Neuromuscular blocking agents exert their effect by modulating the signal transmission in skeletal muscles. An action potential is, in other words, a depolarisation in neurone membrane due to a change in membrane potential greater than the threshold potential leads to an electrical impulse generation. The electrical impulse travels along the pre-synaptic neurone axon to synapse with the muscle at the neuromuscular junction (NMJ) to cause muscle contraction.
When the action potential reaches the axon terminal, it triggers the opening of the calcium ion gated channels, which causes the influx of Ca. Ca will stimulate the release of neurotransmitter in the neurotransmitter containing vesicles by exocytosis (vesicle fuses with the pre-synpatic membrane).
The neurotransmitter, acetylcholine(ACh) binds to the nicotinic receptors on the motor end plate, which is a specialised area of the muscle fibre's post-synaptic membrane. This binding causes the nicotinic receptor channels to open and allow the influx of Na into the muscle fibre.
Fifty percent of the released ACh is hydrolysed by acetylcholinesterase (AChE) and the remaining bind to the nicotinic receptors on the motor end plate. When ACh is degraded by AChE, the receptors are no longer stimulated and the muscle can be repolarised.
If enough Na enter the muscle fibre, it causes an increase in the membrane potential from its resting potential of -95mV to -50mV (above the threshold potential -55V) which causes an action potential to spread throughout the fibre. This potential travels along the surface of the sarcolemma. The sarcolemma is an excitable membrane that surrounds the contractile structures known as myofibrils that are located deep in the muscle fibre. For the action potential to reach the myofibrils, the action potential travels along the transverse tubules (T-tubules) that connects the sarcolemma and center of the fibre.
Later, action potential reaches the sarcoplasmic reticulum which stores the Ca  needed for muscle contraction and causes Ca to be released from the sarcoplasmic reticulum.