Influence of oxygen on the heartbeat rhythm of the leech.

Raising and lowering the oxygen content in the fluid bathing the skin of the leech modified the frequency of the heartbeat. In raised concentrations of oxygen, the period between bursts of impulses in the heart excitor motoneurones (HE cells) was reduced. Conversely, with lowered oxygen concentrations bathing the skin, the heart rate was slowed, with longer periods between bursts of firing in the HE motoneurones. Changes in oxygen concentration did not affect HE motoneurone firing patterns in preparations in which the CNS was dissected from the skin and surrounding body tissues. Various other stimuli were tried, including stroking, pinching, stretching the skin, blood vessels and gut, as well as changing the temperature. None of these stimuli mimicked the specific effects of oxygen on rhythmicity. It is concluded that peripheral receptors sensitive to changes in oxygen tension are able to influence the central neuronal circuits responsible for generating the rhythm of the heartbeat.