The reflex arc

• Nerve cell


  Also called a neuron
• Nerve cell


  • Long
  • Thin
  • Lots of branch connections to either end
  • Adapted to carry electrical impulses from one point to another
• Synapse


  Connection between nerve cells where electrical impulses are converted to chemical signals to pass between cells
• Nerve cell communication


  1. Electrical impulse hits end of nerve

  2. Causes release of chemicals

  3. Chemicals diffuse across gap to next nerve cell

  4. Triggers another electrical impulse

  5. Electrical impulse continues along new neuron
• Central nervous system


  • Made up of brain and spinal cord
  • Where 'thinking' takes place
  • Takes in sensory information, decides what to do, sends out orders to the body
• Sensory neurons


  Carry information from receptors all over the body to the central nervous system
• Motor neurons


  Carry impulses from the central nervous system to effectors (muscles or glands)
• Reflex arc


  Nerve pathway that underlies unconscious reflexes
• Reflex arc


  1. Stimulus detected by receptor cells

  2. Sensory neuron carries impulse to spinal cord

  3. Relay neuron transfers impulse to motor neuron

  4. Motor neuron carries impulse to effector (muscle) to cause movement
• Insignificant stimuli


  Stimuli like the feel of shoes on your feet that are filtered out to prevent overloading of the brain
• Constant stimulation eventually tires out the synapse and the nerve impulse stops
• If the stimulus is too weak

  The impulse is not transmitted, preventing overloading of the CNS
• Anaesthetics and sedatives, including alcohol


  Block the transmission of the nerve impulse, preventing the sensation of pain
• Reflex action


  A quick, automatic response of an effector to a stimulus received by a receptor
• The natural knee-jerk reflex


  • Provides a way of testing a person's reflexes
• Reflexes, such as walking, cycling, writing and reading

  Learnt or conditioned reflexes, learned through repetition
• Reflex arc


  The pathway along which nerve impulses are carried from a receptor to an effector to bring about a reflex action
• Parts of a reflex arc

  • Receptor
  • Sensory neuron
  • Interneuron
  • Motor neuron
  • Effector
• Functioning of a reflex arc

  1. Receptor detects stimulus and converts it into nerve impulse

  2. Sensory neuron conducts impulse to CNS

  3. Interneuron serves as reflex centre and conducts impulse to motor neuron

  4. Motor neuron conducts impulse to effector

  5. Effector responds to original stimulus
• The automatic withdrawal of the hand from a painful heat stimulus (flame) is an example of a reflex arc through the spinal cord, resulting in a protective reflex action