What is a Dendritic Spike?
Channel: Numenta
Date: 2017-12-01

A pyramidal neuron spikes in order to communicate to other neurons. This is called an 'action potential'.

But there's another spike happening inside a neuron called a "Dendritic Spike" and it never even leaves the cell. But it makes a huge difference to the cell's firing behavior.

Pyramidal neurons can have hundreds of dendrites and thousands of synapses. These synapses are connections from other neurons. They are the cell's input. Activity at these synapses increases the local voltage of the cell. The cell tries to keep a constant voltage, so these local pockets of high voltage dissipate quickly. 

Proximal dendrites are close to the cell body. A small amount of synaptic stimulus dissipates too quickly to affect the cell, but if enough distal synapses fire within a short period, this can cause the voltage at the cell body to breach a threshold and cause a chain reaction called an "Action Potential" or spike. The cell has now fired and communicated this state to whatever it is connected to.

Distal dendrites are too far away to affect the voltage at the cell body enough to cause the cell to fire. For years, we didn't understand Why Neurons Had these Thousands of Synapses, but we now know these synapses cause Dendritic Spikes. Much like an action potential, if a local distal segment receives enough synaptic activity, it can again cause a high voltage pulse to travel down the dendrite to the cell body. This is not powerful enough to cause an action potential, but the voltage increase at the cell body has a lasting effect.

The cell is now primed to fire. For some period of time, it will take less proximal stimulus for it to reach action potential.

In HTM, we call this a predictive state of the neuron. We explain its importance in our 2016 paper "Why Neurons have Thousands of Synapses". (https://numenta.com/papers/why-neurons-have-thousands-of-synapses-theory-of-sequence-memory-in-neocortex/)

Collectively, your neurons are constantly modeling the state of the world and predicting multiple possible futures using this and other mechanisms.

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