Ss. Schafer et Ma. Schoppmeyer, TIME-DEPENDENT FUSIMOTOR EFFECTS ON THE DISCHARGE OF CAT PRIMARY MUSCLE-SPINDLE AFFERENTS INDUCED BY A LONG-LASTING SUCCINYLCHOLINE INFUSION, Brain research, 709(1), 1996, pp. 111-121
The responses of 46 Ia afferents from the tibial anterior muscle of th
e cat to repetitive ramp-and-hold stretches were investigated under a
succinylcholine (SCh) infusion of 120 mu g . kg(-1) . min(-1) lasting
15 to 25 min. It was possible to distinguish four consecutive phases o
f the effect of the SCh on the responsiveness of the Ia afferents. The
first three of these four phases have already been described. We anal
ysed in more detail the changes from Phase III to the end of Phase IV.
Static fusimotor effects were dominant in the discharge patterns obta
ined during Phase III; dynamic fusimotor effects prevail at the end of
Phase IV. Our observations were quantified by comparing the mean valu
es of initial activity, final static value, dynamic response and slow
receptor adaptation read from the discharge patterns obtained during P
hase III with the mean values of the same parameters obtained from dis
charge patterns from the end of Phase IV: the two mean values were sig
nificantly different for each of the four measurements. This change fr
om Phase III to the end of Phase IV is highly specific for each spindl
e. To demonstrate this spindle specificity, discharge patterns were se
lected 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 I
I, IV and V display combinations of static and dynamic fusimotor effec
ts with an increasing admixture of static fusimotor effects. The spind
le-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 lowe
r-number category in the case of each of the 46 Ia afferents. In the d
iscussion a combination of activity by the two nuclear bag fibres of a
spindle is deduced from the specific discharge pattern of each catego
ry. The conclusion from these considerations is that results obtained
from the administration of SCh have to be interpreted with great cauti
on in making any statement about the existence of a dynamic bag, fibre
in a spindle.