Functional electrical stimulation and rehabilitation--an hypothesis.

Functional Electrical Stimulation (FES), used to mimic a weak or paralysed movement, sometimes is followed by a specific recovery of voluntary power in that movement. The mechanism by which this occurs is unclear, and the presumption has often been that FES may somehow promote adaptive changes in cortical connectivity. However, the unique feature of electrical stimulation is that it activates nerve fibres both orthodromically and antidromically. The antidromic impulse in motor nerve fibres will reach the anterior horn cell, but it can go no further up the neuraxis. If the corticospinal-anterior horn cell synapse is a Hebb-type modifiable synapse (i.e. one that is strengthened by the coincidence of presynaptic and postsynaptic activity), then FES, combined with coincident voluntary effort through a damaged pyramidal motor system, could help to promote restorative synaptic modifications at anterior horn cell level, by this unique adaptive mechanism.