The Motor Unit

  • John C. Rothwell


The previous chapter discussed some of the properties of whole muscles and the importance of these properties in the overall control of movement. The present chapter is concerned with the internal organisation of single muscles. A muscle does not consist of an homogenous population of muscle fibres: there are several different types of fibre within muscle, each of which have different mechanical properties. Thus the range of operation of the whole muscle is extended beyond that expected from any one fibre type alone.


Firing Rate Motor Unit Medial Gastrocnemius Motor Axon Single Motor Unit 


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References and Further Reading

Review Articles

  1. Buchthal, F. and Schmalbruch, M. (1980) `Motor Unit of Mammalian Muscle’, Physiol. Rev, 60, pp. 90–142Google Scholar
  2. Burke, R.E. (1981) `Motor Units: Anatomy, Physiology and Functional Organisation’ in V.B. Brooks (ed.) Handbook of Physiology, sect. 1, vol. 2, part 1, Williams and Wilkins, Baltimore, pp. 345–411Google Scholar
  3. Desmedt, J.E. (ed.) (1981) Motor Unit Types, Recruitment and Plasticity in Health and Disease. Progress in Clinical Neurophysiology, vol. 9, Karger, BaselGoogle Scholar
  4. Dum, R.P. and Kennedy, T.T. (1980) `Synaptic Organisation of Defined Motor-unit Types in Cat Tibialis Anterior’, J. Neurophysiol, 43, pp. 1631–44Google Scholar
  5. Freund, H.-J. (1983) `Motor Unit and Muscle Activity in Voluntary Motor Control’, Physiol. Rev, 63, pp. 387–436Google Scholar
  6. Garnett, R. and Stephens, J.A. (1981) `Changes in the Recruitment Threshold of Motor Units Produced by Cutaneous Stimulation in Man’, J. Physiol, 311, pp. 463–73Google Scholar
  7. Granit, R. (1970) The Basis of Motor Control, Academic Press, New YorkGoogle Scholar
  8. Henneman, E. and Mendell, L.M. (1981) `Functional Organisation of Motorneuron Pool and Its Inputs’ in V.B. Brooks (ed.), Handbook of Physiology, Sect. 1, vol. 2, part 1, Williams and Wilkins, Baltimore, pp. 423–508Google Scholar
  9. Kanda, K., Burke, R.E. and Walmsley, B. (1977) `Differential Control of Fast and Slow Twitch Motor Units in the Decerebrate Cat’, Exp. Brain Res, 19, pp. 57–74Google Scholar
  10. Kernell, D. (1965) ‘High-frequency Repetitive Firing of Cat Lumbosacral Motoneurones Stimulated by Long-lasting Injected Currents’, Acta Physiol. Scand., 65, pp. 74–86CrossRefGoogle Scholar
  11. Kimura, J. (1983) Electrodiagnosis in Diseases of Nerve and Muscle: Principles and Practice, F.A. Davis, PhiladelphiaGoogle Scholar
  12. Lewis, D.M. (1981) `The Physiology of Motor Units in Mammalian Skeletal Muscle’ in A.L. Towe and E.S. Luschei (eds.), Handbook of Behavioural Neurobiology, vol. 5, Plenum, New York, pp. 1–67Google Scholar

Original Papers

  1. Adrian E.D. and Bronk, D.W. (1929) `The Discharge of Impulses in Motor Nerve Fibres. Part II. The Frequency of Discharge in Reflex and Voluntary Contractions’, J. Physiol, 67, pp. 119–51Google Scholar
  2. Bigland-Ritchie, B., Johansson, R., Lippold, O.C.J. et al (1983) `Changes in Motoneurone Firing Rates During Sustained Maximal Voluntary Contractions’, J. Physiol, 340, pp. 335–46Google Scholar
  3. Burke, R.E. (1975) `A Comment on the Existence of Motor Unit “Types” ’ in D.B. Tower (ed.) The Nervous System. The Basic Neurosciences, vol. 1, Raven, New York, pp. 611–19Google Scholar
  4. Burke, R.E., Levine, D.N., Tsairis, P. et al (1973) `Physiological Types and Histochemical Profiles in Motor Units of the Cat Gastrocnemius’, J. Physiol, 234, pp. 723–48Google Scholar
  5. Burke, R.E. Rymer, W.Z. and Walsh, J.V. (1976) `Relative Strength of Synaptic Input from Shortlatency Pathways to Motor Units of Defined Type in Cat Medial Gastrocnemius’, J. Neurophysiol, 39, pp. 447–85Google Scholar
  6. Burke, R.E. and Tsairis, P. (1973) `Anatomy and Innervation Ratios in Motor Units of Cat Gastrocnemius’, J. Physiol, 234, pp.749–65Google Scholar
  7. Datta, A.K. and Stephens, J.A. (1981) `The Effects of Digital Nerve Stimulation on the Firing of Motor Units in Human First Dorsal Interosseous Muscle’, J. Physiol, 318, pp.501–10Google Scholar
  8. De Luca, C.J., Lefever, R.S., McCue, M.P., et al (1982) `Behaviour of Human Motor Units in Different Muscles During Linearly Varying Contractions’, J. Physiol, 329, pp. 113–28Google Scholar
  9. Denny-Brown, D. (1929) `On the Nature of Postural Reflexes’, Proc. Roy. Soc. B, 104, pp. 252–301CrossRefGoogle Scholar
  10. Desmedt, E. and Godaux, E. (1977) `Fast Motor Units are not Preferentially Activated in Rapid Voluntary Contractions in Man’, Nature, 267, pp. 717–9CrossRefGoogle Scholar
  11. Eccles, J.C., Eccles, R.M. and Lundberg, A. (1957) `The Convergence of Monosynaptic Excitatory Afferents onto Many Different Species of Alpha-motoneurone’, J. Physiol, 137, pp. 22–50Google Scholar
  12. Edstrom, L. and Kugelberg, E. (1968) ‘Histochemical Composition, Distribution of Fibres and Fatiguability of Single Motor Units’, J. Neurol. Neurosurg. Psychiat, 31, pp. 424–33Google Scholar
  13. Garnett, R.A.F., O’Donovan, M.J., Stephens, J.A. et al. (1978) `Motor Unit Organisation of Human Medial Gastrocnemius’, J. Physiol, 287, pp. 33–43Google Scholar
  14. Granit, R., Kernel’, D. and Smith, R.S. (1963) `Delayed Depolarisation and the Repetitive Response to Intracellular Stimulation of Mammalian Motoneurones’, J. Physiol, 168, pp. 890–910Google Scholar
  15. Gustaffson, B. and Pinter, M.J. (1984) `Relations among passive electrical properties of lumbar α-motoneurones of the cat’. J. Physiol, 356, pp. 401–32 and 433–42Google Scholar
  16. Henneman, E., Somjen, G. and Carpenter, D.O. (1965) `Functional Significance of Cell Size in Spinal Motoneurons’, J. Neurophysiol, 28, pp. 560–80Google Scholar
  17. Jami, L. Murthy, K.S.K., Petit, J. et al (1982) `Distribution of Physiological Types of Motor Unit in the Cat Peroneus Tertius Muscle’, Exp. Brain Res, 48, pp. 177–84Google Scholar
  18. Kugelberg, E. and Edstrom, L. (1968) `Differential Histochemical Effects of Muscle Contractions on Phosphorylase and Glycogen in Various Types of Fibres: Relation to Fatigue’, J. Neurol. Neurosurg. Psychiat, 31, pp. 415–23CrossRefGoogle Scholar
  19. Luscher, H.-R., Ruenzel, P.W. and Henneman, E. (1983) `Effects of Impulse Frequency, PTP, and Temperature on Responses Elicited in Large Populations of Motoneurones by Impulses in Single la Fibres’, J. Neurophysiol, 50, pp. 1045–58Google Scholar
  20. Marsden, C.D., Meadows, J.C. and Merton, P.A. (1983) ’ “Muscular Wisdom” that Minimises Fatigue During Prolonged Effort in Man: Peak Rates of Motoneurone Discharge and Slowing of Discharge During Fatigue’ in J.E. Desmedt (ed.), Adv. in Neurol, vol. 39, Raven Press, New YorkGoogle Scholar
  21. Merton, P.A. (1954) `Voluntary Strength and Fatigue’, J. Physiol, 123, pp. 553–64Google Scholar
  22. Milner-Brown, H.S., Stein, R.B. and Lee, R.G. (1974) `Contractile and Electrical Properties of Human Motor Units in Neuropathies and Motor Neurone Disease’, J. Neurol. Neurosurg. Psychiat, 37, pp. 670–6 (and pp. 665–9)Google Scholar
  23. Rall, W. (1967) `Distinguishing Theoretical Synaptic Potentials Computed for Different Somadendritic Distributions of Synaptic Input’, J. Neurophysiol, 30, pp. 1138–68Google Scholar
  24. Rosenfalck, A. and Andreassen, S. (1980) `Impaired Regulation of Force and Firing Pattern of Single Motor Units in Patients with Spasticity’, J. Neurol. Neurosurg. Psychiat, 43, pp. 907–16CrossRefGoogle Scholar
  25. Tang, A. and Rymer, W.Z. (1981) `Abnormal Force-EMG Relations in Paretic Limbs of Hemiparetic Human Subjects’, J. Neurol. Neurosurg. Psychiat, 44, pp. 690–8CrossRefGoogle Scholar
  26. Walmsley, B., Hodgson, J.A. and Burke, R.E. (1978) `Forces Produced by Medial Gastrocnemius and Soleus Muscles During Locomotion in Freely Moving Cats’, J. Neurophysiol, 41, pp. 1203–16Google Scholar
  27. Zengel, J.E., Reid, S.A., Sypert, G.W. et al (1985) `Membrane Electrical Properties and Prediction of Motor-unit Types of Medial Gastrocnemius Motoneurons in the Cat’, J. Neurophysiol., 53, pp. 1323–44Google Scholar

Copyright information

© John C. Rothwell 1987

Authors and Affiliations

  • John C. Rothwell
    • 1
  1. 1.Department of Neurology, Institute of PsychiatryUniversity of LondonUK

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