How Neurons Communicate: The Big Picture

Electrical inside the cell, chemical between cells: a one-page overview of how a neuron receives, decides, fires and passes on a message.

Neural communication has two halves: electrical signalling within a neuron and chemical signalling between neurons. Understanding how they fit together is the key to most of neuroscience.

Step 1: a charged cell

Every neuron keeps a small voltage across its outer membrane — about 70 thousandths of a volt (−70 millivolts), negative inside. This resting potential comes from different concentrations of charged particles, ions, inside and outside the cell, maintained by molecular pumps. It's like a battery, charged and ready.

Step 2: inputs arrive

At synapses, other neurons release chemical neurotransmitters that open channels in the membrane. Some push the voltage up (excitatory inputs), some push it down (inhibitory inputs). Each input is small. The neuron continually sums thousands of them across its dendrites.

Step 3: the decision to fire

If the summed voltage at the axon's starting point reaches a threshold (around −55 mV), voltage-sensitive sodium channels snap open and the neuron fires an action potential: a brief, all-or-nothing spike lasting about a millisecond. Below threshold, nothing is sent at all.

Step 4: down the axon

The spike regenerates itself along the axon like a falling row of dominoes, jumping between gaps in the myelin on insulated axons. Its size doesn't change; stronger stimuli produce more spikes per second, not bigger ones.

Step 5: across the synapse

At the axon terminal, the arriving spike lets calcium in, which makes tiny bubbles — vesicles — fuse with the membrane and release neurotransmitter into the synaptic cleft. The transmitter binds to receptors on the next cell, and the cycle begins again. The whole process, from one neuron's spike to the next cell's response, can take well under a few milliseconds.