The Nervous System: Learn It 3—How Neurons Communicate

How Neurons Communicate

Now that we understand a neuron’s structure, let’s look at how neurons send and receive signals—the process that allows the brain and body to function.

The Resting Potential

  • A neuron’s membrane separates the inside (intracellular fluid) from the outside (extracellular fluid) environment.
  • These fluids have different electrical charges, creating an electrical imbalance across the membrane.
  • When the neuron is not sending a signal, it is in a resting potential—a state of readiness like a stretched rubber band waiting to release.
  • Ions, or electrically charged atoms, line up on either side of the membrane:
    • Sodium (Na⁺) ions are more concentrated outside the cell.
    • Potassium (K⁺) ions and negatively charged proteins are more concentrated inside the cell.
  • This charge difference allows the neuron to respond quickly when activated.

Watch this short video on membrane potential, and why the resting potential of a neuron is negative:

You can view the transcript for “2-Minute Neuroscience: Membrane Potential” here (opens in new window).

Depolarization and the Action Potential

When a neuron receives a signal at its dendrites—usually from neurotransmitters binding to receptors—the following steps occur:

  1. Gates open in the neuron’s membrane, allowing Na⁺ ions to rush in.
  2. The inside of the neuron becomes less negative—this is depolarization.
  3. If depolarization reaches a certain level, the threshold of excitation, the neuron fires an action potential.

The Action Potential

  • The action potential is an all-or-none event:
    • The neuron either fires at full strength or not at all—there’s no partial firing.
    • Once it begins, it propagates down the axon without losing intensity.
  • Think of it like sending a text message:
    • You can type and rethink it, but nothing happens until you hit “send.”
    • Once you do, it delivers completely—you can’t stop it mid-send.
  • Because of this property, your brain interprets a stubbed toe as just as “strong” a signal as a touch on your face—the difference lies in which neurons are firing, not how hard they fire.
A close-up illustration depicts the difference in charges across the cell membrane, and shows how Na+ and K+ cells concentrate more closely near the membrane.
Figure 1. At resting potential, Na+ (blue pentagons) is more highly concentrated outside the cell in the extracellular fluid (shown in blue), whereas K+ (purple squares) is more highly concentrated near the membrane in the cytoplasm or intracellular fluid. Other molecules, such as chloride ions (yellow circles) and negatively charged proteins (brown squares), help contribute to a positive net charge in the extracellular fluid and a negative net charge in the intracellular fluid.

From Electrical to Chemical Signals

When the action potential reaches the axon terminal:

  • Synaptic vesicles release neurotransmitters into the synaptic cleft (the tiny gap between neurons).
  • These neurotransmitters cross the cleft and bind to receptors on the next neuron’s dendrites.
  • If the signal is strong enough, it triggers another action potential in the receiving neuron—and the process continues.

Reuptake

After a neurotransmitter has done its job:

  • Some molecules drift away or are broken down by enzymes.
  • Others are reabsorbed into the releasing neuron through reuptake—a recycling process that:
    • Clears the synapse for the next signal.
    • Helps regulate how much new neurotransmitter the neuron produces.
  • This “clean-up” step ensures crisp, on/off signaling in the brain.
The synaptic space between two neurons is shown. Some neurotransmitters that have been released into the synapse are attaching to receptors while others undergo reuptake into the axon terminal.
Figure 2. Reuptake involves moving a neurotransmitter from the synapse back into the axon terminal from which it was released.

Watch this short video to understand how neurons communicate across the synaptic cleft:

You can view the  transcript for “2-Minute Neuroscience: Synaptic Transmission” here (opens in new window).