SEQUENCE OF EVENTS IN SYNAPTIC ACTIVATION OF A MOTONEURONE

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Abstract
It was observed that, whether a motoneurone is activated antidromically, synaptically or directly (in the last case by an outward current applied to the soma), the intracellularly recorded spike invariably develops in two stages with a transition at a level of about 30 mV. There is a greater delay between the two stages, resulting in a more noticeable step, in the antidromic than in the synaptic or direct spikes. As the first stage of the antidromic spike has already been attributed to the soma, it is concluded that, however the motoneurone is activated, the soma is discharged first and then the dendrites. The failure of the first component of an antidromic spike to abolish a synaptic potential, while the second component does, was confirmed. From the diffuse distribution of the extracellular potential field produced by synaptic activity and other information, it is concluded that synaptic activity occurs to a large extent on the dendrites, but that the spike is, nevertheless, initiated in the soma because of the threshold being lower there.