There are two main types of muscle fibre in the dogfish myotome. Slow fibres make upabout 18 % of the mass of the myotome, the remainder consisting of fastfibres. 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 typesof 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 % ofthe 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 donot change. The slow fibres contain around 4–5 times the amount of fat found in thefast fibres; after long periods of sustained slow swimming, the fat levels inthe 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.