Generation of the respiratory rhythm in fish

Abstract

Gill ventilation in fishes arises from contractions of muscles inserted on the jaws and oro-branchial skeleton, driven by activity in branches of the Vth, VIIth, IXth and Xth cranial nerves. Activity in these nerves is generated by a respiratory rhythm generator (RRG) in the brainstem. Although the isolated medulla oblongata can generate a respiratory rhythm, the site of the primary generator remains poorly defined. Its activity is modulated by centers in the midbrain and stabilized by peripheral inputs from mechanoreceptors and chemoreceptors on the gills. Fish often show markedly reduced ventilation rates when inactive in normoxic or hyperoxic waters. This may interrupt their normal regular rhythm of gill ventilation, causing fish to exhibit episodic (or intermittent) breathing patterns. In contrast, when fish swim rapidly they may switch to ram ventilation, with the increased rates of water flow triggering cessation of the normal respiratory rhythm, as the mouth is held open to allow a continuous stream of water, generated by the swimming muscles, to ventilate the gills. When the oxygen supply is limited by hypoxia or increased following exercise, fish can recruit feeding muscles, innervated by the hypobranchial nerves, into the respiratory cycle to provide active inspiration of water during the mouth-opening phase. Air-breathing fish utilize these same muscles to gulp air at the water surface. There is some evidence of separate RRG's for water and air-breathing, but their properties and locations are unclear.