Mechanoreceptor modulation of endogenous respiratory rhythms in vertebrates.

While pulmonary mechanoreceptors appear to play little or no role in determining the eupneic breathing pattern in some species of vertebrates, they do in others as well as in all species under conditions of elevated respiratory drive. Tonic and phasic inputs from this receptor group have independent roles in determining breathing pattern. Thus withholding lung inflation produces very different results from receptor denervation. There are at least five phases to the respiratory cycle that appear to be under separate control. Tonic receptor input is involved primarily in regulating the length of the respiratory pause, which can occur at the end of inspiration or expiration, depending on the species. Phasic receptor input has different effects during different phases of the cycle as well as different effects at different times during a single phase. This activity contributes to phase switching during the ventilation cycle and thus to the regulation of breathing frequency and tidal volume. The significance of the modulatory effects of phasic input on the duration of different phases of the ventilation cycle is not totally clear, but the evidence suggests that phasic input acts to stabilize the respiratory pattern and may be instrumental in optimizing the breathing pattern in terms of ergometric costs. This appears to be the case in all vertebrate classes, despite dramatic differences in the mechanical events associated with ventilation arising from different respiratory pumps. These receptors also appear to have significant roles other than those associated with modulation of respiratory rhythm, particularly in lower vertebrates. Many of these roles, such as maintaining the integrity of the gill curtain in fish or buoyancy control and regulation of blood flow distribution in reptiles, may be as important as their role in modulating the endogenous rhythm.