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.
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Acknowledgements-This researchw ass upportedb y grants from the National ScienceF oundation( NSF No. 76-10791) and the U.S. Public Health Service( NS 109690-04C)o. m-puterf acilitesw erem adea vailableb y the Air Force Office of Scientific Research,A FSC (Grant AFOSR-1221).W e wish to gratefullya cknowledgeth e technicala ssistanceo f L+ISDAO NALDSOaNnd RICHARDH ANSEN.