Figure 1 shows a specialized myelinated motor neurone and a typical cuboidal epithelial cell.
Which of the following statements correctly identifies a specialized structure present in the neurone but completely absent in the epithelial cell, along with its correct physiological function?
Structure X (myelin sheath), which acts as an electrical insulator to allow fast saltatory conduction of action potentials.
Structure Y (axon), which acts as a site of intensive protein synthesis to generate neurotransmitter peptides locally in its cytoplasm.
Structure Z (Node of Ranvier), which contains specialized mitochondria to power the continuous active transport of calcium ions.
Structure W (dendrite), which actively secretes myelin proteins to insulate adjacent sensory neurons.