Image edit
Depiction of a motor unit, comprised of a motor neuron innervating a muscle fiber, to be added to the Motor unit recruitment page. Created by me:
Neuronal mechanism of recruitment edit
To be added to a new section of Motor unit recruitment page, titled "Recruitment of motor unit types":
Another topic of controversy resides in the way in which Burke and colleagues categorized motor unit types. They designated three general groups by which motor units could be categorized: S (slow – slow twitch), FR (fast, resistant – fast twitch, fatigue-resistant), and FF (fast, fatigable – fast twitch, fatigable).[1][2] These designations have served as the basis for motor unit categorization since their conception, but modern research indicates that human motor units are more complex and possibly do not directly fit this model.[3]
However, it is important to note that Burke himself recognized the risk in classifying motor units:
My friend the late Elwood Henneman told me several times in conversation that he thought classifying motor units into distinct categories was probably a bad idea because, unless used with care, classifications tend to distort reality. I agreed, and still do, that taxonomies can lead to overly rigid thinking (and sometimes even lack of thinking) but they are necessary for communication, which requires that things be named; and scientific communication demands that things be named precisely, according to their attributes.[4]
If a correlation were to be drawn between Henneman's size principle[5] and the motor unit categorization of Burke regarding the order of motor unit recruitment, it would resemble the following order: the smallest units, S (slow), would be recruited first, followed by larger FR (fast, resistant) units, and lastly the largest FF (fast, fatigable) units, reserved for high-energy tasks that require additional motor unit recruitment.[6][7]
Location within 'Neuronal mechanism of recruitment' section of Motor unit recruitment page to be determined:
In their 1982 paper, Burke and colleagues propose that the small cell size and high surface-to-volume ratio of S motor units allows for greater metabolic activity, optimized for the "highest duty cycles" of motoneurons, while other motor unit types may be involved in "lower duty cycles." However, they state that the evidence is not conclusive "to support or deny the intuitively appealing notion that there is a correlation between metabolic activity, motoneuron size, and motor unit type."[2]
References edit
- ^ Burke, R.E.; Levine, D.N.; Zajac, F.E.; Tsairis, P.; Engel, W.K. (1971). "Mammalian Motor Units: Physiological-Histochemical Correlation in Three Types in Cat Gastrocnemius". Science. 174 (4010): 709–712. doi:10.1126/science.174.4010.709 – via American Association for the Advancement of Science.
- ^ a b Burke, R.E.; Dum, R.P.; Fleshman, J.W.; Glenn, L.L.; Lev-Tov, A.; O'Donovan, M.J.; Pinter, M.J. (1982). "A HRP study of the relation between cell size and motor unit type in cat ankle extensor motoneurons". The Journal of comparative neurology. 209 (1): 17–28. doi:10.1002/cne.902090103. PMID 7119171 – via PUBMED.
- ^ Heckman, CJ; Enoka, R.M. (2012). Terjung (ed.). "Motor Unit". Comprehensive Physiology. 2 (4): 2629–2682. doi:10.1002/cphy.c100087. PMID 23720261 – via PUBMED.
- ^ Burke, R.E. (1999). Binder, M.D. (ed.). "Chapter 15 Revisiting the notion of 'motor unit types'". Progress in Brain Research. 123: 167–175. doi:10.1016/S0079-6123(08)62854-X – via Elsevier.
- ^ Henneman, E. "Skeletal muscle: the servant of the nervous system". Medical physiology. 1: 674–741.
- ^ Hodson-Tole, E.F.; Wakeling, J.M. (2009). "Motor unit recruitment for dynamic tasks: current understanding and future directions". J Comp Physiol B. 179: 57–66. doi:10.1007/s00360-008-0289-1. PMID 18597095 – via PUBMED.
- ^ Conwit, R. A.; Stashuk, D.; Tracy, B.; McHugh, M.; Brown, W. F.; Metter, E. J. (1999). "The relationship of motor unit size, firing rate and force". Clinical neurophysiology : official journal of the International Federation of Clinical Neurophysiology. 110 (7): 1270–1275. doi:10.1016/s1388-2457(99)00054-1.