Redirection of the hypoglossal nerve to facial muscles alters central connectivity in human brainstem.

Functional motor control requires perfect matching of central connectivity of motoneurones with their peripheral connections. However, it is not known to what extent central circuitry is influenced by target muscles, either during development or following a lesion. Surgical interventions aimed at restoring function following peripheral nerve lesions provide an opportunity for studying this interaction in the mature human nervous system. We have followed 8 patients in whom the hypoglossal nerve was anastomosed into a lesioned facial nerve, allowing voluntary contractions of the previously paralyzed muscles. We show that, in addition to replacing the facial neurons at peripheral synapses, a new short-latency trigemino-hypoglossal reflex, of the R1 blink reflex type, can be demonstrated in patients showing recovery, implying a sprouting of trigeminal neurons towards hypoglossal motoneurones, over a distance of at least 0.5 cm. These surprising results show an unexpected influence of the periphery in remodelling central connectivity in man.