Nerve endings in the atrial muscle of the heart.
D000818:Animals; D001341:Autonomic Nervous System / Q000033:anatomy & histology; D004285:Dogs; D006325:Heart Atria; D006329:Heart Conduction System / Q000033:anatomy & histology*; D008297:Male; D009046:Motor Neurons / Q000166:cytology; D009411:Nerve Endings / Q000033:anatomy & histology*; D009413:Nerve Fibers, Myelinated / Q000166:cytology; D011817:Rabbits; D011984:Sensory Receptor Cells / Q000033:anatomy & histology