5.2 The nervous system

• What is the function of the nervous system?

  • So organisms can react to changes in their surroundings
  • Coordinate their behaviour
• Describe the overall structure of the human nervous system

  • Central nervous system (CNS) - brain & spinal cord
  • Other nerves run to and from the CNS
• How does information passes along neurones?

  As electrical impulses
• Define the term 'synapse'

  Small gap between two neurones
• Describe how electrical impulses pass across a synapse

  • When an electrical impulse reaches end of one neurone, a chemical is released
  • Chemical diffuses across synapse
  • Chemical attaches to the second neurone
  • Triggering another electrical impulse
• How does the nervous system respond to a stimulus?

  • Receptors
  • Sensory neurones
  • CNS
  • Motor neurones
  • Effectors
• How does the nervous system respond to a stimulus? (detailed)

  • Receptors detect stimulus, initiate an electrical impulse
  • Sensory neurones pass information from receptors to the CNS as electrical impulses
  • The CNS coordinates response of effectors
  • Motor neurones pass information from CNS to the effectors as electrical impulses
  • Muscles contract or glands secrete hormones
• What is a reflex action?

  • Automatic and rapid response
  • Doesn’t involve conscious part of the brain
• Why are reflex actions important?

  • Protective
  • helps minimise damage to body
• Describe how a reflex action occurs via a reflex arc

  • Receptors detect stimulus, initiate an impulse
  • Sensory neurone pass info from receptors to CNS as impulses
  • Relay neurones in spinal cord / brain pass impulses from sensory neurones to motor neurones
  • Motor neurones pass info from CNS to effectors as impulses
  • Muscles contract or glands secrete hormones
• What is the difference between coordination by the nervous system and the reflex arc?

  The reflex arc involves relay neurones in the unconscious brain / spinal cord
• Why does a conscious action take longer than a reflex action?

  • The impulse has to travel to the brain
  • Reflex actions don't involve the conscious part of the brain
• Reflex response to temperature:

  • (Temperature) receptors in the hand detect the heat
  • An impulse travels along the sensory neurone
  • Then travels along the relay neurone
  • And then the motor neurone
  • Chemical moves / diffuses across synapses
• Label the brain

  Use the image:

  A) cerebral cortex

  B) cerebellum

  C) medulla

  
• What is the function of the cerebral cortex?

  Consciousness, intelligence, memory and language
• What is the function of the cerebellum?

  Muscle coordination, movement and balance
• What is the function of the medulla?

  • Regulates unconscious activities
  • eg. heartbeat and breathing
  • Connected to the spinal cord
• Describe how the regions of the brain are mapped

  • Studying patients with brain damage
  • * To observe symptoms / changes in behaviour → indicate function of damaged region
  • Electrically stimulating different parts of the brain
  • * To observe effects on behaviour in live patients
  • Using MRI scanning techniques
  • * To observe which parts most active during different activities
• Explain the difficulties of investigating brain function

  • Brain is very delicate → surgery may cause unintended damage to another area
  • Brain is very complex → difficult to work out exactly which parts carry out specific functions
• Label the eye

  Use the image:

  A) suspensory ligaments

  B) cornea

  C) iris

  D) pupil

  E) lens

  F) ciliary muscles

  G) sclera

  H) retina

  I) optic nerve

  
• Retina

  Contains receptors sensitive to light intensity / colour → send impulses to optic nerve
• Optic nerve

  Made of many neurones → carries electrical impulses from retina to brain
• Sclera

  Tough white outer layer → protects the eye
• Cornea

  Transparent and curved layer at front of eye → refracts light into the eye
• Lens

  Focuses light onto retina
• Iris

  Controls pupil (hole) diameter to control how much light enters eye
• Ciliary muscles

  Contract to change lens shape
• Suspensory ligaments

  Help control lens shape
• How does the eye respond to dim light?

  • Pupil dilates (increase in diameter)
  • Radial muscles in the iris contract
  • Circular muscles in the iris relax
  • So more light enters the eye
• How does the eye respond to bright light?


  • Pupil constricts (decrease in diameter)
  • Radial muscles in the iris relax
  • Circular muscles in the iris contract
  • So less light enters the eye
• Define the term 'accommodation'


  Changing shape of lens to focus on near or distant objects
• How does the eye focus on near objects?

  • Ciliary muscles contract
  • So suspensory ligaments loosen
  • So lens thickens (becomes more curved)
  • So lens refracts light rays more
• How does the eye focus on distant objects?

  • Ciliary muscles relax
  • So suspensory ligaments tighten
  • So lens is pulled thin
  • So lens refracts light rays less
• What is myopia?

  • Shortsightedness
  • Can't focus on distant objects
• What is hyperopia?

  • Longsightedness
  • Can't focus on near objects
• What is the cause of myopia?

  • Rays of light from distant objects focus in front of retina
  • As the eyeball is too long
• What is the cause of hyperopia?

  • Rays of light from near objects focus behind retina
  • As the eyeball is too short
• How is myopia treated?

  • Concave lenses
  • Light rays diverge / bend outwards more
  • So light rays focus on retina
• How is hyperopia treated?

  • Convex lenses
  • Light rays refracted more
  • So light rays focus on retina
• Describe how eye defects can be treated using technology

  • Hard (longer lasting) and soft (more comfortable, disposable) contact lenses
  • Laser eye surgery - changes cornea shape
  • Replacement lenses in eye