Table 3  Comparison of fast (FF, fast-fatigable; FR, fast-resistant) and slow (S) spinal α-motor neurons.
Spinal α-MN
F  S
Target muscle fiber  IIb (FF), IIa (FR)1  I1
Soma size  Similar2,3,4,5,6  Similar2,3,4,5,6
Axon diameter  Larger7,8  Thinner7,8
Dendrite branching  More4,9  Less4,9
Motor unit size (innervation ratio)  Larger1,10  Smaller1,10
Membrane input resistance  Smaller11,12,13  Larger11,12,13
Firing  Phasic14,15  Tonic14,15
Axon conduction velocity  Faster1,13  Slower1,13
Afterhyperpolarization duration  Shorter14  Longer14
Recruitment  Late15  Early15
Affected in ALS  Early16,17,18  Late16,17,18
Affected in aging  Early19,20,21  Late19,20,21
Markers  CHODL22 CALCA22  SV2a23 SK324 ESRRB22 (adult)
CALCA, calcitonin-related polypeptide alpha; CHODL, chondrolectin; SV2A, synaptic vesicle glycoprotein 2a; SK3, postsynaptic Ca2+-activated K+ 3; ESRRB, estrogen-related receptor beta. 1(Burke et al., 1973), 2(Kernell and Zwaagstra, 1981), 3(Burke et al., 1982), 4(Cullheim et al., 1987), 5(Vinsant et al., 2013), 6(Hadzipasic et al., 2014), 7(Burke et al., 1977), 8(Dukkipati et al., 2018), 9(Ulfhake and Kellerth, 1981), 10(Burke and Tsairis, 1973), 11(Bakels and Kernell, 1993), 12(Gardiner, 1993), 13(Zengel et al., 1985), 14(Eccles et al., 1957), 15(Zajac and Faden, 1985), 16(Frey et al., 2000), 17(Hegedus et al., 2007), 18(Pun et al., 2006), 19(Hashizume et al., 1988), 20(Kadhiresan et al., 1996), 21(Kanda and Hashizume, 1989), 22(Enjin et al., 2010), 23(Chakkalakal et al., 2010), 24(Deardorff et al., 2013).