Introduction
In the ‘Earthworm Action Potentials’ practical class described in the companion abstract (Mason), it was clear that the simple preparation provides a stable platform with which to investigate some fundamental questions of nerve conduction. In comparison to amphibian and mammalian nerve preparations, benefits of the earthworm median fibre include single fibre recording, long length and relatively slow conduction, which together permit accurate measurement of conduction velocity. In particular, visual inspection of student recordings showed that the preparation demonstrated strong conduction velocity supernormality, a phenomenon whereby when one action potential immediately follows another, the second shows faster conduction.
Aims
The aetiology of conduction velocity supernormality has received considerable attention over the years but there is no unifying hypothesis that fully describes it. The earthworm nerve preparation offered an opportunity to further investigate the supernormality phenomenon as a final-years Honours research project.
Methods
Earthworms were prepared as described by Mason. They were placed within a chamber and each end was passed through holes in opposite sides that were then sealed with adhesive putty. This allowed the long central length of the worm to be exposed to worm Ringer at different temperatures, while keeping short end lengths of the worm sufficiently dry in the regions of the extracellular stimulation and recording electrodes.
Results
Figure 1A shows an example of two stimuli delivered with a 7 ms interval and the resultant extracellular action potential traces. The measurements of latency from each stimulus to each action potential peak is shown (marked L1 and L2). Pairs of stimuli were delivered with stimulus intervals ranging from less than the refractory period to the point where the first action potential had no influence on the latency of the second. Figure 1B shows the influence of stimulus interval on the latency of the first and second action potentials in each pair. As has been described previously in a range of different preparations, the second action potential of each pair shows a reduction in latency, representing an increase in conduction velocity, over a range of stimulus intervals. Note that even very small changes in latency (<<5%) are consistently detectable in this simple preparation. Exposure of the earthworm median giant fibre allowed an investigation into the effects of changes in temperature and extracellular ion concentrations on the supernormality phenomenon.
Conclusions
The earthworm nerve preparation is quick and easy for a student to set up without the need for continuous academic supervision. As an invertebrate model, research students are able to conduct original experiments on live nerve fibres without any requirement for a Home Office licence. As a robust single-fibre preparation, it lends itself to experiments investigating the effects of extracellular ion concentrations, pH, temperature, osmolality or pharmacological agents for both teaching and research purposes.
Note This submission forms one of a pair of submissions together with the abstract of M. Mason. We are hoping that, if accepted, they could be allocated adjacent slots within a teaching-focused podium session.
