Editing Motor unit (section)

{{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]].