Motor Unit - Motor Unit Types

Motor Unit Types

Motor units possess a range of properties and are generally placed into groups based upon the similarities between those properties. Generally, three or four groups are identified, based upon several factors:

• Physiological
  • Contraction speed in Isometric contractions
    • Rate of rise of force
    • Time to peak of a twitch contraction (re

FF — Fast fatigable — high force, fast contraction speed but fatigue in a few seconds.

FR — Fast fatigue resistant — intermediate force, fatigue resistant — fast contraction speed and resistant to fatigue.

FI — Fast intermediate — intermediate between FF and FR.

S or SO — Slow (oxidative) — low force, slower contraction speed, highly fatigue resistant.

• Biochemical
  • Histochemical (the oldest form of biochemical fiber typing)
    • Glycolytic enzyme activity (e.g. glycerophosphate dehydrogenase (GPD))
    • Oxidative enzyme activity (e.g. succinate dehydrogenase -SDH )
    • Sensitivity of Myosin ATPase to acid and alkali

These generally designate fibers as:

I (Slow oxidative, SO) — Low glycolytic and high oxidative presence. Low(er) myosin ATPase, sensitive to alkali.

IIa (Fast oxidative/glycolytic, FOG) — High glycolytic, oxidative and myosin ATPase presence,sensitive to acid.

IIb (Fast glycolytic, FG) — High glycolytic and myosin ATPase presence, sensitive to acid. Low oxidative presence.

IIi — fibers intermediate between IIa and IIb.

Histochemical and Physiological types correspond as follows:

S and Type I, FR and type IIa, FF and type IIb, FI and IIi.

• • Immunohistochemical (more recent form of fiber typing)
    • Myosin Heavy Chain (MHC)
    • Myosin Light Chain — alkali (MLC1)
    • Myosin Light Chain — regulatory (MLC2)

The Immunohistochemical types are as follows, with the type IIa, IIb and slow corresponding to IIa, IIb and slow (type I) histochemical types:

		Expressed in
Gene family	Developing	Fast fibers (II)	Slow fibers(I)
MHC	Embryonic MHC	MHC IIa	β/slow MHC
	Neonatal MHC	MHC IIb
		MHC IIx
MLC1 (alkali)	Embryonic	1f	1s
	1f	3f
MLC2 (regulatory)	2f	2f	2s

• • Gene characterization of myosins

There are currently about 15 known different types of MHC genes recognized in muscle, only some of which may be expressed in a single muscle fiber. These genes form one of ~18 classes of myosin genes, identified as class II which should not be confused with the type II myosins identified by immunohistochemistry. The expression of multiple MHC genes in a single muscle fiber is an example of polymorphism. The relative expression of these myosin types is determined partly by genetics and partly by other biological factors such as activity, innervation and hormones.

The typing of motor units has thus gone through many stages and reached a point where it is recognized that muscle fibers contain varying mixtures of several mysosin types that can not easily be classified into specific groups of fibers. The three (or four) classical fiber types represent peaks in the distribution of muscle fiber properties, each determined by the overall biochemistry of the fibers.