Fiber size distribution of myelinated and unmyelinated axons in the medial and posterior articular nerves of the cat's knee joint.

The distribution of fiber size in the posterior articular nerve (PAN) and medial articular nerve (MAN) of the cat's knee joint was studied by light and electron microscopy. The myelinated fibers of the PAN ranged from approximately 1 to 18 micron, with maxima at 3-4 micron and 8-9 micron. According to the classification of Boyd and Davey (1968), the PAN contained about 34% Group III fibers, 56% Group II fibers, and 10% Group I fibers. In contrast, the MAN showed a unimodal, skewed distribution, with a range from approximately 1 to 14 micron and a maximum at 3-4 micron. According to the Boyd and Davey classification, the PAN contained about 69% Group III fibers, 30% Group II fibers, and 1% Group I fibers. Unmyelinated fibers examined in the MAN showed a unimodal distribution, with a range of from 0.1 to 1.5 micron and a maximum at 0.4-0.5 micron. To differentiate between afferent and sympathetic fibers, a sympathectomy or ganglionectomy was performed on one side. The fiber size distribution indicated a considerable overlap in the diameter of the afferent and sympathetic unmyelinated fibers. Most sympathetic fibers had a diameter of between 0.8 and 0.9 micron, whereas afferent fibers showed a maximum at 0.3-0.4 micron. When data were combined for myelinated and unmyelinated fibers in the PAN, about 74% of the afferent fibers were found to belong to Groups III and IV; they are thought to terminate in noncorpuscular endings. The other 26% were found to belong to Groups I and II; they terminate in corpuscular receptors and muscle spindle primary and secondary endings.(ABSTRACT TRUNCATED AT 250 WORDS)