Question

what is the immediate effect of acetylcholine binding to its receptors on the motor end plate?
release of more acetylcholine from synaptic vesicles
muscle relaxation
increased membrane permeability to sodium ions
release of myosin cross - bridges from thin filaments
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Explanation:

When acetylcholine binds to its receptors on the motor end - plate, ligand - gated ion channels open. These channels are mainly permeable to sodium ions ($Na^+$). As a result, there is an influx of sodium ions into the muscle cell. This is the immediate event. Muscle relaxation is not the immediate effect as acetylcholine is an excitatory neurotransmitter for skeletal muscles. Release of more acetylcholine from synaptic vesicles is not the immediate effect of acetylcholine binding to its receptors on the motor end - plate. The release of myosin cross - bridges from thin filaments occurs during muscle relaxation and is not the immediate result of acetylcholine binding.

Answer:

Increased membrane permeability to sodium ions