Reflex consequences of muscle afferent input to beta and gamma motoneurones

Abstract

Beta-motoneurons send axonal branches to both intrafusal and extrafusal muscle fibers, but are thought to be similar to alpha motoneurons in most other characteristics including size, input resistance, conduction velocity, extrafusal muscle innervation, and pattern of afferent input (Emonet-Denand et al, ’75, Barker, et al, ’77, Harker, et al, ’77, Burke and Tsairis, ’77). If beta motoneurons receive spindle afferent input of comparable magnitude to that of alphas, and if they have significant effects on the discharge of muscle spindle afferents, a positive feedback loop involving beta motoneurons and muscle spindles would be in operation. That is, muscle spindle afferents responding to stretch would augment beta motoneuron discharge which would then further increase spindle afferent discharge. If the gain of this positive feedback is significant, then the loop can be shown to cause the system to be less dependent on changes in muscle and load properties (Houk, ’72).