Discuss briefly the physiological functions of oxytocin.

Outline:

·        Synthesis and release

·        Afferent pathway for release

·        Actions on:

- uterine smooth muscle

- myoepithelial cells

Essay:

            Oxytocin is known as the milk letdown factor. The primary role of oxytocin is to eject milk from the lactating mammary gland. Oxytocin is synthesized in the cell bodies of the paraventricular nucleus of the hypothalamus. After processing of the preprohormones, oxytocin is packaged together with a protein called neurophysin I in neurosecretory granules and transported down the axons to be stored in the posterior pituitary gland in terminal swellings called Herring bodies.

            Oxytocin is released when a nerve impulse is transmitted from the cell body in the hypothalamus down the axon, where it depolarizes the neurosecretory vesicles within the terminal Herring body. An influx of calcium into the neurosecretory vesicle then results in hormone secretion by exocytosis. During this process, oxytocin dissociates from neurophysin I and enters into the bloodstream by endocytosis into the endothelial cell and then by diffusion through pores in the fenestrated capillary endothelium.

            Oxytocin binds to a G protein-coupled serpentine receptor. Increases in calcium levels and in phosphatidylinositol products mediate oxytocin actions. The unique effect of oxytocin is to cause contraction of the myoepithelial cells of the alveoli of mammary glands. As a result, milk is forced from the alveoli into the ducts, from where it is evacuated by the infant. Estrogens augment and catecholamines block the action of oxytocin.

            Milk ejection is normally initiated by a neuroendocrine reflex. The receptors involved are the touch receptors, which are plentiful in the breast – especially around the nipple. Impulses generated in these receptors are relayed from the somatic touch pathways in the spinothalamic tract to the supraoptic and paraventricular nuclei. Discharge of the oxytocin-containing neurons causes secretion of oxytocin from the posterior pituitary. The infant suckling at the breast stimulates the touch receptors, the nuclei are stimulated, oxytocin is released, and the milk is expressed into the sinuses, ready to flow into the mouth of the waiting infant.

            Oxytocin causes contraction of the smooth muscle of the uterus by acting on them directly and indirectly by stimulating the formation of prostaglandins in the decidua. Prostaglandins exert a paracrine effect on the uterine muscles, increasing myometrial calcium levels and triggers uterine contractions.  The sensitivity of the uterine musculature to oxytocin is enhanced by estrogen and inhibited by progesterone. In late pregnancy, the uterus becomes very sensitive to oxytocin coincident with a marked increase in the number of oxytocin receptors and oxytocin receptor mRNA. Oxytocin secretion is increased during labor. After dilation of the cervix, descent of the fetus down the birth canal initiates impulses in the afferent nerves that are relayed to the supraoptic and paraventricular nuclei, stimulating oxytocin release.