The diagram shows the structure of a myelinated motor neurone with labels P, Q, R, and S.
Which of the following correctly describes the physiological roles of the labelled structures?
P: Conducts graded electrical impulses towards the cell body; R: Concentrates voltage-gated Na+\text{Na}^+Na+ and K+\text{K}^+K+ channels, allowing depolarisation and saltatory conduction.
Q: Site where the lipid-rich myelin sheath is produced and wrapped around the axon; S: Stores calcium ions and releases them into the synaptic cleft to initiate postsynaptic depolarisation.
R: Serves as a high-resistance electrical insulator that prevents any movement of ions across the axon membrane; S: Houses postsynaptic neurotransmitter receptors that trigger ligand-gated sodium channel opening.
P: Conducts action potentials away from the cell body using saltatory conduction; Q: Lacks rough endoplasmic reticulum and ribosomes, requiring proteins to be transported from the synaptic knob.