The primary function of electrical stimulation is to depolarize sensory, motor, or pain nerves. Nerves and muscles respond to electrical stimulation because they are excitable tissues. The nerve or muscle cell's membrane regulates the exchange of substances between the external and internal cell environment. This membrane has voltage sensitive permeability; this means that, based on the electrical charge of ions, the cell membrane will allow negatively charged ions to flow into the cell and will shift positively charged ions out of the cell. The membrane is polarized when more negative ions reside within the cell than positive ions. Depolarization requires more positive ions to reside within the cell than negative ions. As you can see on the chart, electrical stimulation can also be used to control edema, improve wound healing, and help partially paralyzed patients regain some use of their muscles. There's one type of stimulation, called iontophoresis, that can be used to introduce certain medications into the body. Before you use electrical stimulation with patients, you need to screen them to be sure there aren't any contraindications to the treatment or precautions you should take. Because this is so important, I've made up another reference chart for you to have handy whenever you need it. Take a few minutes to look at the chart and print it if you want to, then click Next when you're ready to continue.