PROJECTION OF CAT JAW MUSCLE-SPINDLE AFFERENTS RELATED TO INTRAFUSAL FIBER INFLUENCE

1. A method of classification of muscle spindle afferents using succin ylcholine (SCh) and ramp stretches has recently been described, which appears to estimate separately the strength of influence of bag, (b1) and of bag, (b2) intrafusal fibres. Increase in dynamic difference (DE LTADD) indicates b1 influence whilst increase in initial frequency (DE LTAIF) indicates b2 influence. The significance of this classification has now been examined by correlation with the strength of synaptic pr ojection of jaw muscle spindle afferents to the fifth motor nucleus (M otV) and the supratrigeminal region (STR) in anaesthetized cats.2. Pro jection strength was estimated by computing the extracellular focal sy naptic potential (FSP) from spike-triggered averages of 1024 sweeps at 100 mum intervals along tracks through STR and MotV. Trigger pulses w ere derived from spindle afferent cell bodies of the jaw-closer muscle s recorded in the mesencephalic trigeminal nucleus, and characterized by the effect of SCh on their responses to ramp-and-hold stretches. 3. The maximum size of FSPs in tracks traversing STR and MotV ranged fro m 2.08 to 36.99 muV with a mean of 7-55 muV. The amplitudes were bimod ally distributed into roughly equal-sized groups with high and low amp litude FSPs. 4. Mean values of DELTAIF were significantly greater for the group with large FSPs than for those with small FSPs. There were n o significant differences in ADD. FSP amplitude was significantly posi tively correlated with DELTAIF, but not with DELTADD. 5. Spindle affer ents with high values of FSP amplitude in MotV had a wide range of val ues of DELTADD (b1b2c and b2c groups), while units with large FSPs in STR were all in the b2C category. Some evidence is presented to indica te that this reflects a preferential projection of secondary afferents to the STR. 6. For those units with projection to both STR and to Mot V, there was a significant positive correlation between FSP amplitude in the two nuclei. 7. These results indicate that the extent of the b2 influence on spindle afferents predicts the central projection streng th better than does the b1 influence. This finding is discussed from t he viewpoint of possible developmental and functional issues.