> From a strictly mechanical point of view, the brain is a syncytium. While the electrical component of a spike that invades the synapse is chemically transformed into a vesicle message, the mechanical portion of the spike is transformed and propagated into and across the synapse in a way dictated by the mechanical impedance of the local membrane, cytoskeleton, and synaptic matrix.

I always hear that the brain doesn't do back-propagation the way that NNs are trained, but why are we so sure of that? This reads to me like a potential communication channel for doing backprop.