Anatomy and Physiology of the Peripheral Nervous System
Core concepts
Know the neuron (cell body, axon, myelin), the anterior horn cell and dorsal root ganglion, and how motor versus sensory pathways differ. Motor fibers originate in the anterior horn and travel via the ventral root; sensory fibers have their cell body in the dorsal root ganglion and enter via the dorsal root. This anatomy explains why a lesion before the ganglion (radiculopathy) spares the SNAP while a lesion after it (plexopathy, neuropathy) may reduce it.
Fiber types and physiology
Large myelinated fibers (A-alpha) conduct fastest and are tested first; small and unmyelinated fibers conduct slowly. Saltatory conduction at nodes of Ranvier underlies fast conduction, and myelin integrity determines velocity. Understand the neuromuscular junction and the motor unit (one motor neuron plus its muscle fibers).
Traps
- Forgetting that pre-ganglionic lesions preserve SNAPs.
- Mixing up ventral (motor) and dorsal (sensory) roots.
- Assuming all nerve fibers conduct equally; size and myelination matter.
- Overlooking that the phrenic and other special nerves have distinct anatomy.
Rule of thumb
Use the ganglion as a dividing line: lesions proximal to the dorsal root ganglion (root) spare the SNAP; lesions distal to it (plexus or nerve) can reduce the SNAP. This single rule resolves many localization questions.