heart right atrium morphology trait sinoatrial node morphology trait Any measurable or observable characteristic related to the shape, structure, color or pattern 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. SA node morphology trait Trait.ontology VT:0004115 pacemaker morphology trait sinus node morphology trait impulse conducting system morphology trait