Neuronal connections and pathways underlying sacculocollic reflexes were studied by intracellular recordings from neck extensor and flexor motoneurons in decerebrate cat. Bipolar electrodes were placed within the left saccular nerve, whereas other branches of the vestibular nerve were removed in the 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 postsynaptic potentials (EPSPs) in ipsilateral neck extensor motoneurons and di- or trisynaptic (1.8-4.0 ms) EPSPs in contralateral neck extensor motoneurons, and di- and trisynaptic (1.7-3.6 ms) inhibitory postsynaptic potentials (IPSPs) in ipsilateral neck flexor motoneurons and trisynaptic (2.7-4.0 ms) IPSPs in contralateral neck flexor motoneurons. Ipsilateral inputs were about twice as strong as contralateral ones to both extensor and flexor motoneurons. To determine the pathways mediating this connectivity, the lateral part of the spinal cord containing the ipsilateral lateral vestibulospinal tract (i-LVST) or the central part of the spinal cord containing the medial vestibulospinal tracts (MVSTs) and possibly reticulospinal fibers (RSTs) were transected at the caudal end of the C1 segment. Subsequent renewed intracellular recordings following sacculus nerve stimulation indicated that the pathway from the saccular nerve to the ipsilateral neck extensor motoneurons projects though the i-LVST, whereas the pathways to the contralateral neck extensors and to the bilateral neck flexor motoneurons descend in the MVSTs/RSTs. Our data show that sacculo-neck reflex connections display a qualitatively bilaterally symmetrical innervation pattern 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 scheme of the utriculus system. These results suggest a different symmetry plane along which sacculus postural reflexes are organized, thus supplementing the reference planes of the utriculus system and allowing the gravistatic system to represent all three translational spatial degrees of freedom. We furthermore suggest that the sacculocollic reflex plays an important role in maintaining the relative position of the head and the body against the vertical linear acceleration of gravity.

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1. Connections from the utricular (UT) nerve to motoneurons and interneurons in the ipsilateral abducens (AB) nucleus were studied in anesthetized and decerebrated cats. Bipolar electrodes were fixed on the left UT nerve under visual observation. The other branches of the vestibular nerve and the facial nerve were transected in the left inner ear. 2. Stimulation of the UT nerve evoked a small positive-negative (P/N) deflection and a negative (N1) potential in the vestibular nuclei, with mean latencies of 0.56 and 0.84 ms, respectively. In the AB nucleus a small P/N deflection with a mean latency of 0.72 ms was recorded, which was considered as a incoming volley of the UT nerve. 3. Excitatory postsynaptic potentials (EPSPs) were recorded from AB motoneurons with short latencies after UT nerve stimulation. They were classified into two types, M and D. M-type EPSPs, which followed repetitive high-frequency stimuli and were recorded from the majority of AB motoneurons, had latencies ranging from 0.9 to 1.2 ms. Double shocks to the UT nerve evoked EPSPs that had the same latency. It was suggested that the AB motoneurons had monosynaptic connections with the UT nerve. D-type EPSPs, which were recorded from most of the AB motoneurons, had slightly longer latencies ranging from 1.2 to 1.8 ms. They showed temporal facilitation when double shocks were provided to the UT nerve. They did not follow repetitive high-frequency stimuli (< or = 2.5-ms interval). It was suggested that D-type EPSPs were di-synaptically evoked via secondary vestibular neurons. Interneurons in the AB nucleus had the same characteristics as AB motoneurons.(ABSTRACT TRUNCATED AT 250 WORDS)

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Low-dose AVP therapy should be considered as rescue therapy when high-dose catecholamine therapy and/or steroid administration do not produce sufficient increase in the blood pressure. Further investigations are required to prove the efficacy and safety of AVP infusion therapy in preterm infants.