Editing Motor unit

{{Short description|Combination of a motor neuron and all muscle fibers it innervates}}
In [[biology]], a '''motor unit''' is made up of a [[motor neuron]] and all of the [[Skeletal muscle#Skeletal muscle cells|skeletal muscle fibers]] innervated by the neuron's [[axon terminal]]s, including the [[neuromuscular junction]]s between the neuron and the fibres.<ref name=Buchtal>{{cite journal|last=Buchtal|first=F|author2=H. Schmalbruch|title=Motor Unit of Mammalian Muscle|journal=Physiological Reviews|date=1 January 1980|volume=60|issue=1|pages=90–142|doi=10.1152/physrev.1980.60.1.90|pmid=6766557}}</ref> Groups of motor units often work together as a [[Motor pool (neuroscience)|motor pool]] to coordinate the contractions of a single [[skeletal muscle|muscle]]. The concept was proposed by [[Charles Scott Sherrington]].<ref>{{cite book |last=Kandel |first=Eric |date=2013 |title=Principles of Neural Science, 5th ed.|publisher=McGraw-Hill, New York. |page=768 |isbn=978-0-07-139011-8}}</ref>

Usually muscle fibers in a motor unit are of the same [[Skeletal muscle#Skeletal muscle cells|fiber type]].<ref>{{Citation |last=Purves |first=Dale |title=The Motor Unit |date=2001 |work=Neuroscience. 2nd edition |url=https://www.ncbi.nlm.nih.gov/books/NBK10874/ |access-date=2024-11-10 |publisher=Sinauer Associates |language=en |last2=Augustine |first2=George J. |last3=Fitzpatrick |first3=David |last4=Katz |first4=Lawrence C. |last5=LaMantia |first5=Anthony-Samuel |last6=McNamara |first6=James O. |last7=Williams |first7=S. Mark}}</ref> When a motor unit is activated, all of its fibers contract. In [[vertebrate]]s, the force of a [[muscle contraction]] is 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 more [[muscle fiber]]s, whereas smaller muscles contain fewer muscle fibers in each motor unit.<ref name=Buchtal/> For instance, [[thigh]] muscles can have a thousand fibers in each unit, while [[extraocular muscles]] might 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 in [[invertebrates]]: 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 between [[Excitatory postsynaptic potential|excitatory]] and [[Inhibitory postsynaptic potential|inhibitory signals]].

==Recruitment <small>(vertebrate)</small>==
The central nervous system is responsible for the orderly [[motor unit recruitment|recruitment of motor neurons]], beginning with the smallest motor units.<ref>{{cite journal |vauthors=Milner-Brown HS, Stein RB, Yemm R |title=The orderly recruitment of human motor units during voluntary isometric contractions |journal=J. Physiol. |volume=230 |pages=359–70|date=September 1973 |pmc=1350367 |pmid=4350770 |issue=2 |doi=10.1113/jphysiol.1973.sp010192}}</ref> [[Henneman's size principle]] indicates 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.<ref>{{cite journal |author=Robinson R |title=In mammalian muscle, axonal wiring takes surprising paths |journal=PLOS Biol. |volume=7 |issue=2 |pages=e1000050 |date=February 2009 |pmid=20076726 |pmc=2637923 |doi=10.1371/journal.pbio.1000050 |doi-access=free }}</ref>

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, or [[rate coding]], deals with the frequency of activation of muscle fiber contractions. Consecutive stimulation on the motor unit fibers from the [[alpha motor neuron]] causes 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.

Using [[electromyography]] (EMG), the neural strategies of muscle activation can be measured.<ref>{{cite journal |last=Farina |first=Dario |author2=Merletti R |author3=Enoka R.M. |title=The extraction of neural strategies from the surface EMG |journal=Journal of Applied Physiology |year=2004 |volume=96 |issue=4 |pages=1486–1495 |doi=10.1152/japplphysiol.01070.2003 |pmid=15016793}}</ref> 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.<ref>{{cite journal |author1=Spiegel KM. |author2=Stratton J. |author3=Burke JR. |author4=Glendinning DS |author5=Enoka RM |title= The influence of age on the assessment of motor unit activation in a human hand muscle |journal=Experimental Physiology |volume=81 | pages=805–819 |date=November 2012 |issue=5|doi=10.1113/expphysiol.1996.sp003978 |pmid=8889479 |s2cid=29034955 |doi-access=free }}</ref> 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.<ref name="Openstax Anatomy & Physiology attribution">{{CC-notice|cc=by4|url=https://openstax.org/books/anatomy-and-physiology/pages/10-3-muscle-fiber-contraction-and-relaxation}} {{cite book|last1=Betts|first1=J Gordon|last2=Desaix|first2=Peter|last3=Johnson|first3=Eddie|last4=Johnson|first4=Jody E|last5=Korol|first5=Oksana|last6=Kruse|first6=Dean|last7=Poe|first7=Brandon|last8=Wise|first8=James|last9=Womble|first9=Mark D|last10=Young|first10=Kelly A|title=Anatomy & Physiology|location=Houston|publisher=OpenStax CNX|isbn=978-1-947172-04-3|date=May 14, 2023|at=10.3 Muscle Fiber Contraction and Relaxation}}</ref>

To test motor unit stimulation, [[electrode]]s are 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 an [[action potential]], known as a '''motor unit action potential''' (MUAP). When multiple MUAP’s are recorded within a short time interval, a '''motor unit action potential train''' (MUAPT) is then noted. The time in between these pulses is known as the '''inter-pulse interval''' (IPI).<ref name=MUAP>{{cite journal|last=De Luca|first=Carlo|author2=William J. Forrest|title=Some Properties of Motor Unit Action Potential Trains Recorded during Constant Force Isometric Contractions in Man|journal=Kybernetik|date=December 1972|volume=12|issue=3|pages=160–168|doi=10.1007/bf00289169|pmid=4712973|s2cid=11373497 }}</ref> In medical [[electrodiagnostic testing]] for a patient with [[weakness]], careful analysis of the MUAP size, shape, and recruitment pattern can help in distinguishing a [[myopathy]] from a [[neuropathy]].

==Motor unit types <small>(vertebrate)</small>==
Motor units are generally categorized based upon the similarities between several factors:
*[[Physiology|Physiological]]<ref name="Burke73">{{cite journal |vauthors=Burke RE, Levine DN, Tsairis P, Zajac FE |date=November 1973 |title=Physiological types and histochemical profiles in motor units of the cat gastrocnemius |url=https://physoc.onlinelibrary.wiley.com/doi/10.1113/jphysiol.1973.sp010369 |journal=J. Physiol. |volume=234 |issue=3 |pages=723–48 |doi=10.1113/jphysiol.1973.sp010369 |pmc=1350696 |pmid=4148752}}</ref>
**Contraction speed in [[Isometric exercise|Isometric contractions]]
***Rate of rise of force
***Time to peak of a twitch contraction (response to a single nerve impulse)
****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.
*[[Biochemistry|Biochemical]]
**Histochemical (the oldest form of biochemical fiber typing)<ref name=Burke73/><ref>{{cite journal |vauthors=Collatos TC, Edgerton VR, Smith JL, Botterman BR |title=Contractile properties and fiber type compositions of flexors and extensors of elbow joint in cat: implications for motor control |journal=J. Neurophysiol. |volume=40 |issue=6 |pages=1292–300 |date=November 1977 |pmid=925731 |doi=10.1152/jn.1977.40.6.1292 }}</ref>
***Glycolytic enzyme activity (e.g. [[glyceraldehyde 3-phosphate dehydrogenase|glycerophosphate dehydrogenase (GPD)]])
***Oxidative enzyme activity (e.g. [[succinate dehydrogenase]] -SDH )
***Sensitivity of [[Myosin]] ATPase to acid and [[alkali]]:
****I (Slow oxidative, SO) — Low glycolytic and high oxidative presence. Low(er) myosin ATPase, sensitive to alkali.
****IIa (Fast oxidative/glycolytic, FOG)<ref>{{cite journal |author1=Altshuler D. |author2=Welch K. |author3=Cho B. |author4=Welch D. |author5=Lin A. |author6=Dickinson W. |author7=Dickinson M. |title= Neuromuscular control of wingbeat kinematics in Annas hummingbirds |journal= The Journal of Experimental Biology|volume=213 |issue=Pt 14 |pages= 2507–2514 |date=April 2010 |doi=10.1242/jeb.043497 |pmid=20581280 |pmc=2892424}}</ref> — 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.)
**[[Immunohistochemistry|Immunohistochemical]] (a more recent form of fiber typing)<ref name="Schiaffino">{{cite journal |vauthors=Schiaffino S, Reggiani C |title=Myosin isoforms in mammalian skeletal muscle |journal=J. Appl. Physiol. |volume=77 |issue=2 |pages=493–501 |date=August 1994 |pmid=8002492 |doi=10.1152/jappl.1994.77.2.493 }}</ref>
***Myosin Heavy Chain (MHC)
***Myosin Light Chain — alkali (MLC1)
***Myosin Light Chain — regulatory (MLC2)
{| class="wikitable" style="text-align: center;"
|-
! !! !! Expressed in !!
|-
| '''''Gene family''''' || '''''Developing''''' || '''''Fast fibers (II)''''' || '''''Slow fibers(I)'''''
|-
| MHC || [[Embryo]]nic 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 [[myosin]]s<ref name="Caiozzo">{{cite journal |vauthors=Caiozzo VJ, Baker MJ, Huang K, Chou H, Wu YZ, Baldwin KM |title=Single-fiber myosin heavy chain polymorphism: how many patterns and what proportions? |journal=Am. J. Physiol. Regul. Integr. Comp. Physiol. |volume=285 |issue=3 |pages=R570–80 |date=September 2003 |pmid=12738613 |doi=10.1152/ajpregu.00646.2002 |s2cid=860317 }}</ref>
::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 [[Myosin#Myosin classes|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 (biology)|polymorphism]].<ref name="Caiozzo" /> The relative expression of these myosin types is determined partly by genetics and partly by other biological factors such as activity, innervation and hormones.<ref name="Baldwin">{{cite journal |vauthors=Baldwin KM, Haddad F |title=Effects of different activity and inactivity paradigms on myosin heavy chain gene expression in striated muscle |journal=J. Appl. Physiol. |volume=90 |issue=1 |pages=345–57 |date=January 2001 |pmid=11133928 |doi=10.1152/jappl.2001.90.1.345 |s2cid=9677583 }}</ref>

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:<ref name="Karpati2010">{{cite book | last = Karpati | first = George | title = Disorders of Voluntary Muscle | year = 2010 | publisher = Cambridge University Press | isbn = 9780521876292 | pages = 7 | url = http://assets.cambridge.org/97805218/76292/excerpt/9780521876292_excerpt.pdf }} referenced {{cite journal | last1 = Feinstein | first1 = B | last2 = Lindegard | first2 = B | last3 = Nyman | first3 = E | last4 = Wohlfart | first4 = G | year = 1955 | title = Morphologic studies of motor units in normal human muscles | journal = Acta Anat (Basel) | volume = 23 | issue = 2 | pages = 127–142 | doi=10.1159/000140989| pmid = 14349537 }}</ref>
{| class="wikitable"
|-
! Muscle !! Number of Motor Axons !! Number of Muscle Fibers !! Innervation Ratio !! Reference
|-
| [[Biceps]] || 774 || 580,000 || 750 || Buchtal, 1961
|-
| [[Brachioradialis]] || 315 || 129,200 || > 410 || Feinstein ''et al.''<ref name=":0">{{Cite journal |last1=Feinstein |first1=Bertram |last2=Lindegård |first2=Bengt |last3=Nyman |first3=Eberhard |last4=Wohlfart |first4=Gunnar |date=2008-06-18 |title=Morphologic Studies of Motor Units in Normal Human Muscles |url=https://doi.org/10.1159/000140989 |journal=Acta Anatomica |volume=23 |issue=2 |pages=127–142 |doi=10.1159/000140989 |pmid=14349537 |issn=0001-5180|url-access=subscription }}</ref>
|-
| First [[Dorsal interossei of the hand|dorsal interosseous]] || 119 || 40,500 || 340 || Feinstein ''et al.''<ref name=":0" />
|-
| Medial [[Gastrocnemius muscle|gastrocnemius]] || 579 || 946,000 || 1,634 || Feinstein ''et al.''<ref name=":0" />
|-
| [[Tibialis anterior muscle|Tibialis anterior]] || 445 || 292,500 || 657 || Feinstein ''et al.''<ref name=":0" />
|}

==See also==
* [[MYH1]], [[MYH2]], [[MYH3]], [[MYH4]], [[MYH6]], [[MYH7]], [[MYH7B]], [[MYH8]], [[MYH9]], [[MYH10]], [[MYH11]], [[MYH13]], [[MYH14]], [[MYH15]], [[MYH16]]
* [[tetanic contraction]]
* [[motor unit number estimation]]

==References==
*{{cite journal|last=Altshuler|first=Douglas|author2=K. Welch |author3=B. Cho |author4=D. Welch |author5=A. Lin |author6=W. Dickinson |author7=M. Dickinson |title=Neuromuscular control of wingbeat kinematics in Annas hummingbirds|journal=The Journal of Experimental Biology|date=April 2010|volume=213|issue=Pt 14|pages=2507–2514|doi=10.1242/jeb.043497 |pmid=20581280 |pmc=2892424}}
{{Reflist}}

==External links==
*[http://www.siumed.edu/~dking2/ssb/neuron.htm Neurons and Support Cells]

{{Muscle tissue}}
{{Authority control}}

{{DEFAULTSORT:Motor Unit}}
[[Category:Somatic motor system]]