If receptors in muscle fibers are thought of as locks, the keys are:

If the receptors in muscle fibers are thought of as locks, the keys are NEUROTRANSMITTERS.
Receptors are large protein molecules which are located on the surface of neuron. They have to be opened before a neuron is activated, that is why they are considered as lock. The neurotransmitter molecules are considered as the key, because they bind to the neuron and activate neural firing.

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