Identified Muscle Fibres in a Crab. Differences in Facilitation Properties

1. The superficial muscle fibres in the proximal head of the closer muscle in the crab Eriphia belong to four distinct fibre types (I-IV). By stimulating either the 'slow' or the 'fast' axon, facilitation properties of the EJPs were investigated in eight identifiable muscle fibres which represented the four fibre types. 2. Amount, time course and frequency dependence of facilitation (f) of both 'slow' and 'fast' EJPs differed characteristically among the fibres studied, but were similar within fibres belonging to the same type. 3. In fibres receiving double excitatory innervation through the 'slow' and the 'fast' axon (type I and II fibres), the amount of facilitation of 'fast' EJPs was smaller than for 'slow' EJPs. 4. With the exception of type I fibres, there was an inverse relationship between facilitation and the size of both 'slow' and 'fast' EJPs. Facilitation increased with frequency and time of stimulation. In type I fibres, facilitation of 'slow' EJPs increased with time at low frequencies of stimulation (up to 32 Hz), but decreased at higher frequencies after an initial rise. 'Fast' EJPs in type I fibres showed little, if any, facilitation of frequencies below 8 Hz. At frequencies above 8 Hz, they were always depressed (negative values of f). 5. The functional significance of different facilitation properties is discussed with regard to the participation of different muscle fibre types in the mechanical performance of the closer muscle.