Synapses are crucial structures that enable communication between neurons in the nervous system. They come in two main types: electrical synapses, which allow direct ionic flow through physical connections, and chemical synapses, which use neurotransmitters to transmit signals across a synaptic cleft. These mechanisms ensure efficient and diverse neuronal signaling, supporting essential nervous system functions.

The Synapse

At the synapse, the terminal of a presynaptic cell comes into close contact with the cell membrane of a postsynaptic neuron.

Synapse Types

There are two types of synapses: electrical and chemical.

Electrical

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Electrical synapses are a physical connection between two neurons. Cell membrane proteins called connexons form gap junctions between the neurons. The gap junctions form pores that allow ions to flow between neurons, so as an action potential propagates in the presynaptic neuron, the influx of sodium can move directly into the postsynaptic neuron and depolarize the cell. The response in the postsynaptic cell is almost immediate, with little to no delay between signaling in the pre- and postsynaptic neurons. Electrical synapses play an important role in the development of the nervous system but are also present throughout the developed nervous system, although in much smaller numbers that chemical synapses.

Animation 8.1. Electrical synapse ion flow. Connexons form gap junctions between neurons, allowing direct ion exchange. Sodium influx during a presynaptic action potential flows immediately through gap junctions into the postsynaptic neuron, causing rapid depolarization. ‘Electrical Synapse Ion Flow’ by Casey L. Henley (CC-BY-NC-SA). View static image of animation. View detailed alternative text.

Since the gap junctions allow diffusion of ions without any obstruction, the signal can flow bidirectionally through an electrical synapse. The electrochemical gradients will drive direction of ion flow.

Animation 8.2. Bidirectional transmission in electrical synapses. Gap junctions allow ion flow in either direction based on electrochemical gradients. An action potential in either neuron can depolarize the other, unlike chemical synapses which transmit signals in only one direction. ‘Bidirectional Electrical Synapse’ by by Casey L. Henley (CC-BY-NC-SA). View static image of animation. View detailed alternative text.

Additionally, small molecules like ATP or second messengers can also move through the gap junctions. These signaling molecules play an important role in cellular mechanisms, which we will see in a later chapter.

Animation 8.3. Small molecule transfer through electrical synapses. Gap junctions permit passage of small signaling molecules including ATP and second messengers, enabling metabolic and biochemical coupling between connected neurons in addition to electrical coupling. ‘Electrical Synapse Small Molecules” by Casey L. Henley (CC-BY-NC-SA). View static image of animation. View detailed alternative text.

Chemical

Chemical synapses are the primary synapse type in the developed nervous system and do not form physical connections between the pre- and postsynaptic neurons. Instead, a space called the synaptic cleft exists between the presynaptic terminal and the postsynaptic membrane.

At a chemical synapse, the depolarization of an action potential reaching the presynaptic terminal causes release of neurotransmitters, which act on specialized receptors located in the cell membrane of the postsynaptic neuron. The structure and function of chemical synapses make them slower than electrical synapses and permit signaling in only one direction.

Animation 8.4. Chemical synapse neurotransmitter release. An action potential in the presynaptic terminal triggers calcium influx, causing synaptic vesicles to fuse with the membrane and release neurotransmitters into the synaptic cleft. Neurotransmitters diffuse across the cleft and bind to receptors in the postsynaptic membrane. ‘Chemical Synapse Neurotransmitter Release’ by Casey L. Henley (CC-BY-NC-SA). View static image of animation. View detailed alternative text.

Synapse Location

As we discuss synaptic transmission, we will focus mainly on axodendritic synapses, in which the presynaptic terminal synapses on the dendrites of the postsynaptic cell. But synapses can also be located between the terminal and the cell body of the postsynaptic cell, called axosomatic, or even between the terminal and the axon of the postsynaptic cell, called axoaxonic.

Conclusion

Both electrical and chemical synapses play vital roles in neural communication. Electrical synapses provide rapid, bidirectional signal transfer, important in development and some adult processes. Chemical synapses, being slower and unidirectional, allow for more complex signal modulation and integration, forming the predominant communication method in the developed nervous system.

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