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• Involuntary action


  Action that is not under conscious control of the individual
• Voluntary actions


  • Reading a book
• Involuntary actions


  • Withdrawing hand from a hot object
  • Peristalsis in oesophagus
• The responses of the human body to stimuli can be divided into voluntary actions and involuntary actions
• The human nervous system controls and coordinates organs and parts of the body
• Functions of the human nervous system

  • Detects stimuli
  • Sends information in the form of impulses
  • Interprets impulses
  • Produces appropriate responses
• Daily activities involving detection of stimuli and production of responses

  • Collecting garbage
  • Sneezing
  • Surfing the Internet
• Nervous system


  Controls and coordinates organs and parts of the body
• Functions of the human nervous system

  • Detects stimuli
  • Sends information in the form of impulses
  • Interprets impulses
  • Produces appropriate responses
• Daily activities involving detection of stimuli and production of responses

  • Collecting garbage
  • Sneezing
  • Surfing the Internet
• Voluntary actions


  Conscious actions conducted under one's will, controlled by the brain
• Pathway of impulse in a voluntary action

  1. Stimulus

  2. Affector (receptor)

  3. Nerve impulse

  4. Brain

  5. Nerve impulse

  6. Effector (muscle or gland)

  7. Response
• Involuntary actions are actions that occur immediately without conscious control or prior thoughts
• Involuntary actions


  • Involving medulla oblongata: Heartbeat
  • Breathing
  • Peristalsis
  • Secretion of saliva
  • Involving spinal cord (reflex actions): Withdrawing hand when it accidentally touches a hot object
  • Withdrawing foot when it accidentally steps on a sharp object
  • Sneezing when dust enters the nose
• Pathway of impulse in an involuntary action (reflex action)


  1. Affector (receptor)

  2. Spinal cord

  3. Direction of impulse

  4. Effector
• A damaged nervous system normally causes a person to become temporarily, partially or completely paralysed
• Humans have five sensory organs: eyes, ears, nose, skin and tongue
• Stimuli


  Changes in surroundings
• Parts of the eye

  • Sclera
  • Iris
  • Pupil
  • Cornea
  • Lens
  • Retina
  • Optic nerves
• Parts of the ear

  • Outer ear: Earlobe
  • Ear canal
  • Middle ear: Eardrum
  • Ossicles
  • Oval window
  • Eustachian tube
  • Inner ear: Cochlea
  • Semicircular canals
  • Auditory nerve
• Parts of the nose

  • Nasal cavity
  • Sensory cells for smell
• Sensory cells for smell

  Detect chemical substances in the air and produce nerve impulses
• Parts of the tongue

  • Papillae
  • Taste buds
  • Taste receptors
• Taste receptors


  Detect five basic tastes: sweet, salty, sour, bitter, umami
• Parts of the skin

  • Epidermis
  • Dermis
  • Fat layer
  • Pain receptors
  • Cold receptors
  • Heat receptors
  • Touch receptors
  • Pressure receptors
• The skin has five types of receptors to detect different stimuli
• Taste buds


  Receptors that detect the taste of food
• Tasting food


  1. Food dissolves in saliva

  2. Dissolved chemicals diffuse into taste buds

  3. Taste receptors stimulated

  4. Nerve impulses sent to brain

  5. Interpreted as sweet, salty, sour, bitter, umami
• Parts of the skin

  • Pain receptor
  • Cold receptor
  • Heat receptor
  • Touch receptor
  • Pressure receptor
• Skin receptors


  • Detect different stimuli
  • Send nerve impulses to brain to interpret
• Hearing


  1. Sound waves enter ear

  2. Vibrate eardrum

  3. Amplified by ossicles

  4. Sent to cochlea

  5. Nerve cells convert to nerve impulses

  6. Sent to brain
• Sight


  1. Light rays enter eye

  2. Focused on retina

  3. Photoreceptors produce nerve impulses

  4. Sent to brain

  5. Interpreted as upright image
• Sensitivity of skin


  • Depends on number of receptors and thickness of epidermis
  • Tip of finger very sensitive
  • Elbow, sole of foot less sensitive
• Tasting different tastes


  1. Gargle with water

  2. Apply drop of solution to different parts of tongue

  3. Identify taste
• Tongue sensitivity to tastes

  • Front - sweet
  • Sides - sour, salty
  • Back - bitter
  • Centre - umami
• Sense of smell and taste

  Pinching nose reduces ability to identify flavours
• Limitations of sight include optical illusions and blind spot
• Optical illusions occur when the brain is unable to accurately interpret what the eye sees due to distractions
• Blind spot is an area on the retina with no photoreceptors, so images that fall on it are invisible
• Various devices can be used to overcome limitations of sight