In order to investigate the sensory innervation, the upper cervical spine of a small laboratory marsupial (monodelphis domestica) was examined with serial section light microscopy and re-embedding of selected sections for electron microscopy. Large numbers of free nerve endings supplied by A delta- and C-fibres were found in the longitudinal ligaments and facet joint capsules. Electron microscopically, areas of direct contact between axon and collagen fibres of the surrounding connective tissue separated only by the basal lamina were observed. Such structural adaptations suggest mechanoreceptive or polymodal nociceptive functions. In addition, about 100 small lamellated corpuscles were found in the longitudinal ligaments mainly concentrated around the first intervertebral disk. Electron microscopy shows finger-like processes extending from the axon terminal into the inner core lamellae. These are the likely sites of the mechanoelectric transduction process. Smaller numbers of lamellated corpuscles were seen in the lower intervertebral disks and facet joint capsules. Lamellated corpuscles are known to function as rapidly adapting mechanoreceptors supplementing information supplied by muscle spindles to the CNS about position and movement of the cervical spine.
