Robert S Fitzgerald
The carotid body (CB) is a tiny structure, bilaterally located at the bifurcation of the common carotid artery into its internal and external branches, is innervated by a branch of cranial nerve IX, and sends its neural output to the nucleus tractus solitarius in the medulla. Arterial flow through the CB is the highest of any organ ever measured. The CB is the unique sensor of decreases in PaO2, but is also stimulated by low glucose as well as increases in PaCO2, [H+]a, temperature, and osmolarity. Hypoxia-generated increases in neural activity result from K+ channels in the CB's neurotransmitter-containing glomus (Type I) cells being blocked; this depolarizes the cells allowing calcium to enter, which promotes the movement of the glomus cells' transmitter-containing vesicles to exocytose ACh and ATP into the gap between cell and an abutting afferent fiber, where these agents bind to cholinergic and purinergic postsynaptic receptors. Stimulation of the CB's provokes reflex responses in the respiratory, cardiovascular, endocrine and renal systems. Recent research has identified the CB's malfunctioning in chronic heart failure (CHF). The decreased blood flow in this pathology reduces shear stress on the endothelial cells of the CB's vasculature, reducing the level of critically important molecules, and making the CB hypersensitive. The increased neural output from the CB's promotes an increase in output of the sympathetic nervous system which has a deleterious impact on breathing, cardiovascular and renal function. Techniques have been found in animal models that reduce the CB's impact during CHF. The very large role of this tiny structure in both health and disease underline its organismal importance.