Anatomy and Physiology of the Peripheral Nervous System
Core concepts
Know the structure of a peripheral nerve: axon, myelin sheath (Schwann cells in PNS), endoneurium, perineurium, epineurium, and fascicles. Understand fiber types: Aα (motor/proprioception, largest, fastest), Aβ (touch), Aδ (pain/temperature, thinly myelinated), and C fibers (unmyelinated, slow). Saltatory conduction in myelinated fibers depends on nodes of Ranvier; demyelination slows or blocks conduction.
Understand the brachial and lumbosacral plexuses and common compression sites (median nerve at carpal tunnel, ulnar at cubital tunnel, peroneal at fibular head). Recognize Wallerian degeneration (distal degeneration after axonal injury) versus demyelination (segmental, faster recovery). Know the difference between neurapraxia, axonotmesis, and neurotmesis.
Traps
Questions confuse which fiber type carries which modality, or which cells myelinate PNS (Schwann) versus CNS (oligodendrocytes). Watch for misattribution of compression sites and for conflating nerve injury grades.
Worked example / rule of thumb
A patient with wrist drop and preserved sensation suggests radial nerve involvement; a foot drop with sensory loss over the dorsum of the foot suggests peroneal nerve at the fibular head. Rule of thumb: localize by matching the motor pattern plus sensory territory to a named nerve and its common entrapment site.