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Brains, Biology, and Behavior

Endocrine System

The endocrine system releases hormones, which influence both neural activity and the release of other hormones.
The endocrine and nervous systems work together to control behavior through complex hormonal feedback loops. A hormone is a chemical secreted by an endocrine gland that triggers a response from a particular type of cell or tissue. The endocrine system consists of glands used to produce the hormones that regulate body functions. The glands secrete hormones into the bloodstream. The hormones then travel within the bloodstream to their target tissue, where they can exert their effect. Endocrine glands are ductless organs of many different types. They all function to secrete hormones into the body's fluid to help with systemic regulation and communication. There are eight types of endocrine glands: the hypothalamus, pineal gland, pituitary gland, parathyroid glands, thyroid gland, adrenal glands, pancreas, and reproductive glands (ovaries/testes).

The Glands of the Endocrine System

Endocrine glands secrete hormones that have wide-ranging effects throughout the body, including the stimulation of other endocrine glands.
The hypothalamus, an almond-sized area located deep inside the brain, links the nervous and endocrine systems. It is called neuroendocrine tissue because it receives neural input and releases hormones. The hypothalamus responds to signals from other parts of the brain by releasing several different hormones, collectively called neurohormones. Many of these hormones play a key role in regulating the activity of the pituitary gland. The hypothalamus is essential to maintaining balance throughout the body. It helps controls hunger, weight, body temperature, sleep-wake cycles, sexual responsiveness, and aggression.

The pituitary gland, located just below the hypothalamus, is often called the master gland because its hormones control several other endocrine glands. These include the thyroid, adrenal glands, ovaries, and testes. The pituitary gland consists of two lobes, each producing different hormones. One such hormone is growth hormone, which aids in development and growth.

The pineal gland is a small endocrine gland located in the brain that plays a role in the production of melatonin, a hormone that aids in the regulation of sleep patterns. The thyroid gland is an endocrine gland located in the front of the neck that produces thyroid hormones that help regulate metabolic processes, heat production, and the development and function of other organ systems. The parathyroid glands are four small endocrine glands located near or within the thyroid gland that release hormones that help maintain the levels of essential minerals (such as calcium and phosphorus) in the body. These hormones are important for bone development. Much of the body's phosphorus and nearly all its calcium are in the bones.

The adrenal glands are located on the top of each kidney. Each gland consists of an inner portion, called the medulla, and an outer portion, called the cortex. The adrenals are responsible for three hormones associated with the fight-or-flight response to stressors: adrenaline, norepinephrine, and cortisol. Adrenaline and norepinephrine are immediately triggered by stress. These hormones increase arousal and shift blood flow away from nonessential organs, such as the skin, and toward those that are needed to fight or flee, such as the muscles. The release of cortisol is more complex. When the amygdala recognizes a threat, it sends a message to the hypothalamus, which releases corticotropin-releasing hormone (CRH). CRH then signals the pituitary gland to release adrenocorticotropic hormone, which tells the adrenal glands to produce cortisol. Short-term bursts of cortisol are adaptive in response to immediate stressors. But if cortisol levels remain high for too long, cortisol can suppress the immune system, increase blood pressure and blood sugar levels, decrease libido, and contribute to obesity. Elevated cortisol levels also decrease the body's ability to make serotonin, which can trigger depression. About 50% of clinically depressed individuals have an excess of cortisol in their blood, and cortisol levels return to normal once depression is resolved.

The pancreas is found near the stomach and small intestine. It secretes enzymes that help with digestion. If blood sugar is too low, the pancreas causes the release of stored glucagon, a hormone, to increase blood sugar. Males and females each have reproductive glands. The ovaries aid in the production of estrogen and progesterone hormones in females. Both hormones are important to maintain sexual traits and support pregnancy. In males the testes produce testosterone, which helps with male development and the maintenance of male sexual traits. Insufficient levels of sex hormones can impair sexual desire and sexual function.