How to Dissect the Peripheral Nervous System

Overview

Tools Required

• Scalpels and no. 15 blades
• Blunt dissecting scissors
• Fine-tipped and standard forceps
• Nerve hooks and probes
• For deep structures (brachial/lumbosacral plexus): Kerrison rongeurs, bone chisels, rubber mallet, Dremel tools
• Magnifying glass or dissecting microscope (for fine branches)

General Principles

1. Work from known to unknown - always start at identifiable landmarks (bony prominences, muscle bellies, foramina) and trace nerves outward or inward from there.
2. Use blunt dissection - nerves are delicate; use blunt-tipped probes and forceps to separate surrounding connective tissue rather than cutting.
3. Follow the nerve's innervation pattern - if you identify the muscle a nerve supplies, you can trace it back to its origin.
4. Preserve surrounding structures - avoid cutting nearby blood vessels, since the nerve's vascular supply (vasa vasorum) runs alongside it.

Dissecting Key PNS Regions

1. Brachial Plexus (Upper Limb)

• The brachial plexus arises from nerve roots C5-T1 and is organized into: roots → trunks → divisions → cords → terminal branches.
• Begin in the posterior triangle of the neck: locate the trunks between the anterior and middle scalene muscles.
• For distal branches, use an antero-axillary approach.
• For full plexus exposure with minimal disruption, use a posterior subscapular incision (between the medial border of the scapula and the spine, retracting the scapula laterally).

2. Lumbosacral Plexus (Lower Limb)

• Access requires removal or retraction of overlying abdominal and pelvic wall muscles.
• The lumbar plexus lies within the psoas muscle; carefully split the muscle fibers to reveal the roots.
• The sacral plexus lies on the piriformis muscle on the posterior pelvic wall.
• Trace the sciatic nerve from the greater sciatic foramen distally down the thigh.

3. Spinal Nerves

• Identify the intervertebral foramina by removing the erector spinae muscles from the vertebral column.
• Carefully remove laminae (laminectomy) to expose dorsal and ventral nerve roots.
• Trace each root to its dorsal root ganglion (a visible swelling) just distal to the foramen.

4. Cranial Nerves

• Begin by tracing nerves from identifiable surface landmarks (e.g., the parotid gland for CN VII, the orbital fissures for CN III/IV/VI).
• Identify muscles innervated by the nerve to confirm identity, then trace back to origin.

5. Autonomic Ganglia

• In the aortic plexus and paravertebral chain, ganglia appear as dark spots within nerve tissue - these are clusters of unmyelinated neuron cell bodies, visually distinct from white myelinated nerve trunks.
• Sympathetic chain ganglia run lateral to the vertebral bodies; carefully separate from the pleura and peritoneum.

Nerve Structure to Recognize During Dissection

Safety and Preservation Tips

• Attend a safety briefing before beginning if working in a formal dissection lab.
• Keep tissues moist to prevent drying and distortion.
• For practicing nerve blocks on cadavers: inject 3-5 mL of saline (or methylene blue to trace spread) around target nerves without permanently disrupting the anatomy.
• Work in short sessions to avoid fatigue-related errors with fine structures.