Motor unit

”A motor unit is a single α-motor neuron and all of the corresponding muscle fibers it innervates; all of these fibers will be of the same type (either fast twitch or slow twitch). When a motor unit is activated, all of its fibers contract. Groups of motor units often work together to coordinate the contractions of a single muscle; all of the motor units that subserve a single muscle are considered a motor unit pool. Larger motor units have stronger twitch tensions.[1]

The number of muscle fibers within each unit can vary: thigh muscles can have a thousand fibers in each unit, eye muscles might have ten. In general, the number of muscle fibers innervated by a motor unit is a function of a muscle's need for refined motion. The smaller the motor unit, the more precise the action of the muscle. Muscles requiring more refined motion are innervated by motor units that synapse with fewer muscle fibers.

Nerve cell axons are very thin, about 1 micrometer. However, they are extraordinarily long. For many motor neurons the axon is over a meter long, extending from the spinal column to a muscle cell. They stretch the spinal column to increase height.

In medical electrodiagnostic testing for a patient with weakness, careful analysis of the motor unit action potential (MUAP) size, shape, and recruitment pattern can help in distinguishing a myopathy from a neuropathy.

Motor unit types (mammalian)

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^[1]
  • 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)^[2][3]
    • 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)^[4]
    • 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

Table reproduced from^[4]

• • Gene characterization of myosins^[5]

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.^[5] The relative expression of these myosin types is determined partly by genetics and partly by other biological factors such as activity, innervation and hormones.^[6]

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.

See also

• Myosin
• MYH1, MYH2, MYH3, MYH4, MYH6, MYH7, MYH7B, MYH8, MYH9, MYH10, MYH11, MYH13, MYH14, MYH15, MYH16
• motor unit recruitment
• tetanic contraction
• myopathy (muscle pathology/diseases)
• motor unit number estimation
• Tonic and Phasic Inputs
• Motor pool

References

1. ^ Burke RE, Levine DN, Tsairis P, Zajac FE 3rd. Physiological types and histochemical profiles in motor units of the cat gastrocnemius. J Physiol. 1973 Nov;234(3):723-48.
2. ^ Burke RE, Levine DN, Tsairis P, Zajac FE 3rd.J Physiol. Physiological types and histochemical profiles in motor units of the cat gastrocnemius. 1973 Nov;234(3):723-48.
3. ^ Collatos TC, Edgerton VR, Smith JL, Botterman BR. Contractile properties and fiber type compositions of flexors and extensors of elbow joint in cat: implications for motor control. J Neurophysiol. 1977 Nov;40(6):1292-300.
4. ^ ^{a b} S. and C. Reggiani. Myosin isoforms in mammalian skeletal muscle. Journal of Applied Physiology, Vol 77, Issue 2 493-501, 1994.
5. ^ ^{a b} Caiozzo, V.J., Baker, M.J. , Huang, K., Chou, H., Wu, Y.Z., Baldwin, K.M. Single-fiber myosin heavy chain polymorphism: How many patterns and what proportions? American Journal of Physiology Regul Integr Comp Physiol Volume 285, 2003, Pages R570-R580
6. ^ Kenneth M. Baldwin, and Fadia Haddad. Plasticity in Skeletal, Cardiac, and Smooth Muscle: Invited Review: Effects of different activity and inactivity paradigms on myosin heavy chain gene expression in striated muscle. J Appl Physiol 90: 345-357.

External links

• Neurons and Support Cells