Prolonged potentiation of transmission through a withdrawal reflex pathway after noxious stimulation of the heel in the rabbit.

The sural-gastrocnemius reflex of the spinalized rabbit was potentiated to an average of 3-6 times control levels after the application of noxious mechanical, thermal or chemical stimuli to the skin of the heel. Facilitation of the reflex was maximal within 1 min of the noxious stimulus, and in many cases persisted for more than 1 h. Prolonged increases in the excitability of the sural-gastrocnemius reflex were not seen after innocuous mechanical or thermal stimulation of the heel. Repetitive electrical stimulation of the sural nerve (100 shocks given at 0.5 Hz) caused persistent facilitation of the reflex when small myelinated A delta fibres or non-myelinated C-fibres were recruited by the conditioning stimulus. Such protracted increases in the excitability of the sural-gastrocnemius pathway would enhance the protective functions of this reflex. The mechanisms described here have probably evolved to provide a high level of reflex protection to the heel after tissue damage has occurred at that site.