Feedback Systems

The body can regulate its internal environment through feedback systems. A feedback system is a cycle of events in which the status of a body condition is monitored, evaluated, changed, remonitored, reevaluated and so on. Each monitored variable, such as body temperature, blood pressure, or blood glucose level, is called a controlled condition. Any disruption that changes a controlled condition is called a stimulus

A feedback system has three basic parts: a receptor, a control center, and an effector. A receptor is a body structure such as a nerve ending or sensory receptor that monitors changes in a controlled condition and sends input, usually in the form of a nerve impulse or chemical signal, to a control center, which in the body is typically the brain. A control center sets the range of values within which a controlled condition should be maintained, evaluates the input it receives, and generates output commands, usually in the form of nerve impulses, hormones, or other chemical signals.

An effector is a body structure, such as a muscle or gland that receives output from the control center and produces a response or effect that changes the controlled condition. Almost every organ or tissue in the body can behave as an effector. Specific examples include muscle and glands.

Positive Feedback System 

A positive feedback system strengthens or reinforces a change in one of the body’s controlled conditions. It operates similarly to a negative feedback system, except that the response reinforces the change instead of negating it. The response of a positive feedback system continues until it is interrupted by some mechanism.

Example: 

Childbirth provides a good example of positive feedback. The first contractions of labor (stimulus) push part of the fetus into the cervix, which is the opening of the uterus. Stretch-sensitive nerve cells (receptors) monitor the amount of stretching of the cervix (controlled condition). As stretching increases, they send more nerve impulses (input) to the brain (control center), which in turn releases the hormone oxytocin (output) into the blood. Oxytocin causes muscles in the wall of the uterus (effector) to contract even more, resulting in pushing the fetus further down the uterus, which stretches the cervix even more. This cycle is interrupted only by the birth of the baby, which will stop the stretching of the cervix and the release of oxytocin. Usually, positive feedback systems reinforce conditions that do not happen very often.

Negative Feedback System

A group of receptors and effectors communicating with their control center forms a feedback system that can regulate a controlled condition in the body’s internal environment. In a feedback system, the response of the system to the change in the controlled condition is either to negate the change, which is called negative feedback.

Example:

A negative feedback system reverses a change in a controlled condition. Consider the regulation of blood pressure. Blood pressure is the force exerted by blood as it presses against the walls of blood vessels. When the heart beats faster, the blood pressure rises. If some internal or external stimulus causes blood pressure, which is a controlled condition, to rise, the following sequence of events will occur: baroreceptors (the receptors), which are pressure-sensitive nerve cells located in the walls of certain blood vessels, detect the higher pressure. The baroreceptors send nerve impulses (input) to the brain (control center), which interprets the impulses and responds by sending nerve impulses (output) to the heart (effector). Heart rate decreases, which causes blood pressure to decrease (response), thereby causing the controlled condition (blood pressure) to return back to normal.