8.1.3 Nervous System: Response to a Stimulus

• 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