Effects of stimulating the lumbar sympathetic trunk on cat hindlimb muscle spindles

1. The effect of stimulating the lumbar sympathetic trunk has been observed on cat lumbrical and tenuissimus muscle spindles. 2. Spindle afferent discharges were recorded either from single Ia fibers in teased dorsal root filaments or from a large number of spindles by integrating their discharges led from muscles nerves. 3. Blood flow in small arteries supplying the muscle was observed through a microscope during and after the stimulation of the sympathetic trunk. 4. In some spindles repetitive stimulation of the sympathetic trunk elicited, after a few seconds delay, a small increase in firing rate. This can be ascribed to a direct action of sympathetic axons on the spindles because it precedes by about 20-30 sec the reduction of blood flow observed in the muscle arteries. This effect is not accompanied by a change in dynamic sensitivity of the primary ending. 5. This early effect is followed, after 20-30 sec, by a later rise in firing frequency which still progresses after the end of stimulation and eventually terminates in an abrupt fall in firing often leading to interruption of the ending activity. Recovery takes places at a variable time after the blood flow has bee reestablished. These long lasting effects can be ascribed to reduction of blood flow in muscle spindles since they are always associated with changes in blood flow in muscle arteries and since they are mimicked by occlusion of the muscle circulation. 6. In some spindles, the amplitudes of frequencygrams elicited by stimulation of static gamma axons were slightly increased suggesting a weak facilitatory effect on the contraction of some intrafusal muscle fibers.