E5 Electrical synaptic transmission (gap junctions)

Topic

A gap junction forms direct intercellular channels, known as connexons, that allow for the passive flow of ionic current between two electrically coupled neurons. Unlike chemical synapses, gap junctions do not require chemical mediation or introduce the synaptic delay characteristic of chemical synapses; instead, they transmit ionic current directly and immediately between the two coupled neurons.

The current flowing through the gap junction between the two coupled cells is proportional to the potential difference between them—known as the junctional voltage: the greater this junctional voltage, the greater the current flowing between the two cells. Gap junction conductance can be fixed, in which case this proportionality remains constant, or it can depend on the junctional voltage itself; in the latter case, the conductance follows a sigmoidal curve—increasing slowly at low junctional voltages, rising rapidly within an intermediate range, and finally plateauing at high junctional voltages. This behavior can cause the gap junction to conduct current differently depending on the direction of flow, a phenomenon known as rectification.

Furthermore, each of the two cells coupled by the gap junction is described by the same variables used for a resting neuronal membrane, but the gap junction current is now included as an additional coupling term for each cell. This coupling term has the opposite sign in each of the two cells because, due to the conservation of charge, the charge leaving one cell through the gap junction is exactly the same as the charge entering the neighboring cell.

The gap junction coupling coefficient quantifies the fraction of a voltage disturbance occurring in one of the two cells that is passively transmitted—via the gap junction—to the neighboring cell: the higher the coupling coefficient, the greater the fraction of the voltage disturbance that reaches the neighboring cell. Taken together, it is this very gap junction—with its connexons, its current proportional to the junction voltage, and its coupling coefficient—that enables two neurons to communicate electrically with each other without the chemical mediation or the delay that characterize chemical synapses.

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