Which inhibitory neurotransmitter is primarily involved in regulating muscle tone and is active in the spinal cord?

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The key idea is fast inhibitory control of motor neurons in the spinal cord. Glycine serves as the primary inhibitory transmitter in the spinal cord by acting on glycine receptors to open chloride channels, hyperpolarizing neurons and dampening motor-neuron activity. This glycinergic inhibition is what helps regulate muscle tone and modulate reflexes, keeping motor output balanced. While GABA is also inhibitory and important in the brain (and to some extent in the spinal cord), the direct, fast control of spinal motor neurons and tone is most characteristically glycinergic. Glutamate is the main excitatory neurotransmitter, driving activation, and norepinephrine is more modulatory, not the principal inhibitory regulator of spinal muscle tone. Thus glycine best explains the described role.