Love hormone | oxytocin

Oxytocin- The love hormone

Oxytocin is secreted mainly by paraventricular nucleus of hypothalamus. It is also secreted by supraoptic nucleus in small quantity and it is transported from hypothalamus to posterior pituitary through the nerve fibers of hypothalamo-hypophyseal tract.

In the posterior pituitary, the oxytocin is stored in the nerve endings of hypothalamo-hypophyseal tract.

When suitable stimuli reach the posterior pituitary from hypothalamus, oxytocin is released into the blood.

Oxytocin is secreted in both males and females.

Chemistry of oxytocin

Oxytocin is a polypeptide having 9 amino acids. It has a half-life of about 6 minutes.

Actions in Females of oxytocin hormone

In females, oxytocin acts on mammary glands and uterus.
Action of oxytocin on mammary glands 
Oxytocin causes ejection of milk from the mammary glands. Ducts of the mammary glands are lined by myo-epithelial cells.

Oxytocin causes contraction of the myo-epithelial cells and flow of milk from alveoli of mammary glands to the exterior through duct system and nipple.

The process by which the milk is ejected from alveoli of mammary glands is called milk ejection reflex or milk let-down reflex. It is one of the neuroendocrine reflexes.

Milk ejection reflex

Plenty of touch receptors are present on the mammary glands, particularly around the nipple.
When theinfant suckles mother nipple, the touch receptors are stimulated. The impulses discharged from touch receptors are carried by the somatic afferent nerve fibers to paraventricular and supraoptic nuclei of
hypothalamus.

Now hypothalamus, in turn sends impulses to the posterior pituitary through hypothalamo-hypophyseal tract. Afferent impulses cause release of oxytocin into the blood.

When the hormone reaches the mammary gland, it causes contraction of myoepithelial cells, resulting in ejection of milk from mammary glands.

As this reflex is initiated by the nervous factors
and completed by the hormonal action, it is called a neuroendocrine reflex. During this reflex, large amount of oxytocin is released by positive feedback mechanism.

Action on uterus of oxytocin

Oxytocin acts on pregnant uterus and also non-pregnant uterus.On pregnant uterus Throughout the period of pregnancy, oxytocin secretion is inhibited by estrogen and progesterone.

At the end of pregnancy, the secretion of these two hormones decreases suddenly and the secretion of oxytocin increases. Oxytocin causes contraction of uterus and helps in the expulsion of fetus.

During the later stages of pregnancy, the number of receptors for oxytocin increases in the wall of the uterus. Because of this, the uterus becomes more sensitive to oxytocin.

Oxytocin secretion increases during labor. At the onset of labor, the cervix dilates and the fetus descends through the birth canal.

During the movement of fetus through cervix, the receptors on the cervix are stimulated and start discharging large number of impulses.

These impulses are carried to the paraventricular and supraoptic nuclei of hypothalamus by the somatic afferent nerve
fibers. Now, these two hypothalamic nuclei secrete large quantity of oxytocin, which enhances labor by causing contraction of uterus.

Throughout labor, large quantity of oxytocin is
released by means of positive feedback mechanism, i.e. oxytocin induces contraction of uterus, which in turn causes release of more amount of oxytocin.

The contraction of uterus during labor is also a
neuroendocrine reflex. Oxytocin also stimulates the release of prostaglandins in the placenta.
Prostaglandins intensify the uterine contraction induced by oxytocin.

Effect of Oxytocin On non-pregnant uterus

The action of oxytocin on non-pregnant uterus is to facilitate the transport of sperms through female genital tract up to the fallopian tube, by producing the uterine contraction during sexual intercourse.

During the sexual intercourse, the receptors in the vagina are stimulated. Vaginal receptors generate the impulses, which are transmitted by somatic afferent nerves to the paraventricular and supraoptic nuclei of hypothalamus.

When, these two nuclei are stimulated,
oxytocin is released and transported by blood. While reaching the female genital tract, the hormone causes antiperistaltic contractions of uterus towards the fallopian tube. It is also a neuroendocrine reflex.

Sensitivity of uterus to oxytocin is accelerated by estrogen and decreased by progesterone.

Action of oxytocin in Males

In males, the release of oxytocin increases during ejaculation. It facilitates release of sperm into urethra by causing contraction of smooth muscle fibers in reproductive tract, particularly vas deferens.

Mode of Action of Oxytocin

Oxytocin acts on mammary glands and uterus by activating G-protein coupled oxytocin receptor.