Anatomy and Physiology for Dummies Pdf Download

 OXYTOCIN

Source and Synthesis of Oxytocin

Oxytocin is an oligopeptide containing nine amino acids. The hormone is synthesized in magnocellular neurons of hypothalamus. Oxytocin is mainly synthesized in the paraventricular cells of hypothalamus though synthesis also occurs in supraoptic nucleus.

1. After synthesis, oxytocin is secreted into the posterior pituitary where it is stored. 

2. The precursor molecule is prepro-oxyphysin that contains oxytocin, neurophysin I and a glycoprotein. Otherwise, the steps of synthesis and secretion of oxytocin are similar to that of ADH, as described above.

Regulation of Secretion of Oxytocin

Oxytocin secretion occurs in response to two important physiological stimuli: 

1. Suckling at the time of breastfeeding, and

2. Cervical dilatation at the time of parturition. 

Genital stimulation in females during coitus increases oxytocin release. Stressful stimuli facilitate oxytocin release. Oxytocin secretion is inhibited by alcohol. 

Anatomy and Physiology for Dummies Pdf Download

Functions of Oxytocin

Oxytocin mediates two physiological reflexes:

 1. milk ejection reflex

 2. parturition reflex.

Milk Ejection Reflex 

Discharge or expulsion of milk from the breast of the mother into mouth of the baby when baby suckles during breastfeeding is called milk ejection reflex.

Receptors

Tactile receptors in and around the nipple.

Reflex Arc

Nipple is stimulated by suckling of the baby: 

1. The afferent neural impulses are transmitted from the nipple to the spinal cord and from there via spinothalamic tract to the brainstem. 

2. In the midbrain, the information reaches paraventricular nucleus of hypothalamus via collaterals arising from the ascending sensory pathway. The neurons terminating on hypothalamus are cholinergic. 

3. These fibers stimulate oxytocin secreting magnocellular neurons of hypothalamus. 

4. Via hypothalamopituitary axis, oxytocin is released from posterior pituitary and enters circulation. 

5. Oxytocin causes contraction of myoepithelial cells of the milk-laden alveoli of the mammary gland. Therefore, milk is forced from the alveoli into the ducts and from there to the mouth of the infant.

Multiple Pregnancies and deliveries: Types, Diagnosis, Complications,

Effects

Due to initiation of this reflex, milk from the mother’s breast is forcefully discharged into the mouth of the baby. Therefore, the reflex is called milk ejection reflex.

Physiological Significance

This is an example of neurohumoral reflex. Ovulation that occurs during mating in some animals like rabbit is also an example of neurohumoral reflex.

A neurohumoral reflex: Normally, a reflex is purely a neural phenomenon, the reflex arc having neurons in both afferent and efferent limbs. When a hormone forms part of the efferent limb, the reflex becomes a neurohumoral reflex. Thus, the neurohumoral reflex involves both neurons and hormones in the reflex pathway. Therefore, milk ejection by oxytocin is a neurohumoral reflex.

Parturition Reflex

Oxytocin causes contraction of uterine muscle:

1. Toward term, oxytocin secretion gradually increases and reaches its peak just before parturition. Also, number of oxytocin receptor increases in the uterine muscle. 

2. Due to increased plasma level of oxytocin and increased sensitivity of uterus to oxytocin, uterus contracts vigorously leading to expulsion of the fetus. 

3. Thus, oxytocin initiates and completes parturition. Parturition reflex is also a neurohumoral reflex.

Other functions of oxytocin are as follows:

1. Oxytocin receptors are also present in the ovary. It is believed that oxytocin facilitates degeneration of corpus luteum at the end of the menstrual cycle.

2. Oxytocin secretion increases in males during orgasm, though the exact physiological significance of it is not known. At the time of ejaculation in male, possibly oxytocin causes contraction of smooth muscles of vas deferens that helps in propelling sperms toward the urethra. 

3. In females, following ejaculation of sperms into female genital tract, oxytocin causes contraction of nonpregnant uterus to facilitate sperm transport into fallopian tubes. Though sperm transport to the fallopian tube for fertilization depends on motility of sperm, uterine contraction facilitates the process

Applied Physiology

.

Oxytocics (preparations of oxytocin) are used routinely 

for:

1. Induction of labor, if labor pain does not start effectively at term. 

2. Oxytocic infusion is given to facilitate the progress of labor. 

3. It is routinely injected immediately following delivery of placenta, to prevent excessive postpartum hemorrhage, in which uterus contracts severely in response 

to oxytocic and bleeding vessels are compressedin the 

contracted uterus that prevents bleeding.

Reference Comprehensive Textbook of Medical Physiology.