Neurons are the cells of the nervous system. Many, many neurons make up a nerve or a tract.
Neurons have dendrites, which bring signals to the cell body, and axons, which bring electrical signals away from the cell body.
The cell body integrates all of the inputs of all the different neurons around it. At the end of the axon are axon terminals that connect to the dendrites of the next neuron or to a muscle.
When a neuron is not sending a signal, it is at rest. At rest, the inside of the neuron is negative relative to the outside.
When new information is transmitted by a dendrite, channels open up and positive ions begin to move in, depolarizing (the inside of the cell gets more positive) the neuron. When it reaches a certain threshold, a neuron fires an action potential.
An action potential does not depend on the strength of the input and no action potential will fire if the threshold isn’t met. Therefore, the neuron either reaches threshold or it doesn’t. This is called the all-or-none principle.
It’s sort of like a light switch. It doesn’t matter how much force you use to turn the light switch on or off. As long as you use a specific amount of force, the light switch turns on or off. More force than required doesn’t turn the light “more on.”
The complication: there are an infinite number of combinations of input signals into a neuron that can achieve an output. Neurons create a complex, integrated network.¹
The number of dendrites, neurons, and connections vary from individual to individual and can change at different points in life due to environmental interactions, adding further to the complexity.²