The Nervous System: Learn It 3—How Neurons Communicate

Now that we have learned about the basic structures of the neuron, let’s take a closer look at how the neurons communicate by sending signals from one to the other.

How Neurons Communicate

We begin at the neuronal membrane, essentially the wall of the neuron that separates the membrane from the outside environment. The neuron exists in fluid—it is surrounded by extracellular fluid but also contains intracellular fluid (i.e., cytoplasm). The fluids on the outside and inside have different electrical charges, and the difference in charge across the membrane is what causes signals to be sent through the neurons.

Between signals, the neuron membrane’s potential is held in a state of readiness, called the resting potential. Like a rubber band stretched out and waiting to spring into action, ions, atoms with electrical charges, line up on either side of the cell membrane, ready to rush across the membrane when the neuron goes active and the membrane opens its gates. Ions in high-concentration areas are ready to move to low-concentration areas, and positive ions are ready to move to areas with a negative charge.

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 Action Potential

From this resting potential state, the neuron receives a signal, and its state changes abruptly. When a neuron receives signals at the dendrites—due to neurotransmitters from an adjacent neuron binding to its receptors—small pores, or gates, open on the neuronal membrane, allowing sodium (Na+) ions to move into the cell. With this influx of positive ions, the internal charge of the cell becomes more positive. The process of when the cell’s charge becomes positive, or less negative, is called depolarization. If the charge reaches a certain level, called the threshold of excitation, the neuron becomes active and the action potential begins.

The action potential is an all-or-none phenomenon. In simple terms, this means that an incoming signal from another neuron is either sufficient or insufficient to reach the threshold of excitation. There is no in-between, and there is no turning off an action potential once it starts. Think of it like sending a text message. You can think about sending it all you want, but the message is not sent until you hit the send button. Furthermore, once you send the message, there is no stopping it. Because it is all or none, the action potential is recreated, or propagated, at its full strength at every point along the axon. Much like the lit fuse of a firecracker, it does not fade away as it travels down the axon. It is this all-or-none property that explains the fact that your brain perceives an injury to a distant body part like your toe as equally painful as one to your nose.

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.

When the action potential arrives at the terminal button, the synaptic vesicles release their neurotransmitters into the synapse. The neurotransmitters travel across the synapse and bind to receptors on the dendrites of the adjacent neuron, and the process repeats itself in the new neuron (assuming the signal is sufficiently strong to trigger an action potential).

Reuptake

Once the signal is delivered, excess neurotransmitters in the synaptic cleft drift away, are broken down into inactive fragments, or are reabsorbed in a process known as reuptake. Reuptake involves the neurotransmitter being pumped back into the neuron that released it, in order to clear the synapse. Clearing the synapse serves both to provide a clear “on” and “off” state between signals and to regulate the production of neurotransmitter (full synaptic vesicles provide signals that no additional neurotransmitters need to be produced).

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).