Topic 10: The Nervous System and the Eye

• Reflex action is the shortest pathway from receptor to effector.
• Nerve impulses are transmitted from sensory neurone to relay neurone then to motor neurone.
• Brain, spinal cord and relay neurone are all components of the Central Nervous System (CNS).
• Receptors detects the stimulus and produces nerve impulses.
• What are the components of the CNS?

  Brain, spinal cord and relay neurone
• What is the function of eyelids?

  1. protects cornea from mechanical damage
  2. Squinting prevents excessive light from entering eye
  3. Blinking spreads tears over cornea & conjunctiva and wipes dust particles off the cornea
• What is 1 function of the eyelid?

  protects cornea from mechanical damage
• What is function of the iris?

  control amount of light entering the eyes
• What is the function of the pupil?

  allow light to enter eye
• What is the function of the sclera?

  protects eye from mechanical damage
• What is the function of the conjunctiva?

  secretes mucus to keep front of the eyeball moist
• What is the function of the eyelash?

  to shield eye from dust particles
• What is the function of the tear glands?

  wash away dust particles
• What is pupil reflex?

  Change in pupil due to light intensity
• Pupil reflex from normal to low light intensity:

  1. Light ray enters eye. Photoreceptors in eye detect decrease in light intensity, which are stimulated and produces nerve impulses.
  2. Sensory neurone transmits nerve impulses to relay neurone in brain, then to the motor neurone in the iris muscles.
  3. Radial muscles of the iris contract and circular muscles of the iris relax.
  4. Pupil dilates, increasing amount of light entering eye.
• Pupil reflex from normal to high light intensity:

  1. Light ray enters eye. Photoreceptors in eye detect increase in light intensity, which are stimulated and produces nerve impulses.
  2. Sensory neurone transmits nerve impulses to relay neurone in brain, then to the motor neurone in the iris muscles.
  3. Radial muscles of the iris relax and circular muscles of the iris contract.
  4. Pupil constricts, decreasing amount of light entering eye.
• define accommodation:

  focusing on distant or near objects
• focusing on distant objects:

  1. Light rays enter eye, ciliary muscles relax, pulling on suspensory ligaments.
  2. Suspensory ligaments become taut, pulling on edge of lens.
  3. Lens become thinner and less convex, increasing focal length.
  4. Light rays become sharply focused on fovea of retina.
  5. Photoreceptors of retina stimulated and produce nerve impulses.
  6. Nerve impulses are transmitted via sensory neurone to brain in relay neurone.
  7. Brain interprets the nerve impulses, allowing us to see the distant object.
• focusing on near objects:

  1. Light rays enter eye, ciliary muscles contracts, relaxing pull on suspensory ligaments.
  2. Suspensory ligaments slacken, relaxing pull on edge of lens.
  3. Lens become thicker and more convex, decreasing focal length.
  4. Light rays become sharply focused on fovea of retina.
  5. Photoreceptors of retina stimulated and produce nerve impulses.
  6. Nerve impulses are transmitted via sensory neurone to brain in relay neurone.
  7. Brain interprets the nerve impulses, allowing us to see the near object.