The drug succinylcholine (SCh) is known to produce contracture in two of the three classes of intrafusal muscle fibre whilst paralysing the third. Since the effects on afferent stretch sensitivity of contraction in each class of intrafusal muscle fibre are known, SCh can be used to assess the pattern of intrafusal termination of afferents studied electrophysiologically. This approach has previously been used to differentiate primary afferents from secondaries when alternative means were not applicable, but the aim of the present experiments has been to extend this work to allow the differentiation of primary afferents from classical spindle encapsulations (which have the full complement of intrafusal muscle fibres) and those from tandem encapsulations which lack a bag<SUB>1</SUB> fibre which would otherwise be activated by SCh. Three sets of experiments were performed. In the first, afferents from the cat neck extensor muscle biventer cervicis were studied, since this muscle is known to be particularly rich in tandem spindles. Four types of afferent response to intraarterial infusion of SCh were identified, three corresponding to the behaviour previously reported for hindlimb muscle spindle afferents. The fourth had not previously been seen, and showed features which indicated that the afferents activated by SCh in this way were probably the b<SUB>2</SUB>c primary afferents arising in tandem muscle spindles. Other evidence in the form of the passive properties of these afferents was adduced to support this diagnosis. In the second series of experiments using the medial gastrocnemius muscle of the hindlimb, four patterns of SCh activation were again seen for muscle spindle afferents; these were all very similar to the patterns seen for biventer afferents, including that presumed to belong to b<SUB>2</SUB>c primary afferents from tandem spindles. Additional evidence that b<SUB>2</SUB>c primary afferents from tandem muscle spindles had been correctly identifed came from the measurement of their conduction velocities, which indicated that the majority of presumed b<SUB>2</SUB>c primary afferents from tandem muscle spindles had conduction velocities which overlapped with those of slower b<SUB>1</SUB>b<SUB>2</SUB>c primary afferents from classical encapsulations, as was expected on histological grounds. In the final series of experiments, the spindles of origin of afferents from the tenuissimus muscle which had been studied electrophysiologically were located in the muscle, excised and studied histologically in serial transverse sections. Seven afferents were diagnosed by SCh as primaries arising in classical muscle spindles, and the histological reconstruction indicated that their parent spindles indeed contained all three intrafusal muscle fibre types. In contrast, the single afferent diagnosed by SCh and a b<SUB>2</SUB>c primary arising in a tandem muscle spindle was subsequently found to innervate an encapsulation of a tandem spindle containing only two types of intrafusal muscle fibre. The histological evidence thus supports the SCh-based classification of primary afferents, and in particular indicates that b<SUB>2</SUB>c primary afferents from tandem muscle spindles can be readily identified during electrophysiological experiments.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:660760 |
| Date | January 1990 |
| Creators | Price, Rupert Francis |
| Publisher | University of Edinburgh |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/1842/20120 |
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