Neuronal substrates of spatial transformations in vestibuloocular and vestibulocollic reflexes

Afferent diversity and the organization of central vestibular pathways

This review considers whether the vestibular system includes separate populations of sensory axons innervating individual organs and giving rise to distinct central pathways. There is a variability in the discharge properties of afferents supplying each organ. Discharge regularity provides a marker for this diversity since fibers which differ in this way also differ in many other properties. Postspike recovery of excitability determines the discharge regularity of an afferent and its sensitivity to depolarizing inputs. Sensitivity is small in regularly discharging afferents and large in irregularly discharging afferents. The enhanced sensitivity of irregular fibers explains their larger responses to sensory inputs, to efferent activation, and to externally applied galvanic currents, but not their distinctive response dynamics. Morphophysiological studies show that regular and irregular afferents innervate overlapping regions of the vestibular nuclei. Intracellular recordings of EPSPs reveal that some secondary vestibular neurons receive a restricted input from regular or irregular afferents, but that most such neurons receive a mixed input from both kinds of afferents. Anodal currents delivered to the labyrinth can result in a selective and reversible silencing of irregular afferents. Such a functional ablation can provide estimates of the relative contributions of regular and irregular inputs to a central neuron's discharge. From such estimates it is concluded that secondary neurons need not resemble their afferent inputs in discharge regularity or response dynamics. Several suggestions are made as to the potentially distinctive contributions made by regular and irregular afferents: (1) Reflecting their response dynamics, regular and irregular afferents could compensate for differences in the dynamic loads of various reflexes or of individual reflexes in different parts of their frequency range; (2) The gating of irregular inputs to secondary VOR neurons could modify the operation of reflexes under varying behavioral circumstances; (3) Two-dimensional sensitivity can arise from the convergence onto secondary neurons of otolith inputs differing in their directional properties and response dynamics; (4) Calyx afferents have relatively low gains when compared with irregular dimorphic afferents. This could serve to expand the stimulus range over which the response of calyx afferents remains linear, while at the same time preserving the other features peculiar to irregular afferents. Among those features are phasic response dynamics and large responses to efferent activation; (5) Because of the convergence of several afferents onto each secondary neuron, information transmission to the latter depends on the gain of individual afferents, but not on their discharge regularity.

Postnatal development of spontaneous tonic activity in mouse medial vestibular nucleus neurones

The timecourse of the appearance and maturation of intrinsic tonic activity in medial vestibular nucleus (MVN) neurones was examined using extracellular single unit recording techniques in slices of the dorsal brainstem prepared from balb/c mice at specific stages during the first postnatal month. In slices from animals at postnatal day 5 (P5), the intrinsic spontaneous discharge rate was low (< 5 impulses/s on average). Over the period P10 to P30 this gradually increased to levels comparable to those of adult MVN cells in vitro. There was a rostro-caudal gradient in the time-course of development of tonic activity, such that cells located rostrally within the MVN developed higher frequencies of tonic discharge earlier than caudally located cells. The opening of the eyes around P14 was associated with a significant increase in the mean discharge rate of caudally located, but not rostrally located, MVN cells.

Morphology of single vestibulospinal collaterals in the upper cervical spinal cord of the cat: III collaterals originating from axons in the ventral funiculus ipsilateral to their cells of origin

Some vestibulospinal pathways are composed of a homogeneous collection of axons with similar intraspinal collaterals. Other pathways contain axons whose collaterals vary in terms of shape, distribution, and complexity. The purpose of the present study was to extend the study of homogeneity versus heterogeneity of vestibulospinal axons to vestibulospinal axons that travel in the ventral funiculus ipsilateral to their cells of origin. Collaterals of these axons were stained following extracellular injections of Phaseolus vulgaris-leucoagglutinin in rostral parts of the medial and descending vestibular nuclei. All collaterals found in C2 and C3 were reconstructed. Collaterals arising from small diameter (0.5 to 2.9 microns) axons usually consisted of a single main branch with short side branches. The termination zones of most of these collaterals formed a narrow path in lamina VIII, but the location of this pathway was highly variable. Collaterals arising from large-diameter (3.0-6.1 microns) axons were usually more complex and consisted of many branches with en passant and terminal boutons that were located in motoneuron nuclei as well as laminae VIII and VII. Despite a relationship between termination zone and the position of the parent axon in the ventral funiculus, the variability in collaterals from large-diameter axons precluded a simple classification scheme. These results demonstrate that diversity, instead of homogeneity, is a characteristic feature of vestibulospinal axons that originate from the medial and descending vestibular nuclei and travel in the ipsilateral ventral funiculus. This pathway is therefore composed of multiple anatomical subunits that, as individuals, may selectively coordinate the activity of specific combinations of interneurons and motoneurons.

Effects of chemical deactivation of the interstitial nucleus of Cajal on the vertical vestibulo-collic reflex induced by pitch rotation in alert cats

The basic circuitry for the vestibulo-collic reflex (VCR) is a three-neuron arc, and this reflex requires the temporal and spatial transformation of vestibular signals to activate the appropriate neck muscles. Signals carried by vestibulo-collic neurons are insufficient to explain the responses of neck muscles. However, it is still unknown as to where the additional signal conversion is performed in the vertical VCR. We examined the effects of chemical deactivation of the interstitial nucleus of Cajal (INC) on the responses of biventer cervicis EMG induced by pitch rotation in the dark in alert head-fixed cats, and compared the results with the vertical vestibulo-ocular reflex (VOR) and also with the VCR and VOR induced by horizontal rotation. Muscimol infusion into the bilateral INC resulted in phase advance and gain drop in both the vertical VCR and the VOR, although the change was smaller in the VCR. The response phases of the horizontal VCR and VOR were not affected. Muscimol infusion outside the INC did not affect the phase of the vertical VCR or VOR. These results suggest that the INC is partially involved in temporal conversion of vestibular signals in the vertical VCR as well as in the VOR evoked by pitch rotation in alert cats.