Ultrastructure and synaptology of the initial axon segment of cat spinal motoneurons during early postnatal development.

The initial axon segment (IS) of spinal alpha-motoneurons has been studied in single and serial sections during the first three postnatal weeks in the cat. It was found that boutons of different ultrastructural types made synaptic contact with the IS during the first postnatal week but during the second postnatal week the boutons disappeared. The diameter of the IS was not observed to change from the first to the second postnatal week but a significant increase was noted from the second to the third week. The length and general ultrastructural appearance of the IS did not change significantly over the postnatal period studied. The results are discussed in relation to some aspects of the functional development of spinal motoneurons during the early postnatal period.