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First published online February 6, 2004
Journal of Experimental Biology 207, 983-991 (2004)
Published by The Company of Biologists 2004
doi: 10.1242/jeb.00862

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Muscle activation and strain during suction feeding in the largemouth bass Micropterus salmoides

Andrew M. Carroll

Section of Evolution and Ecology, University of California, One Shields Avenue, Davis, CA 95616, USA

View larger version (31K): [in a new window]   Fig. 1. Action of the sternohyoideus (SH) in medial (A) and ventral (B) views. (A) The SH originates on posterior myosepta and the pectoral girdle. Thick ligaments connect the urohyal to the hyoid structure, so that the primary action of the sternohyoideus is to rotate the hyoid about its articulation on the suspensioria, depressing the buccal floor away from the neurocranium and expanding the buccal cavity. Ventral rotation of the hyoid results in depression of the lower jaw through a linkage between the proximal ends of the hyoid and the mandible. (B) Caudal movement also widens the midventral angle of the hyoid, rotating the suspensoria laterally from the neurocranium and laterally expanding the buccal cavity.

View larger version (23K): [in a new window]   Fig. 2. Lateral view of the anatomy of the sternohyoideus in situ, showing the location of sonomicrometry crystals and EMG electrodes in this study. The sternohyoideus originates on the pectoral girdle and on posterior hypaxial myosepta and inserts on the urohyal bone. There are three myomeres between the pectoral girdle and the urohyal. 1 mm crystals were placed rostal and caudal to the middle myomere. An electrode was placed in the middle myomere. 2 mm crystals were placed on the symphysis of the lower jaw and on the anterior palate just lateral to the vomer. For this figure the hyoid, lower jaw and upper jaw were sectioned, and the gills and part of the suspensoria were removed to expose the sternohyoideus.

View larger version (26K): [in a new window]   Fig. 3. (A–F) Representative profiles of the lower jaw kinematics (thick, upper trace), sternohyoideus fascicle strain (middle trace) and EMG signal (thin, lower trace) are depicted. Lower jaw displacement and fascicle strain were normalized by subtracting by the minimum crystal distance and dividing by the maximum gape distance or resting fascicle length (FL) for a preparation. The drop line indicates the end of fast lower jaw depression, which is defined in this study as peak gape. The timing of fascicle strain and kinematics were closely matched within each strike, but there is variability in the strain profile and in the relationship between activation and strain. (A), (C) and (D) show a `notch' during fast lower jaw depression indicating slight lengthening. Each strike is from a different fish with the exception of (C) and (D),which are from the same fish. l, FL at each sampling time; l0, resting FL; (l–l0)/l0=normalised strain. See Materials and methods for details.

View larger version (16K): [in a new window]   Fig. 4. Timing of kinematic, strain and activation events relative to peak gape. The timing ± S.E.M. (among individuals not within) relative to peak gape are displayed. (The same information is given in row three of Table 1.) The following events were all nearly simultaneous: (1) the onset of slow jaw opening and of fascicle strain; (2) the onset of fast jaw opening and of fast fascicle strain; (3) the onset of jaw closing, peak fascicle strain, and the offset of activity; and (4) the return of the lower jaw and of fascicle to resting length.

View larger version (32K): [in a new window]   Fig. 5. Total shortening velocity (A) and strain (B) before and after peak gape in individuals, showing individual means ± S.E.M. Shortening velocity is expressed as absolute values (FL s–1). An asterisk indicates a significant difference between strain or shortening velocity before and after peak gape (P<0.05). Shortening velocity always increased after peak gape, though the result was not significant in one individual. Strain is presented as percent of total strain. In three individuals the sternohyoideus generated significantly more strain after peak gape, while in the two others the levels were insignificantly different. The two individuals in whom less than 40% fascicle strain occurred before peak gape are the same two that showed isometric contraction or slight lengthening during fast jaw opening.

View larger version (18K): [in a new window]   Fig. 6. Instantaneous shortening velocity relative to peak gape from three individuals are displayed. This figure shows the change in normalized length between each sampling period, averaged within individuals and smoothed. Fish 1 showed roughly isometric contraction during fast lower jaw depression, while fish 5 showed lengthening during fast lower jaw depression. These fish also displayed less than 40% fascicle strain prior to peak gape. Fish 1 showed a gradual increase in shortening velocity prior to peak gape. This pattern was also found in fish 2 and 4. Maximum instantaneous speed was variable between individuals.

View larger version (37K): [in a new window]   Fig. 7. Velocity and strain by individual. Total shortening velocity is the total strain distance divided by the time between strain onset and peak strain. Fast shortening velocity is the total strain between onset of fast shortening and peak strain divided by the time between the onset of fast shortening and peak strain. Strain and shortening velocity are expressed as negatives to facilitate viewing. Mean total shortening velocity (including slow shortening) was –1.3 FL s–1. Mean fast shortening velocity across individuals was –2.5 FL s–1, suggesting that the sternohyoideus functions to produce mechanical power. The majority of strain occurred in the fast portion of the strain cycle in all individuals. l, minimum FL, l0, resting FL.