Driving of spindle primary endings by static β axons
The motor innervation of mammalian spindles is mainly provided by specific fusimotor or γ motoneurones, whose axons supply intrafusal muscle fibres exclusively. In addition, some spindles also receive collateral branches of motor axons supplying ordinary or extrafusal muscle fibres. These axons, termed skeleto-fusimotor or β. axons comprise the same two functional types, dynamic and static, that exist among γ axons. The two types of β axons differ not only by the actions they exert on spindle endings (Fig 1) but also by their conduction velocities, by their intrafusal distribution and by the type of muscle fibres composing the extrafusal portion of their motor units. Dynamic β axons have conduction velocities under 85m/s and innervate intrafusal bag1 fibres as well as extrafusal type 1 fibres whereas static β axons, with conduction velocities above 85m/s, are distributed to intrafusal chain fibres and extrafusal type 2 fibres (see references in Laporte et al. 1981). In the cat peroneus tertius muscle, 30% of motor units are innervated by β axons, of which one third are dynamic and two thirds are static (Jami et al. 1982).
KeywordsMotor Unit Conduction Velocity Instantaneous Frequency Muscle Spindle Muscle Length
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.
Unable to display preview. Download preview PDF.
- Emonet-Denand, F. & Laporte, Y. (1983). Observations on the effects on spindle primary endings of the stimulation at low frequency of dynamic β-axons. Brain Res. 258, 101–104.Google Scholar
- Harker, D.W., Jami, L., Laporte, Y. & Petit, J. (1977). Fastconducting skeletofusimotor axons supplying intrafusal chain fibers in the cat peroneus tertius muscle. J. Neurophysiol. 40, 791–799.Google Scholar
- Jami, L., Murthy, K.S.K. & Petit, J. (1982). A quantitative study of skeletofusimotor innervation in the cat peroneus tertius muscle. J. Physiol. 325, 125–144.Google Scholar
- Jami, L., Lan-Couton, D., Malmgren, K. & Petit, J. (1979). Histo-physiological observations on fast skeletofusimotor axons. Brain Res. 164, 53–59.Google Scholar
- Kucera, J. (1984). Histological identification of (static) skeleto-fusimotor innervation to a cat muscle spindle. Brain Res. 294, 390–395.Google Scholar
- Kucera, J. & Hughes, R. (1983). Ultrastructural study of motor endings on long chain intrafusal fibers in muscle spindles of the cat. Brain Res. 262, 309–313.Google Scholar
- Kuffler, S.W., Hunt, C.C. & Quilliam, J.P. (1951). Function of medullated small nerve fibres in mammalian ventral roots: efferent muscle spindle innervation. J. Neurophysiol. 14, 29–54.Google Scholar
- Laporte, Y., Emonet-Denand, F. & Jami, L. (1981). The skeleto-fusimotor or β-innervation of mammalian muscle spindles. Tins, 4, 97–99.Google Scholar
© I. A. Boyd and M. H. Gladden 1985