With neuraxial anesthesia, which nerve fiber type is blocked first?

With neuraxial anesthesia, which nerve fiber type is blocked first?

• A. A Alpha and A Gamma
• B. B Fibers (Autonomic)
• C. A Delta
• D. A Beta

Answer: (B)

The main idea is differential blockade: in neuraxial anesthesia, smaller, autonomic fibers are more susceptible to local anesthetics than larger somatic fibers. The autonomic preganglionic fibers are small in diameter and lightly myelinated, so they lose function first when the block is established. This means sympathetic transmission is interrupted before pain, temperature, touch, or motor fibers, leading to early sympathetic (vasomotor) blockade with possible hypotension and vasodilation. That’s why the autonomic preganglionic fibers are the best answer: they’re blocked before the somatic fibers, which are larger and require a higher concentration or longer time to block. The other fiber types—pain (A-delta), touch/proprioception (A-beta), and motor (A-alpha)—tend to be affected later in the sequence.

The main idea is differential blockade: in neuraxial anesthesia, smaller, autonomic fibers are more susceptible to local anesthetics than larger somatic fibers. The autonomic preganglionic fibers are small in diameter and lightly myelinated, so they lose function first when the block is established. This means sympathetic transmission is interrupted before pain, temperature, touch, or motor fibers, leading to early sympathetic (vasomotor) blockade with possible hypotension and vasodilation.

That’s why the autonomic preganglionic fibers are the best answer: they’re blocked before the somatic fibers, which are larger and require a higher concentration or longer time to block. The other fiber types—pain (A-delta), touch/proprioception (A-beta), and motor (A-alpha)—tend to be affected later in the sequence.