Differential reaction of fast and slow α‐motoneurones to axotomy

1. The properties of medial gastrocnemius (m.g., fast α) and soleus (sol., slow α) motoneurones of the cat were examined with intracellular electrodes 8–119 days after section of the muscle nerves. 2. The axonal conduction velocity was significantly decreased in both m.g. and sol. motoneurones after chronic section of the muscle nerves. 3. The amplitude of overshoot of action potentials was significantly increased in both m.g. and sol. motoneurones following section of the muscle nerves. 4. No significant changes in the resting membrane potential or the input resistance were observed for sol. motoneurones, whereas m.g. motoneurones showed a slight decrease in the resting potential and a slight increase in the input resistance. 5. The duration of after‐hyperpolarization was significantly decreased in sol. motoneurones, whereas that in m.g. motoneurones remained virtually unchanged or increased slightly following section of the muscle nerves. 6. The changes described above were not seen in the preparations examined 29–46 days after section of the lumbosacral dorsal roots, suggesting that alterations in the motoneurone properties observed after section of the muscle nerves resulted from axotomy of the motoneurones rather than from sensory deprivation. 7. The differences in electrophysiological properties between m.g. and sol. motoneurones were less prominent in axotomized animals than in control, unoperated cats. 8. It is concluded that fast (m.g.) and slow (sol.) α‐motoneurones have qualitatively different properties. A possible ‘dedifferentiation’ of fast and slow α‐motoneurones by axotomy is discussed.