Question: what are the muscle fibers under the control of a single motor axon
[{'answer': 'A motor unit', 'provenance': [{'wikipedia_id': '422470', 'title': 'Motor unit', 'start_paragraph_id': 1, 'start_character': 0, 'end_paragraph_id': 1, 'end_character': 12, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '422470', 'title': 'Motor unit', 'start_paragraph_id': 1, 'start_character': 0, 'end_paragraph_id': 1, 'end_character': 339, 'bleu_score': 0.9663744727315161, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 7187980414064799537}, 'section': 'Section::::Abstract.'}]}]