<p>Partial denervation of a shared target tissue leads to 'collateral' sprouting of the remaining nerve fibres supplying the same region. The mechanisms responsible for this widespread phenomenon are unknown, but it has generally been believed that products from the degenerating fibres constitute the sprouting stimulus. Recently it was shown that a similar sprouting of intact nerves occurred in the salamander hind-limb when the axoplasmic transport in one of the neighbouring nerves was blocked by colchicine. This block did not, however, appear to interfere with the functioning of the treated nerve. On this evidence it was proposed that nerves release factors which are concerned in the regulation of their peripheral fields. The experiments of this thesis test this hypothesis further. In particular this study quantitatively examines the organization of the cutaneous mechanoreceptors of the salamander hindlimb, and the effects of various nerve treatments on this organization.</p> <p>A mechanical prodder of 10-50μ tip diameter was used to determine the sensory threshold of the skin at selected spots. The mechanoreceptors were found to be all rapidly-adapting. An analysis of the distribution of their thresholds across the skin points to the existence of a single population of mechanosensory endings of similar threshold which are fairly uniformly distributed in the plane of the skin. A simple model, based on the results, indicates that these receptors have receptive field sizes of about 50μ, and are spaced approximately 150-200μ apart. Direct inspection of data from systematically surveyed skin areas gave findings which were consistent with these estimates. These quantitative results made it possible to establish that the blocking of fast axoplasmic transport does not affect the sensitivity or distribution of the individual mechanosensory endings of the treated nerve. Nevertheless new sprouts appeared from the adjacent nerves, and it seemed that there was a tendency for these to grow preferentially to the site of the still-functioning endings of the treated nerve. Most significantly, after partial denervation of the skin the number of newly sprouted endings quantitatively matched the number of endings lost by nerve section.</p> <p>It was concluded that there is a control mechanism which continuously regulate the density of the skin innervation. This control system probably involves the mutual interaction of substances continuously secreted from the target tissue, which cause nerves to sprout, and factors released from the nerve endings which offset the effects of the growth stimulus. The nerve field density reflects an equilibrium state between the neural and tissue influences.</p> / Doctor of Philosophy (PhD)
| Identifer | oai:union.ndltd.org:mcmaster.ca/oai:macsphere.mcmaster.ca:11375/7810 |
| Date | 10 1900 |
| Creators | Cooper, John Ellis |
| Contributors | Diamond, J., Medical Sciences |
| Source Sets | McMaster University |
| Detected Language | English |
| Type | thesis |
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