Carotid body containsislands of two types of cells, type I and type II cells, surrounded by fenestrated sinusoidal capillaries. Type I cells are excited byhypoxia, and the principal transmitter appears to be
**Core Concept**
The carotid bodies are chemoreceptors located at the bifurcation of the carotid arteries, responsible for detecting changes in oxygen, carbon dioxide, and pH levels in the blood. They play a crucial role in regulating respiration and cardiovascular function.
**Why the Correct Answer is Right**
Type I cells, also known as glomus cells, are the primary sensory cells in the carotid body. They are excited by hypoxia, which triggers a cascade of events leading to the release of neurotransmitters. The principal transmitter released by type I cells is ATP (adenosine triphosphate), which binds to purinergic receptors on the type II cells, also known as sustentacular cells. This binding causes depolarization of the type II cells, leading to the release of acetylcholine, which ultimately stimulates the respiratory centers in the brainstem.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the primary neurotransmitter released by type I cells is ATP, not acetylcholine.
**Option B:** This option is incorrect because dopamine is a neurotransmitter involved in reward and motivation, but it is not the principal transmitter released by type I cells in response to hypoxia.
**Option C:** This option is incorrect because serotonin is a neurotransmitter involved in mood regulation, but it is not directly involved in the response of the carotid body to hypoxia.
**Clinical Pearl / High-Yield Fact**
The carotid bodies are responsible for detecting changes in oxygen levels in the blood, and their activation leads to the stimulation of respiration. This is why patients with carotid body tumors may experience respiratory disturbances, such as shortness of breath or orthopnea.
**Correct Answer:** A. ATP