To synapse means to transmit a signal from one nerve cell to another across a tiny gap. The word works as both a noun (the junction itself) and a verb (the act of passing that signal). Your brain contains roughly 100 trillion of these connections, each one a precise meeting point where one neuron communicates with the next. Everything you think, feel, remember, and do depends on this process happening billions of times per second.
The Physical Structure of a Synapse
A synapse is not a point of direct contact. It’s a three-part structure: the sending end of one neuron, a gap, and the receiving surface of the next neuron. The sending side, called the presynaptic terminal, sits at the tip of a nerve fiber and is packed with tiny sacs of chemical messengers. The receiving side has a dense cluster of receptor proteins designed to catch those messengers and convert the signal into a response.
The gap between the two cells is called the synaptic cleft, and it’s extraordinarily small. Measurements put it at roughly 20 to 24 nanometers wide, about a thousand times thinner than a human hair. Despite its size, this gap is critical. It separates the electrical activity of one cell from the next and forces communication to happen through a controlled chemical handoff.
How a Chemical Synapse Works
The vast majority of synapses in your brain are chemical synapses, and they follow a specific sequence. It starts when an electrical impulse travels down a nerve fiber and arrives at the terminal. That arrival triggers calcium to rush into the terminal through specialized channels. The influx of calcium causes tiny sacs of chemical messenger molecules (neurotransmitters) to fuse with the cell membrane and spill their contents into the synaptic cleft.
Those neurotransmitter molecules drift across the gap and latch onto receptor proteins on the receiving cell. Depending on the type of receptor, this binding either nudges the receiving cell closer to firing its own electrical impulse or pushes it further from firing. The whole process, from electrical signal arriving to the receiving cell responding, takes roughly 1 to 5 milliseconds. After the signal is delivered, neurotransmitters are either broken down or recycled back into the sending cell so the synapse resets and is ready to fire again.
Excitatory vs. Inhibitory Signals
Not all synapses tell the next cell to “go.” Some tell it to “stop.” The difference comes down to which neurotransmitter is released and what it does to the electrical charge inside the receiving cell.
Excitatory neurotransmitters, like glutamate (the most common one in the brain), open channels that let positively charged ions flood into the receiving cell. This raises the cell’s voltage toward the threshold needed to fire. Inhibitory neurotransmitters, like GABA, do the opposite: they open channels that let negatively charged chloride ions in, pushing the cell’s voltage further from its firing threshold. At any given moment, a single neuron may be receiving both excitatory and inhibitory signals from thousands of synapses simultaneously. Whether it fires depends on which side wins.
Electrical Synapses Work Differently
A smaller number of synapses skip the chemical handoff entirely. At electrical synapses, the two cells are physically linked by protein channels called gap junctions that connect their interiors. Ions pass directly from one cell to the next, carrying electrical current with no delay and no neurotransmitter involved.
Electrical synapses are bidirectional, meaning current flows both ways, and they transmit almost instantaneously. This makes them especially useful in circuits that need speed and coordination, like escape reflexes in animals. They can also synchronize large groups of neurons to fire together. The tradeoff is flexibility: electrical synapses can’t amplify or transform signals the way chemical synapses can. Chemical synapses are slower but far more versatile, capable of strengthening, weakening, or completely changing the nature of a signal.
How Synapses Change Over Time
Synapses are not fixed connections. They strengthen, weaken, form, and disappear throughout your life, and this flexibility is the physical basis of learning and memory.
When two connected neurons fire together repeatedly within a narrow time window of about 50 milliseconds, with the sending cell firing just before the receiving cell, the synapse between them gets stronger. This is called long-term potentiation. The receiving cell becomes more responsive to signals from that particular partner. If the timing reverses, with the receiving cell firing first, the synapse weakens instead, a process called long-term depression. This elegant timing rule means your brain physically rewires itself based on which patterns of activity occur together.
On a larger scale, your brain also prunes entire synapses it doesn’t use. From birth through your first two years, synapse numbers increase dramatically as your brain builds connections in every direction. Starting in childhood and continuing through adolescence into early adulthood, unused synapses are eliminated while frequently used ones are preserved and reinforced. This pruning helps your brain become more efficient, channeling energy toward the connections that matter most for your particular environment and experiences. Synapse numbers remain relatively stable in adulthood, with a slight decline after age 65.
When Synapses Malfunction
Because synapses control virtually all brain communication, problems at the synapse are at the root of many neurological and psychiatric conditions. In Alzheimer’s disease, toxic protein fragments disrupt receptor function at synapses, leading to synapse loss that tracks closely with cognitive decline. In Parkinson’s disease, synapses that release dopamine deteriorate, impairing movement control.
Epilepsy involves a shift in the balance between excitatory and inhibitory synapses, with too much excitation or too little inhibition allowing uncontrolled waves of neural firing. In autism spectrum disorders, researchers have identified alterations in both receptor proteins and the scaffolding molecules that organize the receiving side of the synapse, again pointing to an imbalance between excitation and inhibition. Even a condition as specific as startle disease, where a person has exaggerated startle reflexes, traces back to defective signaling at a single type of inhibitory synapse.
These examples underscore how much depends on synapses working correctly. The 100 trillion connections in your brain aren’t just wiring. They’re dynamic, adaptable junctions that shape every aspect of how you experience and interact with the world.