Observations on the number, distribution and morphological peculiarities of muscle spindles in the tensor tympani and stapedius muscle of man

Although the middle ear muscles have been described for the first time more than four hundred years ago their role in modulation and transmission of s ound is not yet fully understood. Surprisingly very little is known about p roprioceptors in these muscles, especially in man, although this seems to b e the key to the understanding of their various functions. Therefore, the q uestion for proprioceptive sensory organs in these muscles is still relevan t. The tensor tympani and stapedius muscles of four women who had donated t heir bodies to our institute were taken. Complete serial sections of these muscles were made which were either impregnated with silver, stained with f erric oxide for acidic polysaccharides or incubated with antibodies against S-100 protein. Thereby four to eight (mean five) muscle spindles distribut ed along the whole muscle could be detected in the tensor tympani muscles. These spindles contain one to three intrafusal muscle fibres and their leng th ranges from 140 to 4270 mu m (mean 1492.8 mu m). Furthermore, in three s tapedius muscles one to two (mean 1.7) muscle spindles were found. They wer e from 350 to 500 mu m (mean 482 mu m) long and contained only one intrafus al muscle fiber. Regarding the diameter of intrafusal muscle fibers in both , the tensor tympani as well as the stapedius muscle, no difference to extr afusal muscle fibers of these muscles could be detected. The structure of t hese spindles differs considerably from those found in skeletal muscles. Th e morphological findings presented strongly suggest that muscle spindles oc cur regularly in both middle ear muscles. The results presented herein are consistent with clinical findings obtained from electromyographic studies a nd may help to elucidate all functions the middle ear muscles might serve i n man. (C) 1999 Elsevier Science B.V. All rights reserved.