How Neurons Work: Action Potential and Synapse in 3D

The parts of a nerve cell

The picture shows a single neuron, enlarged many thousands of times. Branches called dendrites collect messages from other cells, like antennas. They feed the cell body, or soma, which holds the nucleus and decides whether to send a signal on. The signal leaves along the axon, a thin fibre that can be a metre long in the leg. Many axons are wrapped in myelin, a fatty sheath that is interrupted at small gaps called nodes of Ranvier. At the far end the axon splits into terminals, which pass the message to the next cell.

A battery in the membrane

At rest, a neuron is like a charged battery. Its membrane keeps sodium ions mostly outside and potassium ions mostly inside, and a molecular pump called the sodium-potassium pump works constantly to hold the balance. The result is an electrical difference across the membrane: the inside is about 70 millivolts more negative than the outside. Scientists write this resting potential as roughly −70 mV. Use the voltmeter in the overlay to see it. A millivolt is a thousandth of a volt, so the whole cell runs on a tiny voltage, yet it is enough to carry thought.

The action potential

Now stimulate the cell. A weak push only makes the voltage wobble and then fade. But if the push lifts the voltage to a threshold of about −55 mV, sodium channels snap open, positive ions rush in, and the voltage shoots up to roughly +30 to +40 mV. Then potassium channels open and the voltage falls back. This spike, the action potential, lasts about a millisecond. It is all or nothing: a stronger stimulus does not make a bigger spike, only more spikes per second. Drag the stimulus slider and watch the trace.

Why myelin makes it faster

A spike is not a current flowing down a wire. It is a wave that renews itself along the membrane. On a bare axon it creeps along at only about 1 metre per second. Myelin changes that. The sheath insulates the axon, so the spike jumps from one node of Ranvier to the next, which is called saltatory conduction, from the Latin for 'leaping'. On thick myelinated fibres the impulse reaches up to about 100 to 120 metres per second, more than 400 kilometres per hour. Switch myelin off in the sandbox and see how the pulse slows down.

Crossing the gap

When the spike reaches the terminal, it meets a gap, the synapse, only about 20 to 40 nanometres wide. A nanometre is a millionth of a millimetre, so the gap is thousands of times narrower than a human hair. The electrical pulse cannot jump it. Instead, calcium ions flow in and tiny bubbles called vesicles release a chemical messenger, a neurotransmitter, into the gap. It drifts across and docks on receptors on the next cell, which can make that cell more or less likely to fire. Examples include glutamate, GABA and dopamine.

Learning is rewiring

The last picture zooms out to a web. The brain has about 86 billion neurons, and each can connect to thousands of others, an estimated hundred trillion synapses or more. Signals travel through whole networks at once. The psychologist Donald Hebb suggested in 1949 that connections strengthen when two cells are active together, an idea often summed up as 'cells that fire together wire together'. This is a simplified slogan, yet it captures why practice works: repeating something strengthens the pathways involved, and that is how learning is stored.

Neurons and nerve impulses