The structure and physiology of cephalopod muscle fibres
Bone, Q.; Brown, E.R.; Usher, M. (1995). The structure and physiology of cephalopod muscle fibres, in: Abbott, N.J. et al. (Ed.) Cephalopod neurobiology: neuroscience studies in squid, octopus and cuttlefish. pp. 301-329
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| Authors | | Top |
- Bone, Q.
- Brown, E.R.
- Usher, M.
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| Abstract |
The structure of cephalopod 'skeletal' muscle fibres and radial chromatophore fibres are first described; with the interesting exception of some cross-striated fibres in the tentacles, all are obliquely-striated and of similar structure, divisible into two types according to the volume of the mitochondrial core. In squid mantle, the fibre types are zoned; in the central mitochondria-poor (MP) fibres, mitochondria make up around 6 per cent of the cross-sectional area of the fibre, whereas in the outer layers of mitochondria-rich (MR) fibres, the core occupies around 45 per cent of fibre area. The chromatophore fibres are of the MR type. Cephalopod fibres contract rapidly, but only the responses of the MP type have been examined directly, although what are probably the MR type in the mantle show slower responses. Little is yet known in Sepia or squid of the innervation of the mantle circular fibres (which power jetting) but it is probable that the giant axons only supply the MP fibres, and that these are also innervated by small fibres, since escape jets can be driven either by the small fibre system alone or by a combination of small fibres and giant axons. However, the existence of extensive dye-coupling between fibres suggests that the fibres may be electrically coupled and hence individual fibres are unlikely to be doubly-innervated. The mantle circular fibres and the chromatophore fibres probably are innervated by glutamatergic nerves, whilst the radial fibres of the mantle, the head retractor muscles and some muscles in the tentacles and arms receive a cholinergic innervation. Mechanoreceptors occur in the cephalopod mantle and fins, but their role in motor control is not known. Finally, after a brief consideration of muscle biochemistry, some indications are given of the lacunae remaining in our knowledge of cephalopod muscle. |
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