The hypothalamus is a small but complex region of the brain. It has links with both the nervous system as well as the endocrine system. Although shaped and sized like an almond, there are nuclei within the hypothalamus that are involved in many different functions.

Nuclei of the hypothalamus

The two major nuclei are the supraoptic and paraventricular nuclei.

Regions and the nuclei

• Anterior – Medial area includes nuclei like Medial preoptic nucleus,  Supraoptic nucleus,  Paraventricular nucleus, Anterior nucleus, Suprachiasmatic nucleus. Lateral area includes Lateral preoptic nucleus, Lateral nucleus, Part of supraoptic nucleus
• Tuberal – Medial area includes nuclei like Dorsomedial nucleus, Ventromedial nucleus, Arcuate (infundibular) nucleus. Lateral area includes Lateral nucleus,  Lateral tuberal nuclei
• Posterior – Medial nuclei including Mammillary body, Posterior nucleus. Lateral area includes Lateral nucleus

The cells in the two major nuclei secrete vasopressin (ADH, antidiuretic hormone), oxytocin, and CRH (corticotropin releasing hormone).  The hypothalamus co-ordinates many hormonal and behavioural circadian rhythms, complex patterns of neuroendocrine outputs, complex homeostatic mechanisms, and many important behaviours.

Where do the signals to the hypothalamus come from?

The signals to the hypothalamus come from the nervous system. The nerves from and to the nuclei are connected with many parts of the central nervous system. This includes:

• the brainstem
• reticular system
• autonomic zones
• limbic forebrain
• amygdala
• septum
• diagonal band of Broca
• the olfactory bulbs
• the cerebral cortex etc.

Stimuli to the hypothalamus

The hypothalamus controls the release of 8 major hormones by the pituitary gland, body temperature, food and water intake, sexual behavior and reproduction, control of daily cycles known as circadian rhythm and mediation of emotional responses.

Stimuli to the hypothalamus thus include:

• Smell or olfactory stimuli like pheromones
• Light – This is related to the circadian rhythm
• Autonomic inputs – These arise from vital organs like heart and lungs and reach the brain via the autonomic nervous system.
• Steroids and steroid hormones, including gonadal steroids or sex steroids produced by the testes and ovaries and corticosteroids or the stress hormones produced by the adrenal gland
• Nerve impulses from heart, the stomach, and the reproductive tract
• Hormonal and other signals like leptin, ghrelin, angiotensin, insulin, pituitary hormones, glucose, osmolarity of blood etc. These regulate the feeding behaviour, hunger and satiation
• Emotional stress
• Microbes and their effects on body temperature