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

Electrical synapses outnumber chemical synapses in the developing nervous system

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 than chemical synapses.

Animation 8.1. Connexon proteins bridge the space between adjacent cell membranes to create open gap junction channels. This structural connection allows sodium ions to travel directly from the presynaptic neuron into the postsynaptic neuron, ensuring instantaneous signal transmission. ‘Electrical Synapse Ion Flow’ by Casey L. Henley (CC-BY-NC-SA). 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 determine the direction of ion flow.

Animation 8.2. Because gap junction channels are structurally identical from both sides, they lack a fixed direction. Sodium ions can pass through the pores in either direction, with the net movement determined entirely by the prevailing electrochemical gradient. ‘Bidirectional Electrical Synapse” by Casey L. Henley (CC-BY-NC-SA). 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 detailed alternative text.

Chemical

Chemical synapses outnumber electrical synapses in the fully developed nervous system

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. At a chemical synapse, an action potential reaching the presynaptic terminal triggers synaptic vesicles to fuse with the membrane and release neurotransmitters into the synaptic cleft. The 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 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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