of all back prop demands synaptic symmetry in the forward and backward paths right so basically if you have a neuron and it has output to another neuron what you need to be able to do is to pass back information along that neuron so it kind of has to be a symmetric connection idea of the forward and the backward pass and these need to be exact right and this is just not if you know how neurons are structured they have kind of input dendrites and then there's this accent act action potential and along the axon the signal travels