The respiratory function of gill filament muscles in the carp.

The activity pattern of the adductor muscles of the gill filaments has been determined with E.M.G. techniques and analysed in relation to the activity of the respiratory pump muscles, the respiratory movements and the hydrostatic pressures in buccal and opercular cavities. The gill filament adductor muscles contract twice during a normal respiratory cycle. First during the transition from the contraction to the expansion phase and for a second time at the end of the expansion phase. These two contractions serve different purposes. The first 'primes' the opercular pump for the start of the next expansion phase in the following way. At the end of the contraction phase, the final adduction of the opercula results in a positive pressure in the opercular cavities. If this pressure persisted until the start of the expansion, it would make the opercular suction pump inoperative, because it would blow away the flexible opercular flap which, as a passive valve, seals the widening opercular slit during abduction. Filament adduction at the transition from contraction to expansion, however, by lowering the resistance of the gill curtain, allows water to escape from the opercular cavities through the mouth and so reduces opercular pressure to zero before expansion starts. The second contraction of the filament adductor muscles, at the end of the expansion phase, occurs when the opercular flap separates from the body of the fish, opening the opercular slit. At this moment, there is a considerable negative pressure in the opercular cavity. Nevertheless, inflow of water through the opercular slit is negligible, because flow reversal is counteracted by the kinetic energy of the normal water flow from the buccal to the opercular cavities. This process is significantly facilitated by a reduction in gill resistance through filament adduction. In the cough, a burst of filament adductor activity occurs during the intermediate expansion. It then increases water flow velocity over the gills by lowering the gill resistance and also brings the filaments in such a position that the water flows parallel to their surface, which facilitates the flushing off of foreign matter.