Fibre type distribution in the muscle spindles of cat jaw-elevator muscles.

Small samples of the masseter and temporalis muscles of six adult male cats were dissected free, snap-frozen and cryosectioned. Serial sections from areas with muscle spindles were histochemically stained for myofibrillar ATPase after incubation in acid and alkali buffer, and for reactivity to two different myosin isoform-specific antibodies. Differential reactivity to these four stains identified bag1, bag2 and chain intrafusal fibre types. The most typical distribution of fibres within the spindle was 1 bag1, 1 bag2 and 3-4 chains. Although a few spindles were found in the anterior area of the temporalis that attaches to the coronoid process of the mandible and in the anterior area of the superficial masseter, the vast majority were found in the deepest portion of the masseter. Here spindles were characteristically with a number of simple spindles fused together into complex units.