The Spike: An Epic Journey Through the Brain in 2.1 Seconds

by Mark Humphries

In tracing this feed-forward circuit, from layer four up to layers two and three, and back down to layers five and six, we’ve run into all three types of pyramidal neurons in cortex.9 All three types use glutamate as their molecules, so all excite neurons they connect to; all connect to neurons of the same type within their own layer, but they are separated by where they send the long branch of their axon. In layer five we watch some of the cloned spikes run into pyramidal tract neurons that send a long branch of their axon all the way through the brain, down to the brain stem, and some onward to the spinal cord. In layer six other cloned spikes jump gaps onto cortico-thalamic neurons, their long branch dropping down out of the cortex into the tiny nugget of midbrain called the thalamus, whose own neurons send their axons all over the cortex, creating complex feedback loops. And in all layers (except layer one) are the cortico-cortical neurons, whose long branch of axon connects regions within the cortex, carrying spikes far across the cortex on this left side of the brain, and on to the cortex on the other side of the brain. Neurons just like the one we’ve been dispatched from.