The lobster is an ideal animal for researchers to study if they are interested in neural control: large and robust, these creatures have very few neurons controlling the muscles which move their limbs. The transmission of signals from neuron to neuron and from neuron to muscle is very dependent on temperature, so scientists are keen to know what happens to signal transmission at different temperatures. This is particularly relevant for the lobster, a cold-blooded crustacean that lives at temperatures ranging from a chilly 0°C to a relatively balmy 20°C. But it's not just temperature that affects signal transmission: chemicals called neuromodulators also play a role and can alter muscle function as well. It is the combination of the effects of temperature and the neuromodulator serotonin that interested Mary Kate Worden and colleagues at the University of Virginia, who investigated the role that they play in affecting the function of a lobster's neurons and muscles (p. 1025).
The team chose to investigate one muscle in a lobster's walking leg, the dactyl opener muscle. This is controlled by only one excitatory neuron, which causes the muscle to contract, and two inhibitory neurons which cause the muscle to relax. To find out how temperature alone affected the transmission of neural signals to the muscle, and the muscle's contractions, the team extracted a dactyl closer muscle from a lobster's leg and placed it in a temperature controlled bath, which they could change in temperature from 2–20°C. Stimulating the excitatory neuron to make the muscle contract, the team used microelectrodes inserted into the muscle fibres to record the neural signals transmitted to the muscle. They found that these excitatory signals, called EJPs, were present at all temperatures but were particularly large at 2°C and between 14–16°C.
But what about the contractions caused by the EJPs: would they also differ in size according to the temperature? Using a force transducer attached to the muscle's end via a piece of thread, they found that the muscle contracted at all temperatures, but most strongly in the cold. When the team repeated their experiments by stimulating the inhibitory motor neuron to make the muscle relax, they found that temperature had a different effect. The neural signals,called IJPs, caused the muscle to relax at colder temperatures only, between 0–6°C, but not at warmer temperatures.
Having shown that temperature had a different affect on the excitatory and inhibitory neurons, and on the muscle, the team then added serotonin to the muscle's bath and repeated their measurements. They found that the neuromodulator changed the effect of temperature on the muscle. When they stimulated the excitatory neuron, the EJPs and the muscle contractions were bigger, especially at 18°C. Again, there was a different effect when they stimulated the inhibitory neuron. They found that serotonin both increased muscle relaxation, and also increased the temperature range over which IJPs relaxed the muscle to 0–12°C.
Because serotonin has such a wide range of different effects on muscles at different temperatures researchers should look at the influence of neuromodulators such as serotonin over a range of temperatures, Worden explains. One of the next challenges will be to unravel how temperature and neuromodulators affect lobsters in the wild, altering their behaviour and how their muscles and neurons function.