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Title: Cutaneous C-fibres in the rat and the rabbit : how efferent actions and axonal properties vary with functional class
Author: Gee, Michelle Debra
Awarding Body: University of London
Current Institution: University College London (University of London)
Date of Award: 1997
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The aim of this study was to investigate further some efferent actions and axonal properties of the unmyelinated fibres innervating rabbit and rat skin. This investigation can be separated into two parts. Firstly, single unit studies were carried out to determine which functional class(es) or sub-class(es) of C-fibre are responsible for antidromic vasodilatation in both rabbit and rat skin. The findings of these single unit studies were compared with the flare responses of the skin to noxious mechanical and heat stimuli. Secondly, activity-dependent slowing of conduction velocity and axonal spike shape were examined in identified cutaneous C-fibres in the rat in order to determine whether such axonal properties could be used to identify the different functional classes of C-fibre. For the antidromic vasodilatation study, fine filaments were dissected from the cut proximal end of the saphenous nerve in anaesthetized rabbits and rats. Individual C-fibres (conduction velocity <2m/s) were classified into functional groups according to their responses to mechanical and thermal stimulation. The threshold for electrical stimulation of individual C-fibres was determined using the collision technique. Filaments were antidromically electrically stimulated at intensities sufficient to excite the conducting C-fibres, and skin blood flow was monitored before, during and after filament stimulation using laser Doppler perfusion imaging or laser Doppler flowmetry. In both species, the only C-fibres capable of producing a detectable vasodilator response following antidromic stimulation were nociceptive in nature, and in all cases the area of vasodilatation coincided well with the afferent receptive field. However, not all nociceptors produced a detectable vasodilatation, and it seems that a sub-group of polymodal and heat nociceptors are responsible for the efferent action of antidromic vasodilatation in rabbit and rat skin. Flare responses in rabbit and rat skin were only detected following mechanical and heat stimuli within noxious ranges. The spread of the flare responses, together with the sizes of the afferent and efferent receptive fields of individual C-units, provide support for the axon reflex mechanism for the production of flare and antidromic vasodilatation. The percentage slowing of conduction velocity was calculated following a standard electrical tetanus in identified C-fibres dissected from the saphenous nerve in anaesthetized rats. Nociceptive C-fibres showed a greater slowing of conduction velocity than non-nociceptive fibres, and moreover, one could separate the two classes of non-nociceptive afferent C-fibres found in the rat saphenous nerve (the mechanoreceptors and cold thermoreceptors) on the basis of their conduction velocity slowing. In addition, activity-dependent slowing of conduction velocity could be used to differentiate between the afferent and non-afferent populations of inexcitable C-fibres. Spike shapes of functionally classified C-fibres were recorded extracellularly using standardized filter settings, and some variations in spike shape in relation to receptor type were found. Polymodal nociceptors displayed wider spikes than mechanoreceptors, and cold thermoreceptor units tended to have monophasic spikes. Also, the spontaneously active sympathetic efferent C-fibres tended to have spikes of relatively long duration. The use of axonal properties such as activity-dependent slowing of conduction velocity and spike shape to differentiate nociceptors from non-nociceptors has great potential in experiments where axons are isolated from their terminals.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available