Distribution of recurrent inhibition within a motor nucleus. II. Amount of recurrent inhibition in motoneurones to fast and slow units

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Abstract
The maximal recurrent inhibition was studied by intracellular recording from 43 triceps surae motoneurones (tentatively type‐identified by the biophysical properties of the neurones) and from 67 medial gastrocnemius motoneurones (type‐identified by the muscle unit properties; fast fatiguing, FF; fatigue resistant, FR and slow, S). Maximal homonymous recurrent IPSP (RIPSP) and input resistance (Rn) were measured at ‘resting’ membrane potential and close to firing threshold. The ‘synaptic current’ at the peak of the RIPSP was estimated (RIPSP/Rn). At ‘resting’ membrane potential the RIPSPs increased in the order FF < FR < S (0.9, 1.4, and 2.5 mV, respectively). This order was preserved when the ‘current’ (RIPSP/Rn) rather than voltage was considered, although the overall range was much reduced. When investigated close to firing threshold there were no significant differences in synaptic ‘current’. This apparent paradox may be explained by systematic differences in firing threshold between motor unit types; it increased in the order S < FR < FF (4.6, 8.9 and 13.5 mV, respectively).