The Human Brain : From Neurone to Nervous System


  1. When an injurious stimulus is applied to the skin, there is a reflex withdrawal of the limb away from the stimulus. The mechanism of withdrawal involves a polysynaptic pathway that initiates and maintains contraction of flexor muscles.

  2. The flexor withdrawal reflex has a polysynaptic pathway that initiates withdrawal of a limb from a noxious stimulus and maintains the withdrawn position.

  3. Nociceptors in the skin initiate activity in polysynaptic circuits within the spinal cord that :
    • cause flexor muscles to withdraw the limb and
    • maintain that new position and
    • extend the contralateral limb to support the body
    Although pain normally accompanies injury, the withdrawal of the limb does not depend on the perception of pain. Pain perception depends on higher levels of the CNS, and the flexor reflex persists after spinal transection.

  4. The Crossed Extensor Reflex is part of the flexor withdrawal reflex and ensures that extensors in the opposite limb contract so as to maintain posture and support the weight of the body while the opposite limb is flexed. The pathways that accomplish this are polysynaptic and cross the midline of the spinal cord.

  5. Key Words: nociceptors, flexor withdrawal reflex, crossed extension reflex, polysynaptic pathway