Ligand gated sodium channel:

Here the channel protein is cylindrical which is normally closed. When a physiological ligand (molecule e.g. Acetylcholine) binds with the cylindrical protein at specific position, the channel is opened and sodium ion enters the cell. Here action potential causes the release of these ligands which bind with the channel to activate it.

Fig: Acetylcholine Gated Sodium Channel

Above is an example of ligand gated channel regulated by acetylcholine. The protein receptor is pentameric (has five subunits). It contains two α subunits, one β subunit, one γ subunit and one δ subunit. All of these five subunits cross the cell membrane and form walls of a cylinder. This cylinder has the diameter of 8 nm. Normally this cylinder is closed. But when two acetylcholine molecule bind with the α subunits (in the extracellular region of course) a conformational change occurs and the channel is opened for a brief moment. Then Na⁺ can enter the cell.

Acetylcholine is released from the presynaptic neuron or from neuron of neuro-muscular junction and affects the receptors present in postsynaptic neuron or muscle respectively.

The ligand gated sodium channel is to some extent permeable to K⁺.

Once again, my drawing is not very good, so I am sorry if the picture is not understandable.