The largest 1-22 micrometers in diameter nerve fiber; conducts action potentials the most rapidly (5-120 m/sec). Some A fibers are sensory; others have a motor function.

The largest 1-22 micrometers in diameter nerve fiber; conducts action potentials the most rapidly (5-120 m/sec). Some A fibers are sensory; others have a motor function.

• A. A-delta fiber
• B. A fiber
• C. C fiber
• D. B fiber

Answer: (B)

In peripheral nerves, conduction velocity increases with axon diameter and with myelination, so the fastest signals come from the largest, heavily myelinated fibers. These large-diameter, myelinated fibers—referred to as A fibers—conduct action potentials at the highest speeds, up to about 120 m/s. They include both sensory and motor functions, such as motor commands to skeletal muscles and sensory information like proprioception. Other fiber types are smaller or less well myelinated, so they conduct more slowly: A-delta fibers are smaller and slower than the largest A fibers, C fibers are unmyelinated and very slow, and B fibers are myelinated but smaller than the largest A fibers. Therefore, the correct choice points to the large-diameter, fast-conducting A fiber category.

In peripheral nerves, conduction velocity increases with axon diameter and with myelination, so the fastest signals come from the largest, heavily myelinated fibers. These large-diameter, myelinated fibers—referred to as A fibers—conduct action potentials at the highest speeds, up to about 120 m/s. They include both sensory and motor functions, such as motor commands to skeletal muscles and sensory information like proprioception. Other fiber types are smaller or less well myelinated, so they conduct more slowly: A-delta fibers are smaller and slower than the largest A fibers, C fibers are unmyelinated and very slow, and B fibers are myelinated but smaller than the largest A fibers. Therefore, the correct choice points to the large-diameter, fast-conducting A fiber category.