• The nervous system enables the body to?detect changes in the environment?and brings about?appropriate responses?to ensure its safety
  • Receptor cells?detect changes in the environment, or?stimuli
  • Nerve impulses travel from the receptor cells along?sensory neurones?to the?central nervous system,?or CNS
  • The?CNS?acts as a?coordinating centre?for the impulses that arrive from the receptors, determining which part of the body needs to respond and sending out a new set of impulses along?motor neurones
  • Motor neurones send impulses?to the?effectors?to bring about a?response
    • Effectors may be muscles or glands
• Nerve impulses pass through the nervous system along the following pathway

stimulus → receptor → sensory neurone → CNS →motor neurone →effector

Receptors detect stimuli and impulses are sent through to nervous system to bring about a response in the effector

• An example of a nerve pathway in action is the sequence of events that leads to a change in the diameter of the pupil in the eye
  • Changing pupil diameter enables the eye to control the amount of light hitting the retina
  • The diameter of the pupil in the eye is determined by two sets of muscles
    • The?circular muscles?contract to?constrict?the pupil
    • The?radial muscles?contract to?dilate?the pupil
  • The two sets of muscles work?antagonistically, meaning that when one set of muscles contracts the other relaxes, and vice versa
• In?bright light?the following events occur

bright light → light receptors in eyes → sensory neurone → CNS → motor neurone →?circular?muscles in iris

• • Contraction of the circular muscles in the iris of the eye causes the pupil to?constrict
  • This limits the amount of light entering the eye and?prevents damage to the retina
• In?low light?the following events occur

low light → light receptors in eyes → sensory neurone → CNS → motor neurone →?radial?muscles in iris

• • Contraction of the radial muscles in the iris of the eye causes the pupil to?dilate
  • This?maximises the amount of light entering the eye, improving vision

The muscles in the eye respond to light levels due to nerve impulses generated by light receptors in the eye