Active force and sensory response of single isolated cat muscle spindles in vitro

Active tetanic tension developed by stimulation of a nerve innervating single isolated muscle spindles of the cat was measured. The maximum tetanic force varied among spindles, ranging from 2.3 to 8.5 mg. For four spindles, the maximum tetanic force was converted into the force per unit cross-sectional area of nuclear chain and nuclear bag fibers. The results are discussed in terms of structural and functional complexities of the intrafusal fibers. The length-tension diagram of single isolated spindles showed maximum tetanic tension occurring at a spindle length longer than 1o, which was defined as the length beyond which passive tension starts to develop. This result, which appears to be different from the corresponding diagram for extrafusal muscle, is discussed in relation to the existing reports on the mechanical properties of intrafusal fibers. Spindle sensory response vs. tetanic tension was analyzed for single isolated spindles with two independent nerve supplies, one containing sensory and the other containing fusimotor axons. The results suggest that the static discharge rate of sensory endings may be linearly related, within the range examined, to the tetanic plateau tension of intrafusal fibers.