Properties of fast and slow alpha motoneurones following motor reinnervation.

1. The properties of medial gastrocnemius (fast alpha) and soleus (slow alpha) motoneurones of the cat were examined with intracellular electrodes 33-154 days after self- or cross-union of the muscle nerves.2. The original properties of fast and slow alpha motoneurones measured in terms of the axonal conduction velocity, the duration of after-hyperpolarization and the overshoot amplitude were restored at least in part after self-union of the muscle nerves.3. Slow alpha motoneurones recovered their original properties whether the muscle nerve was united to the original, slow red muscle or to the fast pale muscle. In two animals, some of the fast muscle fibres seemed to be doubly innervated by both fast and slow alpha motoneurones.4. The degree of restoration of the original motoneurone properties was closely related to the degree of motor reinnervation of the muscle.5. At a given stage of motor reinnervation, the degree of recovery of the motoneurone properties was approximately the same regardless of the presence or absence of functional motor connexions of the motoneurone under study.6. When the dorsal root ganglia were removed immediately before self-union of the muscle nerves, recovery of the conduction velocity of motoneurones was impeded without significant effect upon the process of motor reinnervation or upon recovery of the overshoot amplitude and the duration of after-hyperpolarization.7. It is concluded that the ;dedifferentiated' properties of fast and slow alpha motoneurones after nerve section may be ;redifferentiated' following motor reinnervation to any muscle. The possible signal for restoration of the motoneurone properties is discussed.