Hypothalamus

The hypothalamus is a small, cone-shaped region at the base of the brain that tapers downward into the pituitary stalk. It is the anatomical and functional bridge between the nervous system and the endocrine system: it contains clusters of specialised neurosecretory cells that behave like neurons but release hormones instead of (or in addition to) neurotransmitters, directly regulating the secretions of the pituitary gland below it (Figure 11.2). Every hormone the hypothalamus produces is either a releasing hormone, which switches on secretion of a particular pituitary hormone, or an inhibitory hormone, which switches it off — ten of these are listed with their targets in Table 11.2.

The hypothalamus is linked to the two lobes of the pituitary in two completely different ways. In the basal region of the brain, a specialised hypothalamo-hypophyseal portal blood vessel connects the hypothalamus directly to the anterior pituitary, allowing hypothalamic releasing/inhibiting hormones to reach it in high, undiluted concentration. The posterior pituitary, in contrast, is linked to the hypothalamus by a bundle of nerve fibres called the hypothalamo-hypophyseal tract (or axis), which carries nerve impulses (not hormones travelling through blood) that trigger release of the hormones already stored there. …