Editing Motor unit (section)

==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]].