Fibers in monkey posterior articular nerves

Area of Science:

• Neuroscience
• Anatomy
• Histology

Background:

• The composition of articular nerves is crucial for understanding joint sensation.
• Previous studies may have underestimated unmyelinated fiber populations in these nerves.

Purpose of the Study:

• To quantify the distribution and size of myelinated and unmyelinated axons in rhesus monkey posterior articular nerves.
• To determine the ratio of myelinated to unmyelinated fibers and their conduction velocities.

Main Methods:

• Axon counting and diameter measurement using electron microscopy.
• Conduction velocity measurements of nerve fibers.

Main Results:

• Posterior articular nerves contain 2,000–2,200 unmyelinated fibers (0.2–1.2 µm diameter) and 399–478 myelinated fibers (1.4–12.3 µm diameter).
• Unmyelinated fibers constitute 80–85% of the total nerve fibers.
• Maximum conduction velocity was 70–80 m/s, with most fibers conducting slower.

Conclusions:

• The high proportion of unmyelinated axons suggests a significant role in transmitting sensory information, potentially including pain signals from the knee joint.
• These findings correct previous underestimations by incorporating electron microscopy data.
• The unmyelinated group likely includes post-ganglionic sympathetic fibers.