PARTIAL RESTORATION OF BLINK REFLEX FUNCTION AFTER SPINAL ACCESSORY FACIAL-NERVE ANASTOMOSIS

Functional motor control requires perfect matching of the central conn ections of motoneurons with their peripheral inputs. It is not known, however, to what extent these central circuits are influenced by targe t muscles, either during development or after a lesion. Surgical inter ventions aimed at restoring function after peripheral nerve lesions pr ovide an opportunity for studying this interaction in the mature human nervous system. A patient was studied in whom the spinal accessory ne rve was anastomosed into a lesioned facial nerve, allowing voluntary c ontractions of the previously paralysed muscles. This procedure, in ad dition to replacing the facial neurons at peripheral synapses, allowed a new short latency trigeminospinal accessory reflex of the R1 blink reflex type to be demonstrated, implying that trigeminal neurons had s prouted towards spinal accessory motoneurons over a distance of at lea st 1 cm. These results show an unexpected influence of the periphery i n remodelling central connectivity in humans. The motoneuronal excitab ility for this R1 reflex response was therefore studied to compare the convergent properties of facial motoneurons (normal side) with those of the spinal accessory motoneurons (operated side) using a classic do uble shock technique with variable interstimulus intervals (conditioni ng test stimulus). On the normal side, conditioning stimuli (to the ip silateral or contralateral infraliminar supraorbital nerve) produced a clearcut facilitation of the R1 blink reflex when the interstimulus i nterval was 30-80 ms. By contrast, a similar procedure had no effect o n the R1 blink reflex mediated via the trigeminal-spinal accessory ref lex are. These data indicate that despite the heterotopic sprouting of some axons from neurons in the trigeminal principal nucleus towards t he XIth nucleus, those motoneurons involved in the newly formed reflex are remain totally inexcitable by other trigeminal afferents and seem unable to ensure a physiological functioning of the normal blink refl ex. Thus the functional relevance of the recovered R1 blink response r emains unclear.