Charge, voltage and current

Every heartbeat, every thought and every movement you make starts with a small electrical change. Your body has no wires, yet it runs on voltage and current just as surely as a phone does. The difference is what carries the charge: in your body, it is ions dissolved in water. This page builds the electrical ideas you need from the ground up: what it means to separate charge, what voltage is, what an electrical current is in your body, and how an ion responds to both a concentration gradient and a voltage at the same time. Every later topic on the nervous system, muscle and the heart builds on these four ideas.

Separating charge takes energy and stores it

Rub a balloon on a wool sweater, pull it away, then hold it close to the sweater. The wool fibers lift toward the balloon. Rubbing moved some electrons from the wool onto the balloon, so the balloon now carries extra negative charge and the wool carries extra positive charge. Pulling the two apart took a little effort, and the attraction between them is that effort stored.

You met electric charge in the topic on atoms and ions: opposite charges attract, and like charges repel. Because opposite charges attract, pulling them apart takes energy. That energy is not lost. It is stored as potential energy, like the energy in a stretched spring, and it is released when the charges are allowed to come back together.

Charge separation is a state in which positive and negative charges are held apart, so that one region carries extra positive charge and a nearby region carries extra negative charge. Something has to hold them apart, or they would simply move back together. In your body, that barrier is the thin membrane around each cell, which blocks most ions from crossing freely (you met this idea as a semipermeable membrane in the last topic).

Charge separation in your cells

Your cells keep a thin layer of extra negative charge on the inner face of their membrane and a matching layer of extra positive charge on the outer face (Figure 1). Two facts about this layer matter.

• It is tiny. Only a very small fraction of the ions in and around a cell, far less than one in a thousand, are part of the separated layer. They sit right against the membrane.
• Everywhere else is neutral. Away from the membrane, the fluid inside the cell and the fluid outside each have equal amounts of positive and negative charge. The separation exists only across the membrane itself.

Your cells spend energy, from ATP, to set up the ion differences that produce this layer. The next chapter shows how.

Voltage: the push that separated charge creates

An AA battery is labeled 1.5 volts. Inside it, a chemical reaction has piled up extra electrons at one end, the negative terminal, and taken them from the other, the positive terminal. Connect the two ends with a wire and electrons rush through it from the negative end toward the positive end. The 1.5 volts measures how hard the separated charge pushes.

Voltage is the difference in electrical potential energy between two points, for each unit of charge. It is also called the potential difference, because it is always a difference between two places, never a value at one place alone. It is measured in volts (V).

• Voltage exists only where charge is separated. No separation, no voltage.
• The more charge that is separated, the larger the voltage.
• Voltage is a stored push. Nothing has to be moving for a voltage to exist. A battery on a shelf has its full voltage.

Millivolts

The voltages across the membranes around your cells are small, so physiologists use the millivolt (mV; milli- = one thousandth): 1 mV = 0.001 V, and 1 V = 1,000 mV. The voltage across a typical cell's membrane is a few tens of millivolts, usually somewhere between about 50 and 90 mV, with the inside negative compared with the outside. By convention, it is written as the voltage of the inside compared with the outside, so an inside-negative voltage has a minus sign.

A few tens of millivolts sounds weak, but the membrane is extremely thin, only about 8 millionths of a millimeter. An 80 mV difference across that distance is one of the steepest electrical gradients you will find anywhere: equal to about 10 million volts across a meter. That is strong enough to pull on charged parts of the proteins in the membrane and change their shape.

Voltage is an electrical gradient

In the topic on gradients, you saw that a pressure gradient drives fluid flow. Voltage plays the same role for charge. It is an electrical gradient: a difference in electrical potential between two places. Positive charges are pushed from the positive side toward the negative side, and negative charges are pushed the other way.

Electrical current: charge on the move

Switch on a flashlight and electrons flow through the wire and the bulb. The bulb lights only while they are flowing. That flow is the current.

Electrical current is the flow of electric charge. It is measured in amperes (A), usually shortened to amps. In a wire, the moving charges are electrons. In your body, they are ions dissolved in water, so the current in your body is an ion current: sodium, potassium, calcium and chloride ions moving through fluid or across a membrane.

Direction of current

By convention, the direction of a current is the direction positive charge moves. So:

• Positive ions (such as sodium, Na⁺) moving into a cell make an inward current.
• Negative ions (such as chloride, Cl⁻) moving into a cell make an outward current, because bringing negative charge in has the same electrical effect as taking positive charge out.

Check the effect on the inside, not the arrow. Positive ions entering make the inside more positive. Negative ions entering make the inside more negative.

What a current needs

A current needs two things: a voltage to push the charge, and a path the charge can move along. Your body fluids, full of dissolved ions (electrolytes), carry current well. Fat and the lipid core of a membrane carry it very poorly, so they have a high electrical resistance, the opposition to the flow of charge. A membrane carries an ion current only where proteins open a route for that ion. The chapter on the membrane shows those routes.

The electrical version of the flow equation

You already know F = ΔP / R for fluids. Electricity follows the same form, known as Ohm's law:

I = V / R: current (I) equals voltage (V) divided by electrical resistance (R).

This is why wet skin makes an electric shock more dangerous. Dry skin has a high resistance. Wet skin, coated in salty water, has a much lower one, so the same household voltage drives a much larger current through the body.

Current changes voltage

Here is the link that ties this page together. When ions flow across a membrane, they carry charge with them, so they change the charge separation, and with it the voltage.

• Positive ions flowing in cancel some of the negative charge on the inside, so the voltage becomes less negative.
• Positive ions flowing out leave more negative charge behind, so the voltage becomes more negative.

Because the separated layer is so small, a tiny number of ions is enough to change the voltage a great deal. The concentrations of ions inside and outside the cell barely change. That is how your cells can change their voltage within a thousandth of a second, over and over, without running down their ion differences.

The electrochemical gradient: two pushes on one ion

Picture a sodium ion just outside one of your cells. Two things push on it at once.

• There is far more sodium outside your cells than inside, so its concentration gradient pushes it inward.
• It is positive, and the inside of the cell is negative, so the voltage pulls it inward too.

Both pushes point the same way, so the moment a route opens, sodium rushes in.

An ion is a charged particle, so it always feels two gradients:

• its concentration gradient, the chemical part, which pushes it from high to low concentration, as you saw for diffusion; and
• the electrical gradient, the voltage, which pushes positive ions toward the negative side and negative ions toward the positive side.

The electrochemical gradient (electro- = electric, chem- = chemical) is the combination of the two: the total push on an ion. It decides which way the ion moves, and how strongly, when a route across the membrane opens. The two parts can point the same way and add together, or point opposite ways and partly cancel (Figure 2).

When the two pushes oppose

Potassium shows the other case. There is far more potassium inside your cells than outside, so its concentration gradient pushes it outward. But potassium is positive, and the negative inside attracts it, so the voltage pulls it inward. The net push is whichever is larger, and it is weaker than either one alone. In your cells, the outward concentration push on potassium is the larger of the two, so when potassium can cross, it moves out.

If the two pushes on an ion are exactly equal and opposite, the ion has no net movement, even with an open route. It is at equilibrium: ions still cross, but equally in both directions. The topic on the voltage across the membrane returns to this balance point and shows how it sets the voltage of a cell at rest.

Uncharged particles feel only one push

Glucose, oxygen and carbon dioxide carry no net charge, so a voltage does not push them. They follow their concentration gradients alone. Only charged particles, ions, feel an electrochemical gradient.

Putting it together: too much potassium in the blood

Mr. Okoro has kidneys that have stopped removing potassium from his blood, and his blood potassium has risen from a normal level of about 4 mmol/L to nearly 8 mmol/L. Follow what happens to his heart cells.

1. More potassium outside his cells means a smaller difference between the potassium inside and outside. The outward concentration push on potassium weakens.
2. With a weaker outward push, less potassium leaves the cells, so less positive charge is carried out.
3. Less charge is separated across each membrane, and the voltage across it becomes smaller: the inside is less negative.
4. His heart cells depend on that voltage to fire in the right order. With it changed, his heart rhythm becomes irregular and can stop.

Every step uses an idea from this page: a concentration gradient, charge separation, voltage and the electrochemical push on an ion. The chapters on the membrane, the nervous system and the heart build these into a full picture.