Time-dependent fusimotor effects on the discharge of cat primary muscle spindle afferents induced by a long-lasting succinylcholine infusion.

The responses of 46 Ia afferents from the tibial anterior muscle of the cat to repetitive ramp-and-hold stretches were investigated under a succinylcholine (SCh) infusion of 120 micrograms.kg-1.min-1 lasting 15 to 25 min. It was possible to distinguish four consecutive phases of the effect of the SCh on the responsiveness of the Ia afferents. The first three of these four phases have already been described. We analysed in more detail the changes from Phase III to the end of Phase IV. Static fusimotor effects were dominant in the discharge patterns obtained during Phase III; dynamic fusimotor effects prevail at the end of Phase IV. Our observations were quantified by comparing the mean values of initial activity, final static value, dynamic response and slow receptor adaptation read from the discharge patterns obtained during Phase III with the mean values of the same parameters obtained from discharge patterns from the end of Phase IV: the two mean values were significantly different for each of the four measurements. This change from Phase III to the end of Phase IV is highly specific for each spindle. To demonstrate this spindle specificity, discharge patterns were selected from among those produced by each Ia afferent in Phase III and at the end of Phase IV. Each of these discharge patterns was assigned to one of six categories. Category I displays purely dynamic fusimotor effects and Category VI purely static fusimotor effects. Categories II, IV and V display combinations of static and dynamic fusimotor effects with an increasing admixture of static fusimotor effects. The spindle-specific change from Phase III to the end of Phase IV is defined in terms of the specific degree of change from a higher-number to a lower-number category in the case of each of the 46 Ia afferents. In the discussion a combination of activity by the two nuclear bag fibres of a spindle is deduced from the specific discharge pattern of each category. The conclusion from these considerations is that results obtained from the administration of SCh have to be interpreted with great caution in making any statement about the existence of a dynamic bag1 fibre in a spindle.