The distribution and ultrastructure of sensory nerve endings were investigated in the deep lateral elbow region of the rat. Three zones of distribution of mechanoreceptors were distinguished, each in relation to the functional architecture of the connective and muscular tissue in that area: (1) a zone with muscle spindles, Golgi tendon organs, free nerve endings and single small lamellated corpuscles ('muscle-tendon spectrum'), situated in the middle third of the supinator muscle and its superficial aponeurosis; (2) a zone with small lamellated corpuscles and free nerve endings, situated pericapsularly to the humeroradial joint capsule ('shearing spectrum'): this moderately dense, irregular connective tissue is covered by the proximal continuation of the supinator's aponeurosis, and muscle fibers insert from beneath this aponeurosis, which displays, as a part of the joint capsule, a strong collagenous tissue plate; (3) a zone with only free nerve endings within the tendon-like, most proximal part of the supinator's aponeurosis, inserting into the periosteal layer of the lateral humeral epicondyle ('endotenonial spectrum'): it is part of the joint capsule. The ultrastructure of these sensory endings is described and the distribution pattern of the mechanoreceptors observed is discussed in relation to the classification into 'muscle receptors' and 'joint receptors'.
