a) Growth Hormone
d) Follicle-stimulating hormone
The endocrine system is a network of glands and organs that release hormones throughout the body, which regulate and control various functions. The hypothalamus and the pituitary gland are essential structures in the endocrine system because they are involved in the synthesis and secretion of numerous hormones. Figure 1 illustrates how these two structures are linked together. There are two separate pathways involving the hypothalamus and pituitary gland: the hypothalamic-anterior pituitary axis and the hypothalamic-posterior pituitary axis.
The hypothalamic-anterior pituitary axis is the pathway between the hypothalamus to the anterior pituitary gland. Hypothalamic neurons secrete releasing hormones into the hypophyseal portal system. This portal system contains a network of blood vessels that connect the hypothalamus to the anterior pituitary. From the hypophyseal portal system, the releasing hormones can act on the anterior pituitary gland to secrete a variety of different hormones that stimulate other target tissues. In this way, the anterior pituitary gland is separate from the hypothalamus. Like all other endocrine glands, the anterior pituitary is made of glandular tissue.
The second pathway is the hypothalamic-posterior pituitary axis. In this pathway, the axons of the hypothalamic neurons pass directly through the posterior pituitary, releasing hormones into the bloodstream from the latter gland. In this way, the posterior pituitary gland is essentially an extension of the hypothalamus. Therefore, unlike the anterior pituitary, the posterior pituitary is made up of neural tissue rather than glandular tissue. Both oxytocin and vasopressin (ADH) are released into systemic circulation via this pathway. Oxytocin is important for uterine contractions, and it also has a role in lactation after birth. Vasopressin or anti-diuretic hormone (ADH) helps to regulate blood osmolarity by increasing water reabsorption in the renal system.
Recall that the hypothalamus releases various releasing hormones into the anterior pituitary gland in order to stimulate the anterior pituitary to secrete its hormones. One releasing hormone is growth hormone-releasing hormone (GHRH). GHRH is involved in the release of growth hormone (GH) from the anterior pituitary gland. Growth hormone has various functions throughout the body, such as cell growth, regeneration, and reproduction. Somatostatin, which is an inhibitory hormone, is the opposite of GHRH. Somatostatin will inhibit the secretion of growth hormone from the anterior pituitary gland. Another releasing hormone is corticotropin-releasing hormone (CRH). CRH stimulates the anterior pituitary gland to release adrenocorticotropic hormone (ACTH). ACTH, in turn, stimulates the adrenal cortex to release cortisol, which is a steroid hormone involved in the metabolism of glucose and the immune response.
Another releasing hormone is thyrotropin-releasing hormone (TRH), which has a dual effect. First, it will stimulate the anterior pituitary gland to release thyroid-stimulating hormone (TSH). Thyroid-stimulating hormone will act on the thyroid gland to release the thyroid hormones. TRH also stimulates the release of prolactin from the anterior pituitary gland. Prolactin has a vital role in milk production in females. The hypothalamus also secretes dopamine, which inhibits the release of prolactin from the anterior pituitary gland. This is why dopamine is also called prolactin inhibitory hormone (PIH). It is important to note that prolactin is predominantly under inhibitory control, meaning that dopamine has a more substantial effect in inhibiting prolactin release than TRH does in stimulating prolactin release. Much of the time, then, in order to stimulate prolactin secretion, it is necessary to inhibit PIH secretion from the hypothalamus.
Finally, there is gonadotropin-releasing hormone (GnRH). This hormone will stimulate the anterior pituitary to release both follicle-stimulating hormone (FSH) and luteinizing hormone (LH), which both have important roles in regulating the reproductive organs of both males and females.
Dr. Ken Tao is an MCAT guru at MedSchoolCoach and one of the world’s foremost authorities on the MCAT, having a deep understanding over 10+ years and thousands of students. He is the creator of the MCAT Prep App, the #1 Rated MCAT Mobile App. He received his PhD from Harvard University in Neuroscience and has tutored hundreds of students to higher MCAT Scores.