This model of a bacterial sodium channel shows only the pore domain of the protein, not the domain responsible for sensing the voltage across the membrane. 

The pore domain of the sodium channel is similar to the potassium channel in that it is a homo-tetramer in which 4 identical proteins come together to form the ion pore – exposed on the outside surface of the cell.  This pore is much larger than the pore of the potassium channel.  This is surprising because the Na+ ion is smaller than the K+ ion; however, the Na+ ion is transported in its water-hydrated state while the water molecules are stripped off the K+ ion during the transport process, and therefore the pore itself can be smaller. The protein was captured in its open conformation, where your students can easily see how Na+ ions can be released into the cytoplasm after passing through the pore.

This 3.75'' model is made of plaster by rapid prototyping and should be handled with care. Mini models will break if dropped, held tightly or handled roughly.