Individual excitatory post‐synaptic potentials due to muscle spindle Ia afferents in cat triceps surae motoneurones.

Abstract

The monosynaptic projection of single Ia muscle spindle afferents to motoneurons of identified type was examined by spike triggered averaging in barbiturate anesthetized cats. The amplitude of averaged individual monosynaptic e.p.s.p. [excitatory postsynaptic potential] in medial gastroenemius motoneurons from homonymous Ia afferents was clearly related to the motor unit mechanical properties. It was largest in type S units, intermediate in type FR and smallest in type FF. The projection frequency of homonymous Ia afferents to medial gastrocnemius motoneurons followed the same order. Similar trends were evident in both respects for the heteronymous projections of triceps surae. The mechanical property of units which related most directly to e.p.s.p. size was muscle unit contraction strength, a single continuous relation existing throughout all 3 unit types. On the basis of motor axon conduction velocity measurements, it appeared that motoneuron size could provide a simple explanation for e.p.s.p. size in type S and type FR units, but not in type FF. The latter had unduly small e.p.s.p. relative to their axon conduction velocity, indicating a qualitatively different presynaptic organization for FF motoneurons from the others. Ia afferent excitation alone would cause recruitment of units in order of increasing contraction strength; this order would also obtain if a substantial part of the excitation came from other sources with uniform effect on all unit types. A consequence of the observed organization is that the gain of the Ia mediated stretch reflex would be approximately proportional to the developed force.