Hormone Regulation Feedback Mechanisms

What is a Feedback Mechanism ?

A feedback mechanism (also referred to as a 'feedback system' or a 'feedback loop') is a cycle of events in which the state of a specific aspect of the body's condition (called a 'controlled condition'), e.g. temperature, is continually monitored and adjusted to keep the value of that controlled condition within a safe range so the body continues to function successfully, as opposed to sustaining damage e.g. due to over-heating.

The main body systems involved in hormone regulation by feedback mechanisms are:

• the nervous system
  in which signals input to and output from the control centre are either nerve impulses which are part of the nervous system, or chemical signals
• the endocrine system
  which produces and secretes hormones

Terms used to describe feedback systems:

• Controlled Condition - the aspect of the body's condition that the particular feedback mechanism is regulating, e.g. 'level of calcium in blood' - see opposite.
• Stimulus - any disturbance (to the internal or external environment) that causes a change in the controlled condition.
  Some feedback systems involve more than one stimulus, e.g. two stimuli, such as the controlled parameter increases, and the controlled parameter decreases.
  The more stimuli, and other parts of the feedback system, e.g. effectors, the more complicated the feedback system.
• Receptor - a structure of the body that detects changes in the controlled condition and sends information about it (called 'input') to the control centre.
• Control Centre - a processing centre that receives input from receptors (which may be located in one region or throughout the body), compares that information with the range of values of the controlled condition within which the body can operate efficiently, and if necessary sends ('outputs') instructions to effectors - causing them to take specific actions to change the value of the controlled condition, as appropriate.
• Effector - a structure of the body that receives signals output by the control centre and responds to them by taking or producing actions ('effectors' produce effects ) that affect the controlled condition.

Example of a general feedback system regulating a glucocorticoid* hormone

• Controlled Condition
  Level of glucocorticoid in the blood (strictly - the amount or concentration of a glucocorticoid hormone in the blood and extracellular fluid, ECF)
• Stimulus
  Blood level of glucocorticoid decreases
• Receptor
  Neurosecretory cells in the hypothalamus
  send input signals in the form of
  • increased hypothalmic releasing hormone
  • decreased glucocorticoid
    
• Control Centre
  Anterior pituitary gland
  sends output signals in the form of
  • increased adrenocorticotrophic hormone (ACTH)
• Effector
  Adrenal cortex - secretes glucocorticoids*

As a result of the adrenal cortex (the effector) secreting glucocorticoids, the level of glucocorticoid in the blood (controlled condition) is brought back into balance. That is, the effect of the stimulus decreasing the blood level of a glucocorticoid hormone is counter-acted. This is an example of a negative feedback system - see below.

* Glucocorticoid hormones form one of the two main groups of corticosteroids. E.g.s of glucocorticoids include cortisol, cortisone, corticosterone.

Negative Feedback Mechanisms

Produce negative feedback which means that ...

Negative feedback systems reverse changes in the controlled condition.

Therefore negative feedback tends to bring conditions within the body back into balance.

Feedback mechanisms range from relatively simple cycles to more complicated systems, e.g. if many different types of stimuli might disturb the controlled condition, possibly invoking responses from different types of receptors and effectors. In all cases, negative feedback causes responses to counteract (reverse) the initiating change in the controlled condition.

The change(s) from outside the system is the stimulus i.e. the disturbance causing the controlled condition to change - which will, of course, only lead to a response by the body if the change in the controlled condition is sufficient to cause the value of the relevant parameter, e.g. concentration of a specific hormone in the fluid being monitored, to fall outside of the 'acceptable range' set by the control centre.

Negative feedback mechanisms control many long-term (incl. some lifelong) states such as body temperature and hydration. Most of the feedback mechanisms that regulate hormones in the human body are negative feedback systems.

Positive Feedback Mechanisms

Produce positive feedback which means that ...

Positive feedback systems reinforce (increase) changes in the controlled condition.

Therefore positive feedback systems must be such that an event will occur to discontinue the feedback system when appropriate - see examples below.

Positive feedback systems generally control infrequent conditions such as ovulation, childbirth and blood clotting.

Example of a hormone regulated by a positive feedback mechanism : Oxytocin

Two positive feedback mechanisms control release of oxytocin:

• Uterine contractions during childbirth
  When contractions start oxytocin is released which stimulates more contractions and more oxytocin to be released, hence contractions increase in intensity and frequency. Production and release of oxytocin stops after the baby is delivered.
• Secretion of breast milk
  The stimulation of a baby sucking its mother's breast leads to secretion of oxytocin into the mother's blood, which leads to milk being available to the baby via the breast. The mother's production and release of oxytocin ceases when the baby stops feeding.