A 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. We have investigated this behavior in isolated muscle spindles removed from cat tenuissimus muscle. We measured the steady-state strain of intrafusal muscle in sensory and non-sensory regions and found that there is 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.