a series of endocrine glands of vertebrate animals and man that are composed of chromaffin (adrenal) tissue and that secrete adrenaline (epinephrine), noradrenaline (norepinephrine), and possibly other catecholamines into the blood. In man and mammals paraganglia include very small nodules as well as large encapsulated, lobulated aggregates, which have an organ structure. The largest paraganglia are the medullary portions of the adrenal glands; substantially smaller are the carotid paraganglion and the paraganglia located near the heart and aorta. The components of paraganglia are secretory cells, of at least two types, and auxiliary cells, which are enclosed in connective-tissue stroma. The hormone is stored in cytoplasmic granules. Upon excitation (depolarization) of the cell membrane, it is secreted by evacuation of the granule contents into the extracellular medium. The biochemical and pharmacological characteristics of the secretory cells of paraganglia are similar to those of some neurons, especially sympathetic ones, in which noradrenaline is the mediator and which, in the opinion of a number of scientists, share a common embryonic origin with paraganglial cells. The auxiliary, or lining, cells cytologically resemble neuroglia. A unique feature of paraganglia is that their secretory cells are directly innervated, that is, they are innervated by efferent (motor) nerve endings. Functionally and genetically, paraganglia are a single system that cannot be divided into chromaffin (“sympathetic”) and nonchromaffin (“parasympathetic”) parts. The erroneous distinction of nonchromaffin paraganglia led to denial of their secretory function and to the notion that the carotid paraganglion, for example, functions as an organ of chemical sense. It is now established that all paraganglia have a single type of chemistry, which is adrenaline-related, and that the innervation of paraganglial secretory cells is efferent.