Standardized measurements were made of the area and diameter of the muscle spindle, mean cross-sectional area of intrafusal fibers
, number of intrafusal fibers
and thickness of the spindle capsule in the muscle spindle with the largest diameter.
The EMG activity during a stretch is triggered by impulses from the intrafusal fibers
of the muscle spindles.
The therapeutic effects of BTX-A seemed to be due not only to partial denervation of extrafusal muscles but also to fusimotor denervation of intrafusal fibers
that tonically control the sensitivity of spindle sensory afferents [19-21].
The motoneuron plays a role in changing the length of the intrafusal fibers
and adjusting sensitivity of the muscle spindles.
The second type of muscle fiber, intrafusal fibers
, functions within muscle spindles and is innervated by gamma motor neurons and sensory Ia neurons (see Figure 1 ).