Monosynaptic Stretch Reflex Arc

• Monosynaptic Stretch Reflex Arc pt1:

  • reflexes require a sensory stimulus (e.g. muscle spindle for stretch) and a motor response
  • 1A afferents (in blue) conduct action potentials at 150 metres ps up the axon to the spinal cord via dorsal root, past dorsal root ganglion
  • in spinal cord it synapses (synapse & ap lect) with an alpha motor neurone (in red) and its axon comes out of the ventral root to the corresponding muscle, to the neuromuscular junction, causing contraction
  • so this is a reflex loop where lengthening/stretch of muscle causes shortening/contraction to counter the lengthening

  
• Monosynaptic Stretch Reflex Arc pt2:

  • in green is an inhibitory interneuron
  • when 1A afferents take action potentials to the spinal cord, a branch of it synapses to that interneuron, depolarising the interneuron, causing it to generate action potentials which travel down to its inhibitory synapse to inhibit the alpha motor neurone that supplies the antagonist muscle
  • so at the same time as getting excitation of the alpha motor neurone that supplies the agonist, theres inhibition of the alpha motor neurone that supplies the antagonist

  
• Monosynaptic Stretch Reflex Arc pt3:

  • this is the reflex loop that is often affect in many pts, many of which experience spasticity - a velocity dependent increase in the stretch reflex (hyperresponsive stretch reflex)
  • stretch reflex is very vulnerable - some pts (e.g. stroke) will have abnormal gamma motor neurones, so will cause abnormal sensitivity of muscle spindle, causing abnormal outputs from spindle, causing abnormal reflex activity
  • could be due to muscle shortening or positioning of pts has altered length tension relationship at the spindle, inappropriately affecting stretch reflex