Peripheral Nervous System Anatomy and Physiology
Anatomy drives the diagnosis
Know the nerve fiber types: A-alpha fibers are myelinated and fast, serving motor and proprioception; A-beta serves touch; A-delta serves pain and temperature; C fibers are unmyelinated and slow, serving pain and autonomic function. Myelin enables saltatory conduction; demyelination slows conduction, while axonal loss reduces amplitude. The neuromuscular junction uses acetylcholine; presynaptic disorders include Lambert-Eaton, postsynaptic disorders include myasthenia gravis.
Localize by root, plexus, or nerve. The brachial plexus spans C5-T1; the lumbosacral plexus spans L1-S4. Dermatomes and myotomes overlap, so single-root lesions cause myotomal weakness and dermatomal sensory loss. Common compression sites include the median nerve at the carpal tunnel, ulnar nerve at the cubital tunnel, common peroneal nerve at the fibular neck, tibial nerve in the tarsal tunnel, and radial nerve in the spiral groove. Anatomic variants include Martin-Gruber median-to-ulnar forearm connection, accessory phrenic nerve, and pre-fixed or post-fixed plexus.
Traps: assuming foot drop is always common peroneal; L5 radiculopathy also causes foot drop but weakens ankle inversion through tibialis posterior and may cause back pain, whereas common peroneal compression at the fibular neck spares inversion. Ulnar neuropathy at the elbow versus wrist requires testing dorsal ulnar cutaneous and palmar branches. Median neuropathy at the wrist spares the forearm, while more proximal median lesions may not.
Worked example: a patient has foot drop, numbness over the dorsum of the foot, and weak ankle dorsiflexion and eversion but normal inversion. This localizes to the common peroneal nerve at the fibular neck, not the L5 root, which would weaken inversion.
Rule of thumb: 'If inversion is weak, think L5 or sciatic; if inversion is spared, think common peroneal at the fibular neck.'