By Stephanie M. McPherson
One hundred billion neurons fire in our brains, supporting function in systems throughout our bodies. Much of this information is transmitted via chemical synapses, which provoke neurons to release and receive chemical signals.
But a second type of synapse has been neglected in some research circles. Electrical synapses send electrical signals across channels between neurons, called gap junctions. These signals have long been believed to be less dynamic than chemical signals, and therefore less important in neural function.
In a new paper that had its genesis in the Grass Lab at the MBL, Alberto E. Pereda and Pepe Alcami argue that the dynamic contributions of electrical synapses to brain processing deserve a closer look . Their article is the May cover story in Nature Reviews Neuroscience.
“There are properties of electrical synapses that are not shared by chemical synapses,” says Pereda, Professor of Neuroscience at Albert Einstein College of Medicine. “And because [of this] people tend to underestimate electrical synapses.”
“The prevalent ‘chemical synapse-centric’ perspective on synaptic transmission precludes appreciating the powerful computational properties of electrical synapses, which are present in all nervous systems,” adds Alcami, a postdoctoral researcher at Max Planck Institute of Ornithology and Ludwig Maximilian University of Munich.
In the review, Alcami and Pereda outline some of the more interesting properties of electrical synapses. For example, chemical synapses are well-known to adjust their strength based on how frequently their connections are used. This means that certain pathways in the brain can be made stronger if they are used more often—a phenomenon tied to learning and memory. Alcami and Pereda note that, contrary to popular belief, electrical synapses also display degrees of such plasticity.
But electrical synapses are dynamic beyond their ability to adjust their strength.
“While chemical synapse transmission is episodic or intermittent, channels at electrical synapses are open all the time. They are leaking electrical currents into each other, thus dampening their ability of firing in response to some inputs, for example, depolarizing inputs [which lessen the negative potential in the neuron],” says Pereda. “However, this [dampening] effect can be alleviated if neurons are simultaneously depolarized. This property makes electrical synapses very good for detecting coincidence in the arrival of inputs to neural networks.”
This means that when inputs arrive within a few milliseconds of each other at neurons connected by electrical synapses, those neurons have become synchronized.
“Synchronization in the brain is thought to be relevant to sensory processing and cognitive functions,” says Pereda.
MBL has a long history of illuminating the roles of chemical versus electrical synapses. Early data on electrical synapses (in crayfish) made their debut in a presentation by David Potter at MBL’s Monday seminars (a.k.a ‘Monday night fights’) in 1958. Since then, MBL has provided a home for researchers such as Pereda and Alcami to investigate this highly relevant modality of neuronal communication.
The present review article was conceived in 2013 when Alcami was a Grass Fellow at MBL and Pereda was the head of the Grass Lab. The two began discussing their respective work on electrical synapses and started working on a “defense” of the oft-overlooked connections, which evolved into the Nature Reviews Neuroscience paper.
“The summer of 2013 at the MBL as a Grass fellow was one of the best experiences of my life,” says Alcami. “The scientific interactions with MBL researchers, Alberto and the other Grass fellows had a big impact on my research. I look forward to coming back to Woods Hole.”
Citation: Pepe Alcami and Alberto E. Pereda (2019) Beyond Plasticity: The Dynamic Impact of Electrical Synapses on Neural Circuits. Nature Reviews Neuroscience, DOI: 10.1038/s41583-019-0133-5
Photo captions: Group photo of the 2013 Grass Lab at the MBL, where this review article began. Alberto Pereda is front row, center, and Pepe Alcami is in an orange shirt to the right of Pereda. Credit: The Grass Foundation
Cover image of Nature Reviews Neuroscience, illustrating an electrical synapse. Credit: Nature Reviews Neuroscience