Neurons and the All-or-None Principle Explained Simply
A nerve cell (also called a neuron) is specialized for passing along electrical signals - for example, the command from the brain to a muscle to contract. To do that, it has a very specific shape.
The electrical impulse travels from the dendrites through the cell body and along the axon to the axon terminals - there, the signal is passed on to the muscle (or the next neuron).
đź’¬ Mia asks Grandpa Theo
How does a nerve actually send the command "contract the muscle" all the way from the brain to the muscle?
As a tiny electrical pulse called an action potential. It starts in the cell body, races along the axon to the axon terminals, and jumps across to the muscle there.
Can a nerve fire just a little bit if the stimulus is weak?
No - that's exactly the all-or-none principle. Only once a stimulus crosses a certain threshold do many channels in the cell membrane suddenly open, producing a full action potential - always the same size. Below the threshold, nothing happens at all.
But then how can I squeeze with different amounts of force, if every impulse is the same size?
Through frequency, not size: a stronger stimulus doesn't trigger a bigger impulse, it triggers more impulses per second. And in the muscle, more individual muscle fibers get activated at the same time - that's how you get more overall force, even though each single impulse and each single fiber still follows the all-or-none principle on its own.
The myelin sheath insulates the axon much like the plastic coating on an electrical cable. That lets the signal jump from node to node instead of creeping along step by step. Thanks to this, some neurons reach speeds of up to 120 meters per second - faster than a Formula 1 car. Neurons without a myelin sheath, on the other hand, manage only about 1 meter per second, barely faster than walking pace.
The all-or-none principle applies to single neurons and single muscle fibers - it's the frequency of the impulses and the number of fibers activated at once that creates the graded force we actually feel in everyday life.
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