Functional implications of supercontracting muscle in the chameleon tongue retractors
Anthony Herrel1,*,
Jay J. Meyers2,
Peter Aerts1 and
Kiisa C. Nishikawa2
1 Laboratory of Functional Morphology, Biology Department, University of Antwerp (UIA), Universiteitsplein 1, B-2610 Antwerp, Belgium and
2 Functional Morphology and Physiology Group, Biology Department, Northern Arizona University, PO Box 5640 Flagstaff, AZ 86001, USA
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Fig. 1. Frequency/tension relationship of the m. hyoglossus in Chamaeleo calyptratus at resting length. The muscle showed a fused tetanus at 40 Hz, producing a maximal force of 0.173 N.
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Fig. 2. In vivo total tongue retraction forces for an adult female Chamaeleo oustaleti and two adult female C. calyptratus. At least 50 trials were performed per individual, and the largest forces recorded for each distance were plotted against tongue length. All chameleons were able to produce large retraction forces over a wide range of distances.
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Fig. 3. Length/tension diagrams for the m. hyoglossus in Chamaeleo calyptratus. In these experiments, the tongue of the chameleon was attached to a force lever, its length changed, the muscle twitch-stimulated and the forces recorded. As in the in vivo experiments, the forces stay high for a wide range of tongue extensions. The more rapid decline of force at shorter lengths is probably due to the absence of an active hyoid retraction (note that the hyoid was immobilised in these experiments).
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Fig. 4. Length/tension diagrams for the m. hyoglossus in Chamaeleo calyptratus. (A) The results of an experiment similar to that shown in Fig. 3, but with the muscle tetanically (40 Hz) stimulated. (B) The results of a preliminary experiment in which the tongue was attached to a force transducer and stimulated through two bipolar electrodes inserted into the anterior part of the muscle. Here, forces were recorded several times for four discrete tongue extensions.
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Fig. 5. (A) Sagittal (vertical) section through the hyolingual apparatus of a Chamaeleo oustaleti (trichrome stain). Whereas the proximal part of the m. hyoglossus (near its origin) runs approximately straight alongside the second hyoid cornua (not visible in this plane of section), the distal part of the tongue retractor muscle (m. hyoglossus) is folded in a complex manner in three dimensions. Anteriorly, the m. hyoglossus inserts onto the connective tissue sheet surrounding the m. accelerator (not shown here). (B) Frontal (horizontal) section through the hyolingual apparatus in a Chamaeleo jacksonii (Verhoeff’s elastin stain). The folded tongue retractor and hypoglossal nerve are clearly visible. Centrally, the connective tissue sheet sends off branches of elastin fibres running into the folds of the m. hyoglossus. Here too, the folding of the retracted m. hyoglossus can be observed. MHG, m. hyoglossus.
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Fig. 6. Transmission electron micrographs (longitudinal section) through the tongue retractor muscle (m. hyoglossus) in a relaxed (left) and a maximally extended (right) state. Note the perforations in the Z-disks (arrowhead) characteristic of supercontracting muscle. Also note the relatively short I-bands and the large overlap between the thick and thin filaments. The force produced by a muscle is proportional to the number of cross bridges engaged and, thus, to the overlap between the thick and thin filaments.
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© The Company of Biologists Ltd 2001
