The development of alpha and gamma motoneuron fibres in the rat. II. A comparative ultrastructural study of their central and peripheral myelination
Fraher, J.P.; Kaar, G.F.
Journal of Anatomy 141: 89-103
1985
ISSN/ISBN: 0021-8782 PMID: 4077723 Document Number: 257161
The abnodal myelin sheaths of the internodes immediately central and peripheral to the transitional node possess a decremental segment over which sheath thickness gradually decreases in the direction of the paranode. This may represent a sustained morphological immaturity of the sheath. .alpha.- and .gamma.-fiber groups have different sheath thickness to axon circumference relationships at each age during development and at maturity. .gamma.-Fibers have relatively thicker sheaths than .alpha.-fibers. In both groups the relationship is different for central and peripheral fiber segments and also changes during maturation. It is therefore not permissable to make inferences from the adult relationship regarding the relationship during development. Changes in the relationship follow similar patterns for central and peripheral fiber segments, suggesting that the control mechanisms are closely linked in both locations. Sheath thickness growth both peripherally and centrally lags behind axon caliber growth for .alpha.-fibers. The 2 keep pace for .gamma.-fibers. The strength of the correlation between sheath thickness and axon circumference changes little with maturation in either group. The morphological differences between central and peripheral segments of ventral motoneuron fibers suggest that conduction velocity increases when the impulse enters the peripheral nervous system.