Topography and fine structure of proprioceptors in the hagfish, Myxine glutinosa.

Topography and ultrastructural organization of proprioceptors in the striated parietal muscle of the hagfish, Myxine glutinosa, are described. Due to the topographical location of the afferent nerve terminals four subtypes of proprioceptors are distinguished. Type 1-receptor and type 2-receptor are localized in an encapsulated and lamellated corpuscular organ located along and above the myosepta of the parietal muscle. Type 3-receptor is composed of a complex of free en plaque nerve endings within the dense collagen fibre texture at the site where myoseptum, muscle fascia and body fascia fuse to a raphe. Type 4-receptor resembles type 3 but it is localized in the straight collagen fibre bundles of the myoseptum and the body fascia. A proprioceptive function of the nerve terminals is discussed.