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in humans and other mammals, an innate unconditioned reflex that is formed in the fetus during intrauterine development.
The structural and functional mechanisms of sucking develop long before birth, in the early stages of embryogeny. In an 18-week-old human fetus sucking movements of the lips are observed upon the stimulation of the tongue. In the 21st or 22nd weeks spontaneous sucking occurs, and in the 24th week there are distinct seeking and sucking reactions. The sucking function is characteristic even of anencephalics.
Stimulation of the tactile receptors of the lips and the anterior part of the tongue results in the spread of the excitation along the centripetal branches of the trigeminal nerve. The impulses enter the sucking center, which is located in the truncal part of the brain, and are switched to centrifugal nerves, along which the excitation is transmitted to the muscles that perform the sucking (masticatory muscles and the muscles of the lips, mouth, and tongue). The act of sucking begins with the movement of the tongue and lower jaw downward and back, which creates a negative pressure in the oral cavity. The contraction of the lip and mouth muscles causes the squeezing of the nipple of the mother’s mammary gland. The nerve impulses that arise during this process enter the hypothalamus, which the impulses reflexively stimulate to form vasopressin and oxytocin, as well as a releasing factor. The releasing factor enters the pituitary gland and induces the secretion of prolactin, which controls lactation. Oxytocin is transported by the blood to the myoepithelial cells of the mammary gland, inducing the contraction of the glands, the release of milk from the alveoli into the mammary ducts, and the secretion of milk.
REFERENCESShuleikina, K. V. “Fiziologicheskaia kharakteristika sosa-tel’noi funktsii novorozhdennogo v pervye dni ego zhizni.” In Trudy Per-vogo Vserossiiskogo s“ezda detskikh vrachei. Moscow, 1961.
Barabash-Nikiforov, I. I., and A. N. Formozov. Teriologiia. Moscow, 1963.
Villee, C, and V. Dethier. Biologiia: biologicheskie protsessy i zakony. Moscow, 1974. Pages 545–46. (Translated from English.)