Sparkling Chandeliers of Cortex

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What’s in a brain? So many cells; such great variety! When it comes to neurons, they can be broadly categorized as either Excitatory (E) or Inhibitory (I). There are far more E neurons than I ones. Most estimates put the cortical E:I ratio at 4:1. Leading the excitatory charge are Pyramidal cells. Their dendrites are covered in spiny protruded synapses, and their axons encourage post-synaptic partners to generate and send out action potentials of their own.

Inhibitory cells come in many forms. The cortical volume in this post focuses on one, shown in yellow. Does its shape remind you of anything? Perhaps a light fixture in a fancy dining room?  It conjured this image for the researchers who first spotted it, for this one is dubbed a Chandelier Cell.

 

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Chandelier Cells are beautiful, alas, they crush the action potential dreams of the hundreds of pyramidal neurons that they synapse onto. A Chandelier cell’s intent, like all inhibitory cells, is to shut others down.

Unlike the axons of pyramidal neurons that can project to faraway regions of the brain, Chandelier axons stay within a local region of cortex and connect to the initial axon segment of pyramidal neurons. A single Chandelier cell can connect to hundreds of other cells. The diagram above shows one circuit configuration – there are many. In this case, it receives presynaptic input from pyramidal cells in Layers 3 and 5.

Chandeliers have characteristic synaptic configurations called cartridges sprouting from their axons. Cartridges consist of two parallel columns of synapses that strongly inhibit the cells they connect to.

In Neo, we’ll map Chandeliers, Pyramidals, and many other types of cortical cells. It’s possible that we’ll discover previously uncharted circuitry and model new modes of action for these little sparklers.

Keep an eye on the Eyewire blog for more overviews of cortical neuronal diversity!

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Illustration by Daniela Gamba. Produced by Amy Sterling.

For more information on Chandelier Cells, see this article in Frontiers in Neuroscience