Carotid body chemoreceptor function: hypothesis based on a new circuit model.

Integration of our own morphological observations into recent ultrastructural, biochemical, and neuropharmacological results on the carotid body led to a new hypothesis on chemoreceptor function: (i) Glomus cells with small dense-cored vesicles (type IB cells) that store norepinephrine are chemoreceptors. (ii) Glomus cells with large dense-cored vesicles (type IA), which are postsynaptic to the other glomus cell type and presynaptic to afferent nerve endings, are dopaminergic interneurons that suppress the afferent discharge frequency during normoxia by releasing dopamine. (iii) The hypoxic stimulus causes the chemoreceptive cell to release the stored norepinephrine, which in turn brings about disinhibition of the afferent nerve endings by inhibition of the interneuron. (iv) Afferent nerve endings and interneurons interact through reciprocal synapses that form a short inhibitory feedback loop. We propose that information in the carotid body is processed in a fashion graded rather than digital, providing a fine adjusted cooperation of all elements.