Question

The hypothalamus-pituitary complex can be considered as the master control center for the endocrine system.

1. Contrast how the hypothalamus communicates differently with the anterior pituitary gland compared to the posterior pituitary gland.
2. Using a specific example, describe the process by which the hypothalamus can direct the secretion of an anterior pituitary gland hormone and the effects at the target organ or tissue.

Answer 1

1.The main difference between anterior and posterior pituitary gland is that the anterior pituitary gland synthesise it's own hormones where as the posterior pituitary gland only stores the hormones ADH and oxytocin produced by the hypothalamus. In anterior pituitary, hypothalamus regulates the release of hormones produced by the anterior pituitary itself,where as in posterior pituitary it regulates the release of its own hormones stored there. The phenomenon is detailed below.

Anterior pituitary

The release of various hormones produced by anterior pituitary gland is regulated by the releasing hormones of hypothalamus. These hormones are produced by the neuronal cells of hypothalamus and reaches the anterior pituitary gland through blood vessel. This is called hypophyseal portal system.

Posterior pituitary

The cell bodies of paraventricular and supraoptic nuclei resides in the hypothalamus and its axon terminal ends in the posterior pituitary. Supra optic nuclei produce ADH and oxytocin is produced by paraventricular nuclei. The hormones produced by these two neurons passses along the axon and reaches the axon terminal situated in posterior pituitary and is stored there. The signals from hypothalamus inturn regulate the release of these hormones.

2.Example

Anterior pituitary gland produce various hormones such as growth hormone, prolactin, adrenocorticotropic hormone, thyroid stimulating hormone, follicle stimulating hormone, leutinising hormone. Release of all these hormones is under the control of various releasing hormones from hypothalamus. For example let me explain

Hypothalamic pituitary thyroid axis

Thyrotropin releasing hormone(TRH) is released from hypothalamus which stimlates the production of thyroid stimulating hormone(TSH) by anterior pituitary. The thyroid stimulating hormone stimulates the thyroid gland to produce thyroid hormones,T4 and T3. There can be positive and negative feedback control . When the circulating level of thyroid hormone is less it will cause hypothalamus to produce TRH which will inturn produce TSH and then T3 and T4. When the level of thyroid hormone is more the production of TRH and thus TSH will be inhibited.