Beta, or skeletofusimotor, innervation of muscle spindles is now known to be common in a range of different muscles in many animal species, yet its functional role remains poorly understood. The experiments to be described will examine three different aspects of Beta function. First, interactions between Beta and Gamma innervation to muscle spindles will be studied by imposing Beta or Gamma inputs onto quiescent spindles, or onto spindles receiving electrically mediated or reflexively induced changes in Gamma fusimotor input. The hypothesis to be tested is that Beta action is facilitated by concurrent Gamma activity. Second, functional contributions of Beta action will be studied by examining the efficacy of Beta fibers in offsetting the reductions in spindle afferent discharge that normally accompany command-induced muscle shortening. The role of Beta action in compensating for the effects of series compliance (which is largely tendon based) will be studied by comparing spindle afferent responses to force change in deefferented muscle, during Gamma activation, and during combined Gamma and Beta activation. Beta effects in assisting muscle reflex compensation for changes in muscle contractility and load magnitude will also be evaluated. Third, Beta motoneurons will be studied with intracellular recordings from triceps motoneurons in the decerebrate cat preparation, using the finding of tetanically induced late depolarization as a marker of possible Beta identity. The rationale for this approach is that tetanic stimulation of a Beta motoneuron should induce high frequency discharge one or more spindle afferents, giving rise to a delayed depolarization of the stimulated motoneuron. The validity of this approach will be assessed by recording the discharge of spindle afferents from the same region of the muscle, during tetanic intracellular stimulation. Alternatively, glycogen depletion of the associated muscle unit will be induced by prolonged intracellular stimulation of the motoneuron, and Beta motoneurons identified by showing glycogen depletion of intrafusal fibers. Biophysical characteristics of Beta motoneurons and the mechanical properties of the associated muscle units will be compared in a population of Beta and in a population of Alpha motoneurons.
