The development of alpha and gamma motoneuron fibres in the rat. II. A comparative ultrastructural study of their central and peripheral myelination.

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 fibre groups have different sheath thickness to axon circumference relationships at each age during development and at maturity. Gamma fibres have relatively thicker sheaths than alpha fibres. In both groups the relationship is different for central and peripheral fibre segments and also changes during maturation. It is therefore not permissible to make inferences from the adult relationship regarding the relationship during development. Changes in the relationship follow similar patterns for central and peripheral fibre segments, suggesting that the control mechanisms are closely linked in both locations. Sheath thickness growth both peripherally and centrally lags behind axon calibre growth for alpha fibres. The two keep pace for gamma fibres. 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 fibres suggest that conduction velocity increases when the impulse enters the peripheral nervous system.