The Spike
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The Spike

Your Cortex Contains 17 Billion Computers

Neural networks of neural networks

Credit: brentsview under CC BY-NC 2.0
A pyramidal cell — squashed into two dimensions. The black blob in the middle is the neuron’s body; the rest of the wires are its dendrites. Credit: Alain Dexteshe /
The sum-up-and-spit-out-spike model of a neuron. If enough inputs arrive at the same time — enough to cross a threshold (grey circle) — the neuron spits out a spike.
The two coloured blobs are two inputs to a single bit of dendrite. When they are activated on their own, they each create the responses shown, where the grey arrow indicates the activation of that input (response here means “change in voltage”). When activated together, the response is larger (solid line) than the sum of their individual responses (dotted line).
Size of the response in a single branch of a dendrite to increasing numbers of active inputs. The local “spike” is the jump from almost no response to a large response.
Deja vu. A single dendritic branch acts as a little device for summing up inputs and giving an output if enough inputs were active at the same time. And the transformation from input to output (the grey circle) is just the graph we’ve already seen above, which gives the size of the response from the number of inputs.
Left: A single neuron has many dendritic branches (above and below its body). Right: so it is a collection of non-linear summation devices (yellow boxes, and nonlinear outputs), that all output to the body of the neuron (grey box), where they are summed together. Look familiar?



The science of the brain, from the scientists of the brain

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Mark Humphries

Book Author

Theorist & neuroscientist. Writing at the intersection of neurons, data science, and AI. Author of “The Spike: An Epic Journey Through the Brain in 2.1 Seconds”