Post-tetanic changes in the discharge pattern of the extensor alpha motoneurones.

The after effects of orthodromic tetanization on the discharge pattern of gastrocnemius motoneurones were investigated in decerebrate and spinalized cats. tduring stimulation of intact gastrocnemius nerves at 20 Hz, small motoneurones showed ttion for 8 sec, large motoneurones followed the 20 Hz stimulation in 1:1 order, whereas small motoneurones became silent for about 37 sec. Thereafter, the impulse rate of large motoneurone decreased while that of small motoneurone increased, progressively reaching the pre-tetanic values in about 72 sec. When gastrocnemius nerves were tetanized for 20 sec, potentiation of large motoneurones and depression of small motoneurones lasted for about 120 sec. Post-tetanic potentiation of large motoneurone was still observed during stimulation with 40 Hz but not with 80 Hx. Inhibition of a small tonic motoneurone during repetitive activity of a large motoneurone was observed during stimulation of Group I + II afferents following injection of 0.025 mg/kg strychnine. Another strychnine injection of the same dose reduced this inhibition. It was concluded that large-phasic motoneurones may be highly susceptible to post-tetanic potentiation; and that small-tonic motoneurones may be inhibited by large motoneurones through a recurrent circuit.