Intrafusal muscle fiber
Intrafusal fibers are
muscle fibersthat comprise the muscle spindleand are innervated by the gamma motor neurons ( gamma motoneuron). These fibersare walled off from the rest of the muscleby a collagensheath. This sheath has a spindle or "fusiform" shape, hence the name "intrafusal." While the intrafusal fibers are wrapped with sensory receptors, their counterpart, extrafusal muscle fibers are the ones responsible for the power-generating component of muscle and are innervated by motor neurons.
It is by the sensory information from these two intrafusal fiber types that one is able to judge the position of their muscle, and the rate at which it is changing.
Type Ia sensory fiber
Type II sensory fiber
* [http://meded.ucsd.edu/hist-img-bank/chapter_3/Slide_60_artery_vein/pages/b.22.214.171.124.htm Histology at ucsd.edu]
* [http://www3.umdnj.edu/histsweb/lab6/skeletalmuscle/skmotorend.html Histology at umdnj.edu]
* [http://www.blackwellpublishing.com/11thhour/matt/about/u4c12a.html Cartoon of intrafusal vs. extrafusal fibers]
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