Critique of Papers Presented by Kucera et al., Walro & Wang and Barker et al.
The work described here concerns the ontogenetic development of intrafusal muscle fibres and their innervation. The present view, derived from previous work from several laboratories, is that the bag2 fibre of the muscle spindle develops from a primary myotube prenatally, its specialization from extrafusal precursors being associated with, and probably caused by, the termination on it of a primary afferent axon. Bag1 and chain fibres appear later as secondary myotubes in close physical association with the bag2 primary myotube. Temporally the arrival of the motor supply lags slightly behind the afferent supply and is not required for bag2 fibre differentiation.
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