Differences in nerve excitability properties across upper limb sensory and motor axons

Area of Science:

• Neuroscience
• Electrophysiology
• Human Physiology

Background:

• Axonal excitability studies are crucial for understanding nerve function and disease.
• Median nerve neuropathy at the wrist is a common clinical concern.
• Characterizing excitability differences between upper limb nerves is essential for accurate diagnosis.

Purpose of the Study:

• To compare the excitability of motor and sensory axons in major upper limb nerves.
• To identify differences in axonal excitability between median, ulnar, and radial nerves.
• To provide a guide for future research on human diseases affecting peripheral nerves, particularly median neuropathy.

Main Methods:

• Axonal excitability studies were performed on median, ulnar, and superficial radial nerves.
• Specific motor axons studied included abductor pollicis brevis (APB) and abductor digiti minimi (ADM).
• Sensory axons studied included digital nerves 2 (D2), 5 (D5), and digital nerve 1 (D1), using a threshold tracking technique.

Main Results:

• Ulnar motor axons exhibited reduced early depolarizing threshold electrotonus and superexcitability compared to median motor axons.
• Ulnar sensory axons required stronger stimulation and had a larger rheobase than median sensory axons.
• Superficial radial axons showed a 'fanned-in' pattern with a greater resting I/V slope, mathematically modeled to be due to reduced nodal/internodal conductances.

Conclusions:

• Excitability parameters of motor and sensory axons are most comparable between the median and ulnar nerves.
• Axonal excitability recordings are feasible for radial and ulnar nerves, providing normative data.
• Ulnar nerve recordings are suggested as a practical alternative to median nerve recordings in cases of compressive neuropathy at the wrist.