Final answer:
Tetrodotoxin and lidocaine block the voltage-gated sodium channels, leading to fewer propagated action potentials along an axon.
Step-by-step explanation:
When tetrodotoxin (TTX) or lidocaine is applied to an axon, fewer action potentials are propagated because these substances block the voltage-gated sodium channels. These channels are responsible for allowing sodium ions into the axon during depolarization, which triggers an action potential. By blocking these channels, tetrodotoxin and lidocaine prevent the generation and propagation of action potentials along the axon.