Fig 1 -

Direct electrophysiological interrogation of alpha and gamma motor neurons (A-D). (A) Transversal section of P21 (wild-type) mouse lumbar spinal cord ventral horn: motor neurons labeled by retrograde tracer Fluoro-Gold (FG). Putative alpha motor neurons retain low levels of FG in soma (FGlow) (open arrowheads), while putative gamma motor neurons retain high levels of FG in soma (FGhigh) (arrowheads) (scale bar: 50 μm). (B) Imaris 3D reconstruction: VGLUT1+ synaptic varicosities (triangles) associated with FGlow and NeuNhigh alpha motor neuron (open arrowheads), but not with adjacent FGhigh and NeuNlow or negligible gamma motor neurons (arrowheads) (scale bar: 20 μm). (C) Whole-cell patch-clamp recordings: example traces of FGhigh (black) and FGlow (gray) motor neurons upon 900 pA, 1 second square current pulse. (D) Scatter plot: FGhigh (n = 24, N = 16) and FGlow (n = 22, N = 12) motor neurons exhibit divergent gamma and alpha subtype-defining electrophysiological signatures, respectively, including gamma subtype-specific combination with low rheobase and high gain by FGhigh motor neurons (see S1 Table for details). (E) FGhigh motor neurons have significantly lower rheobase (pA) (221.87 ± 31.34), higher firing frequency (Hz) (50.65 ± 3.23), higher gain (Hz/nA) (161.01 ± 9.77), higher input resistance (136.62 ± 14.63), and lower capacitance (76.07 ± 6.01) when compared to FGlow motor neuron rheobase (909.09 ± 82.11), firing frequency (20.41 ± 1.70), gain (32.64 ± 4.02), input resistance (36.55 ± 4.16), and capacitance (239.1 ± 17.17), respectively (see S1 Table for details). Data are presented as mean ± SEM. n = # of neurons, N = # of mice. Statistically significant differences between FGhigh and FGlow neurons are indicated as ***p t test). Data for Fig 1D and 1E can be found in S1 Data.