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Motor unit

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Inbiology,amotor unitis made up of amotor neuronand all of theskeletal muscle fibersinnervated by the neuron'saxon terminals,including theneuromuscular junctionsbetween the neuron and the fibres.[1]Groups of motor units often work together as amotor poolto coordinate the contractions of a singlemuscle.The concept was proposed byCharles Scott Sherrington.[2]

All muscle fibers in a motor unit are of the samefiber type[citation needed].When a motor unit is activated, all of its fibers contract. Invertebrates,the force of amuscle contractionis controlled by the number of activated motor units.

The number of muscle fibers within each unit can vary within a particular muscle and even more from muscle to muscle: the muscles that act on the largest body masses have motor units that contain moremuscle fibers,whereas smaller muscles contain fewer muscle fibers in each motor unit.[1]For instance,thighmuscles can have a thousand fibers in each unit, whileextraocular musclesmight have ten. Muscles which possess more motor units (and thus have greater individual motor neuron innervation) are able to control force output more finely.

Motor units are organized slightly differently ininvertebrates:each muscle has few motor units (typically less than 10), and each muscle fiber is innervated by multiple neurons, including excitatory and inhibitory neurons. Thus, while in vertebrates the force of contraction of muscles is regulated by how many motor units are activated, in invertebrates it is controlled by regulating the balance betweenexcitatoryandinhibitory signals.

Recruitment(vertebrate)

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The central nervous system is responsible for the orderlyrecruitment of motor neurons,beginning with the smallest motor units.[3]Henneman's size principleindicates that motor units are recruited from smallest to largest based on the size of the load. For smaller loads requiring less force, slow twitch, low-force, fatigue-resistant muscle fibers are activated prior to the recruitment of the fast twitch, high-force, less fatigue-resistant muscle fibers. Larger motor units are typically composed of faster muscle fibers that generate higher forces.[4]

The central nervous system has two distinct ways of controlling the force produced by a muscle through motor unit recruitment: spatial recruitment and temporal recruitment. Spatial recruitment is the activation of more motor units to produce a greater force. Larger motor units contract along with small motor units until all muscle fibers in a single muscle are activated, thus producing the maximum muscle force. Temporal motor unit recruitment, orrate coding,deals with the frequency of activation of muscle fiber contractions. Consecutive stimulation on the motor unit fibers from thealpha motor neuroncauses the muscle to twitch more frequently until the twitches "fuse" temporally. This produces a greater force than singular contractions by decreasing the interval between stimulations to produce a larger force with the same number of motor units.

Usingelectromyography(EMG), the neural strategies of muscle activation can be measured.[5]Ramp-force threshold refers to an index of motor neuron size in order to test the size principle. This is tested by determining the recruitment threshold of a motor unit during isometric contraction in which the force is gradually increased. Motor units recruited at low force (low-threshold units) tend to be small motor units, while high-threshold units are recruited when higher forces are needed and involve larger motor neurons.[6]These tend to have shorter contraction times than the smaller units. The number of additional motor units recruited during a given increment of force declines sharply at high levels of voluntary force. This suggests that, even though high threshold units generate more tension, the contribution of recruitment to increase voluntary force declines at higher force levels.

When necessary, the maximal number of motor units in a muscle can be recruited simultaneously, producing the maximum force of contraction for that muscle, but this cannot last for very long because of the energy requirements to sustain the contraction. To prevent complete muscle fatigue, motor units are generally not all simultaneously active, but instead some motor units rest while others are active, which allows for longer muscle contractions. The nervous system uses recruitment as a mechanism to efficiently utilize a skeletal muscle.[7]

To test motor unit stimulation,electrodesare placed extracellularly on the skin and an intramuscular stimulation is applied. After the motor unit is stimulated, its pulse is then recorded by the electrode and displayed as anaction potential,known as amotor unit action potential(MUAP). When multiple MUAP’s are recorded within a short time interval, amotor unit action potential train(MUAPT) is then noted. The time in between these pulses is known as theinter-pulse interval(IPI).[8]In medicalelectrodiagnostic testingfor a patient withweakness,careful analysis of the MUAP size, shape, and recruitment pattern can help in distinguishing amyopathyfrom aneuropathy.

Motor unit types(vertebrate)

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Motor units are generally categorized based upon the similarities between several factors:

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.
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)[11]— 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(a more recent form of fiber typing)[12]
      • 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 EmbryonicMHC 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[12]
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 asclass IIwhich should not be confused with thetype IImyosins identified by immunohistochemistry. The expression of multiple MHC genes in a single muscle fiber is an example ofpolymorphism.[13]The relative expression of these myosin types is determined partly by genetics and partly by other biological factors such as activity, innervation and hormones.[14]

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 myosin 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.

Estimates of innervation ratios of motor units in human muscles:

Muscle Number of Motor Axons Number of Muscle Fibers Innervation Ratio Reference
Biceps 774 580,000 750 Buchtal, 1961
Brachioradialis 315 129,200 > 410 Feinsteinet al.[15]
Firstdorsal interosseous 119 40,500 340 Feinsteinet al.[15]
Medialgastrocnemius 579 946,000 1,634 Feinsteinet al.[15]
Tibialis anterior 445 292,500 657 Feinsteinet al.[15]

Table reproduced from Karpati (2010)[16]

See also

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References

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  • Altshuler, Douglas; K. Welch; B. Cho; D. Welch; A. Lin; W. Dickinson; M. Dickinson (April 2010)."Neuromuscular control of wingbeat kinematics in Annas hummingbirds".The Journal of Experimental Biology.213(Pt 14): 2507–2514.doi:10.1242/jeb.043497.PMC2892424.PMID20581280.
  1. ^abBuchtal, F; H. Schmalbruch (1 January 1980). "Motor Unit of Mammalian Muscle".Physiological Reviews.60(1): 90–142.doi:10.1152/physrev.1980.60.1.90.PMID6766557.
  2. ^Kandel, Eric (2013).Principles of Neural Science, 5th ed.McGraw-Hill, New York. p. 768.ISBN978-0-07-139011-8.
  3. ^Milner-Brown HS, Stein RB, Yemm R (September 1973)."The orderly recruitment of human motor units during voluntary isometric contractions".J. Physiol.230(2): 359–70.doi:10.1113/jphysiol.1973.sp010192.PMC1350367.PMID4350770.
  4. ^Robinson R (February 2009)."In mammalian muscle, axonal wiring takes surprising paths".PLOS Biol.7(2): e1000050.doi:10.1371/journal.pbio.1000050.PMC2637923.PMID20076726.
  5. ^Farina, Dario; Merletti R; Enoka R.M. (2004). "The extraction of neural strategies from the surface EMG".Journal of Applied Physiology.96(4): 1486–1495.doi:10.1152/japplphysiol.01070.2003.PMID15016793.
  6. ^Spiegel KM.; Stratton J.; Burke JR.; Glendinning DS; Enoka RM (November 2012)."The influence of age on the assessment of motor unit activation in a human hand muscle".Experimental Physiology.81(5): 805–819.doi:10.1113/expphysiol.1996.sp003978.PMID8889479.S2CID29034955.
  7. ^This article incorporatestextavailable under theCC BY 4.0license.Betts, J Gordon; Desaix, Peter; Johnson, Eddie; Johnson, Jody E; Korol, Oksana; Kruse, Dean; Poe, Brandon; Wise, James; Womble, Mark D; Young, Kelly A (May 14, 2023).Anatomy & Physiology.Houston: OpenStax CNX. 10.3 Muscle Fiber Contraction and Relaxation.ISBN978-1-947172-04-3.
  8. ^De Luca, Carlo; William J. Forrest (December 1972). "Some Properties of Motor Unit Action Potential Trains Recorded during Constant Force Isometric Contractions in Man".Kybernetik.12(3): 160–168.doi:10.1007/bf00289169.PMID4712973.S2CID11373497.
  9. ^abBurke RE, Levine DN, Tsairis P, Zajac FE (November 1973)."Physiological types and histochemical profiles in motor units of the cat gastrocnemius".J. Physiol.234(3): 723–48.doi:10.1113/jphysiol.1973.sp010369.PMC1350696.PMID4148752.
  10. ^Collatos TC, Edgerton VR, Smith JL, Botterman BR (November 1977). "Contractile properties and fiber type compositions of flexors and extensors of elbow joint in cat: implications for motor control".J. Neurophysiol.40(6): 1292–300.doi:10.1152/jn.1977.40.6.1292.PMID925731.
  11. ^Altshuler D.; Welch K.; Cho B.; Welch D.; Lin A.; Dickinson W.; Dickinson M. (April 2010)."Neuromuscular control of wingbeat kinematics in Annas hummingbirds".The Journal of Experimental Biology.213(Pt 14): 2507–2514.doi:10.1242/jeb.043497.PMC2892424.PMID20581280.
  12. ^abSchiaffino S, Reggiani C (August 1994). "Myosin isoforms in mammalian skeletal muscle".J. Appl. Physiol.77(2): 493–501.doi:10.1152/jappl.1994.77.2.493.PMID8002492.
  13. ^abCaiozzo VJ, Baker MJ, Huang K, Chou H, Wu YZ, Baldwin KM (September 2003). "Single-fiber myosin heavy chain polymorphism: how many patterns and what proportions?".Am. J. Physiol. Regul. Integr. Comp. Physiol.285(3): R570–80.doi:10.1152/ajpregu.00646.2002.PMID12738613.S2CID860317.
  14. ^Baldwin KM, Haddad F (January 2001). "Effects of different activity and inactivity paradigms on myosin heavy chain gene expression in striated muscle".J. Appl. Physiol.90(1): 345–57.doi:10.1152/jappl.2001.90.1.345.PMID11133928.S2CID9677583.
  15. ^abcdFeinstein, Bertram; Lindegård, Bengt; Nyman, Eberhard; Wohlfart, Gunnar (2008-06-18)."Morphologic Studies of Motor Units in Normal Human Muscles".Acta Anatomica.23(2): 127–142.doi:10.1159/000140989.ISSN0001-5180.PMID14349537.
  16. ^Karpati, George (2010).Disorders of Voluntary Muscle(PDF).Cambridge University Press. p. 7.ISBN9780521876292.referencedFeinstein, B; Lindegard, B; Nyman, E; Wohlfart, G (1955). "Morphologic studies of motor units in normal human muscles".Acta Anat (Basel).23(2): 127–142.doi:10.1159/000140989.PMID14349537.
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