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Motor unit recruitment refers to the activation of additional motor units to accomplish an increase in contractile strength in a muscle. A motor unit consists of one motor neuron and all of the muscle fibers it stimulates. All muscles consist of a number of motor units and the fibers belonging to a motor unit are dispersed and intermingle amongst fibers of other units. The muscle fibers belonging to one motor unit can be spread throughout part, or most of the entire muscle, depending on the number of fibers and size of the muscle.〔Ounjian, M., R.R. Roy, E. Eldred, A Garfinkel, J.R. Payne, A. Armstrong, A. Toga, and V.R. Edgerton Physiological and Developmental Implications of Motor Unit Anatomy. J. Neurobiol. 22:547-559, 1991. Motor unit territory.〕〔Bodine-Fowler, S., Garfinkel, A. , Roy, Roland R., and Edgerton, V. Reggie. Spatial distribution of muscle fibers within the territory of a motor unit. Muscle and Nerve 13:1133-1145, 1990. - non-random distribution of fibers.〕 When a motor neuron is activated, all of the muscle fibers innervated by the motor neuron are stimulated and contract. The activation of one motor neuron will result in a weak but distributed muscle contraction. The activation of more motor neurons will result in more muscle fibers being activated, and therefore a stronger muscle contraction. Motor unit recruitment is a measure of how many motor neurons are activated in a particular muscle, and therefore is a measure of how many muscle fibers of that muscle are activated. The higher the recruitment the stronger the muscle contraction will be. Motor units are generally recruited in order of smallest to largest (smallest motor neurons to largest motor neurons (and thus slow to fast twitch) as contraction increases. This is known as Henneman's Size Principle.〔Henneman, E., Somjen, G. & Carpenter, D. O. (1965). Functional significance of cell size in spinal motoneurons. J. Neurophysiol. 28, 560-580.〕 ==Neuronal mechanism of recruitment== Henneman proposed that the mechanism underlying the Size Principle was that the smaller motor neurons had a smaller surface area and therefore a higher membrane resistance. He predicted that the current generated by an excitatory postsynaptic potential (EPSPs) would result in a higher voltage change (depolarization) across the neuronal membrane of the smaller motor neurons and therefore larger EPSPs in smaller motoneurons.〔Henneman, E., Somjen, G. & Carpenter, D. O. (1965). Excitability and inhibitability of motoneurons of different sizes. J. Neurophysiol. 28, 599-620.〕 Burke later demonstrated that there was a graded decrease of both EPSP and inhibitory post synaptic potential (IPSP) amplitudes from small to large motoneurons.〔Burke RE, Rymer WZ & Walsh JR. Relative strength of synaptic input from short-latency pathways to motor units of defined type in cat medial gastrocnemius. J Neurophysiol. 1976 May;39(3):447-58.〕 This seemed to confirm Henneman's idea, but Burke disagreed, pointing out that larger neurons with a larger surface area had space for more synapses. Burke eventually showed (in a very small sample of neurons) that smaller motoneurons have a greater number of synaptic inputs from a single input source.〔Burke RE, Walmsley B, Hodgson JA. HRP anatomy of group Ia afferent contacts on alpha motoneurones. Brain Res. 1979 Jan 12;160(2):347-52〕 The topic is probably still regarded as controversial. Under some circumstances, the normal order of motor unit recruitment may be altered, such that small motor units cease to fire and larger ones may be recruited.〔Garnett, R. & Stephens, JA. The reflex responses of single motor units in human first dorsal interosseous muscle following cutaneous afferent stimulation. J. Physiol. Land. 303: 35 l-364, 1980.〕〔Kanda, K., Burke, R. E., & Walmsley, B. Differential control of fast and slow twitch motor units in the decerebrate cat. Exp. Brain Res. 29:57-74, 1977.〕 This is thought to be due to the interaction of excitatory and inhibitory motoneuronal inputs. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「motor unit recruitment」の詳細全文を読む スポンサード リンク
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