Compensation of the effects of temperature on a motor system in the crab, Cancer borealis.

Understanding how warming affects life-critical physiological processes is increasingly important as ocean temperatures rise at unprecedented rates. We study the stomatogastric system of the crab Cancer borealis. Movement depends on multiple temperature-sensitive processes, from motor pattern generation to neuromuscular transmission and contraction. Previous studies examined these levels separately and showed that warming increased pyloric rhythm frequency until the rhythm crashed at extreme temperatures. Here, we examined how the neuromuscular transform responds to this increase in motor neuron activity. We simultaneously recorded motor neuron activity, muscle fiber membrane potential, and contraction force. As temperature increased, the pyloric rhythm frequency increased, the excitatory junctional potential amplitudes remained constant, and muscle contraction diminished and failed. When muscles were stimulated at a fixed low frequency mimicking neuronal activity at cold temperatures, contraction decreased with acute warming. Increasing motor nerve stimulation frequency to mimic neuronal activity at warm temperatures partially rescued contraction. This shows that the temperature-dependent increase in neuronal firing provides partial compensation but is insufficient to prevent the loss of contraction at high temperatures. Because warming hyperpolarized muscle fibers, it moved the membrane potential further from the contraction threshold, and decreased synaptic integration, thus protecting against overexcitability. However, high-potassium-induced contractures revealed that the contractile machinery remained functional at high temperature, but the nerve-evoked depolarizations were apparently not large enough to cross the threshold for muscle contraction. Thus, a variety of temperature-dependent compensatory mechanisms overlap to extend the temperature range over which the neuromuscular system remains functional over a broad temperature range.