Hypothalamus

A region of the brain, roughly the size of a cherry, situated behind the eyes and the thalamus. The hypothalamus controls the sympathetic nervous system (part of the autonomic nervous system). In response to sudden alarm or excitement, signals are sent from higher regions of the brain to the hypothalamus, initiating sympathetic nervous system activity. This causes a faster heartbeat, widening of the pupils, an increase in breathing rate and blood flow to muscles. Other nerve cells in the hypothalamus are concerned with the control of body temperature, thirst, and appetite for food. The hypothalamus is also involved in regulating sleep, motivating sexual behaviour, and determining mood and emotions. It indirectly controls many endocrine glands through its influence on the pituitary gland. Disorders of the hypothalamus are usually due to an intracerebral haemorrhage or a pituitary tumour. They have diverse effects, ranging from hormonal disorders to disturbances in temperature regulation, and increased or decreased need for food and sleep.