Electrical synapses

Structure

Electrical synapses are formed by two communicating cells whose neighboring cell membranes form an intercellular channel called a gap or communicating junction. Gap junctions allow direct diffusion of molecules and ions between presynaptic and postsynaptic cells. On a molecular level, gap junctions consist of a protein complex, referred to as a connexon.

Each connexon is a hexamer, meaning that it consists of six subunits. The subunits of a connexon are transmembrane proteins called connexins. In each connexon, the connexins are arranged circularly, forming a tubular structure with a hydrophilic pore that spans the whole thickness of a cell membrane.

A pair of connexons, one within the presynaptic membrane and the other within the postsynaptic membrane, connect to form a gap junction, establishing a direct connection between the cells.

There are two configurations of gap junctions, an open pore and a closed pore. Gap junction channels are sensitive to voltage changes and can change their conformation to "gate" (open or close) the channel.

Functions

The main function of electrical synapses is to enable rapid exchange and signal transmission between cells. They enable synchronous and instantaneous responses to stimuli, which is crucial in tissues such as cardiac muscle.

Such rapid communication between the cells is possible because gap junctions are typically non-selective pores. Put simply, they provide continuity between the intracellular environments of the cells that they connect. This direct communication between the presynaptic and postsynaptic cells allows for practically no delay in the transmission of information between them. This immediate synaptic transmission is crucial for the synchronized response of a group of postsynaptic cells. Cardiac muscle contractions, glial cell function, and early neuronal development all heavily rely on the operation of gap junctions.

Since the diameter of the pores in gap junctions typically ranges from 1.2 to 2 nm they allow the passage of ions as well as small to medium signaling molecules, such low-molecular-weight proteins and certain neurotransmitters. The transmission through gap junctions is predominantly bidirectional. A notable exception to this rule is the conduction of action potentials, which is always unidirectional. However, the unidirectional nature of action potential propagation in electrical synapses is primarily regulated by the refractory period of the postsynaptic membrane, rather than the electrical synapse itself. When the electrical potential generated in the presynaptic cell surpasses the threshold potential of the postsynaptic membrane, it triggers an action potential in the postsynaptic cell. So, synaptic transmission in electrical synapses is passively regulated.

Although generally being nonselective and bidirectional, connexons can exhibit more nuanced functional properties. As they are encoded by a family of 21 genes, their genetic diversity accounts for connexons exhibiting different functional properties, including size and charge selectivity and pore conductance. Certain connexins can offer selectivity to the pore and regulate the direction of ion flow.

Electrical vs. chemical synapses

When comparing electrical and chemical synapses, fundamental differences both in structure and function can be defined:

• In electrical synapses, signal transmission occurs through ion currents, whereas in chemical synapses it relies on molecules called neurotransmitters that serve as chemical messengers.
• Chemical synapses demonstrate high specificity due to specialized postsynaptic receptors for particular neurotransmitters, in contrast to electrical synapses that are nonspecific.
• Electrical synapses, as direct continuations of the cytoplasm between adjacent cells involved in the synapse, enable instantaneous signal transmission, while chemical synapses require time to generate an action potential in the postsynaptic cell.
• Within the synaptic cleft of a chemical synapse, neurotransmitters are released from the presynaptic cell and bind to receptors on the postsynaptic cell membrane, calling for unidirectional signal transmission. In contrast, electrical synapses lack any similar mechanism, allowing ion currents to flow bidirectionally.

Sites of electrical synapses

While important, electrical synapses are relatively scarce in the human body compared to the predominating chemical synapses. Gap junctions are present in human cardiac myocytes, as well as in smooth muscle tissue. Cardiac cells need to operate as one, in order for the heart to contract rhythmically and uniformly.

Electrical synapses play a vital role in interconnected cells of the nervous system that require synchronous and instantaneous responses to stimuli.

For example, neurons in the respiratory nuclei of the brainstem utilize electrical synapses to generate rhythmic electrical signals that regulate one's breathing. Similarly, electrical synapses in the hypothalamus connect neurosecretory cells of the same type, facilitating simultaneous secretion of hormones, such as antidiuretic hormone (ADH) or oxytocin (OT), into the circulation.

Glial cells in the nervous system also possess numerous gap junctions, enabling synchronous action and metabolism. The extensive signaling networks formed by gap junction channels in glial cells allow for the establishment of a functional syncytium, which may play a critical role in their maturation.