_________ serves as an example of a rapidly acting neurotransmitter?
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Answer: D. Glycine
Explanation
Glycine functions as an inhibitory amino acid neurotransmitter mediating fast synaptic transmission. It opens chloride channels rapidly in the central nervous system to hyperpolarize postsynaptic neurons. Option D is correct because glycine acts as a fast-acting neurotransmitter in specific pathways. Neurotransmitters are endogenous chemical messengers transmitting signals across chemical synapses. Fast neurotransmitters act via ligand-gated ion channels for immediate postsynaptic membrane effects. Glycine receptors are heavily concentrated within the spinal cord and lower brainstem structures.
Glycine plays a vital role in coordinating motor reflexes and sensory processing within the spinal cord. Strychnine acts as a potent glycine receptor antagonist, causing severe convulsions and muscle spasms. Disruption of glycinergic transmission leads to exaggerated startle disease conditions in humans. Somatostatin is a neuropeptide hormone that acts more slowly via G-protein coupled receptors. Calcitonin functions primarily as a calcium-regulating peptide hormone rather than a fast neurotransmitter. Substance P is a slower-acting neuropeptide involved mainly in pain signal transmission pathways.
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