Repetitive monosynaptic activation of motoneurones

Abstract

Repetitive monosynaptic activation of motoneurones has been set up by maximum repetitive stimulation of the group I afferent fibres of the nerves to various muscles. By studying the effects produced by stimuli of varying strengths and also by comparing the responses of flexor and extensor motoneurones, it has been possible to allow for complications arising from concomitant stimulation of group II and III fibres in these nerves. Motoneurone responses have been recorded either as impulses discharged along the ventral root or as synaptic potentials electrotonically transmitted thereto. The repetitive synaptic potential conforms to a standard pattern for both flexor and extensor motoneurones. With frequencies over 100/sec. there is a brief initial phase of summation, then decline to a plateau at about the height of the initial single potential. On cessation of stimulation there is an immediate decline to a positive after-potential which bears a close resemblance to that observed after repetitive stimulation of peripheral nerve. The repetitive potential is compounded of 'diphasic' potentials generated by each successive volley, an initial negative synaptic potential and a later positive after-potential. At high frequencies of stimulation the successive potentials show an initial rapid decline in size to a low value, e.g. to 50% at 400/sec., which is only in part attributable to a decline in the size of the volleys entering the spinal cord. Focal recording has permitted simultaneous comparison of these two declines. The excitability of the motoneurones during and after repetitive activation was tested heterosynaptically. During the synaptic potential plateau the excitability is raised, with periodic variations in phase with the successive volleys, while depression is observed during the positive after-potential. With extensor muscles the reflex discharge of impulses conforms in general with the repetitive synaptic potential curve, the initial summation giving monosynaptic reflex discharges to the first two or three volleys at high frequency. It is probable that a motoneurone fires only once in this initial burst. Subsequently, the combination of autogenetic inhibitory action of group II fibres (which may however be negligible), diminished synaptic excitatory action, and depressant action of the accumulated positive after-potentials (subsynaptic depression) may prevent all further discharge. Alternatively, after a brief quiescent interval, a small irregular discharge may be set up monosynaptically by subsequent volleys. With low frequencies of stimulation the second and subsequent volleys may all evoke reflex discharges, but they are always smaller than the initial discharge. Repetitive activation of flexor motoneurones differs from extensors in that the autogenetic excitatory action of group II afferent fibres makes both the initial and later bursts of reflex discharge larger and more sustained.