SACCULOCOLLIC REFLEX ARCS IN CATS

Neuronal connections and pathways underlying sacculocollic reflexes we re studied by intracellular recordings from neck extensor and flexor m otoneurons in decerebrate cat. Bipolar electrodes were placed within t he left saccular nerve, whereas other branches of the vestibular nerve were removed in che inner ear. To prevent spread of stimulus current to other branches of the vestibular nerve, the saccular nerve and the electrodes were covered with warm semisolid paraffin-Vaseline mixture. Saccular nerve stimulation evoked disynaptic(1.8-3.0 ms) excitatory p ost synaptic potentials (EPSPs) in ipsilateral neck extensor motoneuro ns and di- or trisynaptic (1.8-4.0 ms) EPSPs in contralateral neck ext ensor motoneurons, and di- and trisynaptic (1.7-3.6 ms) inhibitory pos tsynaptic potentials (IPSPs) in ipsilateral neck flexor motoneurons an d trisynaptic (2.7-4.0 ms) IPSPs in contralateral neck flexor motoneur ons. Ipsilateral inputs were about twice as strong as contralateral on es to both extensor and flexor motoneurons. To determine the pathways mediating this connectivity, the lateral part of the spinal cord conta ining the ipsilateral lateral vestibulospinal tract (i-LVST) or the ce ntral part of the spinal cord containing the medial vestibulospinal tr acts (MVSTs) and possibly reticulospinal fibers (RSTs) were transected at the caudal end of the C-1 segment. Subsequent renewed intracellula r recordings following sacculus nerve stimulation indicated that the p athway from the saccular nerve to the ipsilateral neck extensor motone urons projects though the i-LVST, whereas the pathways to the contrala teral neck extensors and to the bilateral neck flexor motoneurons desc end in the MVSTs/RSTs. Our data show that sacculo-neck reflex connecti ons display a qualitatively bilaterally symmetrical innervation patter n with excitatory connections to both neck extensor motoneuron pools, and inhibitory connections to both neck flexor motoneuron pools. This bilateral organization contrasts with the unilateral innervation schem e of the utriculus system. These results suggest a different symmetry plane along which sacculus postural reflexes are organized, thus suppl ementing the reference planes of the utriculus system and allowing the gravistatic system to represent all three translational spatial degre es of freedom. We furthermore suggest that the sacculocollic reflex pl ays an important role in maintaining the relative position of the head and the body against the vertical linear acceleration of gravity.