Reduction in amplitude of the ventral root elicited small spike by dorsal root stimulation in cat motoneurons

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Ventral root stimulation is known to produce two spikes, large and small, in the α-motoneuron. Since it is the small spike that triggers the outgoing impulse from the cell it is regarded as the key potential in information processing of the cell and referred to either as the IS-spike or as the A-spike depending upon whether one considers it as a purely single spike or as a composite spike. The small spike was elicited by stimulating the L 7 or S 1 ventral root in cats anesthetized with pentobarbital sodium and immobilized with d-tubocurarine. During the later phase of the excitatory prostsynaptic potential and thereto following inhibitory postsynaptic potential (IPSP) produced by stimulating the corresponding dorsal root the amplitude of the small spike was reduced by about 20%. This could be either explained as due to reduction in impedance of the membrane generating the small spike or interpreted as suggesting that the small spike is a composite spike and some of its components were inhibited by the IPSP. This interpretation was favored because the reduction occurred not when the safety factor for antidromic impulse transmission was high, but when it was low. (J Nippon Med Sch 1997; 64: 337-343)