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