C-primary afferent fibre mediated inhibitions in the dorsal horn of the decerebrate-spinal rat

1432-1106

Dorsal horn ; Unmyelinated (C) fibres ; Spinal cord ; Habituation ; Post-tetanic inhibition

Springer Online Journal Archives 1860-2000

Medicine

Summary The effect of repetitive primary afferent C-fibre stimulation on the responses of single wide dynamic range neurones recorded in the dorsal horn of the spinal cord have been exaemined in the unanaesthetised decerebrate-spinal rat. Prolonged stimulation of the sural nerve at 0.5 Hz or higher produced both a progressive increase in the latency of the sural evoked C-responses in all units and a decrease or habituation of the number of C-evoked spikes in the majority of units (62%) recorded in laminae 5 and 6. The shift in latency of the C responses in the dorsal horn neurones was found to be the result of an activity-dependent reduction in the conduction velocity of the primary afferent C-fibres, but such stimulation did not decrease the size of the C compound action potential or produce a block in conduction of single polymodal C-fibres which could account for the habituation of the dorsal horn neurones. A complete habituation or failure of the C-evoked response in a dorsal horn neurone produced by repetitive stimulation of a peripheral nerve was associated with a reduction but not the abolishment of the convergent C-input from an adjacent untetanized nerve, indicating that the habituation did not occur as the result of a postsynaptic inhibition of the dorsal horn neurone. Following a C-tetanus (10 Hz for 10 s) applied to the sural nerve a short and partial homosynaptic post-tetanic inhibition of the subsequent C responses evoked from the sural occurs. Such sural nerve C-tetani however produced a much more powerful and prolonged heterosynaptic post-tetanic inhibition of the C-responses evoked from the common peroneal nerve in those units with convergent C-input from both these two adjacent nerves. None of the effects of repetitive peripheral C-fibre stimulation on the C-evoked responses in the dorsal horn could be produced by A-fibre conditioning stimuli applied at identical frequencies. These results indicate that there is a segmentally organized C afferent-mediated inhibition of C-evoked responses in dorsal horn neurones that is generated both by the C-input that evoked those responses and by adjacent C-inputs.

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