Motor unit
McGraw-Hill Science & Technology Dictionary:
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motor unit
(′mōd·ər ′yü·nət)
(anatomy) The axon of an anterior horn cell, or the motor fiber of a cranial nerve, together with the striated muscle fibers innervated by its terminal branches.
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A single motor neurone and all the muscle fibres it stimulates. Each motor unit supplies from four to more than a hundred muscle fibres. Generally, small muscles capable of precise actions (e.g. intrinsic hand muscles) are composed of motor units with few muscle fibres, whereas trunk and proximal limb muscles contain motor units with a large number of muscle fibres. Each motor unit obeys the all-or-none law programmes to bring about particular movements.
Motor unit
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Mosby's Dental Dictionary:
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motor unit
n
The entity consisting of the lower motor neuron, motor end-plate, and muscle fibers supplied by the end-plate. The final motor activity resulting from a sequence of stimulations to the lower motor neuron is considered a function of the motor unit. The proportion of nerve fibers to the muscle fibers in motor units is designated as the innervation ratio. Motor units may have ratios ranging from 1:4 to 1:150. The closer the ratio approximates unity, the greater the finesse of specificity of the muscular action. The eye muscles have the highest ratio of striated muscles, and the tongue, facial, masticatory, and pharyngeal muscles succeed in that order.
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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.
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Contents
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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
- Contraction speed in Isometric contractions
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- 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
- Histochemical (the oldest form of biochemical fiber typing)[2][3]
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- 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.
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- Histochemical and Physiological types correspond as follows:
- S and Type I, FR and type IIa, FF and type IIb, FI and IIi.
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- Immunohistochemical (more recent form of fiber typing)[4]
- Myosin Heavy Chain (MHC)
- Myosin Light Chain - alkali (MLC1)
- Myosin Light Chain - regulatory (MLC2)
- Immunohistochemical (more recent form of fiber typing)[4]
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- The Immunohistochemical types are as follows, with the type IIa, IIb and slow corresponding to IIa, IIb and slow (type I) histochemical types:
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Table reproduced from[4]Expressed inGene familyDevelopingFast fibers (II)Slow fibers(I)MHCEmbryonic MHCMHC IIaβ/slow MHCNeonatal MHCMHC IIbMHC IIxMLC1 (alkali)Embryonic1f1s1f3fMLC2 (regulatory)2f2f2s
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- 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
- ^ 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.
- ^ 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.
- ^ 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.
- ^ a b S. and C. Reggiani. Myosin isoforms in mammalian skeletal muscle. Journal of Applied Physiology, Vol 77, Issue 2 493-501, 1994.
- ^ 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
- ^ 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.
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