| | Aerial respiration using a primitive lung, central respiratory rhythm generation, and central CO2 chemosensitivity arose early in vertebrate evolution prior to the divergence of sarcopterygian and actinopterygian fish. All vertebrate air breathing, however, is not homologous as this trait evolved independently several times among fish. Two long-standing questions in respiratory physiology are whether or not air breathing in fish is controlled by a central rhythm generator and whether or not air breathing and central CO2 chemosensitivity co-evolved. One means to address these questions is to investigate control of breathing in the brainstem, and thus, we established an isolated brainstem preparation from the Alaska blackfish, Dallia pectoralis, a rare example of an arctic air-breathing fish. In blackfish air breathing consists of gulping and swallowing an air bubble into their esophagus and holding it there with a sphincter that closes off the esophagus from the buccal cavity. Gulping the air bubble is accomplished by the same opercular and mandibular muscles that draw water into the buccal cavity during gill ventilation. Activation of the opercular and mandibular muscles for ventilation is effected by a central rhythm generator in the brainstem that is spontaneously active in the absence of peripheral input. This central rhythm generator, however, is not modulated by central CO2 chemosensitivity. Thus, unless central CO2 chemosensitivity was lost in blackfish, we might conclude that centrally controlled vertebrate air breathing can evolve independent of central CO2 chemosensitivity. |
