J Comp Physiol A (2007) 193:1101–1113 DOI 10.1007/s00359-007-0261-7 123 ORIGINAL PAPER Muscle anatomy is a primary determinant of muscle relaxation dynamics in the lobster (Panulirus interruptus) stomatogastric system JeVrey B. Thuma · Patricia I. Harness · Thomas J. Koehnle · Lee G. Morris · Scott L. Hooper Received: 18 April 2007 / Revised: 30 July 2007 / Accepted: 4 August 2007 / Published online: 21 August 2007  Springer-Verlag 2007 Abstract We stained sarcomere thin Wlaments with Xuo- rescently labeled phalloidin, measured sarcomere and mus- cle length, and calculated sarcomere number in pyloric and gastric mill muscles. A wide range of sarcomere lengths (3.25–12.29 m), muscle lengths (5.9–21.1 mm), and sar- comere numbers (648–3,036) were observed. Sarcomere number diVerences occurred both because of changes in sarcomere length and muscle length, and sarcomere and muscle length varied independently. This independence, the wide range of sarcomere numbers present, and the mus- cles being all ‘slow’, graded muscles allowed us to use these data to test Huxley and Neidergerke’s (1954) hypoth- esis that muscle dynamics depend on sarcomere number. The time constants of exponential Wts to contraction relax- ations were used to measure muscle dynamics, and com- parison of theoretical predictions and experimental results quantitatively conWrm the predicted dependence. The diVering dynamics of the various pyloric muscles are likely functionally important, and the dependence of muscle dynamics on sarcomere number implies that sarcomere number is likely closely regulated in these muscles. The stomatogastric system may thus be an excellent model system for studying the mechanisms regulating muscle sar- comere number. Keywords Confocal microscopy · Invertebrate · Crustacean · Sarcomere · Phalloidin Introduction Understanding how organisms produce behavior is a funda- mental goal of neurobiology. Muscles transform neural activity into movement, and thus a central component of this goal is understanding the often complicated character- istics of this neuromuscular transform. The functional con- sequences of complex neuromuscular transforms have been particularly well studied in two model systems, Aplysia feeding (Weiss et al. 1992; Brezina and Weiss 1997a, b, 2000; Brezina et al. 1996, 1997, 2000a, b, 2005) and the lobster pyloric neuromuscular system (Morris and Hooper 1997, 1998, 2001; Morris et al. 2000; Thuma et al. 2003), which controls the Wltration portion of digestion (Maynard and Dando 1974; Selverston 1974; Selverston et al. 1976; Mulloney 1977; Russell 1979). Slow temporal Wltering by the pyloric muscles transforms rhythmic neural input into tonic, sustained contractions (Morris and Hooper 1998) and results in these muscles contracting in time with neural net- works that do not innervate them (Morris et al. 2000; Thuma et al. 2003). Moreover, the extent to which the mus- cles perform these transformations varies in a muscle-spe- ciWc fashion, with diVerent muscles producing either predominantly tonic or phasic contractions in response to identical bursting neural input (Morris et al. 2000) and expressing to diVerent degrees the output of non-innervat- ing neural networks (Morris et al. 2000; Thuma et al. 2003). J. B. Thuma (&) · P. I. Harness · S. L. Hooper Department of Biological Sciences, Ohio University, 107 Irvine Hall, Athens, OH 45701, USA e-mail: thuma@ohio.edu T. J. Koehnle Department of Neuroscience, University of Pittsburgh, Pittsburgh, PA 15260, USA L. G. Morris Department of Marine Education, Institute for Biomedical Philosophy, 3210 Woodlynne Way, Atlanta, GA 30340, USA