Quantitative studies on the maturation of central and peripheral parts of individual ventral motoneuron axons. II. Internodal length.

The lengths of internodes on peripheral and central stretches of the same ventral motoneuron axons were measured in 6 day rats. The former were more than twice as long as the latter. The distribution of peripheral internodal lengths was broader, and was shifted to the right, relative to that of central internodes. Short internodes were present centrally and peripherally throughout the early period of rapid myelination occurring immediately after birth. All such internodes appeared normal centrally, as did a proportion found peripherally. Some peripheral examples appeared to be degenerating. Evidence is presented suggesting that some short internodes, both peripherally and centrally, increase in length to achieve a normal appearance. Many central stretches of central motoneuron axons which were myelinated at some levels, at other levels possessed unmyelinated segments covered by numberous short glial processes. Such bare stretches commonly occurred immediately deep to the cord surface. The lengths of the unmyelinated stretches decreased as age advanced, perhpas due to the extension of adjacent myelinated segments along them.