Size of myelinated nerve fibres is not increased by expansion of the peripheral field in cats

1. This study tests the hypothesis that target size regulates the size of m yelinated sensory and motor fibres in peripheral nerves. Cat medial gastroc nemius (MG) muscles were partially denervated and the size of the remaining nerve fibres that sprouted was examined 6.4 +/- 0.9 months later to determ ine whether nerve fibre size increased with target size. 2. Electrophysiological and morphometric analyses were used to quantify mye linated nerve fibre size. Charge measurements fr om dorsal and ventral root s were used to electrophysiologically quantify the relative number of cut n erve fibres and the average size of the remaining intact sensory and motor nerve fibres. Medial gastrocnemius muscle and motor unit forces provided in direct measurements of the increase in target size. Conduction velocities a nd amplitude of unitary action potentials of motor nerve fibres innervating single motor units were also measured after partial denervation. 3. Electrophysiological measurements of nerve fibre size and morphometric m easurements of enter fibre perimeters and fibre areas concurred and demonst rated that myelinated nerve fibres supplying partially denervated MG muscle s did not increase in size in parallel with the increase in the target size . 4. Thus, unlike non-myelinated nerve fibres, the size of myelinated nerve f ibres does not increase as target size increases. Retrograde control of siz e in non-myelinated but not in myelinated nerve fibres demonstrates differe nces in plasticity of neurons in the somatic and autonomic nervous systems.