. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 384 T. PREUSS ET AL. 1 week old adult. ~i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—\—i—r 0 6 121824303642485460 video frames [30/s] Figure 7. Manlle kinematics analyzed from video recordings of hovering or slow-swimming adult and juvenile squid. Mantle diameter was measured at the widest point (dorsal view), and the fractional man- tle diameter was calculated by defining the largest diameter in a given measurement sequence as 100''!. Time resolution is 30 frames/s. size of the mitochondrial core. CMP fibers, however, show a selective hyp
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 384 T. PREUSS ET AL. 1 week old adult. ~i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—\—i—r 0 6 121824303642485460 video frames [30/s] Figure 7. Manlle kinematics analyzed from video recordings of hovering or slow-swimming adult and juvenile squid. Mantle diameter was measured at the widest point (dorsal view), and the fractional man- tle diameter was calculated by defining the largest diameter in a given measurement sequence as 100''!. Time resolution is 30 frames/s. size of the mitochondrial core. CMP fibers, however, show a selective hypertrophy of the myofilament and SR area and thus add a disproportional amount of force- generating capability at the individual fiber level. Never- theless, hypertrophy of individual muscle fibers is lim- ited by physiological constraints. For example, fibers re- quiring rapid excitation-contraction coupling in the ab- sence of a transverse tubular system need to be small in diameter (Bone and Ryan, 1973; Bone el at., 1995). This constraint would especially affect the presumptive fast- twitch CMP fibers with their high myofilament volume- to-surface ratio (Fig. 2). Such limitations on fiber diame- ter would indeed necessitate extensive recruitment of new CMP fibers (Fig. 5) to maintain rapid muscular re- sponses as an animal grows throughout development. Two biochemical results described in this paper sup- port the idea that at least some CMP fibers display Na- channel-based excitability. Such excitability is consistent with the hypothesis that these are fast-twitch fibers re- sponsible for all-or-none mantle contractions (Young, 1938; Gilly et a/., 1996). The first supporting evidence is that Na-channel protein is primarily expressed in the CMP fiber layer. Second, the increase of Na-channel pro- tein seen in developing mantle is comparable to the se- lective hypertrophy and hyperplasia of CMP muscle fi- bers as shown by histology. The Na-chan
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