Functional consequences of bag2 and chain fiber coactivation by static gamma-axons in cat spindles

A study of the distribution in cat peroneus tertius spindles of 42 single static gamma-axons was recently carried out with a physiological method for identifying the intrafusal muscle fibers supplied by single gamma-axons. It was found that 35 axons (83%) supplied both slow-contracting bag2 fibers and fast-contracting chain fibers. The distribution of these axons generally varied from one spindle to another among all the spindles that each of them supplied (bag2 and chain fibers together, bag2 alone, chains alone). To find some functional consequences of this coactivation, responses of primary endings to sinusoidal stretch of the muscle (amplitude 0.5-1 mm, frequency linearly increasing from 0.6 to 8-9 Hz in 12 s) were recorded at different average muscle lengths (0.5, 1.0, and 1.5 mm shorter than maximal physiological length) in nembutalized cats during repetitive stimulation at 10, 20, and 30 Hz of single gamma-axons previously shown to supply bag2 and chain fibers in the spindles bearing the primary endings. These responses were compared with responses elicited in passive spindles and during activation of either bag2 fibers or chain fibers alone. Several records of discharge frequency were averaged. During stimulation at 30 Hz of gamma-axons coactivating bag2 and chain fibers, the averaged discharge of primary endings became continuous (that is, without interruption during each shortening phase as occurs in passive spindles) over the whole range of stretch frequencies. The modulation of the discharge was roughly sinusoidal, with an amplitude that increased with the stretch frequency. Stimulation at 30 Hz of gamma-axons activating bag2 fibers alone elicited a modulation of comparable shape and amplitude but only in the range of sinusoidal stretch from 0.6 to 3-4 Hz. Stimulation at 30 Hz of gamma-axons activating chain fibers alone elicited for each cycle in the range of 0.6 to 5-6 Hz a distorted modulation of large amplitude with a minimal frequency close to that of the stimulation. The average muscle length did not significantly influence these various responses. In summary, the coactivation of bag2 and chain fibers, at presumed physiological frequencies, enables primary endings to continuously signal changes of length over a large range of stretch velocities independently of the average muscle length.