Figure 21.36 Negative Feedback Loop Animation Video
In order to maintain homeostasis, most mechanisms of the body work on a negative feedback loop where conditions exceeding a set limit in one direction trigger a negative reaction in the opposite direction. A negative feedback loop restores conditions to a normal set point. For example, when your body temperature falls below 98.6 degrees Fahrenheit, the nervous and endocrine systems work together to increase internal heat production. In this example, the hypothalamus and anterior pituitary target another endocrine gland to release its particular hormone . The hypothalamus sends hypothalamic releasing hormone to the anterior pituitary. This stimulates the anterior pituitary to release its own hormones, which act on the target gland. Once stimulated by the hormone of the anterior pituitary, the target gland releases its unique hormone. As the target gland's hormone is secreted, body systems monitor when enough of this hormone has been released to restore homeostasis. In our body temperature example, the target gland is the thyroid, which secretes the hormone thyroxine. When the body has produced enough heat to return to 98.6 degrees, it is time to stop production of thyroxine, or body temperature may be raised too high. In order to turn off the target gland, a negative feedback loop is begun. The target hormone, thyroxine, begins a negative feedback loop to communicate with the anterior pituitary and hypothalamus. The negative feedback alerts the hypothalamus to stop production of hypothalamic releasing hormone and begin production of hypothalamic non-releasing hormone. Hypothalamic non-releasing hormone inhibits the anterior pituitary's hormone production. Because it's no longer receiving messages from the anterior pituitary, the target gland, or thyroid, ceases production of thyroxine. The negative feedback loop has completed its job and stops operating. The release of proper amounts of the target hormone has restored homeostasis and the negative feedback loop has prevented an imbalance in the opposite direction.
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