Driving of spindle primary endings by static β axons

  • L. Jami
  • J. Petit
  • J. J. A. Scott


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).
Fig 1.

Example of convergence of dynamic and static β axons onto the same spindle, showing the difference of actions exerted on the response of the primary ending to ramp changes in muscle length. Cat peroneus tertius muscle. Upper trace, instantaneous frequency of discharge of the primary ending. Lower trace, muscle length. On the left is shown the response of the passive ending.


Motor Unit Conduction Velocity Instantaneous Frequency Muscle Spindle Muscle Length 
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Copyright information

© I. A. Boyd and M. H. Gladden 1985

Authors and Affiliations

  • L. Jami
  • J. Petit
  • J. J. A. Scott

There are no affiliations available

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