peripheral nerve
Summary
A peripheral nerve is a bundle of axons (motor, sensory, and/or autonomic fibers) outside the CNS that connects the brain and spinal cord to muscles, skin, and organs. Peripheral nerves are enveloped by connective tissue layers (endoneurium, perineurium, epineurium) and myelinated primarily by Schwann cells. They are the anatomical substrate for reflexes, voluntary movement, sensation, and autonomic function.
Detail
Peripheral nerves arise from the ventral (motor) and dorsal (sensory) roots of the spinal cord or from cranial nerve nuclei, and travel to target tissues. Structurally, each axon is surrounded by endoneurium; groups of axons (fascicles) are surrounded by perineurium (blood-nerve barrier); and the entire nerve is surrounded by epineurium. Schwann cells myelinate peripheral axons (contrast with oligodendrocytes in CNS), and each Schwann cell myelinates one axon segment (internode), unlike oligodendrocytes which myelinate multiple axons. Nodes of Ranvier allow saltatory conduction. Peripheral nerve injury classification (Seddon): neurapraxia (myelin damage only, recoverable), axonotmesis (axon damage, endoneurium intact, Wallerian degeneration with regeneration possible), neurotmesis (complete transection, poor prognosis without repair). Wallerian degeneration involves axonal and myelin breakdown distal to injury, macrophage clearance, and Schwann cell proliferation to guide regeneration (~1mm/day). Clinically relevant peripheral neuropathies include diabetic neuropathy (classically symmetric, distal, sensorimotor), Guillain-Barré syndrome (acute inflammatory demyelinating polyneuropathy, ascending paralysis, post-infectious, elevated CSF protein without cellular pleocytosis - albuminocytologic dissociation), Charcot-Marie-Tooth disease (hereditary motor and sensory neuropathy), and compressive neuropathies (e.g., carpal tunnel syndrome affecting median nerve). Nerve conduction studies and EMG are key diagnostic tools distinguishing demyelinating vs axonal, and motor vs sensory involvement. Peripheral nerves are vulnerable to compression, ischemia, metabolic derangement (diabetes), toxins, and autoimmune attack. Understanding peripheral nerve anatomy is essential for localizing lesions (e.g., radiculopathy vs plexopathy vs mononeuropathy vs polyneuropathy) and predicting clinical deficits based on dermatomal/myotomal patterns.
Sources
- Kandel, Principles of Neural Science
- First Aid for the USMLE Step 1
- Guyton and Hall Textbook of Medical Physiology
- Adams and Victor's Principles of Neurology
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