The Hypothalamus

The hypothalamus sits at the base of the diencephalon in the forebrain. Although it is a region of the brain, it also behaves as an endocrine control centre, and it governs a very wide range of body functions.

Neurosecretory cells

Within the hypothalamus lie several clusters of specialised cells called neurosecretory cells, and these clusters are known as nuclei. What makes these cells distinctive is that, like nerve cells, they conduct signals, yet they also produce hormones. The hormones they make have one main job: to regulate how the pituitary gland makes and releases its own hormones.

Two kinds of hypothalamic hormones

The hormones from the hypothalamus fall into two opposite categories, giving it both an accelerator and a brake over the pituitary:

• Releasing hormones — these stimulate the pituitary to secrete its hormones.
• Inhibiting hormones — these hold back or suppress the pituitary's secretions.

Two examples make the contrast clear:

• Gonadotrophin releasing hormone (GnRH) prompts the pituitary to make and release the gonadotrophins.
• Somatostatin, also from the hypothalamus, does the reverse — it blocks the release of growth hormone from the pituitary.

How the signals reach the pituitary …