14.12:
Electrical Synapses
An electrical synapse is like a doorway that facilitates the passive flow of ions and small molecules from the presynaptic to the postsynaptic cell.
Unlike a chemical synapse, the pre- and postsynaptic cells are extremely close to each other, physically connected by gap junctions. In vertebrates, these junctions are made up of channel-forming proteins, connexons, consisting of precisely aligned connexins. These paired channels form a pore, connecting the cytoplasm of both cells.
Ions from the presynaptic cell pass through the pore into the postsynaptic cell, resulting in instantaneous electrical signal transmission. In contrast, the chemical synapses show a characteristic delay due to the involvement of chemical messengers.
Electrical synapses are seen in the heart, intestinal smooth muscle, retina, brain, and spinal cord to aid in rapid and coordinated responses.
For instance, the pacemaker cells initiate the action potential at the hearts’ sinoatrial node, which instantaneously propagates to the adjacent cells coupled with electrical synapses. This rapid cell-to-cell impulse transmission allows for coordinated contraction of cardiac muscles, generating a heartbeat.
14.12:
Electrical Synapses
Electrical synapses found in all nervous systems play important and unique roles. In these synapses, the presynaptic and postsynaptic membranes are very close together (3.5 nm) and are actually physically connected by channel proteins forming gap junctions.
Gap junctions allow the current to pass directly from one cell to the next. In contrast, in the chemical synapse, the neurotransmitters carry the information through the synaptic cleft from one neuron to the next. They consist of two hexameric connexin hemichannels or connexons contributed by each of the adjacent cells. These hemichannels make contact between the two cell membranes by forming a continuous bridge between the cytoplasm of the two connecting cells. The opening of the connexon pore is more like the shutter of a camera where the connexins in the hemichannel rotate slightly with respect to one another for ion passage. In addition to the ions, other molecules, such as ATP, can also diffuse through the large gap junction pores.
Signaling in electrical synapses is virtually instantaneous. Some electrical synapses are bidirectional too. Electrical synapses are not easily blocked and are important for synchronizing the electrical activity of a group of neurons. For example, electrical synapses in the thalamus are thought to regulate slow-wave sleep, and disruption of these synapses can cause seizures. In the intestinal smooth muscle cells, electrical synapses provide electrical rhythmicity contributing to peristaltic intestinal activity vital for normal functioning of the gastrointestinal tract.
Leitura Sugerida
- Openstax, Biology 2e, Section 35.2 How Neurons Communicate.
- Pereda, Alberto E. “Electrical synapses and their functional interactions with chemical synapses.” Nature Reviews Neuroscience 15, no. 4 (2014): 250-263.
- Tse, Gary, Eric Tsz Him Lai, Jie Ming Yeo, Vivian Tse, and Sunny Hei Wong. “Mechanisms of electrical activation and conduction in the gastrointestinal system: lessons from cardiac electrophysiology.” Frontiers in Physiology 7 (2016): 182.