Pituitary gland and Hypothalamus
Both the pituitary gland and hypothalamus works as unit, they regulates the activity of most of the endocrine glands. the pituitary gland lies In the hypophyseal fossa of the sphenoid bone below to the hypothalamus. it weighs about 500 g and consist of two parts; both the parts of this gland originate from different cells. the parts are the anterior pituitary which develops from glandular epithelium and the posterior pituitary which originate from the nervous tissue. there is a network of nerve fibers between the hypothalamus and the posterior pituitary.
the hypothalamus regulates and controls the release of hormones from both anterior pituitary as well as posterior pituitary but in different ways.
The anterior pituitary is supplies blood indirectly from the capillaries that has already passed through the hypothalamus. These blood vessels also provide oxygen and nutrients to this part. This blood transports releasing and inhibiting the hormones secreted by the hypothalamus. these hormones specifically influence the release of hormones from the anterior pituitary.
some of the hormones released by anterior pituitary stimulate and inhibit the secretion of other endocrine glands(target glands), while the other hormones from anterior lobe directly effect the target tissue.
Secretion of hormones follows the stimulation o the gland by the specific releasing hormones from the hypothalamus and are carried out to the gland by the pituitary portal system. when the level of hormones in the blood supplying to the hypothalamus falls, it produces the appropriate releasing hormones that stimulate the release of trophic hormone by the anterior lobe. this in turn stimulates the target gland to produce hormones.
this is the most abundant hormone which is synthesized by the anterior pituitary. it stimulates growth and division of many body cells but specially the cells in the ones and the skeletal muscles. growth is visible during the childhood and adolescence, and after this age group the GH maintains the mass of bones and skeletal muscles. It regulates the metabolism aspects of many organs like liver, intestine and pancreas; stimulates protein synthesis; promotes breakdown of fats and also increases the blood glucose levels.
The secretion is stimulated by the growth hormone releasing hormone(GHRH) and inhibited by growth hormone releasing inhibiting hormone(GHRIH) and both, the GHRH and GHRIH are secreted by hypothalamus. secretion of GH is greater at night during sleep and it also stimulated by hypoglycaemia(low blood sugar), exercise, anxiety etc.
THYROID STIMULATING HORMONE(TSH)
the release of hormone is stimulated by the thyrotrophin releasing hormone(TRH) from hypothalamus, it stimulates growth and activity of thyroid gland which in turn secretes hormones, T3 and T4. when the blood level of thyroid hormone is high, the secretion of TSH is reduced.
the secretion of ACTH is stimulated by the corticotrophin releasing hormone(CRH) from hypothalamus. ACTH increases the concentration of cholesterol and steroids in adrenal cortex and also the secretion of cortisol. secretion is regulated by negative feedback mechanism, when the blood level of ACTH rises, its release is being inhibited.
this hormone is secreted during pregnancy to prepare the breasts from lactation after childbirth. the secretion is stimulated by the prolactin releasing hormone(PRH) from hypothalamus and inhibited by the prolactin inhibiting hormone(PIH). immediately after childbirth, suckling stimulates the prolactin secretion and lactation. prolactin together with oestrogens, insulin, corticosteroids is involved in stimulating and inhibiting the lactation.
it is formed by the nervous tissue and consist of nerve cells. these neurons have their body In the supraoptic and paraventricular regions of the hypothalamus and their axons forms a bundle known as hypothalamohypophyseal tract.
Nerve impulses from the hypothalamus triggers the exocytosis which releases their hormone into the blood. oxytocin and antidiuretic hormone(ADH) are the hormones released by the posterior pituitary.
oxytocin stimulates two target hormones, the uterine smooth muscles and the muscle cells of breasts. during birth, the oxytocin are released into the blood in response to the increasing stimulation of sensory stretch receptors in the uterine cervix when the baby’s head try to dilate it. it is regulated by positive feedback system and this mechanism stops soon after the baby is delivered.
LACTATION also follows positive feedback system. the level of oxytocin increases during sexual awakeness in both males and females. it is believed that the smell of oxytocin can be used to the identify the bond between mother and the baby.
the main effect of ADH is to reduce urine output by increasing the reabsorption. It mainly acts on DCT and collecting ducts. as the osmotic pressure rises, the secretion of ADH also increases, so more water is reabsorbed and then output decreases.
If the osmotic pressure is low because of large fluid intake then secretion of ADH is also reduced hence reabsorption decreases. ADH also causes vasoconstriction during low BP to increase the BP and it also increases the blood volume.
Multiple choice question(MCQs)
1. Where s the pituitary gland situated?
A. below the hypothalamus B. above the hypothalamus
C. behind the hypothalamus D. in front of hypothalamus
2. Secretion of hormones from pituitary gland is regulated by?
A. thalamus B. hypothalamus
C. medulla oblongata D. midbrain
3. Which hormone is controlled by positive feedback mechanism?
A. growth hormone B. thyroid stimulating hormone
C. ACTH d. none of the above
4. What is the main function of ADH?
A. reduce reabsorption B. increase reabsorption
C. lactation D. growth of skeleton
5. Match the following-
a) inhibition of growth hormone 1. PIH
b) stimulation of growth hormone 2. CRH
c)synthesis of ACTH 3. GHRH
d) suppression of prolactin 4. GHRIH
6. What is the function of growth hormone?
A. regulation of metabolism of many organ B. increase blood glucose level
C. protein synthesis D. all of the above
7. Which factor is NOT responsible for the stimulation of secretion of ACTH?
A. exercise B. hyperglycaemia
C. fever D. emotional states
8. The posterior pituitary is formed from?
A. elastic tissue B. epithelium
C. nervous tissue D. muscular tissue
9. Which of the following statement is NOT true?
A. lactation is controlled by positive feedback mechanism
B. prolactin, oestrogens, insulin are involved in regulating lactation
C. oxytocin stimulate uterine muscle and muscle cell of breasts
D. GHRIH stimulates the secretion of TSH and GIT secretions
10. Where ADH works for increasing reabsorption?
A. DCT B. PCT
C. collecting duct D. both A and C
1. below the hypothalamus
3. none of the above
4. increases reabsorption
5. a) – 4 b) – 3 c) – 2 d) – 1
6. all of the above
8. nervous tissue
9. GHRIH stimulates the secretion of TSH and GIT secretions
10. both A and C
REFERENCE: 1. Ross and Wilson-Anatomy and physiology in health and illness; 12th edition; page no.-: 217-220.