Response characteristics and classification of muscle spindles of the masseter muscle in the cat

Abstract

For the classification of primary and secondary spindle endings in jaw-closing muscles, the relationship between physiologic properties of these endings and conduction velocities of their afferent fibers was analyzed. Unitary discharges of afferent fibers from masseter muscle spindles were recorded in themesencephalic trigeminal nucleus (Mes V) by microelectrodes. Conduction velocities of masseter spindle units in the Mes V showed a bimodal distribution with the division at 36 m/s. The units conducting at more than 54 m/s and those conducting less than 36 m/s were categorized as the primary and secondary endings, respectively, on the basis of their responses to muscle stretches, vibratory stimuli, and to suxamethonium (SCh) administration (200 μg/kg, i.v.). On the other hand, spindle units conducting at between 36 and 54 m/s, which were designated as the intermediate spindle unit, showed a great variety in their responses to the same stimuli. In addition, the number of intermediate spindle units was about twice as large as that of the well-defined units. Because the spindle units in the Mes V were rather clearly divied into two groups on the basis of their sensitivity to vibration and to SCh irrespective of their conduction velocities, the intermediate spindle units may effectively be categorized on the same basis. There are several possible reasons to account for the high proportion of intermediate spindles.