DETERMINATION OF STATIC MECHANICAL-PROPERTIES OF INTRAFUSAL MUSCLE IN ISOLATED CAT MUSCLE-SPINDLES

The sensitivity of mammalian muscle spindles to stretch is greater in a stretched muscle than in a slack one. This behavior was investigated in isolated muscle spindles removed from cat tenuissimus muscle. Measurements of the steady-state strain of intrafusal muscle in sensory and non-sensory regions indicated a proportional relationship between sensory strain and receptor sensitivity; both increase with spindle length. By comparing intrafusal strain of sensory and non-sensory areas with and without an intact spindle capsule, we conclude that capsule does not contribute to the non-linear sensitivity. Measurements of steady-state tension indicate that the striated portions of the intrafusal muscle exhibit a non-linear stiffness which can quantitatively account for the observed behavior.