How are sheath dimensions affected by axon caliber and internode length?

The significance of internode length for sheath thickness was analyzed by electron microscopic morphometry in isolated internodes from the human roots C3 and S1. These populations differ in length but have similar caliber. The amount of myelin per internode was in linear relation with the product of axon circumference and the length of the ensheathed axon segment. Neither one of these two vectors was in a statistically significant relationship with sheath thickness. The ratio between the axolemmal area covered by the Schwann cell and the area of the myelin leaflet averaged 1:163 for human root fibers. It was 1:177 for previous data from canine sciatic nerve. The proportions of an internode were defined by an 1/d-quotient, expressing its length as multiples of axon diameter. Relative sheath thickness (g-ratio: diameter axon/diameter fiber) relates inversely with the 1/d-quotient. For a given axon caliber, the g-ratio (sheath thickness) decreases by 0.006 for every 10.0 increase in 1/d-quotient (relative internode length). Thus, internodes relatively long for axon caliber possess slightly thicker sheaths than internodes short for axon caliber. Axon caliber and relative internode length emerge as the two key factors determining the amount of myelin in a sheath.