The hypothalamus is a small almond shaped organ in the lower part of the brain. However, this small organ has numerous connections that help it play an important role in connecting the nervous system with the endocrine system.

The hypothalamus receives several signals and instructions via nerves that help it to function and stimulate the release of hormones and regulatory chemicals from the pituitary as well as other organs. The outpourings of the hypothalamus can be divided into two categories:

• neural projections
• endocrine hormones

Neural projections

There are nerve fibers that run out of the hypothalamus to various control sites. Most fiber systems of the hypothalamus run in two ways. Thus they bring in information and carry out instructions from the organ and are termed bidirectional.

Some of the projections are to areas posterior or below the hypothalamus. These pass via the medial forebrain bundle, the mammillotegmental tract and the dorsal longitudinal fasciculus. Those projections that move to the front part of the hypothalamus are carried by the mammillothalamic tract, the fornix and terminal stria.

There are projections that carry instructions to the areas of the sympathetic motor system (lateral horn spinal segments T1-L2/L3 of the). These are carried by the hypothalamospinal tract and they activate the sympathetic motor pathway.

Regulation of and by hormones

The hypothalamus releases several hormones from its neurons. These include:

• Thyrotropin-releasing hormone (TRH) 
• Gonadotropin-releasing hormone (GnRH) 
• Growth hormone-releasing hormone (GHRH) 
• Corticotropin-releasing hormone (CRH) 
• Somatostatin 
• Dopamine

These are released into the blood in the capillaries. From there they reach the portal veins to another capillary bed in the anterior lobe of the pituitary. The releasing hormones stimulate release of hormones from the pituitary gland.

These hormones are all released in periodic spurts. In patients with deficiencies of these hormones the replacement therapy with external hormones needs to be similar spurts to achieve physiological effects. There are two other hormones Antidiuretic hormone (ADH) and Oxytocin.

Effects of the releasing hormones

1. Thyrotropin-releasing hormone (Prolactin-releasing hormone) – These stimulate the release of thyroid-stimulating hormone (TSH) from anterior pituitary and stimulate prolactin release from anterior pituitary
2. Dopamine (Prolactin-inhibiting hormone) - Inhibit prolactin release from anterior pituitary
3. Growth hormone releasing hormone - Stimulate Growth hormone (GH) release from anterior pituitary
4. Somatostatin (growth hormone-inhibiting hormone) - Inhibit Growth hormone (GH) release from anterior pituitary and inhibit TSH release
5. Gonadotropin-releasing hormone - Stimulate follicle-stimulating hormone (FSH) and luteinizing hormone (LH) release from anterior pituitary
6. Corticotropin-releasing hormone – Stimulate adrenocorticotropic hormone (ACTH) release from anterior pituitary
7. Vasopressin (antidiuretic hormone) – Increases the water permeability in the distal convoluted tubule and collecting duct of nephrons. This leads to conservation of water and reduces urine volume
8. Oxytocin - Uterine contraction and stimulation of milk let down or lactation