Input Resistance , Electrical Excitability , and Size of Ventral Horn Cells in Cat Spinal Cord

Experiments on cat lumbosacral alpha motoneurones showed that, in comparison with cells possessing rapidly conducting axons, the cells with slowly conducting axons have the higher input resistance, that they need weaker stimnulating currents to reach the threshold for repetitive firing, and that they need a relatively larger increment in current strength for a given increase in firing rate. Measurements of the number and diameters of dendritic trunks gave larger values for the larger cell bodies. The discussion deals with the interrelation between cell geometry, electrical properties, and the reflex action of alpha motowas + 0.92 (50 cells, P < .001). The range of variation in input resistance was 0.5 to 8.1 Mohm. Values (Fig. 1) of input resistance determined by the "direct" methods and the "spike" method varied over the same range, and they were related to conduction velocity in a similar manner. The average spike size was 83 ? 10 mv (standard deviation, 50 cells). The membrane potential was monitored during the resistance measurements in 33 of the cells, and averaged 72 ? 5 mv (S.D.). Input resistance in these cells did not correlate with spike amplitude or membrane potential. The relation between input resistance and axonal conduction velocity was not in any obvious way different for motoneurones belonging to different hind limb nerves (soleus, gastrocnemius, popliteal, common peroneal, and hamstring) . Values of input resistance obtained with the "threshold" method were also related in a statistically significant manner to the axonal conduction velocity. However, with this method the values were more scattered than those shown in Fig. 1, probably because of changes in firing level that may occur during the passage of longlasting currents (14). In intracellular work, tonic motoneurones have often been distinguished from the phasic ones by the longer duration of their after-hyperpolarization (2, 6). The input resistance was better correlated with the conduction velocity than with the duration of afterhyperpolarization. However, the latter correlation was also highly significant (r = + 0.79, 50 cells; P < 0.001). Cells with an after-hyperpolarization shorter than 70 msec had an average input resistance of 0.9 ? 0.5 Mohm (S.D., 14 cells), whereas neurones with an after-hyperpolarization exceeding 140 msec had one of 5.2 ? 2.2 Mohm (S.D., 6 cells). The previously published values for the input resistance of motoneurones (0.5 to 2.5 Mohm, 11 and 12) correspond in the present work to those obtained from cells with relatively rapidly conducting (that is, large diameter) axons (Fig. 1). The predominance of such cells in earlier studies was probably due to the fact that good intracellular recordings are most easily obtained from the cells with the largest axons, which are likely to be those with the largest cell bodies. A relation, similar to that of Fig. 1, between the input resistance and conduction velocity has been found to hold for the pyramidal cells in the cat's cerebral cortex (15). Eighteen of the cells (Fig. 1) were stimulated to steady repetitive firing by long-lasting injected currents (16, 17). In Fig. 2A the threshold current strength for steady firing (16) is plotted against input resistance. As expected, the threshold for steady firing varies inversely with the input resistance. The threshold for steady firing for these cells did not correlate with the membrane potential or spike size. The data of Fig 2A indicate that, for motoneurones with comparable membrane potentials, the input resistance is a significant factor determining the current strength needed (synaptic as well as injected) for eliciting a steady repetitive discharge. In Fig. 2A the values are too few and the spread is too large to demonstrate clearly the shape of the curve relating threshold current strength to input resistance. It should be noted that for cells requiring the same threshold depolarization, the threshold current strength should be linearly related to the reciprocal of input resistance rather than to the input resistance itself. For some of the motoneurones (Fig. 2A, filled circles) the slope was determined for the linear relation between steady discharge frequency and current strength within the lower, "primary," range of firing rates (16, 17). In Fig. 2B the increment of current needed for increasing steady firing rate by 1 impulse per second is plotted against neurone resistance. Figure 2B somewhat unexpectedly indicates that more current is needed per unit increase in steady firing rate in cells with a high input resistance than is necessary in cells with a low input resistance. Thus, in spite of their greater input resistance, the firing rate of tonic cells is relatively little affected by variations of