MICRONEUROGRAPHY AND APPLICATIONS TO ISSUES OF MOTOR CONTROL - 5TH ANNUAL REINER,STUART MEMORIAL LECTURE

Among the hypotheses regarding fusimotor functions based on earlier an imal experiments some are inconsistent, others are in conformity with microneurographic observations in man. The human data provide evidence against the following two theories: (1) the length follow-up servo th eory; and (2) the theory that fusimotor neurons can be selectively act ivated to produce spindle sensitization and stretch reflex reinforceme nts. The human data support the theory of alpha-gamma coactivation. In particular, in the early phase of isometric voluntary contractions fu simotor-driven afferent spindle activity assists in autogenetic activa tion of alpha motoneurons and in reciprocal relaxation of antagonists. As muscle fatigue develops, the autogenetic reflex drive via the fusi motor route declines. The fusimotor bias during contraction provides f or maintenance of spindle sensitivity to minute perturbations and for load-compensating reflex adjustments to such perturbations. Reflex ove rcorrections may lead to uncontrollable oscillations of the type seen in enhanced physiological tremor.