What Is a Synapse? How Brain Cells Communicate

A synapse is the specialized junction where a neuron communicates with another cell, whether another neuron, a muscle cell, or a gland cell. Synapses are how information moves through neural circuits, making sensation, movement, learning, memory, and emotion possible. Most communication in the human nervous system happens at chemical synapses, where neurotransmitters carry the signal across a microscopic gap called the synaptic cleft and bind to receptors on the receiving cell.

More than a gap

It is common to describe a synapse as the gap between two neurons, but that’s only part of it. The gap itself is the synaptic cleft. The synapse is the entire communication junction: the signal-sending part of the first cell, the narrow space between the cells, and the signal-receiving membrane of the next cell. The neuron sending the signal is called the presynaptic neuron, and the cell receiving it is the postsynaptic cell. Most synapses connect one neuron to another, but neurons also form synapses with muscle cells, gland cells, and other targets.

Three parts of a chemical synapse

The presynaptic terminal usually sits at the end of an axon. It holds small membrane-bound sacs called synaptic vesicles, which store neurotransmitters. The synaptic cleft is the microscopic space between the two membranes. It is extremely narrow, but the two cells stay structurally separate; at a chemical synapse, neurons do not merge. The postsynaptic membrane carries receptors that recognize particular neurotransmitters.

A postal system offers a rough picture. The presynaptic terminal is the sender holding packaged letters, the cleft is the short distance the mail travels, and the receptors are mailboxes that accept only correctly addressed letters. Real synapses work in a fraction of a millisecond and with chemical rather than written addresses, but the roles line up.

How a message crosses

Communication at a chemical synapse follows a sequence. An action potential, the neuron’s electrical impulse, travels down the axon and reaches the presynaptic terminal. Voltage-gated calcium channels open and calcium flows in. The rise in calcium triggers synaptic vesicles to fuse with the membrane and release neurotransmitters into the cleft. The neurotransmitters diffuse across and bind to receptors on the postsynaptic membrane, changing the receiving cell’s electrical or biochemical activity. The signal ends when neurotransmitters are removed by reuptake, broken down by enzymes, or diffuse away.

Excite, inhibit, or adjust

Neurotransmitters don’t always make the next neuron fire. Binding to receptors may increase the chance that the receiving neuron fires, decrease it, or modify the cell’s activity without immediately producing an electrical impulse. The outcome depends on the neurotransmitter, the receptor type, and the state of the receiving cell. An excitatory synapse makes firing more likely, and an inhibitory synapse makes it less likely. Some synapses are modulatory: rather than directly exciting or inhibiting, they change how strongly the cell responds to other inputs.

A neuron usually receives input from many synapses at once and combines them. A new action potential happens only when the combined input brings the neuron to its threshold.

Chemical and electrical synapses

The nervous system also uses electrical synapses, so not all synapses rely on neurotransmitters. Chemical synapses use neurotransmitters, are highly adaptable, can strengthen or weaken with experience, and usually transmit in one direction, from presynaptic to postsynaptic cell.

Electrical synapses contain specialized channels called gap junctions that connect the interiors of neighboring cells. Ions and small molecules pass straight through, so electrical activity spreads rapidly and groups of neurons can synchronize. Unlike chemical synapses, these channels create a direct pathway between cells. Electrical synapses are generally faster but offer less opportunity to modify the signal. Some transmit in both directions, while others show a directional preference.

Why synapses matter: circuits and plasticity

Synapses organize neurons into working circuits. Without them, the nervous system couldn’t process sensory information, control muscles and movement, regulate organs and glands, form and retrieve memories, support learning, or coordinate emotion and behavior. They also help determine whether information is selected, amplified, weakened, or blocked as it moves through the nervous system.

Synapses are not fixed. Synaptic plasticity is the ability of a synapse to change its strength or effectiveness over time. Some changes last only milliseconds or minutes, while others persist for days, months, or longer, and long-lasting changes are important mechanisms of learning and memory. That doesn’t mean each memory sits in a single synapse. Memories are thought to depend on changes spread across networks of connected neurons.

Related: What Is a Neuron? · What Is an Action Potential? · What Is a Neurotransmitter?

References

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4. Connors BW, Long MA. Electrical synapses in the mammalian brain. Annual Review of Neuroscience. 2004;27:393–418. doi:10.1146/annurev.neuro.26.041002.131128.