Why do sodium ions need a channel to cross the membrane?

Sodium ions are negatively charged and thus cannot directly cross through the hydrophobic lipid bilayer of the cell membrane on their own. They must instead utilize unique channel proteins, known as sodium channels. These channels are proteins with specific structures that allow them to form a pore through which sodium ions can pass through. They are exclusive to sodium ions and block other molecules from passing through. Essentially, they function to give sodium ions a hydrophilic tunnel to cross the membrane that they otherwise would not be able to do. These channels open and allow sodium to rush in when a nerve cell communicates with another. The flooding of sodium ions into the channel triggers the nerve cell to fire. This then causes a series of reactions as the signal is carried from nerve cell to nerve cell until it either travels to the brain or to a muscle for contraction. Sodium moves from areas of higher concentration to areas of lower concentration. These are known as voltage-gated sodium channels. The other type of sodium channel is known as the epithelial sodium channel, in which its main effect is sodium reabsorption in the colon, lungs and in nephrons.