INHIBITORY COMMISSURAL CONNECTIONS OF NEURONS IN THE TRIGEMINAL MOTORNUCLEUS OF THE RAT

Physiological evidence is presented for the existence of commissural f ibres that cross the midsagittal plane in the medulla of the rat at th e level of the trigeminal motor nucleus (Mo5). These fibres, which hav e their origin in the Mo5, terminated in the contralateral Mo5. Small inhibitory postsynaptic potentials were recorded in jaw-closing motone urones by electrical activation of the commissural fibres; jaw-opening and fusimotorneurones as well as the jaw-closing and jaw-opening refl ex were not affected. Electromyographic recordings from jaw-closing an d jaw-opening muscles in the unrestrained rat showed that masseter act ivity was inhibited by the commissural fibres. These trigeminal commis sural connections might play a part in the co-ordination of bilateral activity of the jaw-closing musculature during unilateral chewing.