MOTOR-NEURON BISTABILITY - A PATHOGENETIC MECHANISM FOR CRAMPS AND MYOKYMIA

In three patients suffering from chronic muscle cramps, spasms and myo kymia, these involuntary contractions were triggered in the triceps su rae, quadriceps, flexor carpi radialis or flexor digitorum by means of single or short-train stimulation of homonymous la afferents, elicite d by electrical means or tendon taps. In some cases cramp was induced by the first afferent volleys; more often, however continued stimulati on produced stepwise recruitment of motor units (whose rhythmic firing was visible as myokymia in the muscle) until cramp developed. Cramps and myokymic discharges could usually be terminated by a single maxima l stimulus to the motor axons (producing antidromic invasion and Rensh aw inhibition of the motor neurons), or by short trains of volleys in inhibitory pathways from the skin. The fact that it was possible to in duce myokymia and cramps by brief synaptic excitation and terminate th em by antidromic invasion or synaptic inhibition, suggests that the me chanism generating these disturbances is intrinsic to a-motor neuron s omata. Similar on-off switching of self-sustained motor discharges has been observed in the decerebrate cat and is known to depend on 'bista bility' of the motor neuron membrane. We propose that a similar mechan ism is responsible for discharges that produce cramp.