Editing Motor unit (section)

==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" />
|}