Visual and somatosensory information to tongue motoneurons

The experiments were designed to show whether visual inputs and somatosensory signals travelling along the common radial nerves converge onto the same hypoglossal neurons. The hypoglossal neurons (HN) type I (45.77%) and II (33.89%) responded to both visual and somatosensory stimuli. The HN type III (15.26%) responded only to the retinae and type IV (5.08%) only to the forelimb nerves stimulation. In the convergent neurons, types I and II, the influence of the somatosensory stimulation on the hypoglossal visual response was also analyzed. The conditioning stimulation of the radial nerve significantly increased (31%) the hypoglossal response to the less effective retina and significantly reduced (17%) the response to the more effective retina. The recordings of the electromyographic activity of the genioglossus (GM) and the superior longitudinal muscle (SLM), showed that the somatosensory afferents synergize the excitatory effect of the visual messages on the extrinsic tongue muscles (GM) and antagonize the inhibitory effect that visual messages induce on the intrinsic tongue muscles (SLM). The results suggest that visual messages induce tongue reflex responses functionally directed not only to prepare the oral cavity better for food reception, as previously demonstrated, but also to modulate the postural tone of the tongue together with somatosensory signals.