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).