THE EFFECTS OF TETRODOTOXIN-INDUCED MUSCLE PARALYSIS ON THE PHYSIOLOGICAL-PROPERTIES OF MUSCLE UNITS AND THEIR INNERVATING MOTONEURONS IN RAT

1. Although the inactivity of a slow muscle (cat soleus) induced via n erve impulse blockade has been demonstrated to have some axotomy-like effects (decreased after-hyperpolarization (AHP) duration) on its inne rvating motoneurons, the reported effects of inactivity on motoneurons which innervate fast muscles containing mixtures of motor unit types are equivocal. This study was designed to determine the effect of a pe riod (2 weeks) of complete hindlimb muscle paralysis, via tetrodotoxin (TTX) blockade of sciatic nerve impulses, on the contractile (muscle units) and electrophysiological (motoneurons) properties of motor unit s in the rat gastrocnemius. Motoneuron properties were also compared w ith those of rats subjected to sciatic nerve axotomy 2 weeks earlier. 2. At the time of the terminal experiment (24 h after the removal of t he TTX delivery system) in anaesthetized animals, properties of tibial motoneurons (i.e. rheobase current, input resistance, time course of after-potentials) were determined using conventional microelectrode te chniques. For those tibial motoneurons innervating the gastrocnemius, muscle unit responses (i.e. twitch force and time course, maximum teta nic tension, fatigability) were also recorded in response to current i njection. 3. Consistent with previously reported whole-muscle response s to TTX-induced disuse, the TTX-treated gastrocnemius muscle units sh owed weaker tetanic forces, prolonged twitches and elevated twitch/tet anic ratios. These effects were similar for motor units classified as small, medium and large according to their tetanic tension-generating capacities. Muscle unit fatigue resistances appeared to be unchanged. 4. The mean values, distributions and ranges of tibial motoneuron prop erties were similar between control and TTX-treated groups for rheobas e, input resistance and AHP half-decay time. In the case of the latter , the proportion of motoneurons possessing 'slow' AHP half-decay times (> 20 ms) was not significantly different in control (17%) and TTX-tr eated groups (11%). 5. Motoneurons axotomized 2 weeks earlier had a si gnificantly higher (42-69%) mean input resistance and a longer (34-42% ) mean BHP half-decay time when compared with the control and TTX-trea ted groups. 6. It appears that, for fast muscles containing several di fferent motor unit types, TTX-induced axon blockade does not produce s imilar effects on motoneuron intrinsic properties to those evoked by a xotomy. This lack of effect on the distribution and range of these pro perties of tibial motoneurons indicates that none of the motoneurons w hich innervate muscles of mixed fibre type are particulary susceptible to the decreased activity and the atrophy-associated muscle changes p roduced by this condition. Thus, the apparent 'retrograde signalling' of muscle on motoneuron properties reported previously for the eat sol eus may be specific to this particular muscle or species.