Nervous system and eye

• Nervous system


  Brain, spinal cord and nerves that serve to co-ordinate and regulate bodily functions

  
• Receptors


  Sense organs that detect stimuli
• Sensory neurones


  Transmit nerve impulses from receptors to central nervous system

  
• Relay neurones


  Transmit nerve impulses within the central nervous system

  
• Motor neurones


  Transmit nerve impulses from central nervous system to effectors

  
• Effectors


  Muscles or glands that produce a response
• Reflex action


  1. Receptor stimulated

  2. Sensory neurone transmits impulse to central nervous system

  3. Relay neurone transmits impulse

  4. Motor neurone transmits impulse to effector

  5. Effector produces response

  
• Reflex action


  • Immediate response to a specific stimulus without conscious control
  • Controlled by brain or spinal cord
• Types of reflex actions

  • Cranial reflexes
  • Spinal reflexes
• Synapse


  Junction between two neurones where chemicals are released to transmit nerve impulses

  
• The central nervous system integrates nerve impulses from various parts of the body

  
• The spinal cord is involved mostly in automatic actions known as reflex actions
• The brain integrates visual, auditory, touch, olfactory and taste information from sensory organs
• Stimulus to a sense organ


  Response produced
• Voluntary actions


  • Controlled consciously
• Involuntary actions


  • Not controlled consciously
• Components of the human nervous system

  • Central nervous system (brain and spinal cord)
  • Peripheral nervous system (cranial and spinal nerves, sense organs)

  
• Receptors in sense organs are stimulated to produce nerve impulses, which are transmitted to the central nervous system and then to effectors

  
• The central nervous system works to integrate nerve impulses from various parts of the body

  
• The brain is more sophisticated and integrates information from sensory organs
• Sensory neurone


  • Transmits nerve impulses from receptors to central nervous system

  
• Relay neurone


  • Transmits nerve impulses within the central nervous system

  
• Motor neurone


  • Transmits nerve impulses from central nervous system to effectors

  
• Neurones release chemicals at synapses to transmit nerve impulses

  
• The brain, spinal cord and spinal nerves work together as part of the central and peripheral nervous systems

  
• Pathways of nerve impulses

  • Reflex arc
  • Receptors to brain
  • Brain to effectors (voluntary action)

  
• Reflex actions are immediate responses to specific stimuli without conscious control
• Cranial reflexes are controlled by the brain, while spinal reflexes are controlled by the spinal cord
• The brain and spinal cord are reflex centres
• A reflex arc is the shortest pathway for nerve impulses in a reflex action
• Reflex action of touching a hot object

  1. Thermoreceptors in skin detect heat

  2. Sensory neurone transmits impulse to spinal cord

  3. Relay neurone transmits impulse

  4. Motor neurone transmits impulse to arm muscle

  5. Arm muscle contracts to withdraw hand
• Comparison of voluntary, involuntary and reflex actions

  • Voluntary action: Controlled by will, requires stimulus, regulated by brain, choice-dependent speed
  • Involuntary action: Not controlled by will, does not require stimulus, regulated by brain, slow or quick
  • Reflex action: Sudden response to stimulus, requires stimulus, regulated by brain or spinal cord, always quick
• When the spinal cord is damaged, a person may be paralysed from the neck down
• Brain implant technology enables paralysed people to move their limbs
• The conjunctiva is a mucus membrane covering the sclera and secreting mucus to keep the front of the eyeball moist
• The cornea is a dome-shaped transparent layer that refracts light into the pupil
• The iris contains muscles that control the size of the pupil
• Eyelashes and eyelids protect the eye from mechanical damage and excessive light
• Tear glands secrete tears to wash away dust, keep the cornea moist, and lubricate the conjunctiva
• The sclera is the tough, white outer covering of the eyeball that maintains its shape