Which sensation is lost first during the onset of local anesthesia?

Which sensation is lost first during the onset of local anesthesia?

• A. Pain
• B. Temperature
• C. Touch
• D. Pressure

Answer: (A)

Local anesthetics produce a blockade by blocking voltage-gated sodium channels in nerve fibers, stopping the propagation of impulses. They affect small-diameter fibers more readily than large-diameter ones. Pain fibers are among the smallest (A-delta and C fibers) and are often unmyelinated or lightly myelinated, making them the most susceptible to early blockade. As a result, pain sensation is lost first when anesthesia begins. With increasing effect, other modalities—temperature, then touch, and finally pressure—are blocked as the larger, more heavily myelinated fibers are inhibited. So the first sensation lost during onset is pain.

Local anesthetics produce a blockade by blocking voltage-gated sodium channels in nerve fibers, stopping the propagation of impulses. They affect small-diameter fibers more readily than large-diameter ones. Pain fibers are among the smallest (A-delta and C fibers) and are often unmyelinated or lightly myelinated, making them the most susceptible to early blockade. As a result, pain sensation is lost first when anesthesia begins. With increasing effect, other modalities—temperature, then touch, and finally pressure—are blocked as the larger, more heavily myelinated fibers are inhibited. So the first sensation lost during onset is pain.