Fish Axial Muscle: Structure-Function Relationships On a Micro-Level

Abstract

This paper discusses some examples of strong correlations between functions and structures in axial fish muscle on a micro-level. Muscle tissue needs a certain elasticity to cope with the diverse functional requirements necessary for swimming. During fast-starts of carp, muscles can be stretched up to 40% above their resting length. These muscles need to be designed such that they can endure these strains and simultaneously provide the required force. Posterior fibres of adult carp have a longer phase of eccentric activity (active while being stretched) than anterior ones and will therefore develop greater forces. Posterior and red fibres are subjected to larger strains during continuous swimming and have special adaptations on a microscopical level to meet these functional demands. A linear correlation was found between surface areas of myotendinous junctions (MTJs), connections between muscle fibres and collagen fibres (measure for strength) and the size and duration of load on a junction. Fibres with larger surface areas consequently can bear larger loads. Exactly those fibres subjected to larger loads during swimming, posterior and red fibres, possessed stronger MTJs. Titin and intermediate filaments are important elastic structures in muscle. Titin acts as a dual spring developing passive tension upon sarcomere stretching and shortening. Less passive tension was needed to stretch red posterior fibres, with larger titin isoforms, compared to the less elastic red anterior fibres. Intermediate filaments act as safety devices protecting muscles from permanent damage due to too high strains. Anterior fibres possessed smaller intermediate filaments, thus protecting them at shorter sarcomere lengths from being torn apart due to too large strains. This study corroborates the idea that (small) differences in muscle function during swimming, turning and escaping can be used to predict and possibly explain structural differences between muscle types and at different locations.