Pituitary Gland Introduction .
[1]. Pituitary gland or hypophysis is a small endocrine gland with a diameter of 1 cm and weight of 0.5 to 1 gm .
[2]. It is situated in a depression called ‘Sella turcica’, present in the sphenoid bone at the base of skull. [3]. It is connected with the hypothalamus by the pituitary stalk or hypophyseal stalk.
Divisions of Pituitary Gland .
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| Parts of pituitary gland . |
Pituitary gland is divided into two divisions:
1. Anterior pituitary or adenohypophysis .
2. Posterior pituitary or neurohypophysis.
[1]. Both the divisions are situated close to each other. Still both are entirely different in their development, structure and function.
[2]. Between the two divisions, there is a small and relatively avascular structure called pars intermedia. Actually, it forms a part of anterior pituitary.
Development of Pituitary Gland .
[1]. Both the divisions of pituitary glands develop from different sources.
[2]. Anterior pituitary is ectodermal in origin and arises from the pharyngeal epithelium as an upward growth known as Rathke pouch.
[3]. Posterior pituitary is neuroectodermal in origin and arises from hypothalamus as a downward diverticulum.
[4]. Rathke pouch and the downward diverticulum from hypothalamus grow towards each other and meet in the midway between the roof of the buccal cavity and base of brain. There, the two structures lie close together.
Regulation of Secretion .
Hypothalamo-hypophyseal Relationship .
[1]. The relationship between hypothalamus and pituitary gland is called Hypothalamo-hypophyseal relationship.
[2]. Hormones secreted by hypothalamus are transported to anterior pituitary and posterior pituitary. But the mode of transport of these hormones is different.
[3]. Hormones from hypothalamus are transported to anterior pituitary through hypothalamo-hypophysial portal blood vessels.
[4]. But, the hormones from hypothalamus to posterior pituitary are transported by nerve fibers of hypothalamo-hypophyseal tract (see below for details).
Anterior Pituitary or Adenohypophysis .
Anterior pituitary is also known as the master gland because it regulates many other endocrine glands through its hormones.
Parts of Anterior Pituitary .
Anterior pituitary consists of three parts .
1. Pars distalis .
2. Pars tuberalis.
3. Pars intermedia.
Histology of Anterior Pituitary .
Anterior pituitary has two types of cells, which have different staining properties:
1. Chromophobe cells .
2. Chromophil cells.
Chromophobe Cells .
[1]. Chromophobe cells do not possess granules and stain poorly. These cells form 50% of total cells in anterior pituitary.
[2]. Chromophobe cells are not secretory in nature, but are the precursors of chromophil cells.
Chromophil Cells .
Chromophil cells contain large number of granules and are darkly stained.
Types of chromophil cells .
Chromophil cells are classified by two methods.
1. Classification on the basis of staining property:
Chromophil cells are divided into two types:
[1] . Acidophilic cells or alpha cells, which form 35% .
[2] . Basophilic cells or beta cells, which form 15%.
2. Classification on the basis of secretory nature:
Chromophil cells are classified into five types:
[1] . Somatotrophs, which secrete growth hormone .
[2] . Corticotropes, which secrete adrenocorticotropic hormone .
[3] . Thyrotropes, which secrete thyroid-stimulating hormone (TSH) .
[4]. Gonadotropes, which secrete follicle-stimulating hormone (FSH) and luteinizing hormone (LH) .
[5]. Lactotropes, which secrete prolactin. Somatotropes and lactotropes are acidophilic cells, whereas others are basophilic cells.
[6]. Somatotropes form about 30% to 40% of the chromophil cells. So, pituitary tumors that secrete large quantities of human growth hormone are called acidophilic tumors.
Regulation of Anterior Pituitary Secretion .
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| Blood supply to pituitary gland . |
[1]. Hypothalamus controls anterior pituitary by secreting the releasing and inhibitory hormones (factors), which are called neurohormones.
[2]. These hormones from hypothalamus are transported anterior pituitary through hypothalamo-hypophyseal portal vessels.
[3]. Some special nerve cells present in various parts hypothalamus send their nerve fibers (axons) to median eminence and tuber cinereum.
[4]. These nerve cells synthesize the hormones and release them into median eminence and tuber cinereum.
[5]. From here, the hormones are transported by blood via hypothalamo-hypophyseal portal vessels to anterior pituitary.
Releasing and Inhibitory Hormones Secreted by Hypothalamus .
[1]. Growth hormone-releasing hormone (GHRH): Stimulates the release of growth hormone .
[2]. Growth hormone-releasing polypeptide (GHRP): Stimulates the release of GHRH and growth hormone .
[3]. Growth hormone-inhibitory hormone (GHIH) or somatostatin: Inhibits the growth hormone release .
[4]. Thyrotropic-releasing hormone (TRH): Stimulates the release of thyroid stimulating hormone .
[5]. Corticotropin-releasing hormone (CRH): Stimulates the release of adrenocorticotropin .
[6]. Gonadotropin-releasing hormone (GnRH): Stimulates the release of gonadotropins, FSH and LH .
[7]. Prolactin-inhibitory hormone (PIH): Inhibits prolactin secretion. It is believed that PIH is dopamine.
Hormone Secreted by Anterior Pituitary .
Six hormones are secreted by the anterior pituitary:
1. Growth hormone (GH) or somatotropic hormone (STH)
2. Thyroid-stimulating hormone (TSH) or thyrotropic hormone
3. Adrenocorticotropic hormone (ACTH)
4. Follicle-stimulating hormone (FSH)
5. Luteinizing hormone (LH) in females or interstitial cell-stimulating hormone (ICSH) in males
6. Prolactin. Recently, the hormone β-lipotropin is found to be secreted by anterior pituitary.
Tropic Hormones .
[1]. First five hormones of anterior pituitary stimulate the other endocrine glands.
[2]. Growth hormone also stimulates the secretory activity of liver and other tissues. Therefore, these five hormones are called tropic hormones.
[3]. Prolactin is concerned with milk secretion.
Gonadotropic Hormones .
Follicle-stimulating hormone and the luteinizing hormone are together called gonadotropic hormones or gonadotropins because of their action on gonads.
Growth Hormone .
Source of Secretion of Growth hormone .
Growth hormone is secreted by Somatotropes which are the acidophilic cells of anterior pituitary.
Chemistry, Blood Level and Daily Output .
[1]. GH is protein in nature, having a single-chain polypeptide with 191 amino acids.
[2]. Its molecular weight is 21,500.
[3]. Basal level of GH concentration in blood of normal adult is up to 300 g/dL and in children, it is up to 500 ng/ dL.
[4]. Its daily output in adults is 0.5 to1.0 mg.
Transport of Growth hormone .
Growth hormone is transported in blood by GH-binding proteins (GHBPs).
Half-life and Metabolism .
Half-life of circulating growth hormone is about 20 minutes. It is degraded in liver and kidney.
Actions of Growth Hormone .
[1]. GH is responsible for the general growth of the body.
[2]. Hypersecretion of GH causes enormous growth of the body, leading to gigantism.
[3]. Deficiency of GH in children causes stunted growth, leading to dwarfism.
[4]. GH is responsible for the growth of almost all tissues of the body, which are capable of growing. It increases the size and number of cells by mitotic division.
[5]. GH also causes specific differentiation of certain types of cells like bone cells and muscle cells.
[6]. GH also acts on the metabolism of all the three major types of foodstuffs in the body, viz. proteins, lipids and carbohydrates.
1. On metabolism .
GH increases the synthesis of proteins, mobilization of lipids and conservation of carbohydrates.
a. On protein metabolism .
GH accelerates the synthesis of proteins by:
1. Increasing amino acid transport through cell membrane:
The concentration of amino acids in the cells increases and thus, the synthesis of proteins is accelerated.
2. Increasing ribonucleic acid (RNA) translation:
GH increases the translation of RNA in the cells . Because of this, ribosomes are activated and more proteins are synthesized. GH can increase the RNA translation even without increasing the amino acid transport into the cells.
3. Increasing transcription of DNA to RNA:
It also stimulates the transcription of DNA to RNA. RNA, in turn accelerates the synthesis of proteins in the cells .
4. Decreasing catabolism of protein:
GH inhibits the breakdown of cellular protein. It helps in the building up of tissues.
5. Promoting anabolism of proteins indirectly:
GH increases the release of insulin (from β-cells of islets in pancreas), which has anabolic effect on proteins.
b. On fat metabolism .
[1]. GH mobilizes fats from adipose tissue. So, the concentration of fatty acids increases in the body fluids.
[2]. These fatty acids are used for the production of energy by the cells. Thus, the proteins are spared. [3]. During the utilization of fatty acids for energy production, lot of acetoacetic acid is produced by liver and is released into the body fluids, leading to ketosis.
[4]. Sometimes, excess mobilization of fat from the adipose tissue causes accumulation of fat in liver, resulting in fatty liver.
c. On carbohydrate metabolism .
Major action of GH on carbohydrates is the conservation of glucose.
Effects of GH on carbohydrate metabolism :-
[1]. Decrease in the peripheral utilization of glucose for the production of energy:
1. GH reduces the peripheral utilization of glucose for energy production.
2. It is because of the formation of acetyl-CoA during the metabolism of fat, influenced by GH.
3. The acetyl-CoA inhibits the glycolytic pathway. Moreover, since the GH increases the mobilization of fat, more fatty acid is available for the production of energy.
4. By this way, GH reduces the peripheral utilization of glucose for energy production.
[2]. Increase in the deposition of glycogen in the cells:
1. Since glucose is not utilized for energy production by the cells, it is converted into glycogen and deposited in the cells.
[3]. Decrease in the uptake of glucose by the cells:
1. As glycogen deposition increases, the cells become saturated with glycogen.
2. Because of this, no more glucose can enter the cells from blood. So, the blood glucose level increases.
[4]. Diabetogenic effect of GH:
1. Hypersecretion of GH increases blood glucose level enormously.
2. It causes continuous stimulation of the β-cells in the islets of Langerhans in pancreas and increase in secretion of insulin.
3. In addition to this, the GH also stimulates β-cells directly and causes secretion of insulin. Because of the excess stimulation, β-cells are burnt out at one stage.
4. This causes deficiency of insulin, leading to true diabetes mellitus or full-blown diabetes mellitus. This effect of GH is called the diabetogenic effect.
2. On bones .
[1]. In embryonic stage, GH is responsible for the differentiation and development of bone cells.
[2]. In later stages, GH increases the growth of the skeleton. It increases both the length as well as the thickness of the bones.
In bones, GH increases:
1. Synthesis and deposition of proteins by chondrocytes and osteogenic cells
2. Multiplication of chondrocytes and osteogenic cells by enhancing the intestinal calcium absorption
3. Formation of new bones by converting chondrocytes into osteogenic cells
4. Availability of calcium for mineralization of bone matrix.
[3]. GH increases the length of the bones, until epiphysis fuses with shaft, which occurs at the time of puberty.
[4]. After the epiphyseal fusion, length of the bones cannot be increased. However, it stimulates the osteoblasts strongly. So, the bone continues to grow in thickness throughout the life.
[5]. Particularly, the membranous bones such as the jaw bone and the skull bones become thicker under the influence of GH.
[6]. Hypersecretion of GH before the fusion of epiphysis with the shaft of the bones causes enormous growth of the skeleton, leading to a condition called gigantism.
[7]. Hypersecretion of GH after the fusion of epiphysis with the shaft of the bones leads to a condition called acromegaly.
Mode of Action of GH – Somatomedin .
[1]. GH acts on bones, growth and protein metabolism through somatomedin secreted by liver.
[2]. GH stimulates the liver to secrete somatomedin.
[3]. Sometimes, in spite of normal secretion of GH, growth is arrested (dwarfism) due to the absence or deficiency of somatomedin.
Somatomedin .
[1]. Somatomedin is defined as a substance through which growth hormone acts.
[2]. It is a polypeptide with the molecular weight of about 7,500.
Types of somatomedin .
Somatomedins are of two types:
1. Insulin-like growth factor-I (IGF-I), which is also called somatomedin C
2. Insulin-like growth factor-II.
Somatomedin C (IGF-I) acts on the bones and protein metabolism.
Insulin-like growth factor-II plays an important role in the growth of fetus.
Duration of action of GH and somatomedin C .
[1]. GH is transported in blood by loose binding with plasma protein. So, at the site of action, it is released from plasma protein rapidly.
[2]. Its action also lasts only for a short duration of 20 minutes.
[3]. But, the somatomedin C binds with plasma proteins very strongly. Because of this, the molecules of somatomedin C are released slowly from the plasma proteins. Thus, it can act continuously for a longer duration.
[4]. The action of somatomedin C lasts for about 20 hours.
Mode of action of somatomedin C .
Somatomedin C acts through the second messenger called cyclic AMP .
Growth hormone receptor .
[1]. GH receptor is called growth hormone secretagogue (GHS) receptor.
[2]. It is a transmembrane receptor, belonging to cytokine receptor family.
[3]. GH binds with the receptor situated mainly in liver cells and forms the hormone receptor complex. [4]. Hormone-receptor complex induces various intracellular enzyme pathways, resulting in somatomedin secretion.
[5]. Somatomedin in turn, executes the actions of growth hormone.
Regulation of GH Secretion .
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| Regulation of GH secretion . |
[1]. Growth hormone secretion is altered by various factors.
[2]. However, hypothalamus and feedback mechanism play an important role in the regulation of GH secretion GH secretion is stimulated by:
1. Hypoglycemia .
2. Fasting .
3. Starvation .
4. Exercise .
5. Stress and trauma .
6. Initial stages of sleep.
[3]. GH secretion is inhibited by:
1. Hyperglycemia .
2. Increase in free fatty acids in blood .
3. Later stages of sleep.
Role of hypothalamus in the secretion of GH .
Hypothalamus regulates GH secretion via three hormones:
1. Growth hormone-releasing hormone (GHRH):
It increases the GH secretion by stimulating the Somatotropes of anterior pituitary
2. Growth hormone-releasing polypeptide (GHRP):
It increases the release of GHRH from hypothalamus and GH from pituitary
3. Growth hormone-inhibitory hormone (GHIH) or somatostatin:
It decreases the GH secretion. Somatostatin is also secreted by delta cells of islets of Langerhans in pancreas. These three hormones are transported from hypothalamus to anterior pituitary by hypothalamo-hypophyseal portal blood vessels.
Feedback control .
[1].GH secretion is under negative feedback control .
[2]. Hypothalamus releases GHRH and GHRP, which in turn promote the release of GH from anterior pituitary. GH acts on various tissues.
[3]. It also activates the liver cells to secrete somatomedin C (IGF-I). Now, the somatomedin C increases the release of GHIH from hypothalamus.
[4]. GHIH, in turn inhibits the release of GH from pituitary. Somatomedin also inhibits release of GHRP from hypothalamus.
[5]. It acts on pituitary directly and inhibits the secretion of GH .
[6]. GH inhibits its own secretion by stimulating the release of GHIH from hypothalamus. This type of feedback is called short-loop feedback control.
[7]. Similarly, GHRH inhibits its own release by short-loop feedback control. Whenever, the blood level of GH decreases, the GHRH is secreted from the hypothalamus. It in turn causes secretion of GH from pituitary.
Role of ghrelin in the secretion of GH .
[1]. Ghrelin is a peptide hormone synthesized by epithelial cells in the fundus of stomach.
[2]. It is also produced in smaller amount in hypothalamus, pituitary, kidney and placenta .
[3]. Ghrelin promotes secretion of GH by stimulating Somatotropes directly.
Other Hormones of Anterior Pituitary .
Thyroid-stimulating Hormone (TSH) .
TSH is necessary for the growth and secretory activity of the thyroid gland. It has many actions on the thyroid gland.
Adrenocorticotropic Hormone (ACTH) .
ACTH is necessary for the structural integrity and the secretory activity of adrenal cortex. It has other functions also.
Follicle-stimulating Hormone (FSH) .
[1]. Follicle-stimulating hormone is a glycoprotein made up of one α-subunit and a β-subunit.
[2]. The α-subunit has 92 amino acids and β-subunit has 118 amino acids.
[3]. The half-life of FSH is about 3 to 4 hours.
Actions of FSH .
In males, FSH acts along with testosterone and accelerates the process of spermiogenesis .
In females FSH:
1. Causes the development of Graafian follicle from primordial follicle
2. Stimulates the theca cells of Graafian follicle and causes secretion of estrogen .
3. Promotes the aromatase activity in granulosa cells, resulting in conversion of androgens into estrogen .
Luteinizing Hormone (LH) .
LH is a glycoprotein made up of one α-subunit and one β-subunit. The α-subunit has 92 amino acids and β-subunit has 141 amino acids. The half-life of LH is about 60 minutes.
Actions of LH .
[1]. In males, LH is known as interstitial cell-stimulating hormone (ICSH) because it stimulates the interstitial cells of Leydig in testes.
[2]. This hormone is essential for the secretion of testosterone from Leydig cells .
In females, LH:
1. Causes maturation of vesicular follicle into Graafian follicle along with follicle-stimulating hormone .
2. Induces synthesis of androgens from theca cells of growing follicle .
3. Is responsible for ovulation .
4. Is necessary for the formation of corpus luteum .
5. Activates the secretory functions of corpus luteum.
Prolactin .
[1]. Prolactin is a single chain polypeptide with 199 amino acids.
[2]. Its half-life is about 20 minutes.
[3]. Prolactin is necessary for the final preparation of mammary glands for the production and secretion of milk.
[4]. Prolactin acts directly on the epithelial cells of mammary glands and causes localized alveolar hyperplasia.
β-lipotropin .
[1]. β-lipotropin is a polypeptide hormone with 31 amino acids.
[2]. It mobilizes fat from adipose tissue and promotes lipolysis.
[3]. It also forms the precursor of endorphins. This hormone acts through the adenyl cyclase.
Posterior Pituitary or Neurohypophysis .
Parts of Posterior Pituitary .
Posterior pituitary consists of three parts:
1. Pars nervosa or infundibular process .
2. Neural stalk or infundibular stem
3. Median eminence.
Pars tuberalis of anterior pituitary and the neural stalk of posterior pituitary together form the hypophyseal stalk.
Histology of Posterior Pituitary .
Posterior pituitary is made up of neural type of cells called pituicytes and unmyelinated nerve fibers.
Pituicytes .
[1]. Pituicytes are the fusiform cells derived from glial cells.
[2]. These cells have several processes and brown pigment granules.
[3]. Pituicytes act as supporting cells and do not secrete any hormone.
Unmyelinated Nerve Fibers .
Unmyelinated nerve fibers come from supraoptic and paraventricular nuclei of the hypothalamus through the pituitary stalk.
Other Structures .
Posterior pituitary also has numerous blood vessels, hyaline bodies, neuroglial cells and mast cells.
Hormones of Posterior Pituitary .
Posterior pituitary hormones are:
1. Antidiuretic hormone (ADH) or vasopressin .
2. Oxytocin.
Source of Secretion of Posterior Pituitary Hormones .
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| Hypothalamo-hypophyseal tracts . |
[1]. Actually, the posterior pituitary does not secrete any hormone. ADH and oxytocin are synthesized in the hypothalamus.
[2]. From hypothalamus, these two hormones are transported to the posterior pituitary through the nerve fibers of hypothalamo-hypophyseal tract by means of axonic flow.
[3]. Proteins involved in transport of these hormones are called neurophysins .
[4]. In the posterior pituitary, these hormones are stored at the nerve endings.
[5]. Whenever, the impulses from hypothalamus reach the posterior pituitary, these hormones are released from the nerve endings into the circulation. Hence, these two hormones are called neurohormones.
Experimental Evidence .
[1]. Secretion of posterior pituitary hormones in hypothalamus and their transport to posterior pituitary are proved by experimental evidences.
[2]. When the pituitary stalk is cut above the pituitary gland, by leaving the entire hypothalamus intact, the hormones drip through the cut end of the nerves in the pituitary stalk. This proves the fact that the hormones are secreted by hypothalamus.
Neurophysins .
[1]. Neurophysins are the binding proteins which transport ADH and oxytocin from hypothalamus to posterior pituitary via hypothalamo-hypophyseal tract and storage of these hormones in posterior pituitary.
[2]. Neurophysins I or oxytocin-neurophysins is the binding protein for oxytocin and neurophysins II or ADH-neurophysins is the binding protein for ADH.
Antidiuretic Hormone .
Source of Secretion of Antidiuretic hormone .
[1]. Antidiuretic hormone (ADH) is secreted mainly by supraoptic nucleus of hypothalamus.
[2]. It is also secreted by paraventricular nucleus in small quantity.
[3]. From here, this hormone is transported to posterior pituitary through the nerve fibers of hypothalamo-hypophyseal tract, by means of axonic flow.
Chemistry and Half-life of Antidiuretic hormone .
1. Antidiuretic hormone is a polypeptide containing 9 amino acids.
2. Its half-life is 18 to 20 minutes.
Actions of Antidiuretic hormone .
Antidiuretic hormone has two actions:
1. Retention of water .
2. Vasopressor action .
1. Retention of water .
[1]. Major function of ADH is retention of water by acting on kidneys.
[2]. It increases the facultative reabsorption of water from distal convoluted tubule and collecting duct in the kidneys .
[3]. In the absence of ADH, the distal convoluted tubule and collecting duct are totally impermeable to water. So, reabsorption of water does not occur in the renal tubules and dilute urine is excreted.
[4]. This leads to loss of large amount of water through urine.
[5]. This condition is called diabetes insipidus and the excretion of large amount of water is called diuresis.
Mode of action on renal tubules .
ADH increases water reabsorption in tubular epithelial membrane by regulating the water channel proteins called aquaporins through V2 receptors .
2. Vasopressor action .
[1]. In large amount, ADH shows vasoconstrictor action.
[2]. Particularly, causes constriction of the arteries in all parts of the body.
[3]. Due to vasoconstriction, the blood pressure increases.
[4]. ADH acts on blood vessels through V1A receptors. However, the amount of ADH required to cause the vasopressor effect is greater than the amount required to cause the antidiuretic effect.
Regulation of Secretion ADH .
secretion depends upon the volume of body fluid and the osmolarity of the body fluids.
Potent stimulants for ADH secretion are:
1. Decrease in the extracellular fluid (ECF) volume .
2. Increase in osmolar concentration in the ECF .
Role of osmoreceptors .
[1]. Osmoreceptors are the receptors which give response to change in the osmolar concentration of the blood.
[2]. These receptors are situated in the hypothalamus near supraoptic and paraventricular nuclei.
[3]. When osmolar concentration of blood increases, the osmoreceptors are activated.
[4]. In turn, the osmoreceptors stimulate the supraoptic and paraventricular nuclei which send motor impulses to posterior pituitary through the nerve fibers and cause release of ADH.
[5]. ADH causes reabsorption of water from the renal tubules. This increases ECF volume and restores the normal osmolarity.
Oxytocin .
Source of Secretion of Oxytocin .
[1]. Oxytocin is secreted mainly by paraventricular nucleus of hypothalamus.
[2]. 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.
[3]. In the posterior pituitary, the oxytocin is stored in the nerve endings of hypothalamo-hypophyseal tract.
[4]. When suitable stimuli reach the posterior pituitary from hypothalamus, oxytocin is released into the blood.
[5]. Oxytocin is secreted in both males and females.
Chemistry and Half-life of Oxytocin .
1. Oxytocin is a polypeptide having 9 amino acids.
2. It has a half-life of about 6 minutes.
Actions in Females .
In females, oxytocin acts on mammary glands and uterus.
Action of oxytocin on mammary glands .
[1]. Oxytocin causes ejection of milk from the mammary glands.
[2]. Ducts of the mammary glands are lined by myoepithelial cells.
[3]. Oxytocin causes contraction of the myoepithelial cells and flow of milk from alveoli of mammary glands to the exterior through duct system and nipple.
[4]. The process by which the milk is ejected from alveoli of mammary glands is called milk ejection reflex or milk letdown reflex. It is one of the neuroendocrine reflexes.
Milk ejection reflex .
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| Milk ejection reflex . |
[1]. Plenty of touch receptors are present on the mammary glands, particularly around the nipple.
[2]. When the infant suckles mother nipple, the touch receptors are stimulated.
[3]. The impulses discharged from touch receptors are carried by the somatic afferent nerve fibers to paraventricular and supraoptic nuclei of hypothalamus.
[4]. Now hypothalamus, in turn sends impulses to the posterior pituitary through hypothalamo-hypophyseal tract.
[5]. Afferent impulses cause release of oxytocin into the blood.
[6]. When the hormone reaches the mammary gland, it causes contraction of myoepithelial cells, resulting in ejection of milk from mammary glands .
[7]. As this reflex is initiated by the nervous factors and completed by the hormonal action, it is called a neuroendocrine reflex.
[8]. During this reflex, large amount of oxytocin is released by positive feedback mechanism.
Action on uterus .
Oxytocin acts on pregnant uterus and also non-pregnant uterus.
On pregnant uterus .
[1]. Throughout the period of pregnancy, oxytocin secretion is inhibited by estrogen and progesterone. [2]. At the end of pregnancy, the secretion of these two hormones decreases suddenly and the secretion of oxytocin increases.
[3]. Oxytocin causes contraction of uterus and helps in the expulsion of fetus.
[4]. 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.
[5]. Oxytocin secretion increases during labor. At the onset of labor, the cervix dilates and the fetus descends through the birth canal.
[6]. During the movement of fetus through cervix, the receptors on the cervix are stimulated and start discharging large number of impulses.
[7]. These impulses are carried to the paraventricular and supraoptic nuclei of hypothalamus by the somatic afferent nerve fibers.
[8]. Now, these two hypothalamic nuclei secrete large quantity of oxytocin, which enhances labor by causing contraction of uterus .
[9]. 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 . [10]. The contraction of uterus during labor is also a neuroendocrine reflex.
[11]. Oxytocin also stimulates the release of prostaglandins in the placenta. Prostaglandins intensify the uterine contraction induced by oxytocin.
On non-pregnant uterus .
[1]. 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.
[2]. During the sexual intercourse, the receptors in the vagina are stimulated.
[3]. Vaginal receptors generate the impulses, which are transmitted by somatic afferent nerves to the paraventricular and supraoptic nuclei of hypothalamus.
[4]. 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.
[5]. Sensitivity of uterus to oxytocin is accelerated by estrogen and decreased by progesterone.
Action in Males .
[1]. In males, the release of oxytocin increases during ejaculation.
[2]. 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.
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