The effects of temperature on the threshold of identified neurons in the locust

1. At the preferred body temperature of the locust (30 °C and above), the metathoracic fast extensor of the tibia (FETi) motorneuron will sometimes spike in response to synaptic input from the descending movement detector (DMD) visual interneurons. This does not occur at lower temperatures (Fig. 1). The mechanism of the change in excitability is investigated in FETi and other identified motorneurons over the range 18-35 °C. 2. Action potentials show a reversible decrease in amplitude and duration on heating (Fig. 2). 3. EPSP amplitudes are relatively unchanged by temperature, but their duration decreases slightly on heating (Fig. 3). 4. Membrane potential hyperpolarises on heating and depolarises on cooling (Fig. 4). 5. Membrane resistance shows a transient increase on cooling, and a transient decrease on heating (Fig. 10), but there is usually little steady-state change in resistance with temperature (Fig. 5). 6. Spike threshold shows a transient increase followed by a steady-state decrease on heating, and the opposite on cooling (Fig. 10). This can be demonstrated with injected current (Fig. 6), membrane depolarisation (Fig. 7), spontaneous spike frequency (Fig. 9), and naturally occurring EPSPs (Fig. 8). This change in spike threshold is regarded as the major neural correlate of the change in excitability with temperature.