Coordination

• Reflex Action

  Automatically and rapidly coordinates the stimuli with the responses of effectors (muscles and glands)
• Simple Reflex Arc

  • Involves three neurones: a sensory neurone, a relay neurone and a motor neurone
  • The gap between neurones is called a synapse
• How the simple reflex arc works

  1. Stimulus affects a receptor (cell or organ that converts a stimulus into an electrical impulse)

  2. Sensory neurone carries impulses from the receptor to the CNS

  3. Connector/relay neurone carries impulse slowly across the spinal cord

  4. The motor neurone carries impulses from the CNS to the effector

  5. The effector (either a muscle or a gland) carries out the response
• Cornea

  • Refracts light
• Iris


  • Controls how much light enters the pupil
• Lens

  • Focuses light onto the retina
• Retina

  • Contains light receptors, some sensitive to light of different colours (Rods and cones)
• Optic nerves


  • Carry impulses to the brain
• Low Light Intensity


  • Radial muscles (straight lines) contract
  • Pupil: wider
• High Light Intensity


  • Circular muscles (circular lines) contract
  • Pupil: smaller
• Near Object

  1. Ciliary muscles contract

  2. Suspensory Ligaments slack

  3. The lens becomes short and fat
• Distant Object

  1. Ciliary muscles relax

  2. Suspensory Ligaments tighten

  3. The lens becomes long and thin