What Is Neural Backpropagation?

Neural backpropagation is the name given to the phenomenon of an impulse moving backward through a neural circuit. While action potentials usually travel from the cell — beginning specifically at the point of the axon hillock — down the axon to the terminal boutons that form synapses with receiving cells, a backpropagating action potential actually moves backward by the diffusion of incoming ions, causing voltage-gated ion channels to open up the axon instead of down it. Usually, neural backpropagation has a short range of effect, but it has the potential to travel through an entire neural circuit.

An action potential in a neuron is initiated at the axon hillock, which lies where the axon meets the soma of a neural cell. Most neurons have one single axon that can bifurcate many times. This neurite is the process that sends signals from the cell, while dendrites, which are the other neurites on a neuron, are commonly processes that receive signals. Neural backpropagation is regulated by ion channels in the axon and on the cell body.

An axon functions in its role of conducting action potentials from the axon hillock to the end points of the axon, called terminal boutons, by opening channels in the axonal membrane that allow positively charged ions into the cell, depolarizing it and causing voltage-gated channels to open. Voltage-gated channels allow further positively charged ions into the cell, like calcium and potassium. When a cell loses its resting potential of -70mV and becomes depolarized due to the positive charges of the incoming ions, it "fires" and releases neurotransmitter-filled vesicles from terminal boutons at the end of an axon.

Signal propagation functions as ion channels along an axon cause other nearby channels to open, but this signal propagation can move in the reverse direction and when it does, it is referred to as neural backpropagation. This process occurs when an action potential is initiated at the axon hillock and, while it might proceed down the axon as usual, it also conducts a signal in the opposite direction, causing the cell body to depolarize, including synapses and nearby dendrite segments. When a dendritic segment is depolarized, the post-synaptic densities located within that region respond differently to incoming signals from other neurons. Some possible consequences of neural backpropagation include phenomena like dendro-dendritic inhibition and a membrane potential modification, which can change cellular firing properties.

Synaptic plasticity like long-term potentiation (LTP) and long-term depression (LTD) are associated with neural backpropagation because a back-propagating signal modifies incoming signals. While the concept might seem elementary, the notion of changing future behavior based on past experience is a possible definition of learning. In a way, therefore, neural backpropagation might be said to allow individual cells to "learn" on a molecular level. Neural backpropagation is often seen in the neocortex, hippocampus and other brain regions often associated with memory, learning or a high degree of neural plasticity.