Electrotonic Coupling between Pyramidal Neurons in the Neocortex


Electrotonic couplings (i.e., electrical synapses or gap junctions) are fundamental to neuronal synchronization, and thus essential for many physiological functions and pathological disorders. Interneuron electrical synapses have been studied intensively. Although studies on electrotonic couplings between pyramidal cells (PCs) are emerging, particularly in the hippocampus, evidence is still rare in the neocortex. The electrotonic coupling of PCs in the neocortex is therefore largely unknown in terms of electrophysiological, anatomical and synaptological properties. Using multiple patch-clamp recording with differential interference contrast infrared videomicroscopy (IR-DIC) visualization, histochemical staining, and 3D- computer reconstruction, electrotonic coupling was recorded between close PCs, mainly in the medial prefrontal cortex as well as in the visual cortical regions of ferrets and rats. Compared with interneuron gap junctions, these electrotonic couplings were characterized by several special features. The recording probability of an electrotonic coupling between PCs is extremely low; but the junctional conductance is notably high, permitting the direct transmission of action potentials (APs) and even tonic firing between coupled neurons. AP firing is therefore perfectly synchronized between coupled PCs; Postjunctional APs and spikelets alternate following slight changes of membrane potentials; Postjunctional spikelets, especially at high frequencies, are summated and ultimately reach AP-threshold to fire. These properties of pyramidal electrotonic couplings largely fill the needs, as predicted by simulation studies, for the synchronization of a neuronal assembly. It is therefore suggested that the electrotonic coupling of PCs plays a unique role in the generation of neuronal synchronization in the neocortex.

Wang Y, Barakat A, Zhou H (2010)
Electrotonic Coupling between Pyramidal Neurons in the Neocortex.
PLoS ONE 5(4): e10253. doi:10.1371/journal.pone.0010253

The authors think that these connections have something to do with network synchronization, but I think that they have found the biological basis of mini-columns, for a temporal memory.

They measure Layer 5 Pyramidal cells from the PFC and Visual Cortex and find that:

  • The coupled pyramidal cells are very close together, often the soma’s are touching.
  • The connections are very strong (high conductance).
  • The cells are reciprocally connected.
  • The connection probability is 5%.

I think that these gap junctions are responsible for causing mini-columns to burst after failing to predict the feed forward input. The connection probability (5%) implies that there are about 20 mini-columns per macro-column.