Trigeminal influences on extensor muscles of the neck

Abstract

The aim of this investigation was to analyze in nembutalized cats, lambs and rabbit the relationships between extraocular muscle proprioception and the electrical activity of the extensor muscles of the neck. These neck muscles responded to the stretching of either one or all of the six extraocular muscles with a bilateral inhibition in the cat, with ipsilateral contraction and contralateral inhibition in the lamb, and with no response in the rabbit. In the lamb the threshold value was higher than the one usually required for the activation of the eye muscle spindles. Contraction of the ipsilateral extensor muscles of the neck was also elicited by electrical stimulation of the first or of the second order neurons of the eye muscle proprioceptive pathway. However, similar responses of the neck musculature occurred after pinching and rubbing the skin of the face and after electrical stimulation of the following trigeminal afferents: (i) the medial part of the semilunar ganglion in the cat and in the rabbit (in these animals a cellular pool innervating the eye muscle proprioceptors has not been found); (ii) the ophthalmic and maxillary trigeminal branches; iii) the supraorbital and the infraorbital nerves. Thus, responses of the extensor muscles of the neck to stretching the eye muscles are not a peculiar effect of the extraocular muscle proprioceptors, but they can also be elicited by stimulating other trigeminal afferents.