Reflex actions of muscle afferents on fusimotor innervation in decerebrated cats: an assessment of beta contributions.

The existence of beta innervation in many cat muscles raises the possibility that spindle afferent discharge will excite beta motoneurons, augmenting spindle afferent discharge and thereby closing a positive feedback loop. In order to evaluate the strength of such a loop through beta motoneurons and muscle spindles, the stretch responses of muscle spindle afferents from medial gastrocnemius (MG) and soleus (SOL) muscles were studied in decerebrated cats before and after dorsal root section. If a positive feedback loop were operational, the spindle afferent stretch response should be diminished following dorsal root section by an amount related to the magnitude of positive feedback. After dorsal root section, the static positional sensitivities of our MG spindle afferent sample were significantly reduced for 72% (13/18, p less than 0.05) of the afferents, and dynamic rate/length slopes were decreased for 88% (8/9) of a subset of the afferents studied. Similar reductions for 6 afferents from SOL were not found. To apportion these afferent changes to reflex excitation of either gamma or beta motoneurons, we recorded the stretch responses of gamma and alpha-type fibers in the same preparation. (We assume that the population of alpha-type fibers includes beta fibers). In keeping with other reports, alpha fibers were much more responsive to stretch than gamma fibers. The mean positional sensitivity for alpha fibers (1.29 +/- 0.92 pps/mm, n = 15) was greater (p less than 0.05) than that of gamma fibers (0.49 +/- 0.93 pps/mm, n = 18). Because of these differences in sensitivity, beta motoneurons are more likely (than gamma motoneurons) to be involved in a positive feedback loop, although some gamma contribution is also likely. Using equations based on a beta position regulating scheme, differences in spindle positional sensitivity were used to estimate beta loop gain. The average loop gain was estimated to be 0.41 (n = 18). The contribution of such a beta configuration to reducing the sensitivity of muscle to changes in load and muscle properties is evaluated.