontal lobes. Cortical associative zones are part of two thalamo-cortical associative systems:

The thalamo-parietal system is responsible for components of higher nervous system functions: it participates in the formation of orienting reflexes, forms a representation of the body's spatial schema, and is involved in short-term memory mechanisms.

The thalamo-frontal system is connected with structures of the limbic system. It forms the acceptor of action (compares at the level of cortical forms how well the result corresponds to the adaptation).

The hominid type of brain organization is characterized in higher primates by a six-layered neocortex. All functional zones of the cortex do not overlap with each other, forming three integrative systems:

The thalamo-parietal system, which includes fields 5 and 7 of the associative cortex, performs the following functions: initiating the orienting reaction, forming short-term memory, and sensing the body's spatial schema.

The thalamo-frontal system provides assessment of the situation: the acceptor of the result of action, provides the mechanism for long-term memory, and the formation of fine temporal connections as a result of behavioral actions (higher forms of inhibition).

The thalamo-temporal integrative system provides higher regulation of vegetative functions from the cortex through integrative centers of the central nervous system, primarily the hypothalamus, amygdala complex, and basal ganglia.

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