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It is reported that cardiac afferent information is transmitted through at least three different pathways to the CNS: sympathetic, parasympathetic, and somatic; however, there are few studies concerning the role of afferent fibers of vagus nerves for eliciting cardiac sensation including pain. Especially, receptive field properties innervated by single vagal nerve fiber and mechanical threshold of nociceptors on the cardiac surface are not yet quantitatively studied. Therefore, in this study, we systematically investigated characteristics of vagal units innervating cardiac nociceptors in rats. Using anesthetized and artificially ventilated rats, 37 single unit recordings were made from fine nerve filaments of the left vagal nerve. For quantitative mechanical stimulation, the cardiac surface was stimulated by a von Frey type device. In addition, bradykinin was used for checking the chemical sensitivity of the nociceptor. Electrical …