Gap junctions can couple cells directly electrically. Cell types electrically coupled via gap junctions include neurons, the pancreatic islets of Langerhans (Andreu et al, 1997) and cardiac cells (Fig. 1.). In contrast to chemical synapses, information transfer via electrical "synapses" (gap junctions) is nearly instantaneous. In chemical information flow, the synapse is the rate limiting step, because it depends on the passive diffusion of neurotransmitters across the synaptic cleft.
Fig. 1. The pacemaker potential in cardiac muscle cells spreads rapidly across the surface of the heart to generate synchronous muscle contraction. Source: Austin Community College.
Moreover, in a structure such as a brain nucleus many cells can be coupled. In other words, if all cells in a tissue contain gap junctions, all these cells are directly electrically coupled, not just those cells directly touching each other. In a way, all cells in such a tissue are in open connection with each other. This means that when one cell fires, it can theoretically activate all the connected cells to fire an action potential in near-synchrony.
Hence, gap junctions allow for synchronous activation of inter-connected neurons, both in the spatial and temporal domain.
While this can be the preferred mode of operation of firing in some neuronal structures such as the olivary nucleus (Leznik & Llinás, 2005), it can theoretically lead to trouble in case pathological synchronization is established, leading to epileptic activity (Dudek, 2002).
Whether we know little, or much of gap junctions is subjective. However, it is certainly true that way more research is available on chemical neurotransmission than on electrical transmission. Likewise, many more medications target chemical transmission than electrical neurotransmission.
References
- Andreu et al, J Physiol (1997), 498(3): 753-61
- Dudek, Epilepsy Curr (2002); 2(4): 133–6
- Leznik & Llinás, J Neurophysiol (2005); 94(4): 2447-56
