Sacculo-ocular reflex connectivity in cats

The otolith system contributes to the vestibulo-ocular reflexes (VOR) when the head moves linearly in the horizontal plane or tilts relative to gravit y. The saccules are thought to detect predominantly accelerations along the gravity vector. Otolith-induced vertical eye movements following vertical linear accelerations are attributed to the saccules. However, information o n the neural circuits of the sacculo-ocular system is limited, and the effe cts of saccular inputs on extraocular motoneurons remain unclear. In the pr esent study, synaptic responses to saccular-nerve stimulation were recorded intracellularly from identified motoneurons of all twelve extraocular musc les. Experiments were successfully performed in eleven cats. Individual mot oneurons of the twelve extraocular muscles - the bilateral superior recti ( SR), inferior recti (IR), superior obliques (SO), inferior obliques (IO). l ateral recti (LR), and medial recti (MR)were identified antidromically foll owing bipolar stimulation of their respective nerves. The saccular nerve wa s selectively stimulated by a pair of tungsten electrodes after removing th e utricular nerve and the ampullary nerves of the semicircular canals. Stim ulus intensities were determined from the stimulus-response curves of vesti bular N-1 field potentials in order to avoid current spread. Intracellular recordings were performed from 129 extraocular motoneurons, The majority of the neurons showed no response to saccular-nerve stimulation. In 17 (30%) of 56 extraocular motoneurons related to vertical eve movement (bilateral S R and IR), depolarizing and/or hyperpolarizing postsynaptic potentials (PSP s) were observed in response to saccular-nerve stimulation. The latencies o f PSPs ranged from 2.3 to 8.9 ms, indicating that the extraocular motoneuro ns received neither monosynaptic nor disynaptic inputs from saccular affere nts. The majority of the latencies of the depolarization, including depolar ization-hyperpolarization, were in the range of 2.3-3.3 ms, Latencies of hy perpolarizations were typically longer than those of depolarizations, Only one contralateral SO motoneuron of 43 recorded oblique extraocular motoneur ons (bilateral SO and 10) showed a depolarization-hyperpolarization in resp onse to saccular-nerve stimulation at a latency of 2.5 ms, None of 30 recor ded horizontal extraocular motoneurons (bilateral LR and MR) responded to s timulation of the saccular nerve. The neural linkage in the sacculo-ocular system is relatively weak in comparison to the utriculo-ocular and sacculo- collic systems, suggesting that the role of the sacculo-ocular system in st abilizing eye position may be reduced when compared with utriculo-ocular an d semicircular canal-ocular reflexes. (C) Springer-Verlag 2000.