Differential blocking of myelinated nerve fibres by transient depolarization
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Conduction was blocked in the large myelinated fibres (Group II) of a cutaneous nerve by applying a transient polarizing current to the nerve. By appropriately adjusting the polarizing current conduction was not affected in most of the thin myelinated fibres (Group III) when all group II fibres were blocked. This selective action was achieved in 11 out of 15 experiments. Thus the method enables afferent volleys to be set up selectively in Group III fibres.
Recording single unit action potentials from the nerve revealed that the Group II compound volley is a reliable indicator for the extent of the block. As a prerequisite for this finding it was established by unit analysis that temporal dispersion due to the polarizing current did not occur within both Groups II and III fibre populations.
The membrane mechanism of the block was examined by testing the excitability changes in single fibres produced by the polarizing current. The findings indicate that the block was achieved by a membrane depolarization, causing inactivation of the sodium system.
Asynchronous firing produced by the blocking current itself, known to be a disturbing factor in such experiments, was shown to be virtually absent during the block. Discharges in Group II fibres evoked by the initial rise of the depolarizing current were prevented from conditioning the CNS effects of the selective Group III volley by appropriately advancing the onset of the current.
A complete and selective Group II block could also be performed during prolonged stimulation. In particular the method was shown to be able to ensure afferent activity being confined to small fibres during adequate stimulation of skin receptors.
Key wordsMyelinated Nerve Fibres Selective Afferent Stimulation Nerve Block by Transient Polarization
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