Difference between revisions of "Resources:Two Forms of Electrical Transmission Between Neurons - en"

It has been argued that the function of the nervous system is to support movement and that it evolved because of its usefulness to organisms in navigating their environment (Llinás, 2001<ref>Llinás, R. (2001). ''I of the Vortex: from Neurons to Self.'' Cambridge: MIT Press. Google Scholar</ref>). Early observations established that nerves were required for muscle contraction. However, the mechanism underlying this interaction was unknown. An old, predominant, idea embraced by Rene Descartes was that muscle contraction resulted from the action of “animal spirits” running through hollow nerves (Piccolino, 1998<ref name=":0">Piccolino, M. (1998). Animal electricity and the birth of electrophysiology: the legacy of Luigi Galvani. ''Brain Res. Bull.'' 46, 381–407. doi: 10.1016/s0361-9230(98)00026-4</ref>; Finger, 2005<ref name=":1">Finger, S. (2005). ''Minds Behind the Brain.'' USA: Oxford University Press.</ref>). This and other speculative ideas were later disproved, leading to the consideration of alternative mechanisms. One of them was electricity (Franklin, 1751<ref>Franklin, B. (1751). ''Experiments and Observations on Electricity Made at Philadelphia in America.'' Printed and sold by E. Cave, at St. John’s Gate. doi: 10.5479/sil.211644.39088000092304</ref>). The use of electricity for therapeutic purposes was popular in the second part of the 18th century, and electricity was capable of eliciting muscle contraction. In addition, because of its high travel velocity, electricity was ideally suited to be the agent responsible for nerve action, as some hypothesized (Finger, 2005<ref name=":1" />). Furthermore, experimental evidence showed that certain fish were capable of generating electricity. All this preceding work and speculations paved the way to the studies conducted by Galvani (1791<ref>Galvani, L. (1791). ''Aloysii Galvani De viribus electricitatis in motu musculari commentarius.'' Bononiae: Ex Typographia Instituti Scientiarum, 1791. doi: 10.5479/sil.324681.39088000932442</ref>) which demonstrated that nerves and muscles generate electricity (“bioelectricity”) and, therefore, that electricity was the mysterious fluid or “animal spirit” responsible for nerve conduction and muscle contraction (Piccolino, 1998<ref name=":0" />; Finger, 2005<ref name=":1" />). We know now that these electrical currents result from the movement of charged ions across the cellular membrane following their electrochemical gradient (Hodgkin and Huxley, 1952<ref>Hodgkin, A. L., and Huxley, A. F. (1952). Currents carried by sodium and potassium ions through the membrane of the giant axon of loligo. ''J. Physiol.'' 116, 449–472. doi: 10.1113/jphysiol.1952.sp004717</ref>; Armstrong, 2007<ref>Armstrong, C. M. (2007). Life among the axons. ''Annu. Rev. Physiol.'' 69, 1–18. doi: 10.1146/annurev.physiol.69.120205.124448</ref>). Galvani’s seminal studies led to the foundation of electrophysiology and to the discovery that brain function and, hence, animal behavior, depends upon electrophysiological computations, the only operational mode fast enough to support the required time frame of decision making by neural circuits. In other words, as emphasized by Llinás, electricity makes us who we are (Sohn, 2003<ref>Sohn, E. (2003). “Electricity’s spark of life,” in ''Sci. News Students 1.'' Available online at: www.sciencenewsforstudents.org</ref>).

The discovery that the brain is constructed from networks of individual cells that generate electrical signals raised the question of how electrical currents “jump” from one cell to another. The most hotly debated question in Neuroscience during the 20th century was whether synaptic transmission, which is the currency of the brain, is mediated electrically or chemically. In fact, this might have been the major point of dispute in the biological sciences in that era, with advocates on both sides avidly defending their positions with data—based and theoretical models. Each side advanced its favored mechanism on the basis of its assumed advantages for the operation of neural networks in the central nervous system (CNS). Thus, a great deal of effort was devoted to determining whether there was a delay of 1–2 ms between a presynaptic action potential and the start of a postsynaptic response (chemical) or not (electrical), and to the corresponding functional consequences of these alternatives. In this review article, we briefly describe the critical elements of the debate between electrical and chemical modes of transmission, which seemed to tilt strongly in favor of the latter once it emerged that synaptic inhibition in the spinal cord was mediated by an ionic conductance change. This was particularly compelling in view of the difficulties in determining a satisfying mechanism for electrical inhibition. However, in recent years, electrical transmission has regained recognition and relevance. Rather than occurring via a single mechanism, electrical transmission operates in two ways: via pathways of low resistance between neurons (gap junctions) or as a consequence of extracellular electric fields generated by neuronal activity. Thus, we focus not only on the differences between these modes of operation, but also on the concept they share some operational characteristics. Far from providing an extensive review on the topic, we center here on a number of classic and recent examples that we believe illustrate these properties.