has part has part has_part characteristic of part of has modifier abnormal heart right atrium morphology abnormal sinoatrial node morphology any structural anomaly of the small mass of modified cardiac muscle fibers located at the junction of the superior vena cava and the right atrium that is the source of contraction impulses for the heart abnormal sinus node MPheno.ontology MP:0004115 abnormal sinu-atrial node abnormal pacemaker abnormal SA node abnormal sinuatrial node abnormal SAN morphology abnormal impulse conducting system morphology morphology abnormal sinoatrial node