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How do hypoxia and hypercapnia trigger signals from carotid body type I cells?

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A pine grosbeak with pink and gray plumage perches on a branch, eating red berries among autumn foliage. — Chemoreception Animals, such as this pine grosbeak, use chemoreception to respond to chemicals that can be tasted and smelled in their environments. Many of these chemicals affect behaviors such as food preference and defense.

The carotid body contains specialized type I cells that function as peripheral chemoreceptors to detect changes in arterial blood chemistry.1

When the body experiences hypoxia—a reduction in oxygen supply—or hypercapnia—an excess of carbon dioxide—these type I cells are stimulated to release stored neuroactive substances. These substances, which include acetylcholine, catecholamines, and various neuropeptides like enkephalins, are contained within electron-dense vesicles inside the type I cells.1

Upon release, these chemicals act on the fine sensory nerve fibers located in close proximity to the type I cells. These fibers then transmit signals through the carotid sinus nerve to medullary respiratory neurons to adjust ventilation. Evidence suggests that the mechanisms for sensing and transmitting oxygen versus carbon dioxide changes are independent, as they can be interfered with separately.1