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How organisms respond to changes in their environment

An animal’s response to a stimulus is coordinated by their central nervous system (CNS).

Responding to a stimulus

A stimulus is a change in the environment of an organism.

Animals respond to a stimulus in order to keep themselves in favourable conditions.

Examples of this include:

  • moving to somewhere warmer if they are too cold

  • moving towards food if they are hungry

  • moving away from danger to protect themselves

Animals that do not respond to a stimulus do not survive for long.

An animal’s response to a stimulus is coordinated by their central nervous system (CNS). The CNS consists of the brain and the spinal cord. It gathers information about, and responds to, changes in the environment.

Receptors respond to a stimulus and send impulses along sensory neuronssensory: Sensory neurones carry messages from sense organs into the CNS to the CNS. The CNS coordinates the information and sends impulses along motor neuronsmotor: motor neurones carry messages out of the CNS to effector organs to the effectors, which bring about a response. The sequence is as follows:

  1. Stimulus
  2. Receptor
  3. Sensory neuron
  4. Central nervous system
  5. Motor neuron
  6. Effector
  7. Response

There are light receptors in the eye

Some receptors are found in the skin. Other receptors can form part of complex organs, such as:

  • light receptor cells in the retina of the eye
  • hormone-secreting cells in hormone glands
  • muscle cells
  • position receptors in the inner ear
  • sound receptors in the ear
  • touch, pressure, temperature and pain receptors in skin
  • chemical receptors in the nose and tongue

The peripheral nervous system

The peripheral nervous system (PNS) consists of motor and sensory neurons that carry information from the receptors to the CNS, as well as instructions from the CNS to the effectors.

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