FIGURE 4-26 Muscle Contraction (A) When a motor neuron’s axon collaterals contact a muscle fiber end plate, (B) acetylcholine attaches to receptor sites on the end plate’s transmitter-sensitive channels, opening them. These large membrane channels allow simultaneous influx of Na⁺ and efflux of K⁺, generating current sufficient to activate voltage-sensitive channels, triggering action potentials, and causing the muscle to contract.