Flashcards for AQA A Level Biology Skeletal muscles are stimulated to contract by nerves and act as effectors (A-level only), covering the key terms, definitions and processes you need to recall for Paper 1, Paper 2 and Paper 3. 24 cards drawn from Skeletal muscles are stimulated to contract by nerves and act as effectors (A-level only), matched to the AQA A Level Biology (7402) specification.