Muscle Spindle Responses During Contractions of Extrafusal Muscle Fibers in the Frog

The effects exerted on muscle spindles in the frog sartorius during contraction of the extrafusal muscle fibers were studied in comparison with those during contraction with participation of the intrafusal fibers.2, The discharges were suppressed without appreciable early discharges during the isometric twitch and tetanic contractions of the extrafusal muscle fibers, whilst some early discharges appeared during the contractions with participation of the intrafusal muscle fibers.3. During shortening of the extrafusal muscle fibers against loads, the spindle discharges were facilitated only at the second upward deflection of the muscle tension.The shortening with participation of the intrafusal muscle fibers resulted often in a few additional discharges at the first upward deflection.4. These results suggest that the early discharge of the spindle receptor may be due to contractions of the intrafusal muscle fibers (a serial force with the receptor) in relation to those of the extrafusal muscle fibers (a parallel force).