Membrane electrical properties [time constant, action potential afterhyperpolarization (AHP), rheobase, input resistance, and axonal conduction velocity] were measured in motoneurons of cat medial gastrocnemius (MG) motor units. Motor units were classified on the basis of their mechanical responses as fast twitch, fast fatiguing (FF); fast twitch with intermediate fatigue resistance (FI); fast twitch, fatigue resistant (FR); or slow twitch, fatigue resistant (S; 11, 22). Motoneuron membrane time constant, estimated from the voltage response at the onset or termination of long (50-100 ms) current pulses and corrected for voltage-response nonlinearities (32), was found to differ significantly among the major motor-unit types, increasing in the order FF less than FR less than S. Afterhyperpolarization magnitude, half-decay time, and duration were all significantly greater for the fast (FF + FI + FR) versus the slow (S) motor units. The AHP half-decay time was correlated with muscle unit twitch time over the entire motoneuron population and within the type S motor-unit population. There was no significant correlation between twitch time and AHP half-decay time among the types FF and FR motor-unit populations. In agreement with previous studies, we found a significant difference in both rheobase and input resistance among the major motor-unit types, with rheobase increasing in the order S less than FR less than FF and input resistance decreasing in that order (S greater than FR greater than FF). The differences in input resistance were present both before and after correcting for voltage-response nonlinearities (32). Also in agreement with previous studies, the mean axonal conduction velocity was significantly faster among the fast (FF + FI + and FR) compared with the slow (S) motor units. These data were used to examine the properties alone to determine motor-unit type, which has traditionally been defined on the basis of the muscle unit's mechanical properties (11, 22). We used a discriminant analysis program to classify 73 mechanically typed motor units for which we had measures of rheobase, input resistance, membrane time constant, and AHP half-decay time. This model was able to properly classify 71 of the 73 motor units of this data set, indicating that the motor units of this data set could be grouped into three categories representing the three major motor-unit types (FF, FR, and S) on the basis of their rheobase, input resistance, membrane time constant, and AHP half-decay time.(ABSTRACT TRUNCATED AT 400 WORDS)

SUMMARY1. End-plate potentials (e.p.p.s) were recorded from frog neuromuscular junctions bathed in Ringer solution containing increased Mg and decreased Ca to reduce transmitter release. Conditioning and testing stimulation was applied to the nerve to study a previously uncharacterized process which acts to increase e.p.p. amplitudes. We will refer to this process as augmentation.2. Following repetitive stimulation augmentation decayed approximately exponentially over most of its time course with a mean time constant of about 7 see (range 4-10 see) which is intermediate in duration between the time constants for 'the decay of facilitation and potentiation.3. The magnitude of augmentation increased with the duration of the conditioning stimulation. Assuming a multiplicative relationship between augmentation and potentiation, values of the magnitude of augmentation ranged from 0 3 to 0-6 following 50 impulses at 20/sec to 0-5-7-8 following 600 impulses at 20/sec. (An augmentation of 0 3 and 7-8 would increase e.p.p. amplitudes 1-3 and 8-8 times, respectively.) 4. The time constant characterizing the decay of augmentation remained relatively constant as the duration of the conditioning stimulation was increased.5. Augmentation as well as facilitation and potentiation resulted from an increase in the number of quanta of transmitter released from the nerve terminal.6. Augmentation decayed faster at higher temperatures with a mean temperature coefficient, Q10, of about 3-8. The corresponding Q10 for the decay of potentiation was found to be about 2'4. 7. It is concluded that augmentation can be a significant factor in increasing transmitter release and will therefore have to be accounted for when studying the effects of repetitive stimulation on the function of the nerve terminal or when formulating models of transmitter release.

Endplate potentials were recorded from frog sartorius neuromuscular junctions under conditions of greatly reduced quantal contents to develop a quantitative description of stimulation-induced changes in transmitter release.Four general models relating potentiation, augmentation, and the first and second components of facilitation to transmitter release were developed . These models were then tested by incorporating equations for the kinetic properties of the four components of increased transmitter release and examining the ability of the resulting sets of equations to predict stimulation-induced changes in transmitter release . Three of the models were essentially consistent with the observation that augmentation had a multiplicative type relationship to facilitation . These models could also predict the effect of frequency and duration of stimulation on endplate potential (EPP) amplitude during and after prolonged (40 s) trains including the response to step changes in stimulation rate . These models extend by about two orders of magnitude the duration of stimulationinduced changes in transmitter release that can be accounted for, and show that the combined kinetic properties of potentiation, augmentation, and the two components of facilitation are generally sufficient to account for these changes .

Endplate potentials were recorded from frog and toad sartorius neuromuscular junctions under conditions of greatly reduced quantal contents .The magnitude of augmentation increased with the duration and frequency of stimulation, often increasing at an accelerating rate during 10-20-s conditioning trains . The magnitudes of the first and second components of facilitation also increased, but reached apparent steady state values within the first few seconds of stimulation . These observations could be accounted for by assuming (a) that augmentation and the first and second components of facilitation arise from underlying factors in the nerve terminal that act to increase transmitter release ; (b) that each nerve impulse adds an increment to each of the underlying factors ; (c) that the magnitude of the increment typically increases during the train for augmentation but remains constant for the components of facilitation ; and (d) that the underlying factors decay with first-order kinetics with time constants of -7 s for augmentation and 60 and 500 ms for the first and second components of facilitation, respectively . The increments of facilitation added by each impulse were about twice as large in the toad as in the frog . Facilitation was described better by assuming a power relationship between the underlying factor and the observed facilitation than by assuming a linear relationship. Augmentation was described by assuming either a linear or power relationship .