Biological actions in response to internal and external changes

Nervous system

The nervous system allows animals to detect environmental changes (stimuli) and rapidly respond to them. The nervous system is made up of nerve cells. Nerve cells allow different parts of the body to rapidly communicate with each other by transmitting signals called nerve impulses.

Body structures that can detect environmental changes are called receptors. Parts of the body that can make responses are called effectors.

Examples of stimuli that can be detected by the human nervous system:

  • chemicals
  • light
  • pressure (touch)
  • sound

Examples of responses that can be made:

  • muscle contraction
  • release of hormones by glands


The maintenance of a constant internal environment by the body is called homeostasis.

Factors that are kept at a set point include:

  • body temperature
  • blood glucose level
  • body water content


This is the regulation of body temperature so that it stays at a set point of 37°C. If the body temperature falls too low then biological reactions may happen too slowly for cells to survive. If the body temperature rises too high then enzymes and other cell proteins may be damaged.