Question: where does the heart's electrical stimulus originate from
[{'answer': 'sinoatrial node', 'provenance': [{'wikipedia_id': '650897', 'title': 'Electrical conduction system of the heart', 'start_paragraph_id': 1, 'start_character': 89, 'end_paragraph_id': 1, 'end_character': 104, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '650897', 'title': 'Electrical conduction system of the heart', 'start_paragraph_id': 1, 'start_character': 0, 'end_paragraph_id': 1, 'end_character': 528, 'bleu_score': 0.976043556018343, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 7233267979465026364}, 'section': 'Section::::Abstract.'}]}]