The extensibility of Aplysia nerve and the determination of true axon length.

1. Characteristics of Aplysia nerves in response to extension were examined in comparison with nerves of the cat, frog and lobster. 2. Only the Aplysia nerve was easily elongated up to about 5 times its resting or relaxing length without impairing propagation of the action potential along the axon in the nerve. 3. The conduction velocity along the elongated nerve increased linearly in proportion to the nerve length in the range from the relaxing length to about 1.2-1.5 times extension (the first phase). However, upon further extension it stayed constant regardless of the nerve length (the second phase). 4. In the relaxed nerve bundle the course of the axons was zigzagging and the axon membrane had numerous foldings or wrinklings. 5. The true length of the zigzagging axon was measured by analysing the length of intra-axonal diffusion following intracellular injection of radioactive acetylcholine ([3H]ACh) whose diffusion kinetics in the axoplasm have been studied in detail (Koike & Nagata, 1979). The length of the axon coincided with the nerve length at which the first phase shifted to the second phase. 6. Cat and frog nerves shrank after dissecting out from the body. When extended, they showed the first phase from the shortened length to their original length in the body. These nerves lacked the second phase. The lobster nerve did not shrink and lacked both phases. 7. Thus the zigzagging course of the axon is suggested to be responsible for the first phase during which the axon length did not change but took a straight course upon nerve extension. 8. On the other hand, the second phase appears to be caused by actual extension of the axon itself and could be explained by reduction of the wrinklings of the membrane.