Reflex and Reflex Arcs (after Stim-elicit)

• Reflex arc


  A sensory neuron, interneurons, and a motor neuron

  
• Stimulus driven reflex responses

  1. Stimulus

  2. Sensory neuron activation

  3. Interneuron activation

  4. Motor neuron activation
• Reflex arc


  • All synapses are excitatory
  • Cartoon representation, not the full reality
  • Only one of each neuron type shown, reality has many
  • Connections shown as single synapses, reality has many
• Somatosensory neuron


  Sensory neuron that innervates bodily tissues, likely nociceptive, chemoreceptive or thermoreceptive

  
• Trisynaptic reflex arc


  Reflex arc with two interneurons
• Disynaptic reflex arc


  Reflex arc with one interneuron
• Polysynaptic arc


  Reflex arc with more than three interconnections between neurons
• Monosynaptic skeletomotor reflex


  Simplest reflex arc, sensory neuron connects directly to motor neuron

  
• Phasic muscle stretch reflex


  One type of muscle stretch reflex
• Phasic stretch reflex arc


  • Sensory neuron cell bodies and axons in body, motor neuron cell bodies in spinal cord
  • Spinal reflex, brain not involved
• Sensory neuron cell bodies are in dorsal root ganglia within spaces formed by vertebral notches

  
• Muscle spindles


  Contain two types of sensory endings: primary (type Ia) and secondary (type II)
• Type Ia endings


  Primarily responsive to speed of muscle lengthening
• Type II endings


  Primarily responsive to amount of muscle stretch (slow adapting)
• Type II response to stimulation


  1. Sustained stimulus evokes sustained response

  2. Stronger stimulus evokes stronger response
• Type Ia response to stimulation


  1. Rapid stretch evokes stronger response than slow stretch

  2. Little response when length is not changing
• Tendon tap


  Elicits knee-jerk response
• Tendon tap produces hardly any change in muscle length, too small to be detected by spindle afferents
• The monosynaptic stretch reflex has an unclear purpose
• 6 Common Misconceptions about Reflexes


  • Simple responses
  • Stereotyped responses
  • Mediated by spinal circuits
  • Mediated by reflex arcs
  • Repeatable
  • Not acquired or modified by learning
• No reflex has all 6 misconception properties, many have none
• The phasic stretch reflex and corneal eye blink reflex have 4 of the 6 misconception properties
• Reflex responses are often not simple, involving many coordinated muscles