Electron microscopy of the normal canine heart reveals neural elements in the intercellular spaces of the atria and ventricles, including the papillary muscles. These neural elements are C fibers containing vesicles and electron dense granules. After total extrinsic cardiac denervation some C fibers remain in the atria and ventricles. These must arise from ganglion cells within the heart, and are thus by definition intrinsic and postganglionic. Whether the activity of such nerve fibers is related to adrenergic, cholinergic, or other neurohumoral substances is uncertain.
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Napolitano et al. (1965) studied this question.
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