Responding to changes in environment

Organisms need to detect and respond to changes in their internal and external environment. This is because the conditions inside our body must be carefully controlled for it to function effectively and survive. The control systems that allow organisms to respond to changes are incredibly important.

All control systems include:

  • Cells called receptors, which detect stimuli (changes in the environment).
  • The coordination centre, such as the brain, spinal cord or pancreas, which receives and processes information from receptors around the body.
  • Effectors which bring about responses, and restore optimum levels, such as core body temperature and blood glucose levels. Effectors include muscles and glands, and so responses can include muscle contractions or hormone release.

The nervous system

The nervous system allows an organism to respond quickly to changes in the internal or external environment. The responses to the stimuli are short-lasting.


Receptors are groups of specialised cells. They detect a change in the environment stimulus. In the nervous system this leads to an electrical impulse being made in response to the stimulus. Sense organs contain groups of receptors that respond to specific stimuli.

Sense organStimuli receptors respond to
SkinTouch, temperature, pain
TongueChemicals (in food and drink, for example)
NoseChemicals (in the air, for example)
EarSound, position of head


Effectors include muscles and glands - that produce a specific response to a detected stimulus.

For example:

  • a muscle contracting to move an arm
  • muscle squeezing saliva from the salivary gland
  • a gland releasing a hormone into the blood

Receptors are connected to effectors as follows:

Sensory receptorsThese detect the changes in the environment (stimulus)
Sensory neuronsNerve cells that send a signal from the sensory receptors to the CNS
CNSThe central nervous system (CNS), which is the brain and the spinal cord. This coordinates a response and sends a signal down a motor neuron.
Motor neuronsNerve cells that receive a signal from the CNS and transfer it to the effector
EffectorsThe muscles or glands that produce a response to the stimulus
Diagram showing how receptors connect to effectors