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Location and structure of sensory nerve endings in the periodontium of the third premolar in Monodelphis domestica have been investigated by means of light and electron microscopy. The periodontal cleft of the tooth is apically enlarged. The number of nerve endings increases towards apex. Three types of sensory nerve endings have been observed: free nerve endings, Ruffini nerve endings and lamellated corpuscles. Free nerve endings could only be identified by electron microscopy. Ruffini nerve endings are only incompletely surrounded by lamellae of the terminal Schwann cell. Protrusions of nerve terminals of the Ruffini corpuscles are anchored between bundles of collagen fibers. Small lamellated corpuscles occur exclusively in the apical portion of the periodontium. Ruffini and lamellated corpuscles are considered as part of a masticatory reflex feedback control system. Ruffini corpuscles detect tension, rapidly adapting lamellated corpuscles detect pressure and vibration in the periodontium. Free nerve endings may function as thermoreceptor or nociceptor.

The location and structure of sensory nerve endings was examined in the mucosa and in the gingiva propria adjacent to the third premolar of Monodelphis domestica by light and electron microscopy. The mucous membrane of the gingiva propria is covered with a stratified keratinized squamous epithelium. The mucous membrane of the adjacent inner aspect of the lip and of the vestibulum oris were covered with a nonkeratinized squamous epithelium. Free nerve endings, Merkel nerve endings and lamellated corpuscles were found in all the examined areas. Free nerve endings were located in the connective tissue papillae between the epithelial cones, in the basal layer of the epithelium and directly under the epithelium of the cones. They were innervated by myelinated A-delta- and nonmyelinated C-fibers. The basal layer of the first three to four epithelial cones of the gingiva propria contained single or groups of Merkel nerve endings. The epithelial cones of the oral mucous membrane contained five times more Merkel nerve endings than those of the vestibular mucous membrane. Lamellated corpuscles were mainly found in the plica sublingualis. In the connective tissue below the epithelial cones of the mucous membranes they usually occur with a perineural capsule, in the connective tissue papillae between the cones they occur without a perineural capsule. In the latter position they resemble the Meissner corpuscles of glabrous skin. The density of receptors in the oral mucous membrane of the lower jaw of M. domestica implies that this richly innervated mechanoreceptive area acts as an antagonist of the tongue and provides important information during mastication.