The sphincter of the efferent filament artery in teleost gills: I. Structure and parasympathetic innervation

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Abstract
Previous studies have shown the existence of a sphincter in the efferent filament artery of the teleost gill and its constrictory response to acetylcholine (ACH) and vagal stimulation. This study deals with the muscular organization of this sphincter and the distribution of its innervation as elucidated by degeneration methods and cytochemistry. The sphincter innervation is supplied by the protrematic vagus nerves. Nerve endings filled with cholinergic‐type vesicles are located in close association with the adventitial smooth muscle cells and display a strong acetylcholinesterase (ACHE) activity. Section of the protrematic vagus nerve induces a nearly complete degeneration of the sphincter innervation. ACHE‐positive nerve cell bodies are present both in the sphincter area and in the protrematic vagus nerve. These results suggest that innervation of the sphincter in the efferent filament artery is cholinergic through the activity of postganglionic axons of the parasympathetic system.