Developmental morphometry and physiology of the rabbit vagus nerve

The morphological and physiological features of the rabbit vagus nerve were studied at different ages after birth. The total fibre count is about 37,500 of which at birth 1-2% and in the adult animal approximately 10% are myelinated. In the postnatal period the cross-sectional area of the vagus grows to 5 times its perinatal size due to an increase of endoneural collagen, fibre growth and myelinization. The myelinization is most pronounced in the first 2 weeks after birth, axonal growth is predominant thereafter. The available data suggest that the begin of myelinization as well as the subsequent development of the myelin sheath are not dependent on axonal size. There seems to be no fundamental difference between the morphological development of the vagus and other peripheral nerves, e.g. the sciatic nerve of the rat. At birth the vagus nerve contains 2 fibre groups as can be measured from the compound action potential with conduction velocities of 11.4 and 0.9 m.s-1 respectively. Upon subsequent development the conduction velocity of these fibres increases to 31.9 and 1.2 m.s-1 in full-grown animals. THe compound action potential of the adult nerve implies 2 additional fibre groups with conduction velocities of 12.3 and 4.6 m.s-1 respectively. These two fibre populations develop gradually from 1 to 2 weeks after birth and arise probably from the slowest conducting, non-myelinated or C-fibres. It is concluded that the functional innervation of the sinoauricular node may be operational at birth as far as the cervical vagus nerve is concerned.