# Peripheral Nerve Anatomy and Electrodiagnosis

## Introduction

A thorough understanding of peripheral nerve anatomy is fundamental to neurosurgical practice, from diagnosing entrapment neuropathies to planning surgical explorations and repairs. Electrodiagnostic studies, including nerve conduction studies and electromyography, provide critical physiologic information about nerve injury localization, severity, and prognosis. Neurosurgeons must integrate anatomic knowledge with electrodiagnostic data to guide surgical decision-making.

## Peripheral Nerve Microanatomy

### Structural Components

The axon is the nerve fiber itself and may be myelinated or unmyelinated. The endoneurium is the connective tissue surrounding individual axons. A fascicle is a bundle of axons grouped together, and the perineurium is the dense connective tissue sheath surrounding each fascicle that provides the blood-nerve barrier. The epineurium is the outer connective tissue layer surrounding the entire nerve trunk and contains the vasa nervorum. Internal epineurium refers to the connective tissue between fascicles within the nerve trunk.

### Nerve Fiber Classification

Motor alpha fibers are large and myelinated with fast conduction velocities of 50 to 70 meters per second. Sensory fibers vary in size, with large myelinated fibers carrying proprioception and vibration, small myelinated fibers carrying pain and temperature, and unmyelinated C fibers carrying pain and autonomic information. Autonomic fibers include small myelinated preganglionic fibers and unmyelinated postganglionic fibers. Myelinated fibers conduct via saltatory conduction at the nodes of Ranvier, which is faster and more efficient than continuous conduction.

### Nerve Injury Classification

The Seddon classification divides injuries into three categories. Neurapraxia involves focal demyelination causing conduction block with full recovery expected within weeks to months. Axonotmesis involves axonal disruption with intact endoneurium; Wallerian degeneration occurs distally, but recovery is possible via axonal regeneration at approximately 1 millimeter per day. Neurotmesis involves complete disruption of the nerve including connective tissue with no spontaneous recovery, requiring surgical repair.

The Sunderland classification expands this system into five grades. Grade I corresponds to neurapraxia. Grade II involves axon disruption with intact endoneurium, equivalent to axonotmesis. Grade III involves disruption of both axon and endoneurium with intact perineurium. Grade IV involves disruption of axon, endoneurium, and perineurium with intact epineurium. Grade V is complete transection, equivalent to neurotmesis. Mackinnon added Grade VI to describe a mixed injury pattern within the same nerve.

| Sunderland Grade | Seddon Equivalent | Structure Disrupted | Intact Structure | Recovery |
|-----------------|-------------------|--------------------|--------------------|----------|
| I | Neurapraxia | Myelin (focal demyelination) | Axon, endoneurium, perineurium, epineurium | Complete; weeks to months |
| II | Axonotmesis | Axon | Endoneurium, perineurium, epineurium | Complete; months (1 mm/day) |
| III | — | Axon + endoneurium | Perineurium, epineurium | Variable; may need surgery |
| IV | — | Axon + endoneurium + perineurium | Epineurium | None without surgery |
| V | Neurotmesis | All structures (complete transection) | None | None without surgery |
| VI (Mackinnon) | — | Mixed pattern within same nerve | Variable | Requires surgical exploration |

## Electrodiagnostic Studies

### Nerve Conduction Studies (NCS)

Motor nerve conduction studies involve stimulating a nerve at two points and recording the compound muscle action potential from the target muscle. Measurements include distal latency, conduction velocity, and CMAP amplitude. Reduced CMAP amplitude indicates axonal loss, while prolonged latency and slowed conduction velocity indicate demyelination. Conduction block, defined as significant reduction in CMAP amplitude with proximal versus distal stimulation, indicates focal demyelination.

Sensory nerve conduction studies stimulate and record sensory nerve action potentials. Reduced SNAP amplitude indicates sensory axonal loss. A key principle is that SNAPs are preserved distal to a root avulsion because the dorsal root ganglion is proximal and the injury is preganglionic, whereas SNAPs are absent with postganglionic lesions at the plexus or peripheral nerve level.

F-waves are late motor responses that evaluate proximal nerve segments including roots and proximal nerve trunk. The H-reflex is the electrical equivalent of the ankle jerk and evaluates the S1 nerve root.

### Electromyography (EMG)

Needle EMG uses an intramuscular needle electrode to record muscle electrical activity. Insertional activity is increased in denervation and decreased in chronic fibrosis. Spontaneous activity at rest includes fibrillation potentials and positive sharp waves, which indicate active denervation and appear 2 to 3 weeks after nerve injury, and fasciculation potentials, which may be benign or pathologic as seen in motor neuron disease.

Motor unit action potential analysis provides further information. Increased duration and amplitude with polyphasia indicates reinnervation or chronic neurogenic changes. Decreased duration and amplitude with early recruitment indicates a myopathic pattern. The recruitment pattern is assessed by evaluating how many motor units fire and at what rate; reduced recruitment with fewer MUAPs firing faster indicates neurogenic injury.

### Timing of Electrodiagnostic Studies

The optimal timing for electrodiagnostic studies is 3 to 4 weeks after injury, allowing denervation changes to develop on EMG. Nerve conduction studies can show conduction block immediately in neurapraxia. Wallerian degeneration takes 7 to 10 days, and distal NCS changes appear after this period. Serial studies at 3 to 6 month intervals help track reinnervation and guide surgical timing.

## Clinical Applications in Neurosurgery

### Localization

Combining NCS and EMG findings localizes the lesion to the root, plexus, or peripheral nerve level. Paraspinal EMG showing denervation in paraspinal muscles indicates root-level pathology proximal to the plexus. Sensory NCS helps distinguish preganglionic injuries such as root avulsion from postganglionic injuries.

### Prognostication

The presence of motor unit recruitment on volitional EMG indicates some intact axons and a favorable prognosis. Nascent MUAPs, which are small polyphasic units appearing during reinnervation, represent a positive prognostic sign. Complete absence of voluntary MUAPs at 3 to 6 months indicates poor spontaneous recovery, and surgical intervention should be considered.

### Intraoperative Monitoring

Intraoperative nerve conduction studies help identify functional fascicles during nerve exploration. Direct nerve stimulation with proximal stimulation and distal recording determines which fascicles conduct across an injury zone. For a neuroma-in-continuity, intraoperative NCS helps determine whether to resect and graft or perform neurolysis.

## Clinical Pearls

Preserved SNAPs in the setting of clinical sensory loss indicate a preganglionic lesion such as root avulsion, which has critical surgical implications because direct nerve repair is not possible. Fibrillation potentials on EMG take 2 to 3 weeks to develop after injury, and testing too early may yield false-negative results. The Sunderland classification correlates injury severity with prognosis: grades I and II recover spontaneously, grade III is variable, and grades IV and V require surgery. Axonal regeneration occurs at approximately 1 millimeter per day or 1 inch per month, and this rate guides expected recovery timelines after repair. Serial electrodiagnostic studies are essential for monitoring recovery and determining the optimal timing for surgical intervention.

## References
1. Dumitru D, Amato AA, Zwarts MJ. Electrodiagnostic Medicine. 2nd ed. Hanley & Belfus; 2002.
2. Mackinnon SE. Nerve Surgery. Thieme; 2015.
3. Robinson LR. Traumatic injury to peripheral nerves. Muscle Nerve. 2022;66(6):661-670.
4. Campbell WW. Evaluation and management of peripheral nerve injury. Clin Neurophysiol. 2008;119(9):1951-1965.