Characteristics of Mechanoreceptors in the Air-Breathing Organ of the Holostean Fish, Amia Calva

Single nerve fibre discharge was recorded from mechanoreceptors associated with the air-breathing organ in double-pithed specimens of the bowfin, Amia calva L. These receptors were innervated by the vagus nerve and although their exact location was difficult to determine, most appeared to be located along the anterio-ventral wall of the single lung. All receptors increased tonic discharge with step increases in lung volume, above a threshold level, and were slowly adapting. There was a dynamic, rate-sensitive burst of activity associated with lung inflation and a dynamic, rate-sensitive inhibition of discharge associated with deflation. These responses were qualitatively similar to those of the tonic stretch receptors found in fish swimbladder and mammalian gut. All receptors were insensitive to changes in intrapulmonary partial pressures of oxygen and carbon dioxide. These observations suggest that receptors capable of transducing the rate, as well as the degree, of inflation and deflation are associated with primitive lungs, and may have arisen from tonic gut receptors.