The hypothalamus is a small, yet vitally important, brain region that integrates the body's two communication systems: the endocrine and nervous systems. It links the two by sending and receiving signals from other regions of the nervous system while also controlling the body's "master gland"—the pituitary gland. The pituitary, in turn, controls most other endocrine organs of the body.
The interaction between the hypothalamus, pituitary, and other endocrine glands is known as the hypothalamic–pituitary–endocrine axis. In one animation, we examine the hypothalamic control of the pituitary gland, and we show the endocrine glands that the pituitary controls. In another, we examine a phenomenon called a negative feedback loop, in which hormones from endocrine glands influence the action of the hypothalamus.
Through its release of hormones, the hypothalamus controls reproduction, growth, metabolism, water conservation, blood pressure, lactation, childbirth, and responses to stress. Through its connections with other regions of the nervous system, the hypothalamus controls many other bodily functions.
The hypothalamus receives information about the state of the body and sends instructions—either through synaptic communication with other neurons or through the release of hormones—to keep the body's internal environment within a narrow operating range.
The negative feedback loops that operate in the hypothalamus–pituitary–endocrine axis provide some insight into how the hypothalamus maintains the body's internal environment. There are many examples of such feedback loops. For instance, the hypothalamus controls metabolic rate in part through the control of the thyroid gland. The hypothalamus releases thyrotropin-releasing hormone (TRH), which triggers the anterior pituitary to release thyroid-stimulating hormone (TSH), which in turn stimulates the thyroid gland to release thyroid hormones. The thyroid hormones travel throughout the body to stimulate cellular metabolism. In addition to their action throughout the body, the thyroid hormones also act on the hypothalamus, inhibiting it from releasing additional TRH. When the thyroid hormones drop to low levels—during which metabolism slows—the hypothalamus is released from inhibition and begins to release TRH again. The metabolic rate of the body thereby remains within a narrow range.
Textbook Reference: Concept 35.3 The Vertebrate Hypothalamus and Pituitary Gland Link the Nervous and Endocrine Systems