A synapse is a specialized junction where one neuron communicates with another cell. In the nervous system, synapses allow neurons to pass information to other neurons, muscle cells, or gland cells. They are essential for nearly everything the nervous system does, from sensing and movement to learning, memory, and regulating internal organs.
A synapse is not usually a simple physical connection between two neurons. In the most common type, the cells are separated by a microscopic gap, and communication occurs through chemical signals released from one cell and detected by the next.
Understanding a synapse means following that process from one end to the other: an electrical signal reaches the end of a neuron, triggers the release of chemical messengers, those messengers cross the gap, and the receiving cell responds.
What exactly is a synapse?
A synapse is a communication point between a presynaptic cell, which sends a signal, and a postsynaptic cell, which receives it.
The presynaptic cell is often a neuron, but the receiving cell does not have to be. A neuron can form synapses with another neuron, a muscle cell, or a secretory cell such as one involved in gland function.
In a typical chemical synapse, three structures are particularly important:
- Presynaptic terminal: The end of the signaling neuron where chemical messengers are released.
- Synaptic cleft: The tiny space separating the two cells.
- Postsynaptic membrane: The receiving cell’s membrane, containing proteins that detect the released chemical messenger.
The chemical messenger released at a synapse is called a neurotransmitter. Common neurotransmitters include glutamate, GABA, acetylcholine, dopamine, serotonin, and norepinephrine. They have different effects depending on which receptors detect them and where those receptors are located.
Not all synapses use neurotransmitters. Some neurons communicate through electrical synapses, in which specialized channels called gap junctions provide a direct route for electrical current between cells.
How does a chemical synapse work?
Chemical synaptic transmission occurs through a sequence of tightly coordinated events.
1. An electrical signal reaches the neuron’s end
Neurons transmit information along their membranes using changes in electrical potential called action potentials.
When an action potential reaches the axon terminal—the end of the neuron—it changes the electrical state of the presynaptic membrane. This change activates voltage-sensitive calcium channels.
2. Calcium enters the presynaptic terminal
The opening of these channels allows calcium ions (Ca²⁺) to enter the axon terminal.
The rise in calcium concentration inside the terminal is the key trigger for neurotransmitter release. It causes small membrane-bound structures called synaptic vesicles to fuse with the presynaptic membrane.
3. Neurotransmitter is released
Synaptic vesicles contain neurotransmitter molecules. When a vesicle fuses with the membrane, it releases its contents into the synaptic cleft through a process called exocytosis.
The neurotransmitter then diffuses across the narrow gap and reaches the postsynaptic membrane.
4. Neurotransmitter binds to receptors
The neurotransmitter attaches to specific receptors on the postsynaptic cell.
A receptor is a protein that recognizes particular chemical signals. Different receptors can produce very different effects even when they respond to the same neurotransmitter.
Some receptors are themselves ion channels. When the neurotransmitter binds, the channel opens or closes, allowing particular ions to move across the membrane. These are called ionotropic receptors.
Other receptors work indirectly through intracellular signaling pathways rather than forming an ion channel themselves. These are called metabotropic receptors. Their effects generally develop more slowly but can last longer and influence broader aspects of cell activity.
5. The receiving cell changes its activity
The movement of ions or activation of intracellular signaling pathways changes the postsynaptic cell.
A synaptic signal may make the receiving neuron more likely to produce an action potential or less likely to produce one.
This is why it is misleading to think of a neurotransmitter as simply an “on” or “off” signal. The effect depends on the neurotransmitter, the receptor, the ion channels involved, and the state of the receiving cell.
Excitatory and inhibitory synapses
One of the most useful distinctions between synaptic signals is whether they tend to increase or decrease the likelihood that a neuron will fire an action potential.
An excitatory postsynaptic potential (EPSP) makes the postsynaptic neuron more likely to reach the threshold required for an action potential. Glutamate is the major excitatory neurotransmitter in much of the brain.
An inhibitory postsynaptic potential (IPSP) makes the neuron less likely to fire. GABA is the major inhibitory neurotransmitter in the adult brain.
These categories describe the effect of a particular synaptic connection, not an unchanging property of the neurotransmitter itself. The same chemical messenger can have different effects in different cells because the cells may express different receptor types.
A neuron typically receives input from many other neurons at once. It combines these excitatory and inhibitory signals before determining whether to generate an action potential. This process is called synaptic integration.
What happens to a neurotransmitter after it is released?
Synaptic signaling has to be brief and controlled. Neurotransmitters therefore do not simply remain in the synaptic cleft indefinitely.
Depending on the neurotransmitter and synapse, the signal can be terminated in several ways. The neurotransmitter may be taken back into the presynaptic neuron through reuptake transporters, broken down by enzymes, or removed by nearby cells, including glial cells. Some neurotransmitters can also diffuse away from the synapse.
For example, acetylcholine in many synapses is rapidly broken down by the enzyme acetylcholinesterase. Other neurotransmitters, such as dopamine and serotonin, are commonly cleared in part through reuptake.
These mechanisms allow the nervous system to control both the duration and strength of synaptic signals.
Electrical synapses work differently
Chemical synapses are the dominant form of neuron-to-neuron communication in many parts of the nervous system, but electrical synapses provide another mechanism.
At an electrical synapse, gap junctions directly connect the interiors of neighboring cells. Small ions can pass through these channels, allowing changes in electrical potential to spread directly from one cell to another.
Electrical synapses can transmit signals extremely quickly and can synchronize the activity of groups of cells. Unlike chemical synapses, they generally provide less opportunity to modify or amplify a signal at the point of transmission.
Chemical and electrical synapses therefore serve different purposes. Chemical synapses offer considerable flexibility: they can excite or inhibit cells, amplify signaling through intracellular pathways, and change their strength over time.
Synapses can change with experience
Synapses are not fixed wiring. Their strength can change in response to patterns of activity, a property known as synaptic plasticity.
Some synapses become more effective at influencing the receiving cell, while others become less effective. These changes can involve alterations in neurotransmitter release, receptor number or activity, or the structure of the synapse itself.
Two widely studied forms of long-lasting synaptic change are long-term potentiation (LTP) and long-term depression (LTD). LTP strengthens certain synaptic connections after particular patterns of activity, whereas LTD weakens them.
Synaptic plasticity is an important mechanism underlying the nervous system’s ability to adapt. It plays a central role in how neural circuits develop and is strongly implicated in learning and memory.
A synapse is more than a gap between neurons
The synaptic cleft is tiny, but the machinery surrounding it is complex. The presynaptic terminal contains proteins that control vesicle docking and neurotransmitter release. The postsynaptic membrane contains receptors and signaling proteins that determine how the incoming message affects the cell.
Glial cells also participate in synaptic function. They can help maintain the chemical environment around neurons, influence neurotransmitter levels, provide metabolic support, and interact with synapses.
Synapses can also vary greatly in their structure and function. Some are highly specialized for rapid signaling, while others engage signaling pathways whose effects develop more gradually and persist longer.
Why synapses matter
A single neuron does not normally make decisions in isolation. Its behavior emerges from the combined activity of many synaptic inputs.
Across neural circuits, billions of synapses allow electrical activity to be transformed, combined, filtered, strengthened, weakened, and routed to different destinations. This organization enables the nervous system to coordinate movement, process sensory information, regulate bodily functions, and support complex processes such as attention, learning, and memory.
In simple terms, a synapse is the communication junction of the nervous system. In a chemical synapse, an electrical signal arriving at one neuron causes neurotransmitter release; the neurotransmitter crosses the synaptic cleft and activates receptors on another cell; the receiving cell then changes its activity. The enormous variety of receptors, neurotransmitters, synaptic structures, and plastic changes gives the nervous system the flexibility needed to turn individual cellular signals into coordinated behavior.
