Question: where are the group of cells that control the natural resting heart rate found
[{'answer': 'sinoatrial node', 'provenance': [{'wikipedia_id': '598577', 'title': 'Pacemaker potential', 'start_paragraph_id': 1, 'start_character': 48, 'end_paragraph_id': 1, 'end_character': 63, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '598577', 'title': 'Pacemaker potential', 'start_paragraph_id': 1, 'start_character': 0, 'end_paragraph_id': 1, 'end_character': 1217, 'bleu_score': 0.5944672451288301, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 9635285242483749962}, 'section': 'Section::::Abstract.'}]}]