On the function of the two types of myotomal muscle fibre in elasmobranch fish.
Bone, Q. (1966) On the function of the two types of myotomal muscle fibre in elasmobranch fish. Journal of the Marine Biological Association of the United Kingdom, 46 (2). pp. 321-349.
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There are two main types of muscle fibre in the dogfish myotome. Slow fibres make up about 18% of the mass of the myotome, the remainder consisting of fast fibres. The slow and fast fibres have a different pattern of innervation, and differ in such respects as their fat and glycogen contents, and in abundance of mitochondria. The two types of fibre show different responses to depolarizing agents. Extracellular recordings from the myotomal muscle taken during the swimming of spinal fish show that only slow muscle fibres are active when the fish swims slowly, whereas during vigorous movements, the fast fibres are active. Although both types of fibre contain glycogen, the fast fibres contain only about 50-60 % of the amount found in the slow fibres. After a period of vigorous movement, the glycogen levels in the fast fibres fall markedly, whilst those in the slow fibres do not change. The slow fibres contain around 4-5 times the amount of fat found in the fast fibres; after long periods of sustained slow swimming, the fat levels in the slow fibres fall, and there also may be a slight decrease in the glycogen levels in the slow fibres. Long periods of slow swimming do not alter the glycogen levels found in the fast fibres. It is concluded that the slow and fast muscle fibres in the dogfish myotome represent two separate motor systems, which operate independently and utilize different metabolites. It is suggested that this conclusion also holds for other kinds of fish, but that such an arrangement is not economical for terrestrial animals.
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