Fibre population of the phrenic nerve of rat: changes of myelinated fibre dimensions along the nerve and characteristics of axonal branchings.

All myelinated fibres along the unbranched pant of the phrenic nerve were teased and measured at regular intervals. At the proximal level, the size frequency distribution is bimodal with peaks at 2 and 7 micrometers. Fibres in most size classes exhibit a slight progressive tapering along their unbranched course in the nerve trunk. The degree of tapering is not identical in fibres of various calibre so that at the distal end of the nerve the pattern of size frequency distribution is altered. The volume of axoplasm of the total phrenic fibre population per unit length of the nerve decreases at the distal level to about 80 percent and the sum of axolemmal surface areas to about 90 percent of those found at the proximal level. Relevance of these results for interpretation of biochemical gradients of axonal components with various subcellular localizations is discussed. Below the sites of branching of individual fibres the total cross-sectional area of axoplasm in branches is practically equal to that of the parent fibre, whereas the sum axolemmal surface areas of branches is markedly increased.