What Is a Synapse?

A synapse is a small gap at the end of a neuron that allows a signal to pass from one neuron to the next. Neurons are cells that transmit information between your brain and other parts of the central nervous system. Synapses are found where neurons connect with other neurons.

Synapses are key to the brain's function, especially when it comes to memory.

What Synapses Do

Synapses connect neurons and help transmit information from one neuron to the next. When a nerve signal reaches the end of the neuron, it cannot simply continue to the next cell. Instead, it must trigger the release of neurotransmitters which can then carry the impulse across the synapse to the next neuron.

Once a nerve impulse has triggered the release of neurotransmitters, these chemical messengers cross the tiny synaptic gap and are taken up by receptors on the surface of the next cell.

These receptors act much like a lock, while the neurotransmitters function much like keys. Neurotransmitters may excite or inhibit the neuron they bind to.

Parts of the Synapse

Synapses are composed of three main parts:

• The presynaptic ending that contains neurotransmitters
• The synaptic cleft between the two nerve cells
• The postsynaptic ending that contains receptor sites

An electrical impulse travels down the axon of a neuron and then triggers the release of tiny vesicles containing neurotransmitters. These vesicles will then bind to the membrane of the presynaptic cell, releasing the neurotransmitters into the synapse.

These chemical messengers cross the synaptic cleft and connect with receptor sites in the next nerve cell, triggering an electrical impulse known as an action potential.

Types

There are two main types of synapses:

• Chemical synapses
• Electrical synapses

Chemical Synapses

In a chemical synapse, the electrical activity in the presynaptic neuron triggers the release of chemical messengers, the neurotransmitters. Most synapses are chemical.

The neurotransmitters diffuse across the synapse and bind to the specialized receptors of the postsynaptic cell.

The neurotransmitter then either excites or inhibits the postsynaptic neuron. Excitation leads to the firing of an action potential while inhibition prevents the propagation of a signal.

Electrical Synapses

In electrical synapses, two neurons are connected by specialized channels known as gap junctions.

Electrical synapses allow electrical signals to travel quickly from the presynaptic cell to the postsynaptic cell, rapidly speeding up the transfer of signals.

The special protein channels that connect the two cells make it possible for the positive current from the presynaptic neuron to flow directly into the postsynaptic cell.

Comparing the Types

The gap between electrical synapses is much smaller than that of a chemical synapse (about 3.5 nanometers compared to 20 nanometers).

Electrical synapses transfer signals much faster than chemical synapses. While the speed of transmission in chemical synapses can take up to several milliseconds, the transmission at electrical synapses is nearly instantaneous.

While electrical synapses have the advantage of speed, the strength of a signal diminishes as it travels from one cell to the next. Because of this loss of signal strength, it requires a very large presynaptic neuron to influence much smaller postsynaptic neurons.

Chemical synapses may be slower, but they can transmit a message without any loss in signal strength. Very small presynaptic neurons are also able to influence even very large postsynaptic cells.

Where chemical synapses can be excitatory or inhibitory, electrical synapses are excitatory only.