How do ions cross the membrane?

Ions carry a charge, which prevents them from directly traversing the hydrophobic regions of the cell membrane. These charged ions depend on specialized channel proteins that form a hydrophilic or water-loving tunnel across the membrane. These hydrophilic trans-membrane tunnels are known as ion channels.  

Some ion channels allow different types of ions to pass through, while others are highly selective for specific types of ions. 

Some ion channels open only in response to specific signals and remain closed in resting neurons. Other channels, known as leak channels, remain open in resting neurons too.