The nervous system and the eye

• Central Nervous System (CNS): Brain and Spinal Cord

  Peripheral Nervous System (PNS): Cranial Nerves and Spinal Nerves
• Sensory neurons transmit nerve impulses from receptors to CNS.

  A) synapse

  B) nerve fibre

  
• Relay neurone transmits nerve impulses from the sensory neurone to the motor neurone. They are found within the central nervous system.

  A) synapse

  
• Motor neurones transmit nerve impulses from the central nervous system (CNS) to the effectors.

  A) synapse

  
• Neurone similarity and difference:

  
  

  Similarities:

  1. They all have a cell body

  
  

  Difference:

  1. The cell body of a sensory neurone is in the middle, along the nerve fibre while the cell body for the relay and motor neurones are at the end of the cell.
  2. Sensory neurones and Motor neurones have myelin sheath while relay neurone does not.
• Reflex action is the immediate response to specific stimuli and cannot be consciously controlled or prevented. It is immediate because the nerve impulses are transmitted along the reflex arc, the shortest nervous pathway from receptor to effector.
• Knee Jerk Reflex:

  1. The tap in the knee stimulates the stretch receptors.
  2. Stretch receptor generate nerve impulses.
  3. The nerve impulse is transmitted along the sensory neurone
  4. across a synapse
  5. onto a relay neurone in the spinal cord.
  6. Nerve impulses are transmitted across ANOTHER synapse to the motor neurone.
  7. Nerve impulses are transmitted along the motor neurone to the effector, the leg muscles.
  8. The leg muscles then contract, straightening the leg to create the knee jerk effect.
• Hand Withdrawl Reflex: (eg. bee sting)

  1. Receptors in the skin detect the bee sting.
  2. The receptors generate nerve impluse.
  3. The nerve impulse is transmitted along the sensory neurone
  4. across a synapse
  5. onto a relay neurone in the spinal cord.
  6. Nerve impulses are transmitted across ANOTHER synapse to the motor neurone.
  7. Nerve impulses are transmitted along the motor neurone to the effector, the arm muscles.
  8. The arm muscles then contract, moving the hand away from the bee.
• Voluntary actions are responses that can be either slow or fast, meaning that it is consciously controlled and do not require a stimulus.
• Pupil Reflex: (eg. increase in light intensity)

  1. Photoreceptors in the retina detect an increase in light intensity and generate nerve impulses.
  2. The nerve impulses are transmitted along the sensory neurone in the optic nerve
  3. across a synapse
  4. onto the relay neurone in the brain.
  5. Nerve impulses are transmitted across another synapse onto the motor neurone
  6. which transmit nerve impulses to the effectors, the iris muscle.
  7. The circular iris muscles contract, radial muscles relax, and the pupil constricts to allow less light to enter the eye.
• Iris muscles contract in bright light to constrict the pupil and reduce light entering the eye and relax in dim light to allow more light to enter the eye.
• Radial muscles relax in bright light to allow more light to enter the eye and contract in dim lights to constrict the pupil and reduce light entering the eye.
• Focusing on distance objects:

  1. Ciliary muscles relax, pulling on suspensory ligaments.
  2. Suspensory ligaments become taut, pulling on the edge of the lens.
  3. Lens becomes thinner and less convex.
  4. Light rays from the distant object are sharply focused on the retina.
  5. Photoreceptors are stimulated.
  6. Nerve impulses produced are transmitted by the optic nerve to the brain. The brain interprets the nerve impulses and the person sees the distant object.
• Focusing on near objects:

  1. Ciliary muscles contract, relaxing their pull on suspensory ligaments.
  2. Suspensory ligament slackens, relaxing their pull on the lens.
  3. The lens becomes thicker and more convex.
  4. Light rays from near object are sharply focused on the retina.
  5. Photoreceptors are stimulated.
  6. Nerve impulses produced are transmitted by the optic nerve to the brain. The brain interprets the nerve impulses and the person sees the near object.
• Focusing is the adjustment of the lens of the eye so that clear images of objects at different distances are formed on the retina.
• How do we see:

  1. The light rays are refracted through the cornea and the aqueous humour onto the lens.
  2. The lens causes further refraction and the rays are brought to a focus on the retina.
  3. The image on the retina stimulates either the rods or cones (photoreceptors), depending on the intensity of the light. The image formed is inverted, laterally inverted and diminished.
• struture of eye

  A) tear glands

  B) conjunctiva

  C) eyelashes

  D) cillary body

  E) suspensory ligaments

  F) aqueous chamber

  G) blind spot

  H) fovea

  I) vitreous chamber

  
• Sclera:

  • The white part of the eye that surrounds the iris
  • Protects the eye from mechanical damage
• Conjunctiva:

  • A thin transparent membrane covering the sclera
  • Secretes mucus to keep the front of the eyeball moist
• Eyelashes:

  • Shield the eye from dust particles
• Tear glands:

  • Secret tears that wash away dust particles
  • Keep the cornea moist for atmospheric oxygen to dissolve, allowing dissolved oxygen to diffuse into the cornea
  • Lubricate the conjunctiva, reducing friction when the eyelid moves
• Eyelid:

  • Protects the eye from mechanical damage
  • Can be partly closed (squinting) to prevent too much light from entering the eye and damaging the retina
  • Blinking spreads tears over the cornea and conjunctiva, wiping dust particles off the cornea
• Iris:

  • A circular sheet of muscles that controls the size of the pupil
  • Regulates the amount of light entering the eye
• Pupil:

  • A hole in the center of the iris that allows light to enter the eye
• Retina:

  • Innermost layer of the eyeball
  • Contains photoreceptors connected to nerve endings from the optic nerve
• Lens:

  • Transparent, circular, and biconvex structure
  • Elastic and changes shape or thickness to focus light onto the retina
• Blind spot:

  • Region where the optic nerve leaves the eye
  • Does not have cones or rods, so it is not sensitive to light
• Optic nerve:

  • Transmits nerve impulses to the brain when photoreceptors in the retina are stimulated
• Vitreous chamber:

  • Space behind the lens filled with a transparent, jelly-like substance called vitreous humor
  • Keeps the eyeball firm and helps refract light onto the retina
• Ciliary body:

  • Thickened region at the front end of the choroid
  • Contains ciliary muscles that control the curvature or thickness of the lens
• Suspensory Ligaments:

  • Connective tissue that attaches the edge of the lens to the ciliary body
• Cornea:

  • Dome-shaped transparent layer continuous with the sclera
  • Refracts or bends light rays into the eye, causing the greatest refraction of light
• Aqueous chamber:

  • Space between the lens and the cornea filled with aqueous humor, a transparent and watery fluid
  • Keeps the front of the eyeball firm and helps refract light into the pupil
• Choroid:

  • Middle layer of the eyeball
  • Prevents internal reflection of light
  • Contains blood vessels that bring oxygen and nutrients to the eyeball and remove metabolic waste products
• Fovea:

  • Small depression in the retina directly behind the lens
  • Contains only cones, enabling detailed colored vision in bright light