Experiments on whole crab, Carcinus maenas, as well as on two types of isolated perfused preparationswere performed to locate the origin of the O2 chemosensitivity which drives hyperventilation in hypoxia and hypoventilation in hyperoxia.

Electromyographic recordings from the scaphognathite muscles confirmed the existence of strong ventilatory responses in the whole animal exposed to various water oxygen levels. Furthermore, surgical section of the circumoesophageal connectivesdid not suppress these responses, thereby excluding the cerebral ganglia as the only site or relay for the O2 chemosensitivity.

In semi-isolated preparations in which the thoracic ganglion and anterior arterial system were perfused by saline at various PO2 values, extracellular recordings of the motor output in the peripheral ventilatory nerves showed respiratory responses qualitatively similar, but quantitatively weaker, than those observed in the whole animal. Theseresponses were suppressed by bilateral section of the ventilatory nerves or ligation of the anterior sternal artery.

In perfused preparations of the completely isolated thoracic ganglion, respiratory frequency was reduced under hypoxia. This is consistent with a purely metabolic response and excludes a central O2 chemosensitivity at the level of the respiratory oscillator itself.

We conclude that a population of peripheral O2-sensitive chemoreceptors is present within the arterial systemin the ventral anterior region, probably around the scaphognathites. These receptors are reversibly stimulated by potassiumcyanide, lobeline and almitrine bismesylate, as also are peripheral O2 chemoreceptors in vertebrates.

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