Seasonal variation in conduction velocity of action potentials in squid giant axons

To determine whether the electrical properties of the squid giant axon are seasonally acclimated, action potentials, recorded at different temperature s, were compared between giant axons isolated from Loligo pealei caught in May, from relatively cold waters (similar to 10 degrees -12 degreesC), and in August, from,relatively warm waters (similar to 20 degreesC). Parameters relating to the duration of the action potential (e.., maximum rate of ris e, maximum rate of fall, and duration at half-peak) did not change seasonal ly. The relationship between conduction velocity and temperature remained c onstant between seasons as well, in spite of the fact that May axons were s ignificantly larger than August axons. When normalized to the fiber diamete r, mean May conduction velocities were 83% of the August values at all temp eratures tested, and analysis of the rise time of the action potential foot suggested that a change in the axoplasmic resistivity was responsible for this difference. Direct measurements of axoplasmic resistance further suppo rted this hypothesis. Thus seasonal changes in the giant axon's size and re sistivity are not consistent with compensatory thermal acclimation, but ins tead serve to maintain a constant relationship between conduction velocity and temperature.