1. The main purpose of this study was to quantify the adaptation of spinal motoneurons to sustained and intermittent activation, using an extracellular route of stimulating current application to single test cells, in contrast to an intracellular route, as has been used previously. In addition, associations were tested between firing rate properties of the tested cells and other type (size)-related properties of these cells and their motor units. 2. Motoneurons supplying the medial gastrocnemius muscle of the deeply anaesthetized cat were stimulated for 240 s with microelectrodes which passed sustained extracellular current at 1.25 times the threshold for repetitive firing. Many cells were also tested following a rest period with intermittent 1 s current pulses (duration 600 ms) at the same relative stimulus strength. Cell discharge was assessed from the EMG of the motor unit innervated by the test neuron. The motoneurons and their motor units were assigned to four categories (i.e. types FF, FR, S and F; where F = FF + FR) based on conventional criteria. In all, twenty F (16 FF, 4 FR) and fourteen S cells were studied with sustained stimulation. Thirty of these cells (17 F, 13 S) and an additional two cells (1 F, 1 S) were studied with intermittent stimulation. 3. The mean threshold current required for sustained firing for a period of greater-than-or-equal-to 2 s was not significantly different for F and S cells. However, most of the other measured parameters of motoneuron firing differed significantly for these two cell groups. For example, at 1-25 times the threshold current for repetitive firing, the mean firing duration in response to 240 s of sustained activation was 123 +/- 88 S (+/- S.D.) for F cells vs. 233 +/- 19 s for S cells. These values were significantly longer than those from a comparable, previously reported study that employed intracellular stimulation. With intermittent stimulation, the firing durations of F and S cells were not significantly different from each other. 4. All cells exhibited a delay from the onset of current to the first spike, followed by a brief accelerating discharge that was followed by a slower drop in firing rate. Some cells (21 of 34 with sustained activation; 20 of 32 with intermittent) exhibited doublet discharges (interspike intervals less-than-or-equal-to 10 ms) that were intermingled with the more predominant singlet discharges. Doublets were more common in the S cell type. 5. With sustained activation, the mean delay from the onset of current to the first spike was 2-6 +/- 1.1 s for F cells, and 3.2 +/- 1.9 s for S cells. The time required to reach peak frequency of singlet discharge following repetitive firing onset was significantly shorter for F than S cells (7.0 +/- 5.0 vs. 14.3 +/- 13.6 s) and the peak singlet frequencies also differed significantly (F, 28.0 +/- 7.7 Hz vs. S, 15.6 +/- 2.5 Hz). Subsequently, the mean magnitude of firing rate reduction from the peak to 24 s later was significantly greater for F cells than that for S cells (16.2 +/- 6 Hz vs. 5.8 +/- 3 Hz). These gradual reductions in firing frequency for both F and S cells during the course of their sustained stimulation were qualitatively similar to the late adaptation observed in previous studies that had employed intracellular stimulation. 6. The time course of firing frequency for each unit with sustained activation was fitted with a double-exponential equation: the first time constant (tau1) for the initial increase in frequency was relatively short (F, 2.5 +/- 2.1 s vs. S, 3.7 +/- 4.1 s). The second time constant (tau2) was significantly shorter for F than S cells (130.7 +/- 98.4 s vs. 750.0 +/- 402.4 s). It is argued that the tau2 values provided a quantitative description of the type of adaptation termed 'late' in previous studies. 7. The responses to intermittent stimulation were qualitatively similar to those seen with sustained activation. Cells responded to intermittent stimulation with trains of impulses, which began after several cycles of stimulation, with a mean frequency per train that first increased and then declined slowly over time (termed between-train adaptation). The thirty cells which received both stimulation protocols were compared for differences in their discharge under the two conditions of activation. For these comparisons, doublet intervals were included in the frequency calculations in each case. The time required to reach peak firing frequency was significantly different for intermittent vs. sustained stimulation (F cells, 19.9 +/- 18.1 s vs. 7.9 +/- 9.9 s; S cells, 78.3 +/- 64.7 s vs. 39.0 +/- 43.6 s). Peak firing frequencies were not significantly different in the two protocols (F, 39.4 +/- 14 Hz vs. 35.9 +/- 15.8 Hz; S cells, 26.4 +/- 10.1 Hz vs. 25.4 +/- 7.8 Hz). The magnitude of firing frequency reduction from the peak to 24 s later was not significantly different for the F and S cells between protocols (F cells, 12.0 +/- 12.4 Hz vs. 17.0 +/- 15.7 Hz; S cells, 4.3 +/- 3-1 Hz vs. 6.3 +/- 5-7 Hz). 8. Associations were tested between firing rate properties of the test cells brought out by their extracellular activation, and other well-known, type (size)-related properties of motoneurons and motor units. For sustained activation, peak singlet firing frequency and tau2 were each significantly associated with axonal conduction velocity, motor unit twitch contraction time, and peak tetanic force. The extent of between-train adaptation (intermittent activation), as quantified by the drop in mean frequency per train from its peak to 24 or 58 s later, was significantly correlated to axonal conduction velocity and peak tetanic force. The peak firing frequency at the onset of the adaptive process was correlated with the extent of adaptation for both sustained and intermittent activation. 9. The overall activity of the F and S cell populations were compared using an ensemble average of firing frequency. The population mean is influenced by both adaptation, and discharge duration, of individual cells. Over 240 s of sustained stimulation there was a 94% reduction in the ensemble mean frequency for the F cell population, compared to a 24% reduction for S cells. For intermittent stimulation, the population of type F cells showed a smaller reduction of ensemble mean firing frequency (64%), while the reduction in ensemble mean firing frequency for S cells (20%) was similar to that seen with sustained activation. These findings emphasize the advantages of the S cell type for prolonged discharge with sustained (continuous) activation. 10. These results show that the extracellular activation technique is a reliable means with which to study motoneuron adaptation in the mammalian spinal cord. The associations demonstrated between peak firing frequency and measures of adaptation with other type (size)-related properties of the motor units extend the consideration of Henneman's size principle to include the active repetitive firing properties of mammalian motoneurons.
