Relation to extrafusal fibre-type composition in muscle-spindle structure and location in the human masseter muscle

Muscle-spindle density and size, in relation to extrafusal fibre-type, were examined in different portions of the masseter of six normal males by enzyme histochemistry. A direct relationship between spindle density, intrafusal fibre density and spindle size, and proportion of type I muscle fibres was found in the deep portion. Out of 410 muscle spindles in random samplings, 74 per cent were in the deep portion which is predominantly composed of type I fibres. Spindle density was three times greater than in the other portions, and spindles were commonly clustered, sometimes sharing capsule tissue. The deep masseter spindles were distinctive; they had the largest diameter and the most intrafusal fibres, the fibre density being four times greater than in other portions. Eleven per cent of the spindles, mainly in the deep portion, contained an unusually high number of intrafusal fibres (15 or more fibres). The findings imply that individual portions of the masseter have specialized functions. An especially powerful proprioceptive reflex mechanism is suggested for the deep portion. Thus, the human masseter may be part of an intricate, evolutionarily advanced, motor system also connected with speech.