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First published online December 14, 2006
Journal of Experimental Biology 210, 118-128 (2007)
Published by The Company of Biologists 2007
doi: 10.1242/jeb.02600

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Electromyography of the buccal musculature of octopus (Octopus bimaculoides): a test of the function of the muscle articulation in support and movement

Theodore A. Uyeno^* and William M. Kier

Department of Biology, CB# 3280 Coker Hall, University of North Carolina at Chapel Hill, NC 27599-3280, USA

View larger version (14K): [in this window] [in a new window]   Fig. 1. An oral view of O. bimaculoides in a vertical hanging position. The buccal mass is located in a sinus formed by the base of the arms and includes an upper (blue) and lower (red) chitinous beak embedded in the mandibular muscles. Also shown are the esophagus and the buccal membrane lips, a ring of tissue that seals the sinus and covers the beaks.

View larger version (40K): [in this window] [in a new window]   Fig. 2. Diagram of the upper (blue) and lower (red) beaks of O. bimaculoides. Both beaks are U-shaped in cross section and the anterior end is folded over itself to form the biting surfaces (i.e. the rostrum and the jaw angles). The upper beak has enlarged lateral walls that fit within, but do not contact the lower beak. The lower beak has enlarged wings and a reduced hood relative to the upper beak.

View larger version (37K): [in this window] [in a new window]   Fig. 3. Schematic diagram showing the beaks and musculature of Octopus bimaculoides. (A) The beaks and musculature of a complete buccal mass. (B) The upper beak and lower beak separated to show the underlying muscles. (C) The beaks and musculature separated to show the shape of the beaks and the shape of the individual muscles. AMM, anterior mandibular muscle (yellow); LMM, lateral mandibular muscle (purple); PMM, posterior mandibular muscle (blue); SMM, superior mandibular muscle (green).

View larger version (19K): [in this window] [in a new window]   Fig. 4. Schematic diagram of the isolated buccal mass preparation showing electromyography electrode locations in the musculature, and movement monitor electrode locations on the beak rostra. Also shown are the esophagus, the ligated interbuccal connectives and the approximate location of the inferior buccal ganglion.

View larger version (49K): [in this window] [in a new window]   Fig. 5. Schematic diagram illustrating the movement of the upper beak (opaque blue) relative to the lower beak (semi-transparent red) during the bite cycle [description follows Kear (Kear, 1994) and is redrawn from Uyeno and Kier (Uyeno and Kier, 2005)]. (A) Closed in its resting position (beaks relatively enlarged to illustrate pivot area); (B) opening; (C) fully opened; (D) closing; (E) closed with the upper beak rostrum retracted behind the lower beak rostrum. The axis around which the beaks pivot was observed to be located anywhere in the pivot area indicated in A (resting).

View larger version (31K): [in this window] [in a new window]   Fig. 6. A left side schematic diagram of the hypothesized muscle action during (A) closing and (B) opening states of the superior (SMM, green) and lateral (LMM, purple) mandibular muscles showing dimensional changes seen in the experimental preparation (lower beak, red; upper beak, blue). The cylindrical lateral mandibular muscle is illustrated in the central diagram with the three fiber orientations: Lat, lateral fibers (red); AP, anterior-posterior fibers (green); DV, dorsal-ventral fibers (blue).

View larger version (17K): [in this window] [in a new window]   Fig. 7. Graph of duration of the bite cycle as a buccal mass preparation ages. The movement monitor traces on the insets represent the relative distance between the upper and lower beaks with respect to time. A rise in the inset traces indicate opening. Although the cyclical beak movements became smaller and the duration became longer as time progressed, the beak was capable of powerful stochastic bites up to the end of the life of the preparation (in this case 2100 s).

View larger version (21K): [in this window] [in a new window]   Fig. 8. Plots of rectified, averaged electromyograms of the right superior mandibular muscle (A) and the right lateral mandibular muscle (B) correlated with the output of the monitor of beak position (C) during the cyclical biting of a freshly excised beak preparation. The muscle activation patterns are complex and difficult to correlate with beak movements.

View larger version (38K): [in this window] [in a new window]   Fig. 9. Plots of the raw electromyograms (top) with rectified and then averaged signals plotted successively below from the left superior mandibular muscle (A) and left lateral mandibular muscle (B). The bottom traces are the correlated output from the monitor of beak position. Note that beak opening is correlated with activity of the left lateral mandibular muscle.

View larger version (17K): [in this window] [in a new window]   Fig. 10. Plots of rectified, averaged electromyograms of the left superior mandibular muscle (A) and the right lateral mandibular muscle (B) correlated with the output of the monitor of beak position (C). Note that beak closing is correlated with superior mandibular muscle activity.