Final answer:
The myelin sheath is the primary restraint to tensile forces across the axon, providing insulation and allowing nerve impulses to travel faster.
Step-by-step explanation:
The primary restraint to tensile forces across the axon is the myelin sheath.
The myelin sheath is composed of the tightly wrapped plasma membrane of a Schwann cell and acts as an insulator to keep the electrical signal from dissipating as it travels down the axon.
The myelin sheath allows nerve impulses to travel more rapidly down the axon.