A synapse is a small gap at the end of a neuron that allows a signal to pass from one neuron to the next. The synapses are found where nerve cells connect with other nerve cells . The term ‘ synapse’ was first introduced in 1897 by the physiologist Michael Foster in his Textbook of Physiology and is derived from the Greek synapsis , which means “set”.
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What do synapses do?
When a nervous signal reaches the end of the neuron, it cannot simply continue to the next cell.
Instead, it must trigger the release of neurotransmitters that 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 pass through the small synaptic space and are absorbed by specialized receptors on the surface of the next cell. These receptors act like a lock and neurotransmitters work a lot like a key. This process then converts the chemical signal back to an electrical signal. If the signal is strong enough, it will be propagated by the next neuron for an action potential until it reaches a synapse again and the process is repeated again.
The parts of the synapse
Synapses are composed of three main parts:
- The presynaptic end that contains neurotransmitters
- The synaptic cleft between the two nerve cells
- The postsynaptic termination containing local receptors
An electrical impulse travels to the axon of a neuron and then triggers the release of small vesicles containing neurotransmitters.
These vesicles will then bind to the presynaptic cell membrane, releasing neurotransmitters to the synapse. These chemical messengers cross the synaptic cleft and connect with receptors in the next nerve cell, causing an electrical impulse known as an action potential. .
What types of synapse?
There are two main types of synapses: chemical synapse and electrical synapse .
The first is the chemical synapse , in which the electrical activity of the presynaptic neuron triggers the release of chemical messengers. Neurotransmitters diffuse across the synapse and bind to specialized postsynaptic cell receptors. The neurotransmitter then excites or inhibits the postsynaptic neuron. Excitation leads to the triggering of an action potential while inhibition prevents the propagation of a signal.
However, there are also electrical synapses in which two neurons are connected by specialized channels known as gap junctions. Electrical synapses allow electrical signals to move quickly from the presynaptic cell to the postsynaptic cell, quickly accelerating signal transfer. The difference between electrical synapses is much smaller than that of a chemical synapse (about 3.5 nm compared to 20 nm). The special protein channels that link the two cells make it possible for the positive current from the presynaptic neuron to flow directly into the postsynaptic cell.
Differences between chemical synapse and electrical synapse
Electrical synapses transfer signals much faster than chemical synapses. While the transmission speed at chemical synapses can take up to several milliseconds, transmission at electrical synapses is almost instantaneous. Where chemical synapses can be either excitatory or inhibitory, electrical synapses are only excitatory.
While electrical synapses have the advantage of speed, the strength of a signal decreases as it travels from one cell to the next. Due to this loss of signal strength, it requires a very large presynaptic neuron to influence a much smaller postsynaptic neuron. Chemical synapses can be slower, but they can transmit a message without losing signal strength. Very small presynaptic neurons are also able to influence even large postsynaptic cells.