Alterations of synaptic action in chromatolysed motoneurones of the cat

Abstract

1. Monosynaptic EPSPs in lumbosacral motoneurones undergoing chromatolysis were studied by intracellular recording from 7 to 20 days after section of the appropriate ventral roots of the cat. 2. The maximum monosynaptic EPSPs evoked in chromatolysed motoneurones by afferent volleys from the biceps‐semitendinosus or the triceps surae muscles ranged from 1·0 to 9·5 mV in amplitude. The time‐to‐peak of these EPSPs was 1·7 msec on the average. These values were significantly smaller and longer, respectively, than the amplitude and the time‐to‐peak of monosynaptic EPSPs observed in normal motoneurones. The long time‐to‐peak of EPSPs in chromatolysed motoneurones could not be accounted for by asynchronous transmitter release. 3. The mean number of unit EPSPs responding to a single afferent impulse (m) in chromatolysed motoneurones was comparable to that found in normal motoneurones. 4. The amplitude of unit EPSPs estimated from the mean EPSP amplitude and the m value following stimulation of a single afferent fibre was significantly smaller in chromatolysed motoneurones than in normal motoneurones. This difference was attributed to a difference in synaptic location. 5. The shape of monosynaptic EPSPs evoked in chromatolysed motoneurones by stimulation of single afferent fibres was analysed on the basis of Rall's compartment model. The analysis suggested that there is a lack of the excitatory synaptic input to the cell body in chromatolysed motoneurones. 6. Similar alterations were also found in IPSPs. The degree of change in synaptic responses evoked by stimulation of various pathways appears to depend on the synaptic location. 7. Following the study of the interaction of several inputs on the motoneurone and of their dependence on membrane potential, a tentative model of the synaptic distribution of different pathways is proposed.