Sacculocollic Reflex Arcs in Cats

Abstract

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.

Acoustic Clicks Activate both the Canal and Otolith Vestibulo-Ocular Reflex Pathways in Behaving Monkeys

“…In this study, the minimum latency of crossed, excitatory response was 1.02 ms, which was plausible for a response mediated by a disynaptic circuit given the small difference in the vestibular afferent minimum latency to electric pulses and clicks (0.33 vs. 0.4 ms). Third, as shown by several earlier studies (Uchino et al 1994(Uchino et al , 1996(Uchino et al , 1997Isu et al 2000), the latency analysis involves not only the minimum latency but also the range and distribution of the latency. According to Broussard et al (1995), the median latency of crossed, excitatory response was 1.5 ms.…”

“…The otolith-ocular pathways, however, have been shown to have a very different circuitry. Specifically, the utricule projections make monosynaptic and disynaptic excitatory connections to the ipsilateral abducens neurons and trisynaptic inhibitory connections to the contralateral abducens neurons (Imagawa et al 1995;Schwindt et al 1973;Uchino et al 1994Uchino et al , 1996Uchino et al , 1997. The saccular projections make no connections to the horizontal extraocular motoneurons.…”

Posture, Head Stability, and Orientation Recovery During Vestibular Regeneration In Pigeons

“…Some of these vestibulospinal cells also carry eye movement information, suggesting a role in gaze saccades (Boyle 1983;Boyle et al 1992). Selective stimulation of ei-ther the otolith or the semicircular canal nerves has been shown to provide specific activation patterns in different neck muscles subserving head posture, orientation relative to gravity, and rotational head motion (Bolton et al 1992;Isu et al 1988;Suguichi et al 1995;Uchino et al 1990Uchino et al , 1997Wilson and Maeda 1974). The functional significance of these activation patterns has not been clearly established; however, their involvement in the recovery of posture, head stability, and gaze control seems likely.…”

Vestibular evoked myogenic potentials: optimal stimulation and clinical application

“…VEMPs are proposed to be generated via a disynaptic pathway, beginning in the saccular macula, then via the inferior vestibular nerve, lateral vestibular nucleus, medial vestibulospinal tract, and finally terminating on the motor neurons of the SCM muscle [4][5][6]. Through the efforts during past decade, VEMPs have been validated to reflect the sacculo-collic reflex, and widely used clinically.…”