Proceedings of the Symposium From Osseointegration to Osseoperception: The Functional Translation PHYSIOLOGICAL CHARACTERISTICS OF LOW-THRESHOLD MECHANORECEPTORS IN JOINTS, MUSCLE AND SKIN IN HUMAN SUBJECTS

SUMMARY 1. The development of microneurography, in which an insu-lated tungsten microelectrode is inserted into an accessibleperipheral or cranial nerve in awake human subjects, hasallowed detailed analyses of the signalling capacities of singlemechanoreceptive afferents from the skin, muscles and joints.For example, we know much about how the two classes ofrapidly adapting (Meissner and Pacinian) and two classes ofslowly adapting (Merkel and Ruffini) cutaneous mechano-receptors encode forces applied normal or tangential to theskin of the hand and the similarities and differences inglabrous versus non-glabrous skin (and receptors associatedwith hairs). We also know about stretch- and force-sensitiveendings in muscle (the muscle spindle and Golgi tendon organ,respectively) and how they behave during passive or activemovements or during isometric contractions. In addition, wehave characterized the firing properties of mechanoreceptorsin the joint capsules of the fingers. However, we know littleabout sensory nerves in the periosteum, other than thatnociceptors and Pacinian corpuscles exist.2. In addition to studies on the physiology of sensoryendings in human subjects, microstimulation through therecording microelectrode has revealed how the brain dealswith the sensory information conveyed by a single afferent.From this work, we know that there is specificity in the sensorychannels: electrical stimulation of a single Meissner orPacinian corpuscle generates frequency dependent illusions of‘flutter’ or ‘vibration’, whereas microstimulation of a singleMerkel afferent can produce a percept of ‘pressure’ andstimulation of a single joint afferent can evoke a sensation of‘joint rotation’. Interestingly, the input from a single Ruffiniending in the skin cannot be perceived and the same is true ofmuscle spindle afferents. So, where does this leave us withosseoperception from the mouth? Given that the periodontalreceptors in the vicinity have been lost, which mechano-receptive endings could encode forces applied to a bone-implanted prosthesis?3. Meissner and Merkel endings have very small receptivefields and respond only to local forces. Pacinian corpuscleshave an exquisite sensitivity to brisk mechanical events andcould respond to such stimuli transmitted through the bone toa remote receptor, but would not be able to encode sustainedforces. Ruffini endings also respond to forces applied remote tothe receptive field and, unlike the Pacinian corpuscles, respondin a sustained fashion, but would their signals be perceived?Like muscle spindles, it is possible that the coactivation ofmany Ruffini endings could provide meaningful information.Finally, as we have seen, the input from a single joint receptorcan be perceived, but they mostly respond at the limits ofjoint rotation, so it is unlikely that any associated with thetemporomandibular joint could contribute to osseoperception.Key words: Golgi tendon organs, joint afferents, musclespindles, osseoperception, tactile afferents.