J. M. Fredrickson

Projection of the vestibular nerve to the area 3a arm field in the squirrel monkey (Saimiri Sciureus)

SummaryA projection of the vestibular nerve to the anterior bank of the central sulcus was identified. The zone is small and located within the arm field. Surface positive potentials and negative field potentials in deeper cortical layers were evoked within this field by isolated stimulation of the vestibular nerve. Field potentials after isolated stimulation of the facial and auditory nerves were recorded from distinct cortical locations clearly separate from the vestibular field. The tracks of electrodes which recorded the vestibular negative field potentials were histologically located within area 3a. This cytoarchitectonic area extends from the fundus of the central sulcus onto the cortical surface anterior to this sulcus.

Nucleus ventroposterior inferior (VPI) as the vestibular thalamic relay in the rhesus monkey I. Field potential investigation

SummaryIn order to investigate the thalamic relay of the vestibulo-cortical pathway, field potentials were recorded in the rhesus thalamus under pentobarbital anesthesia. Short latency responses (2.5 msec on the average) upon stimulation in isolation of the vestibular nerve were recorded in the inferior ventroposterior nucleus (VPI). These potentials were abolished after transection of the vestibular nerve but were not affected by total cerebellectomy. Projection of VPI neurons to the primary vestibular cortex was demonstrated by antidromic stimulation. Field potentials with latencies of those observed in the vestibular cortex (about 5 msec) in response to vestibular nerve stimulation were recorded in other areas of the thalamus (ventrobasal, ventrolateral, posterior group, including magnocellular medial geniculate nuclei). Thus, the VPI rather than the other nuclei with long latency responses is likely to be the thalamic relay in the vestibulo-cortical path. The close topographical relationship between vestibular and somatic areas in the cortex is parallelled in the thalamus, the VPI being closely related to VPL and VPM nuclei.

Vestibular and somatosensory interaction in the cat vestibular nuclei

SummaryThe vestibular nuclei of cats were explored extracellularly with micropipettes to locate units with a resting discharge rate which responded to rotation in the horizontal plane. These units were examined for somatosensory input from neck and limbs. Fewer than half responded to somatosensory stimulation. The neck region was the body area most effective in influencing unitary activity. The response pattern most often noted was an increase and decrease in discharge frequency when the body was moved towards and away from the recording electrode respectively. Change in discharge rate was observed to be primarily dependant upon neck velocity and not upon absolute neck position. Half of the somato-sensory units received input from either the forelimbs or the hindlimbs, while the remaining half responded to both.

Vestibular responses in the Rhesus monkey ventroposterior thalamus. II. Vestibulo-proprioceptive convergence at thalamic neurons

SummaryThe vestibular thalamic relay in the Rhesus ventrobasal complex, identified in a previous field potential study (part I, Deecke et al., 1974), has now been investigated with neuronal recordings in the thalamus in order to clarify its functional role. In part I, short latency responses (2.5 msec) were found in the corner between VPL, VPM and VPI nuclei, largely including dorsal portions of the VPI nucleus. Field potentials of somewhat longer latency (4–5 msec) were recorded in VPL and in other thalamic nuclei, including the posterior nuclear group.Neuronal responses were recorded in thalamic nuclei of awake flaxedilized Rhesus monkeys. Cells not responding to vestibular stimulation (round window polarisation of either labyrinth) were ignored. The great majority (80%) of those neurons responding to labyrinth polarisation showed convergence with deep somatic (proprioceptive) input from joints and muscles of vertebral column and limbs. 60% of these bimodal neurons responded to movement of cervical joints. Very few vestibularly responsive cells received cutaneous (6.6%), non-optokinetic visual or auditory (2.6% each) input. Proprioceptive fields tended to be large, frequently involving more than one joint, and could be even bilateral. For a few cells the pattern of vestibulo-proprioceptive convergence could be fitted to a coordinated body position that might occur during normal locomotion. 78% of the cells responded to polarisation of both labyrinths, indicating strong bilateral projection.

Vestibular and auditory cortical projection in the guinea pig (cavia porcellus)

SummaryIsolated electrical stimulation of the vestibular nerve in the guineapig yielded surface positive evoked potentials within the rostral portion of the SI forelimb field. The locus of negative field potential reversal in deeper cortical layers was small. The vestibular field is distinct from those of the auditory and facial nerves. Comparative aspects of vestibular cortical location are discussed. The auditory field corresponds with that of other rodents.

Fine structure of the medial and descending vestibular nuclei in normal rats and after unilateral transection of the vestibular nerve.

Cells and neuropil are of similar structure in the descending and medial vestibular nuclei. Two cell types were found: small neurons and larger cells. Three types of axon terminals have been defined: small and large terminals containing spherical vesicles (SV) and terminals with elongated vesicles (EV). Small SV terminals contact perikarya and dendrites, whereas large SV terminals contact fine dendritic branches. EV terminals contact neurons at perikarya and large as well as small dendritic profiles. SV terminals can be presynaptic to EV terminals. Vestibular nerve transection resulted in degeneration of small SV terminals which were found from the first to fifth postoperative day in the ipsilateral nuclei. Glycogen granules were found in various types of axon terminals bilaterally in the vestibular nuclei 3-5 days after right vestibular transection. They were not observed in normal animals or in degenerating vestibular afferent terminals.

Quantification of the vestibulo-ocular reflex and visual-vestibular interaction for the purpose of clinical diagnosis

Pseudorandom vestibular rotatory stimuli covering the normal head movement range (0–6 Hz) and power spectrum analysis techniques are used to clinically evaluate the vestibulo-ocular reflex (v.o.r.). Measurements of compensatory eye movements are recorded during fixation of a stationary target, during fixation of a target moving with the subject and in darkness. The gain above 3 Hz quantifies vestibular function under all these conditions. A frequency-dependent v.o.r. asymmetry indicates the side of a peripheral lesion. The fixation suppression curve at medium frequencies quantifies visual-vestibular interaction. Thus the new test is a powerful diagnostic tool applicable to disease of the labyrinth and central vestibular pathways. This test is evaluated on monkeys before unilateral labyrinthectomy and for an extended period of time after.

Projection of the vestibular nerve to the SI arm field in the cerebral cortex of the cat

Evoked cortical focal potentials from electrical vestibular nerve stimulation were recorded in the Pcd-area in cats anaesthetized with Chloralose or Nembutal. For comparison, additional cortical projections were located for n. rad. superficialis and group Ia muscle afferents from n. rad. prof., n. fibularis prof., n. femuralis ramus muscularis and the motor nerve to the trapezoid muscle. Surface positive potentials, which reversed to negativity in middle cortical layers, were for vestibular nerve stimulation recorded in the S I forelimb field in a small area close to Pcd in the posterior medial part of the deep radial nerve projection field. The location of this field is compared with the vestibulo-cortical projections described earlier for rodents, squirrel monkey, and rhesus monkey. The histology shows that the field was within the cytoarchitectonic 3a area.

Labyrinthine And Somatosensory Convergence Upon Vestibulo-Ocular Units

The vestibular nuclei were investigated in 18 adult cats. Vestibulo-oculo-motor neurons were identified by antidromic stimulation of the ascending medial longitudinal fasciculus (MLF). The neurons were subjected to various stimuli: vestibular, neck, forelimb and hindlimb nerve stimulation on both sides. The recording was extracellular with micropipettes containing Fast Green. Only the medial and the superior vestibular nuclei were found to project to the MLF. All projecting units had input from the labyrinths. Excitatory response latencies to ipsilateral labyrinth stimulation never exceeded 3 msec. Both excitatory and inhibitory response latencies could be distributed into different categories. The majority of the neurons did not receive a somatosensory input, and surprisingly few convergent units could be seen. Peripheral somatosensory information apparently plays a minor role in vestibulo-ocular relations.

Efferent Vestibular Neurons: Electrophysiological Evidence for Axon Collateralization to Cristae Ampullares in the Pigeon Columha Livia)

Extracellular spikes were recorded under general anaesthesia from the cell bodies of efferent vestibular neurons located in the caudal pontine reticular nucleus of the pigeon. Discrete electrical stimuli, applied directly to the three ampullary nerve branches in one labyrinth and to the anterior ramus of the vestibular nerve trunk in the other labyrinth, evoked antidromic spikes which served to identify efferent neurons. Most cells could be antidromically driven only by stimuli to the vestibular nerve trunk (anterior ramus). The majority of cells exhibiting direct axonal connections to one individual semicircular canal crista ampullaris showed axon collateralization to one or two other cristae as well. Sixty percent of the efferent neurons responded with antidromic spikes to ipsilateral labyrinthine stimuli, 34% to contralateral stimuli, and 6% to both. Synaptic activation was observed in a few efferent and adjacent unidentified neurons. It is concluded that efferent neurons often send collaterals to various cristae in one labyrinth, and less frequently, to both labyrinths. Such projections are incompatible with the assumption that vestibular efferents provide a simple control mechanism which is related to the direction of head movement.