How Can the Brain Use Both Electricity and Chemicals?

Descriptions of the nervous system can sound contradictory.

Neurons are often said to carry electrical signals. But we also hear constantly about neurotransmitters like dopamine, serotonin, glutamate, and GABA: chemicals that neurons use to communicate with one another.

So which is it?

The useful answer is: both.

A typical neural signal repeatedly changes form. Within a neuron, information can travel as an electrical change in the cell membrane. At most synapses, the junctions between neurons, that electrical signal is converted into a chemical one. The receiving neuron then converts the chemical message back into an electrical change of its own.

The nervous system is therefore not purely electrical or purely chemical. It is an enormous network built around translating between the two.

Within a Neuron and Between Neurons

A useful first approximation is:

Within a neuron, signaling is electrical. Between neurons, signaling is usually chemical.

That is not a perfect rule. Some neurons communicate through electrical synapses, and the electrical activity inside a neuron is itself inseparable from chemistry. But as a mental model, it gets surprisingly far.

Imagine one neuron receiving signals from many others. Neurotransmitter molecules bind to receptors on its surface and alter what electrically charged particles, called ions, can move through the neuron's membrane.

Those changes affect the electrical voltage across the membrane.

If the combined effect is strong enough, the neuron may generate an action potential: a rapid, self-propagating electrical event that travels down the axon toward the neuron's output terminals.

When it reaches the end, the story becomes chemical again.

A typical neural signal alternates between chemical and electrical forms: electrical activity travels within a neuron, while neurotransmitters carry the signal across most synapses.

What Does "Electrical" Mean Inside a Neuron?

Calling an action potential an electrical signal can produce the wrong picture.

An axon is not simply a tiny copper wire carrying electricity from one end to the other.

Instead, neurons maintain a difference in electrical charge between the inside and outside of their membranes. This difference is called the membrane potential.

The membrane can control the movement of ions such as sodium and potassium through specialized protein channels. Because ions carry electrical charge, changing which ions can cross the membrane changes the membrane's voltage.

That is the heart of neuronal electricity.

When a neuron is at rest, its membrane maintains a characteristic voltage. If incoming signals change that voltage enough to reach a threshold, voltage-sensitive ion channels begin opening in a coordinated sequence.

The result is an action potential.

What Is an Action Potential?

An action potential is a brief, dramatic change in the voltage across a neuron's membrane.

During its rising phase, channels allow sodium ions to enter the cell rapidly. The membrane becomes much less negative and briefly positive relative to the outside.

Then sodium channels stop conducting while potassium channels allow potassium ions to leave. The membrane voltage falls again and eventually returns toward its resting state.

What matters is that this process can reproduce itself in the neighboring section of the axon.

One patch of membrane changes voltage. That electrical change influences nearby membrane, causing its voltage-sensitive channels to open. That region then undergoes its own action potential, which triggers the next region, and so on.

The signal therefore travels because the neuron continually regenerates it along the axon.

It is less like electricity shooting through a metal wire than a coordinated wave of membrane activity moving through the cell.

This also explains why an action potential does not simply fade away as it travels. Each section of membrane produces the event anew.

Then the Signal Reaches a Synapse

Eventually the action potential reaches the axon terminal, where the neuron communicates with another cell.

Here the electrical signal triggers another type of ion channel to open, allowing calcium ions into the terminal.

Calcium acts as a signal for tiny membrane-bound packages called vesicles to release neurotransmitter molecules.

The neurotransmitter enters the synaptic cleft, the microscopic space separating the two cells.

It then diffuses across that gap and binds to receptors on the next neuron.

At this point, the message becomes electrical again.

Depending on the receptor involved, neurotransmitter binding may open or close ion channels directly, or it may trigger a chain of molecular events that affects them indirectly.

Either way, the result can be a change in the receiving neuron's membrane voltage.

And now the cycle can begin again.

Chemical → electrical → chemical → electrical.

But If Synapses Are Chemical, How Can the Brain Be Fast?

This is one of the more intuitive objections to the whole arrangement.

Electrical signals feel fast. Chemical diffusion feels slow. So if every neuron has to release molecules and wait for them to cross a gap, shouldn't neural communication crawl along?

The key is the scale involved.

The synaptic cleft is extraordinarily narrow. Neurotransmitters are not drifting across anything remotely comparable to the length of a neuron. They are crossing a microscopic gap.

Chemical synapses do introduce a small delay. Releasing neurotransmitter, allowing it to cross the cleft, and activating receptors takes time.

But the delay is generally measured in fractions of a millisecond to a few milliseconds, not seconds.

The nervous system combines two useful strategies.

Over longer cellular distances, action potentials propagate rapidly along axons. At the tiny junction between cells, chemistry provides an extremely flexible way to control how strongly one neuron influences another.

The chemical step costs a little time, but it gives the nervous system enormous control.

A synapse can strengthen or weaken. It can excite a cell or inhibit it. Its effects can depend on which receptors are present. Its properties can change with experience.

The slight delay is therefore not simply a flaw in the system. It comes packaged with much of the flexibility that makes neural circuits capable of learning, adapting, and computing.

Are All Synapses Chemical?

No.

Some cells communicate through electrical synapses, in which neighboring cells are connected by structures called gap junctions. These allow electrical current to pass much more directly from one cell to another.

Electrical synapses can transmit signals extremely rapidly and can help groups of neurons synchronize their activity.

But chemical synapses are far more common in the human nervous system, and they provide much greater flexibility in how signals are modified.

So even the simple rule that signals are electrical within neurons and chemical between them has an exception.

Still, it remains a useful way to understand the basic architecture.

The Main Idea

When someone says that neurons use electricity, that is true.

When someone says that neurons communicate using chemicals called neurotransmitters, that is also true.

The apparent contradiction disappears once the signal is followed through the whole circuit.

A neurotransmitter changes the electrical state of a neuron. That electrical state can produce an action potential. The action potential travels along the neuron and triggers neurotransmitter release at a synapse. The neurotransmitter changes the electrical state of the next neuron.

The brain works not by choosing between electricity and chemistry, but by moving constantly between them.