The ultrastructure of Ruffini and Herbst corpuscles in the articular capsule of domestic pigeon.

The present study identifies two types of sensory nerve endings in the articular capsule of the shoulder joint of domestic pigeons: Ruffini corpuscles (spray-like endings), and Herbst corpuscles. Ruffini corpuscles occur in the fibrous membrane of the articular capsule and consist of two to four branched cylindrical segments within a network of fascicles of collagen fibers. At the terminal ends of the cylinders the perineural sheaths of the capsule are deficient and surround the fascicles of collagen fibers. The axon terminals in each cylindrical segment of a Ruffini corpuscle repeatedly ramify, giving rise to delicate neurite profiles. These neurites and associated Schwann cells envelope small fascicles of collagen fibrils. Schwann cells cover only a part of the neurite profiles. The myelinated afferent axon enters the midregion of the cylinder and has a diameter of approximately 3 micrometer. Herbst corpuscles are situated in the subsynovial connective tissue and in the transition zone between the fibrous membrane and the muscular fascia. They appear as elongated ovals in longitudinal section and round in cross section. Small corpuscles measure approximately 5 micrometer x 200 micrometer in length and large ones approximately 100 micrometer x 600 micrometer. Each has a myelinated afferent axon (diameter 2.5--7.5 micrometer) that terminates in one to three inner cores. The inner core contains the nonmyelinated receptor portion of the nerve fiber surrounded by numerous cytoplasmic lamellae, a subcapsular connective tissue space, and a perineural capsule of eight to 12 layers. Avian joint receptors are similar to those present in the skin of various birds and Ruffini corpuscles resemble in fine structure equivalent receptors in joint corpuscles of the domestic cat.