Age-related features of the brain

The brain of a newborn is relatively large, its average weight is 390 g (340-430 g) in boys and 355 g (330-370 g) in girls, which is 12-13% of the body weight (in adults, approximately 2.5%). The brain weight in relation to the body weight of a newborn is 5 times greater than that of an adult, and is determined by the ratio 1:8 (in an adult, this ratio is 1:40). By the end of the 1st year of life, the brain weight doubles, and by 3-4 years it triples. Subsequently (after 7 years), the brain weight increases slowly and by 20-29 years it reaches its maximum value (1355 g in men and 1220 g in women). In subsequent age periods, up to 60 years in men and 55 years in women, the brain weight does not change significantly, and after 55-60 years, some decrease is noted.

In a newborn, the phylogenetically older parts of the brain are better developed. The brainstem weighs 10.0-10.5 g, which is approximately 2.7% of the body weight (in an adult, about 2%), and the cerebellum weighs 20 g (5.4% of the body weight). By 5 months of life, the cerebellum weighs 3 times, by 9 months - 4 times (the child can stand and begins to walk). The cerebellar hemispheres develop most intensively. The telencephalon in a newborn is also relatively well developed. The frontal lobe of the cerebrum is strongly convex and relatively small. The temporal lobe is high. The insular lobe (islet) is located deep. Up to 4 years of life, the child's brain grows evenly in height, length and width. Subsequently, the brain grows in height. The frontal and parietal lobes grow most rapidly.

In a newborn, there are already grooves and convolutions on the surface of the cerebral hemispheres. The main grooves (central, lateral, etc.) are well expressed, and the branches of the main grooves and small convolutions are weakly expressed. Later, as the child ages, the grooves become deeper, the convolutions between them are more prominent. Myelination of nerve fibers in phylogenetically older parts of the brain begins and ends earlier than in newer parts. In the cerebral cortex, nerve fibers that conduct various types of sensitivity (general), as well as those that communicate with the subcortical nuclei, are myelinated earlier. Myelination of afferent fibers begins at about 2 months and ends by 4-5 years, and efferent fibers somewhat later, in the period from 4-5 months to 7-8 years.

The relationships of the grooves and convolutions with the bones and sutures of the skull roof in a newborn are somewhat different than in an adult. The central groove is located at the level of the parietal bone. The inferolateral part of this groove is 1.0-1.5 cm cranial to the squamous suture. The parieto-occipital groove lies 12 mm anterior to the lambdoid suture. The relationships of the grooves, convolutions of the brain and sutures characteristic of an adult are established in children aged 6-8 years.

The corpus callosum in a newborn is thin and short, since simultaneously with the development and enlargement of the cerebral hemispheres, the corpus callosum grows predominantly in the cranial and caudal directions, being located above the cavity of the diencephalon (above the third ventricle). As the hemispheres develop, the thickness of the trunk of the corpus callosum (up to 1 cm in an adult) and the splenium of the corpus callosum (up to 2 cm) increases, which is due to the increase in the number of commissural nerve fibers.

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