Tonic and Phasic Ventral Horn Cells Differentiated by Post‐Tetanic Potentiation in Cat Extensors.

Abstract

Summary.: In decerebrate cats stretch reflexes from the ankle extensors have been studied in 100 fibres from dissected ventral root filaments. These reflexes have been maximally facilitated by post‐tetanic potentiation, as describthd by G ranit (1956 a). In the post‐tetanic or potcwtiated state the reflexes to a constant 10 mm stretch haw been found to fall into two main categories: (i) tonic reflexes which go on firing for a long duration in maintained stretch and (ii) phasic ones which only fire one or two spikes on the rising phase of stretch, in spite of any amount of post‐tetanic potentiatioit. Analysis of spike size showed that the tonic ventral horn cells tended to fire smaller spikes than the phasic ones. From this it was concluded that the tonic motoneurones tend to group themselves among the smaller ventral horn cells. The difference between tonic and phasic ventral horn cells was maintained when the extensor tonus was raised by decortications of the anterior lobe of the cerebellum.