An animal's response to a stimulus is coordinated by its central nervous system (CNS). A simple reflex is an involuntary response to a stimulus. Humans use reflex actions in only some of their behaviour, for example controlling the eye's pupil size.

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.

Receptors respond to a stimulus and send impulses along sensory neurons to the CNS.

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 a gland.
• 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.

Simple reflexes.

Simple reflexes produce rapid involuntary responses to a stimulus. This ensures an animal responds in the way most likely to result in its survival.

Examples include:

• Moving towards and finding food
• Moving away and sheltering from predators
• Moving towards and finding a mate

Simple animals use reflex actions for the majority…