Question 15

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

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College Answer

A good answer required a description of the sensory pathway(s) (eg nociceptors, Aδ and C 
fibres, spinal dorsal horn, spinothalamic, thalamic and cortical pathways), reflex arc (nocioceptive 
sensory fibres synapse with spinal inter-neurons that in turn synapse with peripheral motor 
neurons supplying the lower limb as well as inter-neurones that also synapse with motor neurons 
on the contra-lateral lower limb producing a crossed extensor response), central integration and 
the motor pathways (fibers from the contra-lateral motor (and pre-motor) cortex pass through the 
posterior internal capsule forming the lateral and ventral cortico-spinal tracts, the cortico-spinal 
tracts pass through the anterior brainstem, the lateral tract decussating in the caudal medulla, 
continue to synapse with spinal motor neurons in the ipsilateral lumbosacral anterior horn cells, 
passage via peripheral nerves and flexor muscle stimulated, extensors inhibited, resulting in 
withdrawal of the limb) Common mistakes included errors in naming nerve pathways and 
receptors. Another error was to confuse the polysynaptic pain response pathways with the 
monosynaptic stretch reflex. Very few candidates mentioned feedback regulation via cerebellar 
input and at spinal level from muscle spindles and Golgi tendon organs 

Discussion

This college answer is excellent, insofar as it clearly demonstrates what was expected. It does not, of course, create any impression in the mind of the revising exam candidate as to what sort of structure was expected for the answer. Certainly unstructured prose would not be an acceptable solution, as it would be hard to mark, and moreover a waste of the exam candidates' precious time (all those useless adverbs). A pointform scaffold with some headings is called for. What follows is just one possible variant:

  • Sensory organs
    • Nociceptor nerve endings
  • Afferent pathways
    • Peripheral sensory nerves (C-fibres)
    • Dorsal root of the spinal cord
    • Ipsilateral dorsal horn neurons 
    • Projections decussate to the contralateral side, then ascend the spine via the spinothalamic tract
    • Terminate in the thalamus
    • Projections then ascend to the sensory cortex
  • Central processor organ
    • Sensory and motor cortex 
  • Efferent pathway
    • ​​​Fibres from the contralateral motor cortex collect into the corona radiata
    • Then down through the anterior brainstem as the pyramides
    • Decussate at the braisntem 
    • Descend ipsilaterally as the corticospinal tract
    • Synapse with lower motor neurons at the level of the spinal nerve root
      (anterior horn)
    • Lower motor nerve fibres project via the anterior 
  • Effector organs
    • ​​​​​​​Lower limb flexor and extensor muscle groups
    • Flexor muscle groups contract, extensor groups relax
  • Feedback pathways
    • Proprioception and muscle stretch feedback to the cerebellum results in regulatory contraction of the extensors and moderation of the withdrawal response

The best reference to describe this reflex is Chew et al (2010), and they definitely felt that the cortex was involved, which means one cannot answer this question without involving the cortical (i.e. suprathalamic) processing organs. That paper is also distinguishable by the researcher's decision to test the foot withdrawal reflexes of medical students by using massive weird larvae to elicit a ticklish sensation. "Subjects were required to inform if they were fearful of the worm", the authors chuckled.  Surely, surely there must have been another way. 

References

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

Chew, K. S., et al. "Withdrawal response in healthy adults." Neurol Asia 15.2 (2010): 159-65.