How do type I cells and type II cells differ in the carotid body?

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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.

In the carotid body, type I and type II cells serve distinct structural and functional roles in chemoreception:

Type I Cells: These cells are arranged in groups and are the primary sensory units. They contain electron-dense vesicles filled with neuroactive substances, including acetylcholine, catecholamines, and various neuropeptides. It is believed that when these cells detect hypoxia or hypercapnia, they release these substances to stimulate adjacent sensory nerve fibers.1
Type II Cells: These cells surround the groups of type I cells. Unlike type I cells, they are generally not considered to play a direct role in the chemoreception process.1

The physical proximity of type I cells to fine sensory nerve fibers facilitates the transmission of signals to the brain, allowing for the regulation of breathing in response to changes in blood gas levels.1