Synaptic Transmission: How Neurons Talk to Each Other
- Pamela Brown
- Nov 20, 2025
- 5 min read
If action potentials are the electrical language of the brain, synaptic transmission is the grammar that gives those signals meaning. Because the real magic taking place in the brain doesn’t happen inside neurons. It happens between them and FAST — in about 1/10000th of a second.
Action potentials are what sends electricity down a neuron, but synaptic transmission is what lets one neuron send a message to another. This “handoff” is how thoughts form, habits solidify, memories strengthen, and movements are triggered.
We often talk about “signals traveling down nerves,” but nerves are basically bundles of axons wrapped together, similar to how a cable contains multiple wires. Those bundles run throughout your body, carrying electrical impulses just like you imagine, but the moment the signal reaches its destination, it changes form. Electricity alone can’t jump from neuron to neuron. For that, the brain relies on synaptic transmission, the tiny junction-based handoff that makes thought possible. Without synapses, your brain would be a silent electrical dead-end.
What Is a Synapse?
A synapse, or synaptic cleft, is the microscopic gap between the end of one neuron and the beginning of another. Neurons do not physically touch, even though the saying “neurons that fire together wire together” gives the illusion that they fuse together into one circuit. But they don’t touch. They communicate across space.
The presynaptic neuron sends the signal.
The postsynaptic neuron receives it.
Most synapses are chemical synapses, meaning the presynaptic neuron releases neurotransmitters (messenger molecules) into the space between neurons. There are also electrical synapses, where ions flow directly from one neuron to another through gap junctions (tiny protein tunnels connecting the cells). These are incredibly fast, but rare in humans compared to chemical synapses.
Chemical synapses dominate because they allow something electrical synapses do not: flexibility. You can strengthen them, weaken them, block them, or rewire them. That’s the basis of learning.
How a Signal Moves Across a Synapse
Here’s what happens in less than a blink:
An action potential arrives at the axon terminal.
Voltage-gated Ca²⁺ channels open.
Calcium rushes into the neuron (ions always want to move from crowded to a less crowded area).
Calcium triggers vesicles filled with neurotransmitters to fuse with the cells' membrane.
The fusion allows the vesicles to open up, spilling neurotransmitters into the synaptic cleft.
The neurotransmitters bind to receptors on the postsynaptic neuron.
Ion channels open and contribute toward creating a new electrical response.
What happens to the neurotransmitter molecules that don't bind to postsynaptic receptors? They are removed through reuptake (taken back up by the presynaptic neuron), broken down by enzymes, or simple diffuse away from the synaptic cleft.
So the simplified version is that one neuron fires. → Neurotransmitters are released. → Another neuron fires. Electrical → Chemical → Electrical. Except it’s not happening in a straight line. It’s happening across billions of synapses at once, and the net result of all those signals is what becomes a thought, a movement, a decision, or a memory.
Not All Neurotransmitters Act the Same
You’ve probably heard of two neurotransmitters: dopamine and serotonin. But those are just two of over 100 neurotransmitters your brain uses. What matters most is this: Some neurotransmitters make the next neuron more likely to fire by adding positive charge and moving the membrane potential closer to the threshold needed for an action potential. Some make it less likely to fire by doing the opposite - adding negative charge.
These effects are called:
EPSPs (excitatory postsynaptic potentials) → move the neuron closer to firing.
IPSPs (inhibitory postsynaptic potentials) → push it further away from firing.
Below are some common neurotransmitters and their effects:
Neurotransmitter | Type | Function Example |
Glutamate | Excitatory | Learning, memory |
GABA | Inhibitory | Reducing anxiety |
Dopamine | Modulatory | Motivation, reward |
Serotonin | Modulatory | Mood, appetite |
Neurons sum up the inputs of all the neurotransmitters its receives. If the excitatory signals outweigh the inhibitory ones enough to reach threshold, another action potential begins. If the inhibitory signals outweigh the excitatory ones, the neuron will stay at rest. Your mind is essentially a running vote tally of thousands of synaptic decisions per millisecond.
Synaptic Plasticity
Every time you learn something, your synapses change - physically, chemically, and structurally. This strengthening process is called long-term potentiation (LTP) and it can involve:
Adding more receptors to the postsynaptic neuron.
Making those receptors more sensitive.
Releasing more neurotransmitters.
Even growing more dendritic spines so the synapse has a larger “docking area”.
That old phrase “neurons that fire together wire together” is the summary of this.
The opposite also happens. If a pathway isn’t used, the brain doesn’t waste resources maintaining it. As a result, the pathway weakens and connections shrink. That process is called long-term depression (LTD), and its the neurological version of “use it or lose it.” The difference between remembering someone’s name next year or forgetting it tomorrow often comes down to whether that synapse was strengthened or allowed to decay.
Research Spotlight
A landmark review by Roger Nicoll (2017, Neuron)* traces how decades of research uncovered the mechanism behind LTP. Nicoll highlights that repeated stimulation leads to the insertion of additional AMPA receptors into the postsynaptic membrane, literally making the neuron more responsive to the same signal in the future. This discovery linked a physical change in synapses to the psychological reality of memory, establishing that learning is not just metaphorical “growth”. It is measurable structural change.
*Nicoll, R. A. (2017). A brief history of long-term potentiation. Neuron, 93(2), 281-290. https://doi.org/10.1016/j.neuron.2016.12.015
When Synaptic Transmission Goes Wrong
Because synapses are so foundational, even small disruptions can have huge effects. For example, depression often involves dysfunctional serotonin transmission, not necessarily “low serotonin,” but impaired signaling. Many antidepressants work by blocking reuptake so serotonin stays in the synapse longer. These are called SSRIs, which stands for “selective serotonin reuptake inhibitor.”
Synaptic failure also plays a major role in Alzheimer’s disease. In Alzheimer’s, synapses start disappearing long before the neurons themselves die. In fact, the amount of synapse loss often predicts memory problems better than the well-known protein changes in the brain.
A different kind of breakdown happens in Parkinson’s disease. Here, the brain slowly loses the neurons that make dopamine. With less dopamine to help signals flow, the circuits that control movement can’t communicate properly, leading to the shaking and stiffness often seen in Parkinson’s. In epilepsy, the disruption goes in the opposite direction - too much signaling. Excess excitation (glutamate) and not enough inhibition (GABA) leads to runaway firing, which overwhelms the brain and produces seizures.
Practical Neuroscience Moment
You’ve probably heard of these practical tips before. They are encouraged over and over again, but now you can have some insight into WHY they work.
Spaced repetition - Reviewing material over time repeatedly triggers LTP. Your brain decides, “This signal keeps returning, so we had better make it easier to fire.”
Caffeine - Increases neurotransmitter release and blocks adenosine (a signal that slows neural firing). Your neurons literally fire faster.
Exercise - Boosts BDNF, a growth factor that supports new synapse formation and helps maintain existing ones, vital for learning and memory.
Sleep - Neurons reset their synaptic loads, clear excess neurotransmitters, and consolidate learning. Skipping sleep is almost like not bothering to save everything you wrote on a document. It will make it harder to retain anything you have learned or studied that day.
Final Thoughts
Every time you remember, laugh, study, fall in love, or rewrite your sense of self, synapses change. They are the microscopic machinery behind becoming who you want to be. If you change your synapses, you literally change who you are. This is why habits matter. Every repetition strengthens a pathway. Every avoided behavior lets a synapse fade.
Your brain is not fixed. It is sculpted, one signal at a time. So be aware of your actions. You don’t just have a brain. You are building it one repetition, one thought, one synapse at at time.
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