Motor neuron substances have to move across the membrane to ensure the correct functioning of the nervous system. How is the movement of Na+/K+ (charged ions) different from the movement of neurotransmitters (large protein molecules) across the cell membrane? Explain.

The difference is that one process is dependent on the inflow of sodium ions inside the nerve membrane, while the other is dependent on the inflow of calcium ions inside the nerve membrane. Let us see how this works.
The nerve membrane has a sodium-potassium pump. In a resting state, the pump sends three sodium ions out of the cell for every two potassium ions that it brings in. Stimulation of the nerve membrane increases its permeability to sodium ions. As sodium ions rush into the cell, a positive charge develops inside the nerve membrane. This creates an action potential.
The action potential travels down the axon to its terminal portion. The axon terminal contains synaptic vessels filled with neurotransmitter molecules. The action potential activates the calcium channels in the nerve membrane. Calcium ions rush into the cell to release the neurotransmitter molecules from the synaptic vesicles. The neurotransmitter molecules move across the synapse and bind to the receptors (proteins) on the cell membrane that are supposed to receive the impulse.
https://mcb.berkeley.edu/courses/mcb135e/nervous.html