Termination of vestibulospinal fibers arising from the spinal vestibular nucleus in the mouse spinal cord.

The present study investigated the vestibulospinal system which originates from the spinal vestibular nucleus (SpVe) with both retrograde and anterograde tracer injections. We found that fluoro-gold (FG) labeled neurons were found bilaterally with a contralateral predominance after FG injections into the upper lumbar cord. Anterogradely labeled fibers from the rostral SpVe traveled in the medial part of the ventral funiculus ipsilaterally and the dorsolateral funiculus bilaterally in the cervical cord. They mainly terminated in laminae 5-8, and 10 of the ipsilateral spinal cord. The contralateral side had fewer fibers and they were found in laminae 6-8, and 10. In the thoracic cord, fibers were also found to terminate in bilateral intermediolateral columns. In the lumbar and lower cord, fibers were mainly found in the dorsolateral funiculus bilaterally and they terminated predominantly in laminae 3-7 contralaterally. Anterogradely labeled fibers from the caudal SpVe did not travel in the medial part of the ventral funiculus but in the dorsolateral funiculus bilaterally. They mainly terminated in laminae 3-8 and 10 contralaterally. The present study is the first to describe the termination of vestibulospinal fibers arising from the SpVe in the spinal cord. It will lay the anatomical foundation for those who investigate the physiological role of vestibulospinal fibers and potentially target these fibers during rehabilitation after stroke, spinal cord injury, or vestibular organ injury.

Dendrodendritic connections between the cochlear efferent neurons in guinea pig.

The outer hair cells of organ of Corti are innervated by the efferent neurons of medial olivocochlear neurons (MOC) of the brainstem which modify the cochlear auditory processing and sensitivity. Most of the MOC neurons are excited by a dominant ear and only a small portion of them is excited by both ears resulting in a binaural facilitation. The functional role of the feedback system between the organ of Corti and the cochlear efferent neurons is the protection of the ear from acoustic injury. The rapid impulse propagation in the bilateral olivocochlear system is suggestive of an electrotonic interaction between the bilateral olivocochlear neurons. The morphological background of the MOC pathway is not yet completely characterized. Therefore, we have labeled the bilateral cochlear nerves with different neuronal tracers in guinea pigs. In the anesthetized animals the cochlear nerves were exposed in the basal part of the modiolus and labeled simultaneously with different retrograde fluorescent tracers. By using confocal laser scanning microscope we could detect close appositions between the dendrites of the neurons of bilateral MOC. The distance between the neighboring profiles suggested close membrane appositions without interposing glial elements. These connections might serve as one of the underlying mechanisms of the binaural facilitation mediated by the olivocochlear system.