Two Biological Properties
The first key to understanding the power of the human brain comes when the result of two basic biological properties of neuron are considered.
- Each neuron has an electrical threshold. When the neural body receives electrical inputs, that when summed exceeds its threshold, the neuron sends a signal down its axon to all receiving neurons.
- The All-or-None principle. When the neuron fires, it always fires the same signal. There is no gradation of electrical signal. It’s a full signal or there is no signal.
For simplicity, this discussion assumes all inputs have the same weighting across the synaptic gaps. That does not affect the existence of the phenomenon, only the specific values for which it occurs.
These two properties combine to imply that a neuron fires with the same output under different conditions. The neuron treats difference inputs as the same. Consider a neuron which has 100 inputs (the typical cortical neuron has about 10 thousand connections). With the cortical neuron electrical charge exceeds 85 mV, it fires. This can happen due to different inputs. In the example figure, 12 excitatory and 4 inhibitory inputs cause the neuron to signal downstream that it has been activated.
As you can well imagine, these particular 12 excitatory and 4 inhibitory inputs are not the only combinations of inputs that cause the neuron’s threshold to be exceeded. Since actual neurons have 10 thousand interconnections, 7 thousand excitatory and 3 thousand inhibitory, there are a vast multitude of combinations which can cause the neuron to fire.
Hebb’s LawHowever with Hebb’s Law (neurons that fire together will fire together more easily in the future) in conjunction with a gross stability in our environment, inputs cluster in their firing.
And of the input sets than exceed the 85 mV neural threshold, small changes between the input sets are not passed to downstream neurons, but all present the same effect downstream.
In this way, the neuron’s threshold and its all-or-none signaling results in almost matches being treated equally downstream in the brain. The Almost Gate lets many inputs over its barrier. Henceforth they are treated the same.