Summary
The hypothalamus is located obliquely below the thalamus ('hypo' meaning 'under' or 'low') and is involved in many different functions of the autonomic nervous system. The primary function of the hypothalamus is to regulate homeostasis. The hypothalamus ensures that the body remains in balance, for example by regulating needs such as hunger, thirst, sleep and sex, but also emotions. The connections with the pituitary gland are important. The hypothalamus ensures that this gland produces certain hormones. In stressful situations, for example, the hypothalamus controls the pituitary gland, which releases the hormone cortisol via the adrenal cortex, making us alert and ready for action. This system is called the HPA axis. The H stands for hypothalamus, the P for pituitary gland and the A for adrenal gland.

Motivation
The hypothalamus contains axons that connect the brainstem to the basal ganglia. If these axons are lesioned in rats, a lack of motivation arises. It is as if their drive to eat or mate, for example, is lost. Electrical stimulation of these axons causes the rat to become active. If food or water is available the rat will eat or drink and if a rat of the opposite sex is present it will mate. The axons that run through the hypothalamus thus play an important role in the general motivation of all kinds of different behaviors.

Hunger
The hypothalamus also plays a more specific role in motivating behavior, namely eating. The lateral part is important for the feeling of hunger. When this part is destroyed, rats will still eat, but only just enough to stay alive. The ventromedial part of the hypothalamus has the opposite function. When this part is damaged, food is digested faster than normal and mainly converted and stored in fat. This leaves little fuel for the body. The body therefore starts sending signals to the brain that more food is needed because there is not enough energy available. When the ventromedial hypothalamus does not function properly, the feeling of hunger is not extinguished after eating a meal. People with a tumor in this part of the hypothalamus therefore often develop obesity.

Sex
The ventromedial hypothalamus is also important for the sex drive of females. It contains receptors for the hormones estrogen and progesterone. These hormones are released during ovulation, increasing the sex drive when the female animal is fertile. In humans, this influence is much smaller. 

Sleep
The hypothalamus is also important for the sleep/wake rhythm. When the front part of the hypothalamus is removed in rats, this leads to enormous sleepiness and eventually even death. Connections to the thalamus play an important role here.

Author: Bart Aben (translated by Pauline van Gils)