Mel Robertson got interested in how temperature affects an insect’s nervous system one hot day almost 20 years ago. The lab’s air conditioning had broken down, and as the temperature rose, the experiments worked better than they ever had at room temperature! He realised that this made perfect sense, because cold-blooded insects keep flying, even at temperatures that no human nervous system could survive. Since then, Robertson has concentrated on finding out what protects the insect’s wiring against naturally rising temperatures. He has now added anoxia to the list of protective shocks that prime the insect’s nervous system to survive rocketing temperatures (p. 815).
All life depends on the ATP that is generated by aerobic respiration to drive its key processes, and the nervous system is no exception. But at times of physical stress, locusts conserve their nervous system’s valuable ATP stores by reducing the nervous system’s potassium currents, saving the huge amounts of energy required to pump potassium ions back into the cell after the nerve has fired. Robertson wondered whether depriving the insect of oxygen would reset the nervous system’s protection mechanisms to conserve ATP so that it could survive even higher temperatures than it could have before with-holding oxygen.
After two hours in a nitrogen environment, he raised the temperature to 53°C to see how well they survived the baking temperatures. After half an hour, six of the original 15 insects survived, while all of the insects that were heat shocked without preparation died! The insect’s nervous system was also able to stand even higher temperatures than before; the locust’s nerve cells were able to generate an action potential during a heat shock that destroyed unprepared neurons. Robertson realised that the insect reset the nervous system’s potassium channels so that it continued to send nerve signals at very high temperatures. But it didn’t work the other way around. An insect that had been heat shocked first was just as vulnerable to anoxia as before.
But would insect’s muscles survive as well? It’s no use having a nervous system that survives boiling temperatures if the signals it sends fall on deaf muscles. When Robertson looked at the insect’s muscle function under different stresses, he discovered that the heat pre-stress, which protected the insect’s nervous system, disrupted its muscular function! The insect could still fly, but the damaged muscle uses more energy.
Although some stresses were able to improve some component’s performance, an improvement in the nervous system’s function wasn’t always matched by an improvement in other essential flight systems. So it’s not enough to look at an individual component in isolation, biology is complex, and an animal’s ability to survive a variety of stresses can only be assessed by looking at the entire animal’s response. Even if the insect can tell its wings to flap, its damaged muscles might not be able to lift it from the ground.
Robertson has reason to believe that the locust’s flight control system could have parallels with the human respiratory system that may help us to understand situations when our breathing fails, such as hyperthermia. Robertson hopes that learning how a simple motor control system failure keeps an insect grounded could help us to understand how the human respiratory system collapses as temperatures rise.
- © The Company of Biologists Limited 2002