Nigral output neurons are engaged in regulation of static fusimotor action onto flexors in cat.

Picrotoxin and muscimol were unilaterally injected into the postero-lateral part of the reticular zone of substantia nigra (plSNR) through chronically implanted guide cannulae in ketamine-anesthetized cats. Afferent activity of pretibial flexor muscle spindle primary and secondary endings was recorded before and after drug administration, and spindle sensitivity monitored during both sinusoidal and ramp stretch of the receptor-bearing muscle. From changes in spindle sensitivity after drug injection it is deduced that unilateral block by picrotoxin of the action of GABA on postsynaptic receptors in plSNR removes tonic static fusimotor action from flexor muscle spindle primary endings. Secondary endings seemed largely unaffected. The effect on primary endings is reversed by a subsequent injection of muscimol. It is concluded that the central nervous system, through GABA-modulated nigral output neurons, can control static fusimotor action onto flexor muscle spindle primary and secondary endings separately to some extent.