Cutaneous fiber groups involved in the inhibition of fictive locomotion in the rabbit.

Decorticate, paralyzed unanaesthetized rabbit preparations can display motoneuron discharges (spontaneous or elicited through various somatic stimulations) which are related to locomotion. These activities are suppressed when manual pressure is exerted on the dorso-lumbar skin. The present study indicates that: (1) locomotor discharges can also be inhibited through repetitive electrical stimulation of one of the skin nerves belonging to dorsal dermatomes (TH5 to L5); (2) other skin nerves, especially those of the limbs, do not present the same inhibitory properties; (3) among the various fiber groups composing these dorsal skin nerves, only A delta are inhibitory; other groups, on the contrary, display moderate (A alpha and A beta) or strong (unmyelinated C fibers) excitatory actions; (4) the inhibitory action of A delta fibers counteracts the excitatory action of C fibers, when both groups are stimulated together. The relationship between this form of motor inhibition and the well known phenomenon of "reflex immobility" is briefly considered.