Neurons and the Nerve Net

How do neurons work? Max Bennett explains in A Brief History of Intelligence. Edgar Adrian made 3 key findings on the topic in the 1920’s, for which he would win the Nobel Prize.

 

First, he found that neurons don’t send electric signals continuously. Rather, they fire all-or-nothing responses. On or Off, nothing in between. This raised a problematic question. Our senses can differentiate between levels of volume, strength of smells, amount of light etc. How could a simple On/Off-only mechanism convey shades?

 

To answer that question, Adrian took a muscle from the neck of a dead frog and attached a recording device to a single stretch-sensing neuron in that muscle. Then he did his experiment: how would the neuron convey different weights? Here is what his recording device noted: 

The strength of the spike (On) did not vary with the weight. But the frequency of the spikes was proportional to the weight – higher the weight, higher the frequency of the spikes. This was Adrian’s second finding, aka rate coding. What is being conveyed varies (sound level, weight, luminosity level etc) but the principle was universal categories of information.

 

But that still didn’t answer the question entirely. Neurons can fire a maximum of 500 spikes per second. This is not sufficient to cover all possible ranges of values. This is called the squishing problem. How do neurons solve it?

“Neurons do not have a fixed relationship between natural variables and firing rates.”

Instead, neurons adjust their firing rate based on the environment. Say, you attached a weight and the neuron fired 100 times per second. Remove the weight and soon after, attach the same weight. This time, the neuron only fires 80 times. The more times you repeat this, the fewer the times the neuron fires to convey the same information (weight in this case). 

The term for this is adaptation, and this was Adrian’s third discovery. By allowing the same firing rate to mean different values (based on prior experience), neurons solve the squishing problem.

 

Next, scientists found tiny gaps between neurons, called synapses. Spikes in the input neuron trigger the release of chemicals called neurotransmitters.

“While neural communication within a neuron is electrical, across neurons, it is chemical.”

 

In the 1950’s, John Eccles discovered that neurons come in two flavours – excitatory neurons and inhibitory neurons. The former trigger spikes in other neurons while the latter suppress spikes in other neurons.

 

These 4 – all-or-nothing spikes, rate coding, adaptation and chemical synapses with excitatory and inhibitory neurotransmitters – are universal across all animals, even those without a brain. Animals without a brain have nerves arranged in what is called a nerve net.

 

Once this was in place, evolution and the eternal arms race would transform the nerve net into the brain. But that’s the topic of another blog.