Figure 2. Motor unit classification based on TMN membrane properties. A, Scatterplot matrices of membrane properties including rheobase, Rinp and Cm showing the result of k-means clustering based classification of motor units with PS in green, PFR in blue, and PFF in red, in both WT (left) and SOD1G93A (right) TMNs. The diagonal cells show the normal curves for each MU group. In WT the F-ratios were 130.2, 26.4, and 12.9, and in the mutant these values were 163.2, 36.6, and 3.7, respectively, for rheobase, input resistance, and membrane capacitance. B, Comparison of membrane properties of MU groups between WT (black) and SOD1G93A (magenta) TMNs. Rheobase values were significantly lower among SOD1G93A PFR and PFF units compared with WT (Student's t test, *p = 0.0002 for PFR, **p = 0.0001 for PFF), whereas Rinp values were significantly greater (Student's t test, *p = 0.0009 for PFR, **p = 0.0028 for PFF). Error bars indicate SD. A, B, Units of rheobase, Rinp, and Cm are pA, MΩ, and pF, respectively. C, Chi-square test of distribution of the three MU types in WT and SOD1G93A Mot V yielded significant difference (p = 0.0014) for the presented dataset. D, Distribution of cells across the age range considered (P8–P12). Data from 28 WT mice (n = 39) and 26 SOD1G93A mutant mice (n = 36), where n is the number of cells.