Question 1

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

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College Answer

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


It seems disingenuous to expand on the question "outline the motor and sensory pathways" by explaining that candidates were expected to "outline both motor and sensory pathways". 

Sensory pathway:

  • Nociceptor transduces a painful stimulus into an action potential
  • This action potential travels along a pain fibre (Aδ fibre or C fibre)
  • Past the body of the neuron, which lies in the dorsal root ganglion
  • Into the spinal cord via the dorsal root
  • Up (or down) the tract of Lissauer
  • Into the dorsal horn grey matter
  • Here, the primary afferents synapse with dorsal horn neurons
  • Dorsal horn neurons then send projections across midline to the contralateral spinal cord, where their fibres form the ascending spinothalamic tract
  • The spinothalamic tract fibres synapse with tertiary neurons in the thalamus

Motor pathway:

  • Interneurons connect the dorsal horn neurons with anterior horn neurons (specifically,  efferent γ-motor neurons) creating an involuntary fast reflex arc.
  • Both ipsilateral and contralateral projections are required
  • Ipsilateral fibres cause contraction of flexors and relaxation of extensors of the affected limb
  • Contralateral fibres cause relaxation of flexors and contraction of extensors of the opposite limb
  • The net effect is a flexion withdrawal of the affected limb from the painful stimulus
  • Extensor contraction of the opposite limb is a "bracing" response
  • More complex interneuron reflex activity occurs to affect the upper limbs, so that balance can be maintained

Conscious pathway:

  • Following the central processing of pain in the sensory cortex, a conscious movement may be initiated in response to the painful stimulus
  • From the motor cortex upper motor neurons send projections which decussate at the level of the pyramids
  • Down the spine via the contralateral lateral corticospinal tract
  • Synapse with a lower motor neuron in the anterior horn
  • Via the lower motor neuron, innervate selected muscle groups involved in the conscious response


Derderian, Celena, and Prasanna Tadi. "Physiology, withdrawal response." StatPearls [Internet] (2019).

J.A. Simpson, W. Fitch, "Chapter 17 - Reflex activity". Applied Neurophysiology, Butterworth-Heinemann, 1988

Almeida, Tatiana F., Suely Roizenblatt, and Sergio Tufik. "Afferent pain pathways: a neuroanatomical review.Brain research 1000.1-2 (2004): 40-56.