Synapses are the tiny junctions where one nerve cell passes a signal to another. Your brain contains roughly 164 trillion of them in the outer layer alone, and every thought, movement, memory, and sensation depends on signals crossing these gaps. They are the fundamental communication points of your nervous system.
How a Synapse Is Built
A synapse has three parts. The sending side (the tip of one nerve cell’s long fiber) stores chemical messengers in small bubble-like packages. The receiving side (the surface of the next nerve cell) is studded with receptors designed to catch those messengers. Between the two sits a narrow gap called the synaptic cleft, typically 12 to 50 nanometers wide. That’s roughly a thousand times thinner than a human hair.
This gap is what makes synapses interesting. Nerve cells carry electrical signals along their length, but at most synapses the electricity can’t jump directly to the next cell. Instead, the signal has to be converted into a chemical one, ferried across the cleft, and then converted back to an electrical signal on the other side. That conversion step gives the brain something it wouldn’t have with simple wiring: the ability to adjust, filter, and fine-tune every message.
What Happens When a Signal Arrives
When an electrical impulse reaches the sending end of a synapse, it opens channels that allow calcium to rush into the cell. This calcium surge is the critical trigger. Within a few hundred microseconds, calcium activates molecular machinery that forces the bubble-like packages to merge with the cell membrane and spill their chemical messengers into the cleft.
Those messengers drift across the gap in microseconds and latch onto receptors on the receiving cell. Depending on which receptors are activated, ion channels open in the receiving cell’s membrane, generating a new electrical signal. The whole process, from electrical impulse to chemical release to new electrical response, takes just a fraction of a millisecond.
Excitatory vs. Inhibitory Signals
Not every synapse tells the next cell to fire. Some synapses encourage activity, and others suppress it. The difference comes down to which channels the chemical messenger opens on the receiving side.
At excitatory synapses, the messenger opens channels that let positively charged ions flood in, pushing the receiving cell closer to the threshold where it will fire its own electrical signal. At inhibitory synapses, the messenger opens channels that let negatively charged ions in (or positive ones out), pulling the cell further from that firing threshold. A single nerve cell can receive thousands of excitatory and inhibitory inputs at once, and whether it fires depends on the running total. This balance between excitation and inhibition is one of the brain’s most important regulatory mechanisms.
Electrical Synapses: The Fast Alternative
Not all synapses use chemical messengers. A smaller number of synapses, called electrical synapses, connect two nerve cells directly through specialized pores known as gap junctions. These pores are large enough for ions and small molecules to pass straight from one cell’s interior to the next, so the electrical signal doesn’t need to be converted into a chemical one and back again.
The result is speed. Transmission at an electrical synapse is virtually instantaneous, with essentially no delay between the signal in one cell and the response in the other. The tradeoff is flexibility. Electrical synapses are simple on-off connections. They can’t amplify a weak signal, change from excitatory to inhibitory, or strengthen over time the way chemical synapses can. The brain uses them where speed and synchrony matter most, such as coordinating groups of cells that need to fire together.
How Synapses Change With Experience
One of the most important properties of chemical synapses is that they aren’t fixed in strength. When two connected nerve cells are repeatedly activated together, the synapse between them can become more efficient, a process called long-term potentiation. Discovered in the early 1970s by Timothy Bliss and colleagues, this phenomenon showed that a few seconds of rapid stimulation could enhance a synapse’s signaling strength for days or even weeks.
Long-term potentiation has several features that make it a plausible basis for learning and memory. It is input-specific, meaning only the synapses that were actually active get strengthened, not every synapse on the cell. It is also associative: a weak input that wouldn’t normally trigger strengthening can be boosted if it fires at the same time as a strong input on the same cell. This mirrors the basic logic of associative learning, where two experiences become linked because they occur together. The strengthening requires that the sending and receiving cells are active within about 100 milliseconds of each other.
Synapses can also weaken through a complementary process. Together, strengthening and weakening allow the brain to continuously update its wiring based on what’s relevant and what isn’t.
Synaptic Pruning During Development
Babies are born with far more synapses than they’ll ultimately keep. Synaptic density in the brain’s outer layer increases rapidly after birth, peaking at one to two years of age at roughly 50% above adult levels. From there, unused connections are progressively eliminated, a process that continues at least through adolescence before stabilizing in adulthood.
This pruning isn’t a sign of decline. Computational research suggests that networks built through initial overabundance followed by selective removal end up more robust and efficient than networks built any other way. The synapses that get used and reinforced by experience survive. The ones that don’t are cleared away. It’s the brain’s way of sculpting a precise, efficient set of circuits from a rough draft.
When Synapses Malfunction
Because synapses are so central to brain function, problems at the synapse can have serious consequences. Researchers increasingly use the term “synaptopathy” to describe brain disorders rooted in synaptic dysfunction, and the list spans both developmental and degenerative conditions.
In autism spectrum disorders, evidence points to an imbalance between excitatory and inhibitory signaling, with genetic variants affecting key receptors and structural proteins at the synapse. In Down syndrome, an over-inhibition of synapses by increased inhibitory circuitry appears to disrupt the excitation-inhibition balance, alongside problems with the molecular machinery that recycles signaling components. Epilepsy involves a similar but opposite imbalance, with enhanced excitatory transmission and reduced inhibitory signaling leading to runaway neural activity and seizures.
In Alzheimer’s disease, the connection between synapses and symptoms is especially direct. Small clumps of a toxic protein fragment disrupt synaptic plasticity and cause synapse loss, and the severity of dementia correlates more closely with the levels of these clumps than with the larger plaques visible on brain scans. These fragments interfere with the very same strengthening processes that underlie learning, which helps explain why memory is typically the first casualty. Some researchers have also hypothesized that excessive synaptic pruning during adolescence could contribute to the onset of schizophrenia, which typically emerges during exactly that developmental window.