Synapse
A synapse is a specialized connection where a neuron communicates with another neuron, muscle cell, or gland cell. Synapses convert activity in one cell into signals that can excite, inhibit, modulate, or coordinate activity in another cell.
What a synapse is
A synapse is the contact zone where a signaling cell influences a target cell. In the nervous system, that usually means a neuron sending a signal to another neuron, a muscle fiber, or a gland cell. Synapses let nervous systems build circuits instead of acting as one continuous wire.
Presynaptic and postsynaptic sides
The sending side is called presynaptic, and the receiving side is called postsynaptic. In many chemical synapses, the presynaptic side is an axon terminal filled with synaptic vesicles. The postsynaptic side contains receptors and signaling machinery that translate the incoming message into a change in cell activity.
The synaptic cleft
The synaptic cleft is the tiny gap between the presynaptic and postsynaptic membranes. It is small, but it matters: neurotransmitters must cross this space, bind receptors, and then be cleared or broken down so the signal does not continue indefinitely.
Chemical synapses
At a chemical synapse, an action potential reaches the axon terminal and triggers calcium-dependent release of neurotransmitters. Those molecules diffuse across the cleft and bind receptors on the target cell. Depending on the transmitter, receptor, and circuit, the result may make the target cell more likely to fire, less likely to fire, or change how it responds to later signals.
Electrical synapses
Electrical synapses pass current more directly between cells, often through gap junctions. They can be very fast and can help synchronize groups of cells. Chemical synapses are more common in many vertebrate nervous systems, but electrical synapses are important in particular circuits where speed or coordination matters.
Excitation, inhibition, and modulation
Synapses are often described as excitatory or inhibitory, but the real effect depends on receptors, ion channels, timing, and the cell's current state. Some synapses push a neuron toward firing an action potential. Others move it away from firing. Neuromodulatory synapses can tune circuit behavior more gradually rather than sending a simple on-or-off message.
Plasticity and learning
Synapses can change strength over time. Activity can strengthen some connections, weaken others, or reshape which cells influence which circuits. This synaptic plasticity is central to learning, memory, development, adaptation, and recovery after some injuries. It is also one reason experiences can leave durable traces in the nervous system.
Clearing the signal
After neurotransmitters are released, the signal must end. Molecules may be taken back into the presynaptic cell, absorbed by nearby cells, broken down by enzymes, or diffuse away. Signal termination keeps synapses ready for the next event and prevents one release from overwhelming later communication.
Synapses and disease
Many neurological and psychiatric conditions involve disrupted synaptic signaling, even when the problem is not limited to one synapse. Changes in neurotransmitter release, receptor function, synapse number, immune signaling, protein buildup, myelin, metabolism, or inflammation can alter how circuits communicate.
Why it matters
Synapses matter because they are where nervous-system signals become relationships between cells. A neuron can only shape behavior, sensation, memory, or movement by influencing other cells. Understanding synapses helps explain neurotransmitters, medications, learning, addiction, motor control, brain development, and why small molecular changes can affect whole-body function.