Cross-modal interactions of auditory and somatic inputs in the brainstem and midbrain and their imbalance in tinnitus and deafness.
OBJECTIVE
This review outlines the anatomical and functional bases of somatosensory influences on auditory processing in the normal brainstem and midbrain. It then explores how interactions between the auditory and somatosensory system are modified through deafness, and their impact on tinnitus is discussed.


METHODS
Literature review, tract tracing, immunohistochemistry, and in vivo electrophysiological recordings were used.


RESULTS
Somatosensory input originates in the dorsal root ganglia and trigeminal ganglia, and is transmitted directly and indirectly through 2nd-order nuclei to the ventral cochlear nucleus, dorsal cochlear nucleus (DCN), and inferior colliculus. The glutamatergic somatosensory afferents can be segregated from auditory nerve inputs by the type of vesicular glutamate transporters present in their terminals. Electrical stimulation of the somatosensory input results in a complex combination of excitation and inhibition, and alters the rate and timing of responses to acoustic stimulation. Deafness increases the spontaneous rates of those neurons that receive excitatory somatosensory input and results in a greater sensitivity of DCN neurons to trigeminal stimulation.


CONCLUSIONS
Auditory-somatosensory bimodal integration is already present in 1st-order auditory nuclei. The balance of excitation and inhibition elicited by somatosensory input is altered following deafness. The increase in somatosensory influence on auditory neurons when their auditory input is diminished could be due to cross-modal reinnervation or increased synaptic strength, and may contribute to mechanisms underlying somatic tinnitus.
Dehmel
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Kresge Hearing Research Institute, 1150 West Medical Center Drive, Room 5434A, Ann Arbor, MI 48109-5616, USA.
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Shore
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D000161:Acoustic Stimulation; D000344:Afferent Pathways / Q000503:physiopathology; D000818:Animals; D001288:Attention / Q000502:physiology*; D001306:Auditory Pathways / Q000503:physiopathology; D001307:Auditory Perception / Q000502:physiology*; D001931:Brain Mapping; D001933:Brain Stem / Q000503:physiopathology*; D017626:Cochlear Nucleus / Q000503:physiopathology; D003638:Deafness / Q000503:physiopathology*; D004558:Electric Stimulation; D005727:Ganglia, Spinal / Q000503:physiopathology; D006801:Humans; D007245:Inferior Colliculi / Q000503:physiopathology; D008636:Mesencephalon / Q000503:physiopathology*; D009433:Neural Inhibition / Q000502:physiology; D009473:Neuronal Plasticity / Q000502:physiology; D009474:Neurons / Q000502:physiology; D012677:Sensation / Q000502:physiology*; D012684:Sensory Thresholds / Q000502:physiology; D009435:Synaptic Transmission / Q000502:physiology; D014012:Tinnitus / Q000503:physiopathology*; D012668:Trigeminal Ganglion / Q000503:physiopathology