The hypothalamus and the pituitary gland serve as the link between the nervous system and the endocrine system. We also said that if there's anything in charge of the endocrine system, it's the hypothalamus and the pituitary gland. Specifically, the anterior pituitary releases hormones that affect the release of other hormones throughout the body. Before we get going, let's orient ourselves to our picture here. Here we have the pituitary gland, color-coded in light brown for the anterior pituitary, the front part of the gland we are discussing. You can also see some blood vessels here, and upward, the hypothalamus in gray. We can also observe some neurons in the hypothalamus, that part of the brain that run downward, and we'll discuss how they function shortly.
We'll start by mentioning that the anterior pituitary is stimulated by tropic hormones from the hypothalamus. Remember, a tropic hormone is one that causes the release of other hormones. The way the hypothalamus directs the anterior pituitary is by releasing its hormones that have target cells in the anterior pituitary. However, this system works differently from the general hormone release. Typically, hormones enter the bloodstream and spread throughout the body. Not here. Here, they're released into what's called a portal system. To understand a portal system, consider our drawing: the first capillary bed where these hormones are released funnels into veins, which unusually lead to more capillaries rather than going straight to the heart. This is called the hypothalamic-hypophyseal portal system because it connects capillaries from the hypothalamus to those of the anterior pituitary.
Looking again, remember the hypothalamus comprises nervous tissue. Neurons at the top have axons running down to the base of the hypothalamus or the top of the infundibulum, where these hormones enter the portal system efficiently reaching the anterior pituitary without dispersing into the overall body's circulation. We want to focus on what hormones the anterior pituitary releases and their effects. The table's middle column highlights likely important hormones released by the anterior pituitary, noting generally what they do based on their target cell locations. This quick review provides an understanding of the hormones released from the anterior pituitary and their functions.
In the first column, we see hormones released by the hypothalamus that either stimulate or inhibit the anterior pituitary. Names like growth hormone-releasing hormone, thyrotropin-releasing hormone, corticotropin-releasing hormone, and gonadotropin-releasing hormone are self-explanatory. They stimulate the anterior pituitary to release growth hormone (GH), thyroid stimulating hormone (TSH), adrenocorticotropic hormone, luteinizing hormone, and follicle-stimulating hormone. Moreover, prolactin inhibiting hormone does the opposite by blocking the release of prolactin (PRL), which has target cells in the breasts and is involved in milk production.
Remembering these hormones can be challenging, so a mnemonic device could be helpful. For the anterior pituitary hormones, consider 'FLAT PEG': F for follicle-stimulating hormone, L for luteinizing hormone, A for adrenocorticotropic hormone, T for thyroid-stimulating hormone, P for prolactin, and G for growth hormone. 'FLAT' refers to the tropic hormones, while 'PEG' denotes hormones with direct physiological actions. Next, we will discuss the posterior pituitary, which has fewer hormone releases, but first, review the examples and try the practice problems.