Questions: In myelinated axons, action potentials occur only at the nodes of Ranvier.

The answer is the second one: occur only at the nodes of Ranvier.

Explanation for each option:

• Occur along the entire length of the axon: This is incorrect because in myelinated axons, action potentials do not occur continuously along the entire length. Instead, they jump from one node of Ranvier to the next in a process called saltatory conduction.
• Occur only at the nodes of Ranvier: This is correct. In myelinated axons, the myelin sheath insulates the axon and prevents ion flow across the membrane. Action potentials are generated only at the nodes of Ranvier, where the axon membrane is exposed and ion channels are concentrated.
• Occur only where the myelin is in contact with the axon: This is incorrect because the myelin sheath itself does not allow for the generation of action potentials. Action potentials occur at the nodes of Ranvier, where there is no myelin.
• Travel more slowly than on unmyelinated axons: This is incorrect. Myelinated axons conduct action potentials more rapidly than unmyelinated axons due to the saltatory conduction mechanism.

Summary: In myelinated axons, action potentials occur only at the nodes of Ranvier.