Knee Jerk Reflex Lecture 1

• The 'knee jerk reflex'


  A simple neural circuit
• Key learning objectives


  • Outline the basic input/output features of a 'typical neurone'
  • Define the major components of a typical reflex arc
  • Describe the function and properties of the 'knee jerk reflex'
  • Discuss how the structure of the muscle spindle is related to its function
  • Compare and contrast the muscle spindle and Golgi tendon organ
• Information flow in the Central Nervous System

  1. Sensory receptor

  2. CNS (spinal cord & brain)

  3. Effector
• Neurone


  Functional unit of the nervous system
• Neurone


  • Dendrites
  • Cell body (soma)
  • Synapse (presynaptic bouton, nerve terminal)
  • Axon
• Real neurons: morphology related to function
• Simple reflex arc


  1. Sensory receptor

  2. CNS (spinal cord & brain)

  3. Effector
• Reflex


  Stereotypical motor response to a stimulus independent of conscious thought
• Delay between sensing and motor action: REACTION TIME
• A simple neural circuit: the 'knee jerk' reflex arc

  Stretch of muscle activates reflex arc to cause same muscle to contract
• Patellar (knee jerk) reflex examination

  • Normal vs hyperreflexia (patient with stroke affecting right, but not left, leg)
• Components of a monosynaptic reflex arc: 'knee jerk' reflex

  • Skeletal muscle (Effector)
  • Spinal cord
  • Tendon
  • Sensory neurone (afferent)
  • Muscle spindle (Sensory receptor)
  • Motor neurone (efferent)
  • Synapse
• Muscle spindle


  A sensory receptor responsive to stretch
• Structure of the muscle spindle

  • Modified muscle fibres 10mm long, 10μm diameter in parallel to contractile fibres
  • Highly innervated
  • Monitors muscle length
• Muscle length changes


  Spindle signals to CNS
• Stretch muscle


  Contract muscle via changes in action potential frequency
• α-motorneurones


  Decrease during contraction, activate to reset spindle tonic frequency and increase dynamic range
• Muscle spindles maintain posture in response to stimulus, allow compensatory contraction to support increasing load
• Components of a polysynaptic reflex arc: Golgi tendon organ (GTO) reflex


  • Skeletal muscle (Effector)
  • Spinal cord
  • Golgi tendon organ (Sensory receptor)
  • Sensory neurone (afferent)
  • Inhibitory interneurone
  • Motor neurone (efferent)
• Golgi tendon organ


  Prevents damage if load too great by relaxing muscle
• Antagonistic muscle regulation to allow movement: divergence to mono- and poly- synaptic reflex arcs