As you read in Chapter 4 of Biopsychology (9th Edition), neurons can send messages to each other at chemical synapses, via neurotransmitters, and at electrical synapses through gap junctions. But can neurons communicate in other ways?
Recent research suggests that neurons transmit electronic signals through the extracellular space, and that such signals can affect the functioning of nearby cells. More specifically, it appears to be the case that the electrical current from an action potential in one neuron can spread through the extracellular space and affect the membrane voltage of nearby cells. This form of cell-to-cell signalling has been termed ‘Ephaptic Coupling.’
Neuroscientists have long recognized the possibility that the electrical current generated by one neuron (during the production of an action potential, for example) might spread through the extracellular fluid (which is an electrically conductive medium) to adjacent cells (<insert ref>). However, until very recently, there was no convincing evidence that such current spread could significantly affect the functioning of other cells. X et al. showed that such ephaptic communication is not only possible, but it also has functional consequences on neural functioning.
Using an intricate experimental setup (see the image below–every one of those navy blue and pink triangles represents one recording electrode; the green blobs are neuron cell bodies), Anastassiou et al. (2011) showed that electrical fields generated by the firing of one cortical neuron could spread through extracellular space and entrain the firing of nearby cortical neurons. This could be one mechanism by which groups of cells come to exhibit synchronous firing.
It appears that the functional ephaptic coupling demonstrated by Anastassiou et al (2011) is not limited to cortical neurons. For example, a more recent study by Su et al. (2012) showed that fruitfly olfactory receptor neurons (ORNs) communicate via ephaptic coupling. The following figure , from their paper, depicts inhibitory ephaptic coupling when two cells are presented with two scents simultaneously. In short, the sustained response of one ORN to one scent can be temporarily inhibited by the activation of a neighbouring ORN through ephaptic coupling.
References and Additional Readings
Shimizu, K., and Stopfer, M. (2012). Olfaction: Intimate neuronal whispers. Nature, 492, 44–45. DOI: 10.1038/nature11757
Su, C-Y., Menuz, K., Reisert, J., & Carlson, J. R. (2012). Non-synaptic inhibition between grouped neurons in an olfactory circuit. Nature, 492, 66–71. DOI: 10.1038/nature11712