112.2 Saturday, Jan. 7 Non-uniform strain in squid mantle muscle: relating the length-tension curve to in vivo muscle performance THOMPSON, J.T.*; SHELTON, R.M.; KIER, W.M.; Franklin and Marshall College; University of North Carolina; University of North Carolina firstname.lastname@example.org
Non-uniform strain occurs during contraction and re-expansion of the hollow, muscular mantle of squids, with up to 1.8-times greater circumferential strain at the inner lumen surface than at the outer surface during a single jet. This gradient of strain may result in muscle fibers near the inner and outer surfaces of the mantle operating over different regions of the length-tension curve during the same mantle contraction. We investigated this phenomenon in long-finned squid (Doryteuthis pealeii) by developing a method to relate the length-tension curve for sheets of circular muscle fibers to the in vivo circumferential strains experienced by those fibers during swimming and ventilation. The mean ranges of excursion lengths (i.e., full extension to full contraction) for the circular fibers near the inner surface were 1.0-0.67 L0, 0.94-0.79 L0, and 0.93-0.87 L0 for escape jets, slow swimming, and ventilation, respectively (n=10 squid, 60 jets/squid; L0, preparation length at peak isometric force), but only 0.95-0.71 L0, 0.92-0.77 L0, and 0.88-0.83 L0 (n=12, 60 jets/squid) for the circular fibers near the outer surface. In addition to being lower, the ranges of excursion lengths for the outer fibers were shifted about 0.05 L0 to the left on the length-tension curve. Our data show that the obliquely striated circular muscle fibers operate along the ascending limb of the length-tension curve for most jets, reaching L0 and the descending limb only during large hyperinflations. The leftward shift on the length-tension curve for the circular fibers near the outer surface is intriguing and may imply that resting sarcomere lengths change with depth in the mantle wall. Funded by NSF grants IOS-0950827 and IOS-0951067.