Fluorescence image showing two nerves (stained in red and green),
which are responsible for transmitting information from the hair
cells to the brain and from neurons (small green dots) that alter
hair cell sensitivity, respectively.
(September 4, 2015) LMU researchers have shown how signals from the spinal cord adjust the sensitivity of hair cells in the inner ear to accommodate shifts in head position associated with active locomotion – thus ensuring that balance is maintained.
When a jogger sets out on his evening run, the active movements of his arms and legs are accompanied by involuntary changes in the position of the head relative to the rest of the body. Yet the jogger does not experience feelings of dizziness like those induced in the passive riders of a rollercoaster, who have no control over the abrupt dips and swoops to which they are exposed. The reason for the difference lies in the vestibular organ (VO) located in the inner ear, which controls balance and posture. The VO senses ongoing self-motion and ensures that, while running, the jogger unconsciously compensates for the accompanying changes in the orientation of the head. The capacity to adapt and respond appropriately to both slight and substantial displacements of the head in turn implies that the sensory hair cells in the inner ear can react to widely varying stimulus intensities.
In collaboration with Dr. John Simmers at the Centre national de la recherche scientifiqu (CNRS) at the University of Bordeaux, neurobiologists Dr. Boris Chagnaud, Roberto Banchi and Professor Hans Straka at LMU’s Department of Biology II, have now shown, for the first time, how this feat is achieved. Their findings reveal that cells in the spinal cord which generate the rhythmic patterns of neural and muscle activity required for locomotion also adaptively alter the sensitivity of the hair cells in the VO, enabling them to respond appropriately to the broad range of incoming signal amplitudes. The results are reported in the online journal “Nature Communications”. As Boris Chagnaud points out, “we are not really aware of what movement actually involves because our balance organs react immediately to alterations in posture and head position. The hair cells, which detect the resulting changes in fluid flow in the semicircular canals in the inner ear, enable us to keep our balance without any conscious effort.”