Hippocampus Biology: The Hypothalamus and the Pituitary Gland

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In order for our body to function properly, it has to respond to external stimuli and internal signals. Some are nerve signals while others are chemical signals. The hypothalamus, a region of the brain, serves as the main integrator of these signals. It receives signals and responds by producing hormones. Many of these hormones regulate the synthesis and secretion of hormones by the glands of the endocrine system.

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Hormones produced by the hypothalamus are released into the pituitary gland, located at the base of the hypothalamus. The pituitary gland has two distinct parts: the anterior pituitary and the posterior pituitary. Some hormones from the hypothalamus are secreted into the blood. This is the job of the posterior pituitary gland. Other hypothalamic hormones control the production and release of hormones from the anterior pituitary gland.

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The hypothalamus synthesizes several hormones that are secreted into the blood via the posterior pituitary gland. Oxytocin is produced and released in females in response to suckling, or breast-feeding. Suckling stimulates nerves that send signals to the hypothalamus. The hypothalamus synthesizes oxytocin, which is released from the posterior pituitary gland. When the oxytocin reaches the mammary gland, the breast, it signals the breast to eject milk. During childbirth, the release of oxytocin causes the muscles of the uterus to contract.

One hormone can have various effects. This is due in part to different target cells expressing the hormone receptor on their surface only under particular conditions; so they respond to the hormone only at certain times.

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Antidiuretic hormone, or ADH, is also produced by the hypothalamus and released by the posterior pituitary gland. ADH helps control the concentration of solutes in the blood. When the concentration of solutes is too high, nerve cells in the hypothalamus signal to secretory cells in the hypothalamus to synthesize ADH. When the ADH, released by the posterior pituitary, reaches the kidney, it signals target cells to release water into the blood to lower the solute concentration. The solute concentration is lowered, and the nerve cells stop signaling the production of ADH.

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This type of negative feedback is common in maintaining homeostasis. Unlike the posterior pituitary, the anterior pituitary doesn't release hormones produced by the hypothalamus. The anterior pituitary is made up of endocrine cells that synthesize and secrete hormones into the blood, under the control of the hypothalamus. The hypothalamus produces and releases two classes of hormones into the blood supply of the anterior pituitary: releasing hormones and inhibiting hormones.

Releasing hormones cause the anterior pituitary to secrete particular hormones. Inhibiting hormones make it stop secreting particular hormones. Each anterior pituitary hormone is controlled by at least one releasing hormone, and some hormones are controlled by both releasing and inhibiting hormones.

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Some anterior pituitary hormones are tropic, meaning they cause the synthesis and secretion of hormones by other endocrine glands. For example, the anterior pituitary gland secretes several hormones, including follicle-stimulating hormone, or FSH, and luteinizing hormone, or LH, which stimulate activities of the male and female gonads. The anterior pituitary secretes some nontropic hormones that directly affect the target tissue, including endorphins, which inhibit the perception of pain by the brain. Some of the secreted hormones are both tropic and nontropic. Growth hormone, or GH, is tropic because it promotes the production of growth factors, such as insulinlike growth factor by the liver. Growth hormone also directly stimulates the growth of bones.

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Now that we've explored the hypothalamus and the pituitary gland, let's consider the hormone products of some other endocrine glands, and see how they interact with the hypothalamus.