K3i / 14A01: Motor + sensory pathways involved in withdrawal reflex

Outline the motor and sensory pathways involved in withdrawing the lower limb from a painful stimulus.

26% of candidates passed this question.

It was expected that candidates would outline both motor and sensory pathways and mention a reflex arc and conscious pathways.


Reflex = involuntary and instantaneous movement in response to a stimulus

Withdrawal Reflex = A polysynaptic Reflex involving contact with a noxious stimulus resulting in flexion of limb in & inhibition of extensors in the same limb with simultaneous stabilisation of contralateral limb by extensor activation & flexor inhibition

Pain = an unpleasant sensory & emotional experience associated with actual/potential tissue damage

Physiology of pain involves:

  • Peripheral detection
  • Transmission (central + peripheral)
  • Modulation
  • Perception
  • Reflex responses

Peripheral Detection

  • Painful stimulus stimulates nociceptive aff n. fibres
  • Aδ/C fibres (fast + slow pain)
  • Release of NTs & neuropeptides by damaged tissue → K+, BK, leukotrienes, 5HT, histamine, Subs P
  • Mediate inflammation + further nociceptive activation


  • Activated nociceptor with DRG cell body enters SC via Dorsal Horn
  • Synapse with 2nd order neuron
  • In Dorsal Horn Laminae II, I, V
  • 2nd order neurons ascend in 2 tracts:

Spinothalamic (discriminates)

  • Crosses ML
  • Localises stimulus
  • Type of pain

Spinoreticular (behavioural)

  • Ascends ipsilateral to brainstem
  • Integrates signals
  • Emotional response
  • 2nd order neurons synapse in THALAMIC NUCLEI with 3rd order neurons
  • 3rd order neurons project through Internal Capsule to Cerebral (behavioural) + Somatosensory Cortex (discrimination)


Peripheral sensitisation

  • Tissue injury → NT/neuropeptide/inflammatory mediator release
  • P4 synthesis → lowering of threshold to activate nociceptors
  • ↑response to firing
  • Activation of silent nociceptors

Central sensitisation

  • Persistent stimulation of 2nd order neurons
  • ↑ membrane excitability
  • NMDA rec closely involved; ↑sensitivity to NTs
  • Prolonged C-fibre input with ↑rate of firing for same stimulus or even after stimulus has stopped

Descending inhibitory pathways

  • Originate in Somatosensory Cortex + Hypothalamus
  • Activate Periaqueductal Grey Matter → disinhibits GABA neurons
  • Activation of Nucleus Raphe Magnus → releases 5HT which activates inhibitory interneurons
  • Both descend to Medulla + SC
  • Inhibit asc. signals → provide analgesia


  • 2nd order neurons travel in Neospinothalamic & Paleospinothalamic tracts
  • Lateral spinothalamic tract → primarily responsible for transmitting to Somatosensory Cortex
  • Type of pain, location of stimulus

Reflex Response

  • Sensory neuron sends excitatory postsynaptic potential (EPSP) to somatic motor neuron originating in ventral horn
  • EPSP makes postsynaptic neuron more likely to activate/depolarise
  • 1° order neuron enters SC & diverges
    • Somatic motor neuron exits ventral horn to activate motor neurons to flexor muscles → eliciting flexion + withdrawal of affected muscles (IPSILAT)
    • Sensory neuron activates inhibitory interneuron
    • Inhibits Motor neurons to extensor muscles via inhibitory interneurons (IPSILAT)
    • Sensory neuron also activates an interneuron that decussates and crosses Midline of SC so that;
    • Interneuron synapses and excites somatic motor n stimulating CONTRALAT extensor muscles
    • Same interneuron inhibits flexors of CONTRALAT limb

∴ Withdrawal away from stimulus + crossed extensor reflex supports as body weight shifts (i.e. stepping on a pin)

The withdrawal Reflex