Method: Undamaged LPMs were extracted from two donated bodies. Selection criteria comprised no general abnormalities and no specific abnormalities in the masticatory muscle group. The collected lateral pterygoid and masseter muscles were fixed in 10% formalin solution. After fixation, specimens were separated into superior and inferior head sections and embedded in paraffin. Slices of 12 and 8 μm were prepared and every 10th slice was stained with hematoxylin-eosin. Slides were observed consecutively under an optical microscope in order to view entire spindles. Muscle spindle markers comprised distinct spindle capsules and sites thought to be intrafusal muscle fibers within the muscle. Immunostaining with anti-S-100 antibodies in the equatorial region was also performed to identify MS. This study was approved by the ethics committee of Nihon University School of Dentistry at Matsudo (EC-09-0011).
Result: In specimens from both donors, multiple MS covered by connective tissue were observed in the inferior head of the LPM, and on immunostaining, muscle spindle nerve endings were observed extending into the equatorial region. These results confirm the presence of MS in the inferior head of the LPM.
Conclusion: MS were confirmed in the inferior head of the human LPM disseminated near the muscle belly with approximately two intrafusal muscle fibers per spindle. Intrafusal muscle fibers were mainly inserted onto the spindle capsule and the spindle lumen in the equatorial region was narrower than in the masseter.