Compare conduction rates in nerve-fibre data, then explain how a larger axon diameter or thicker myelin sheath relates to faster impulse transmission.
Earthworms (Lumbricus terrestris) have a nerve cord on the ventral (underside) side of their body. It contains three nerve fibres (axons), each with a very large diameter, which extend along the whole length of the nerve cord. There is one medial giant fibre (MGF) and two lateral giant fibres (LGFs). The diameter of the MGF is 1.4 times larger than the LGF.
Scientists developed a non-invasive method to record giant nerve fibre activity. The response from the MGF was measured by the electrical activity produced when touching the front segments of the earthworm while the LGF was measured when touching the back segments.
The table shows the mean conduction rates, standard deviation (SD) and number of individual measurements of MGF and LGF action potentials in each earthworm. At the end of the table, the mean and standard error (SE) for 9 of the earthworms are shown.
Calculate the ratio of the mean conduction rate in the MGF to the mean conduction rate in the LGF.
Suggest one possible reason that the conduction rate in the MGF is much faster than in the LGF.