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

This study tests the hypothesis that target size regulates the size of myelinated sensory and motor fibres in peripheral nerves. Cat medial gastrocnemius (MG) muscles were partially denervated and the size of the remaining nerve fibres that sprouted was examined 6.4 +/- 0.9 months later to determine whether nerve fibre size increased with target size. Electrophysiological and morphometric analyses were used to quantify myelinated nerve fibre size. Charge measurements from dorsal and ventral roots were used to electrophysiologically quantify the relative number of cut nerve fibres and the average size of the remaining intact sensory and motor nerve fibres. Medial gastrocnemius muscle and motor unit forces provided indirect measurements of the increase in target size. Conduction velocities and amplitude of unitary action potentials of motor nerve fibres innervating single motor units were also measured after partial denervation. Electrophysiological measurements of nerve fibre size and morphometric measurements of outer fibre perimeters and fibre areas concurred and demonstrated that myelinated nerve fibres supplying partially denervated MG muscles did not increase in size in parallel with the increase in the target size. Thus, unlike non-myelinated nerve fibres, the size of myelinated nerve fibres does not increase as target size increases. Retrograde control of size in non-myelinated but not in myelinated nerve fibres demonstrates differences in plasticity of neurons in the somatic and autonomic nervous systems.