2.3 Changes in diameter

As you may have noted in your experiments with a uniform patch of membrane, the shape of the action potential is remarkably resistant to relatively changes in the ionic conductance parameters. This is the result of a "cool" feedback design where there is ample reserve power for the purpose of generating an impulse under a variety of degradations such as reductions in the ionic conductances. This design uses only the energy necessary for generation of a a robust spike, whatever the conditions.

Similarily, the squid axon is elegantly designed so as to balance the need for speedy transmission of impulses with a minimum disappation of energy However there is ample reserve energy to take care of contingincies such as local injury, mechanical restriction, partial channel blockage by anesthetics or toxins, as well as an abrupt increase in diameter. In such areas these changes cause an increase in the sodium current flow so as to generate as large an impulse as possible under the adverse circumstances. When the impulse proceeds past the region of abnormality, the normal parameters of the spike are restored and propagation continues apace.

Here we are concerned only with the problem of a change in diameter. To explore this problem, many experiments have been carried out in my lab, using squid axons with diameter changes both

In both cases we found the resulting changes in sodium current noted above and the resilience of the impulse generating feedback system. We found that propagation in the large- to-small direction, where there was a reduction in velocity of propagation, presented no problem. However in the small-to-large direction, the impulse seemed to have to "struggle" to overcome the difficulties of initiating a speedier impulse in the larger axon. This required a larger disappation of energy which was provided by the increase in sodium ion entry. We also noted that there was an upper limit of the diameter ratio through which transmission could take place. Furthermore, this ratio was quite temperature sensitive.

A simulation program has been set up with a junction between a small diameter axon and a larger one for your exploration.