The constancy of action potential amplitude along the axon due to regenerative sodium channel opening is a fundamental principle of neurophysiology established by the Hodgkin-Huxley model and standard textbook biology, requiring no citation.
The claim describes the standard, foundational mechanism of action potential propagation (regenerative sodium influx maintaining spike amplitude) taught in introductory biology and neuroscience. Under the established rules, well-known empirical facts of everyday observation or basic scientific definition are classified as COMMON_KNOWLEDGE, meaning no citations are required and paper indices are left empty.