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Thalamic components of the ascending vestibular system
EXPERIMENTAL
NEUROLOGY
Thalamic P. S. BLUM, Departments
64,587-603
Components
(1979)
of the Ascending System
M. J. DAY, M. B. CARPENTER,
of Neurology
and Anatomy, and Surgeons, New Received
Vestibular
AND S. GILMAN’
Columbia University, College York, New York 10032
January
of Physicians
22, 1979
A combined anatomical and physiological approach was used to identify the tha!amic nuclei that relay vestibular activity to the cerebral cortex at short latency in the cat. For the anatomical experiments, electrical stimulation was applied to the vestibular nerve, the cortical sites showing maximal amplitude responses were defined, and horseradish peroxidase was injected in these sites. Two days later, the animals were killed and brain sections were processed to localize enzyme reaction products in thalamic neurons. After either anterior suprasylvian injection or posterior cruciate region injection, most labeled neurons were in the ventral posterolateral nucleus. A few labeled neurons were found in the intralaminar and posterior groups of nuclei. In separate physiological experiments, responses to vestibular nerve stimulation and cerebral cortical stimulation were recorded from the thalamus. Short-latency (~3.5 ms), large-amplitude evoked potentials from vestibular nerve stimulation and antidromic field potentials from cortical stimulation were recorded within the ventral basal complex and the most rostra1 portions of the posterior group of thalamic nuclei. These data indicate that neurons in the ventral basal complex and the region between the ventral basal complex and the posterior group relay vestibular activity to both the anterior suprasylvian and posterior cruciate regions of the cerebral cortex. Abbreviations: AS-anterior suprasylvian, PC-posterior cruciate, VPM-ventral posteromedial nucleus, VPL-ventral posterolateral nucleus, MGmc-magnocellular portion of the medial geniculate nucleus, HRP-horseradish peroxidase, PO-posterior nucleus. r This work was supported in part by U.S. Public Health Service grants NS11307 and 01538. The present address of Dr. Blum is Department of Physiology, Thomas Jefferson University, Philadelphia, PA 19107; of Dr. Day, the Department of Physical Therapy, University of Pennsylvania, Philadelphia, PA 19174; of Dr. Carpenter, the Department of Anatomy, University of the Uniformed Services Health Sciences, Bethesda, MD 90014; and of Dr. Gilman, the Department of Neurology, University of Michigan, Ann Arbor, MI 48109. 587
0014-4886/79/060587-17$02.00/O Copyright 0 1979 by Academic Press, Inc. All rights of reproduction in any form reserved.
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INTRODUCTION Activity originating in the vestibular nerve of the cat reaches two small sites in the cerebral cortex after a latency of 4 to 5 ms. These sites are in the anterior fold of the anterior suprasylvian (AS) gyrus and in the posterior sigmoid gyrus immediately rostra1 to the posterior cruciate (PC) dimple (1, 5, 12, 15, 19). Equivalent sites have been described for other animals (8, 16). Although the vestibular projections to the cerebral cortex have been delineated, the thalamic nuclei that relay vestibular activity to these sites in the cortex at short latency have not been identified definitively. Mickle and Ades (15) presented evidence that the thalamic relay to cortex is in a small region between the medial geniculate nucleus (MG) and ventral posterolateral nucleus (VPL). Subsequently, the magnocellular (mc) portion of the MG was implicated because it contained potentials evoked by vestibular stimulation at short latency (1, 5, 17, 22). In addition, lesions destroying MGmc and other portions of the posterior nucleus (PO) of the thalamus eliminated vestibular responses recorded in the cortex (17, 22). Other thalamic nuclei have been implicated as well. Short-latency potentials evoked by vestibular nerve stimulation were recorded in the ventral posterior inferior, ventral posteromedial (VPM), ventral lateral, and intralamellar nuclei (5,7, 20, 22). In a recent anatomical study, the retrograde transport of the enzyme horseradish peroxidase (HRP) was used to examine the sites of thalamic neurons projecting to the vestibular receptive area of the cortex (13). It was concluded that the MGmc projects to the AS and that the VPL projects to the PC regions of the cerebral cortex. Characterization of the thalamic nuclei that relay vestibular activity to the cerebral cortex has been hampered by many problems. Those problems include (i) the inadvertent stimulation of ascending systems other than vestibular (5), (ii) the small and variable amplitude of thalamic potentials evoked by vestibular stimulation (3, 5, 19), and (iii) the difficulty in interpreting function from anatomic findings alone. The present study was designed to delineate the thalamic components of the ascending vestibular projections with a combination of anatomic and physiologic techniques. Initially, HRP was injected into regions of cerebral cortex showing electrophysiologic responses to vestibular nerve stimulation. The retrograde transport of this enzyme made it possible to identify the regions of thalamus projecting to the areas of vestibular representation in the cortex. In separate experiments, potentials evoked by electrical stimulation of the vestibular nerve were recorded in each experiment from the entire region of thalamus identified in anatomical studies as potentially vestibular relay sites. As a result, it was possible to
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determine the distribution of amplitudes and latencies of the responses recorded in the thalamus after vestibular nerve stimulation. In the same animals, thalamic regions were explored for an antidromic field potential after stimulation of the vestibular projection areas of the cerebral cortex. Data from these experiments suggest that neurons responsible for relaying vestibular information to the cortex are found in the VPL, VPM, and the most rostra1 portion of the PO. A preliminary account of these data was published (6). Data on the sensory properties of neurons in these thalamic nuclei are presented separately (4). METHODS Anatomicaf Studies. Adult cats anesthetized with sodium pentobarbital (35 mg/kg, i.p.) were mounted in a head holder. The utricular and superior canal branches of the vestibular nerve (2) were exposed by microdissection through the right bulla. A stainless-steel bipolar electrode was positioned on the vestibular nerve, the bulla was filled with petroleum jelly, and the electrode was cemented to the cut edge of the bulla. Two areas of cerebral cortex were exposed contralateral to the implanted electrodes, the auditory projection area in the middle ectosylvian gyrus and the vestibular projection area in either the AS or PC gyri. To monitor current spread during electrical stimulation of the vestibular nerve, one monopolar ball electrode was positioned on the cortex of the auditory projection area and another on the cortex of the vestibular projection area. The vestibular nerve was stimulated at 0.3 Hz initially with sufficient intensity to record short-latency (~5 ms) responses in both areas of cortex. Because there is no known vestibular projection to the auditory receptive area of the cortex (5, 17), it was assumed that the response recorded in the auditory projection area resulted from current spread within the bulla from the vestibular to the nearby cochlear nerve. The stimulus intensity then was lowered progressively until a response could be detected in the vestibular projection area but not in the auditory projection area. At this stimulus intensity there was no longer stimulus spread to the cochlear nerve. The electrode on the vestibular projection area of the cortex was repositioned until a response with maximum amplitude was found. In experiments involving injections into both cerebral hemispheres, the procedure described above was followed separately for each side (Fig. 1A). Horseradish peroxidase (Sigma Type VI) was injected in volumes of 0.2 ~1 (30% solution) into the cerebral cortex at a depth of approximately 1 mm at the site of the maximum amplitude response to vestibular nerve stimulation. The total volume of enzyme was injected during 25 min, and the needle remained in place an additional 30 min. After the needle was
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removed, the muscles and skin were closed. After 2 days, the animals were anesthetized deeply, and the brain was perfused with an intracardiac infusion of 6% dextran in saline followed by a fixative consisting of 2.5% glutaraldehyde and 0.5% paraformaldehyde in phosphate buffer atpH 7.4. The brain was removed and cut in the coronal plane into blocks 5 to 10 mm thick. These blocks were placed in fixative plus 30% sucrose for 1 day and then transferred to phosphate buffer (pH 7.4) containing 30% sucrose for 1 to 2 days before frozen sections were cut at 40 pm. Sections were made through the injection site, basal ganglia, and thalamus. Two sections in five were treated with diaminobenzidine in the presence of hydrogen peroxide to visualize HRP within neurons. Sections stained with cresyl violet and unstained sections were used to identify neurons labeled with enzyme. One control brain was examined to determine if endogenous peroxidase activity was present in thalamic neurons (23). All procedures described above were carried out except for the substitution of saline solution for HRP. No evidence was found for the presence of endogenous peroxidase
A
FIG. 1. A-representation of the injection sites in the cerebral cortex. Dotted lines in the anterior suprasylvian (AS) and posterior cruciate (PC) regions of the cerebral cortex show the approximate region where horseradish peroxidase was injected. Typical averaged evoked potentials are shown that were recorded in AS and PC cortex in response to vestibular nerve stimulation. B-distribution of labeled neurons in the thalamus after injection of the AS (left) and PC (right) regions of the cerebral cortex. Abbreviations in text and (10) for this and succeeding figures.
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activity in the neurons of the thalamus. The positions of labeled neurons were determined using the atlases of Jasper and Ajmone-Marsan (lo), Snider and Niemer (21), and Rinvik (18). Electrophysiological Studies. Adult cats were anesthetized with (Ychloralose (60 mg/kg, i.v.), and cannulas were placed in the trachea, a femoral vein, and a femoral artery. Arterial blood pressure was monitored, and experiments were terminated if it decreased to less than 80 mm Hg. Body temperature was monitored rectally and maintained above 37°C with a thermostatically controlled heating pad. Microdissection of the right bulla exposed the cochlear nerve (12) and the utricular and superior canal branches of the vestibular nerve. A stainless-steel bipolar electrode was positioned on the vestibular nerve and an identical electrode on the cochlear nerve. The bulla was filled with petroleum jelly, and the electrodes were cemented to the cut edge of the bulla. To monitor current spread during electrical stimulation of those nerves, bipolar concentric electrodes were inserted stereotaxically in the ipsilateral lateral vestibular and dorsal cochlear nuclei. The following procedure was used to determine the appropriate stimulus intensity to stimulate the vestibular nerve without current spread to the cochlear nerve. Vestibular nerve stimulation was applied with sufficient intensity to yield an evoked response detected with a signal averager in both vestibular and cochlear nuclei. The stimulus voltage then was lowered progressively until a response persisted in the vestibular nucleus with no response recorded in the cochlear nucleus. An analogous procedure was followed to determine the stimulus intensity required to stimulate the cochlear nerve without current spread to the vestibular nerves. Recordings were taken from the cochlear and vestibular nuclei at regular intervals throughout each experiment, and stimulus intensities were adjusted as necessary to prevent spread of current. Square wave (0. 1-ms duration, 1 to 10 V) electrical stimuli were applied to sites on the AS and PC regions of cerebral cortex from which maximum amplitude responses to vestibular nerve stimulation had been recorded in the same experiment. These sites were identified by exploration of the surface of the cortex for a maximum amplitude response to stimulation of the contralateral vestibular nerve. Recordings were taken from the thalamus with bipolar concentric electrodes. To facilitate the exploration of a large region of thalamus in each experiment, an array of four electrodes was used, with the electrodes spaced 2.0 mm apart, and recordings were taken simultaneously from the four sites. The signal from each electrode was fed into a preamplifier with a bandpass of 0.3 to 1000 Hz, viewed with an oscilloscope, and recorded on FM magnetic tape. In each recording site the responses to 16 vestibular nerve stimuli at 0.3 Hz were averaged.
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The electrode array was oriented in the frontal plane contralateral to the vestibular nerve stimulated, with the most medial electrode 3 mm and the most lateral electrode 9 mm from the midline. Recordings were taken from sites in the frontal plane at 1.5-mm intervals between anterior +3 and + 11. In half the experiments the recording array was introduced first into the most anterior site and subsequent sites were sampled by proceeding sequentially in a caudal direction. In the remaining experiments the order of recording was reversed, with the electrode array progressing in a rostra1 direction from the most caudal site. In each frontal plane, recordings were made at preselected horizontal positions from +5 to -3. This recording procedure resulted in a three-dimensional matrix of 120 to MO recording sites per animal distributed throughout the ventral, intralaminar, and posterior groups of nuclei. At each recording site the response was obtained to 0.3-Hz vestibular stimulation and l- to IOOO-Hz stimulation of the AS and PC cortex. The cortical response to vestibular nerve stimulation was recorded both before and after the thalamic recordings to ascertain that the pathway from vestibular nerve to cortex remained functional throughout the experiment. Experiments were included for analysis only if the potentials evoked by vestibular nerve stimulation could be obtained from the cortex both before and after thalamic recordings. After recordings were completed from the most ventral row of sites in the last frontal position, a small electrolytic lesion was made with each thalamic electrode and with the electrodes in the vestibular and cochlear nuclei by passing 10 to 20pA of current for 10 to 20 s. The animal then was perfused intracardially with saline followed by 10% buffered formalin. Frozen sections were cut at 40 to 50pm and stained with cresyl violet to reconstruct the position of the electrode tracts in the thalamus and brain stem. RESULTS Anatomical Experiments. Forty-eight hours after HRP had been injected into either the AS or PC site, the enzyme spread in the cortical mantle varied in diameter from 0.2 to 2.0 mm (Figs. 2A, B). Within this region thin strands of fibers were stained and many of the neurons contained HRP granules. No neurons were labeled at the AS site after injection in the PC region or in the PC region following injection at the AS site. Subcortically, labeled neurons were found only within the ipsilateral thalamus. The majority of these labeled neurons was assembled in a cluster running throughout the rostral-caudal extent of the VPL. Injection in the anterior suprasylvian cortex. Injections were made into the vestibular projection area of the AS region of six cerebral hemispheres. Labeled neurons were located in clusters in the dorsolateral region of the
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FIG. 2. A-photomicrographs of the site of injection in the posterior cruciate cortex of 0.2 ~130% horseradish peroxidase in saline. B-photomicrograph of the site of injection in the anterior suprasylvian (AS) cortex. C- dark-field photomicrograph of a cluster of labeled neurons in the ipsilateral ventral posterolateral nucleus after injection of the anterior suprasylvian region. Calibration bar is 2 mm in A and B.
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FIGURE 2 (Continued)
VPL (Figs. lB, 2C). These clusters could be found throughout the anterior posterior extent of this nucleus. A few single, labeled neurons also were found in the VPM, lateral posterior, ventral lateral, and central medial nuclei. A few scattered neurons were found in various regions of the PO, including the MGmc. There was great consistency between animals in the distribution of labeled neurons within the VPL after injection in the AS region. Injection in the posterior cruciate cortex. Injections were made into the vestibular projection region of the PC gyrus of seven cerebral hemispheres. Again, the majority of labeled neurons was found in the VPL; however, these neuronal clusters were situated more rostrally, ventrally, and medially than those observed after AS injection. Labeled neurons also were found in the VPM, ventral lateral, lateral posterior, central medial, and posterior nuclei (Fig. 1B). There was greater variability in the distribution of labeled neurons in the VPL nucleus after PC injection than after AS injection. Znj’ection in the anterior suprasylvian andposterior cruciate cortex in the same experiment. In three animals, injections were made in the AS region
of one hemisphere
and in the PC region of the opposite hemisphere.
This
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permitted a comparison in the same animal of the sites of labeled neurons in corresponding thalamic nuclei on either side of the midline. The sites found in these experiments were consistent with the results obtained after unilateral injection. In Fig. 3 for example, neurons in the left thalamus labeled by an injection in the left AS region are in an analogous area with neurons in the right thalamus labeled by an injection in the right PC region. Electrophysiological Experiments. Potentials evoked by vestibular nerve stimulation varied considerably in latency and amplitude. The latencies to onset of responses were 1.5 to 25 ms and the amplitudes 5 to 200 PV (Fig. 4). During several electrode penetrations there was a reversal in polarity of the evoked potential as the recording electrode was advanced vertically between two recording sites (Fig. 4, traces A, B). Vestibular responses could be recorded throughout the region of thalamus sampled (Fig. 5). Short- (~3.5 ms) and long-latency evoked potentials were found in similar thalamic sites, including the ventral, intralaminar, and posterior groups of nuclei. Within each experiment the amplitudes of the short- and long-latency evoked potentials were placed in two separate
FIG. 3. Photomicrograph of a frontal section through the thalamus. Two injections of horseradish peroxidase were made in a single preparation, one in the left anterior suprasylvian region of the cortex and the other in the right posterior cruciate region of the cortex. Labeled neurons can be seen within an analogus region of the ventral posterolateral nucleus on each side of midline. Calibration bar is 2 mm.
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pvolts I 10 c-l
5 mscc FIG. 4. Potentials recorded within the thalamus after vestibular nerve stimulation. A and B-potentials recorded within the ventral basal complex. A was recorded 1 mm dorsal to site of recording of potential in B. C-potential recorded within the posterior nucleus.
categories and ranked from highest to lowest. Sites containing evoked potentials in the 50th percentile or greater in amplitude are enclosed in dotted lines in Fig. 5. Although short-latency evoked potentials were located throughout the regions of thalamus investigated, the largestamplitude potentials were found in a much smaller region. Large-amplitude, short-latency potentials evoked by vestibular nerve stimulation were found in the VPL, VPM, and the most rostra1 portions of the PO. Stimulation of the AS and PC regions of the cerebral cortex evoked antidromic field potentials in the thalamus. These potentials were recognized by their short (0.5 to 0.7 ms) latencies and their persistence during highfrequency (500 to 1000 Hz) stimulation. The potentials were found most frequently in the VPL and VPM and less frequently in the PO nucleus (Fig. 6). Figure 7 contains a representation of the sites showing potentials evoked at short latency by vestibular nerve stimulation and also shows an antidromic field potential evoked by stimulation of either the AS or PC region of the cortex. These sites include the ventral basal portion of the thalamus and the region between the ventral basal part of the thalamus and the most anterior portion of the posterior group of nuclei. Thirty-five percent of these sites showed a large-amplitude response as defined above. No sites were found in the MGmc, the lateral posterior, ventral lateral, or mtra-
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latency
X
short
0
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FIG. 5. Sites of recording of potentials evoked by vestibular nerve stimulation in five experiments. A line encloses sites in which the recorded potentials were in the upper 50th percentile of amplitude in each experiment. Section A is at the most rostra1 site, section D is at the most caudal site.
laminar nuclei with both short-latency antidromic field potentials.
vestibular
evoked potentials
and
DISCUSSION
This investigation represents a combined anatomical and physiological approach to the identification of the thalamic nuclei that relay vestibular
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FIG. 6. Sites of recording of antidromic field potentials after stimulation of anterior suprasylvian (AS, A) and posterior crucidate (PC, B) regions of the cerebral cortex in five experiments.
information to the cerebral cortex at short latency. Anatomical procedures were used to identify thalamic neurons with axons coursing to the vestibular projection regions in the cerebral cortex. Electrophysiological procedures were used to identify the thalamic regions showing responses evoked by stimulation of the vestibular nerve and antidromic field potentials evoked by stimulation of the vestibular projection regions in the cerebral cortex.
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FIG. 7. All individual sites showing both short-latency potentials evoked by vestibular nerve stimulation and an antidromic field potential from cortical stimulation.
The anatomical studies demonstrated that the vestibular projection regions in the cerebral cortex receive inputs from a number of thalamic nuclei, including the VPL, VPM, PO, and intralaminar nuclei. Projections to the anterior suprasylvian region were found to originate from more dorsolateral and caudal regions within the VPL than the projections to the PC region. Nevertheless, projections to both cortical vestibular projection regions appeared to originate in adjacent and probably overlapping sites in the VPL. There was no evidence to indicate that projections to the AS region originate in a site separate from projections to the PC, as reported by
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Liedgren et al. (13). Data by Hand and Morrison (9) support our conclusion that neurons in the common region of the ventral thalamus may send axons to both vestibular projection regions of the cortex because they found degenerating fiber terminals in both the AS and PC regions of the cerebral cortex after making small lesions within the VPL. The anatomical studies also demonstrated that there is no direct interconnection between the two vestibular-responsive regions of the same cerebral hemisphere, as labeled neurons were not found in the AS region after injection of the PC or in the PC region after injection of the AS region. This information supports the notion that there are two direct and possibly parallel pathways mediating vestibular information from thalamus to the cortex without direct connections between the two cortical sites. The electrophysiological studies demonstrated that potentials evoked by vestibular nerve stimulation can be recorded at short latency from numerous thalamus nuclei, including VPL, VPM, PO, and intralaminar nuclei. These findings are compatible with those reported previously (5, 19, 20, 22) and indicate that many thalamic nuclei receive vestibular information. The sites having short-latency evoked potentials with the greatest amplitude, however, are located in the VPL and VPM and in the region between the VPL and PO. Attempts to identify the thalamic nuclei that transmit vestibular information to the two cortical projection regions at short latency were carried out by stimulating the two cortical projection sites and recording antidromic field potentials in these nuclei. Similar potentials, recorded at short latency and persisting at high stimulus frequencies, were interpreted by Deecke et al. (7) and by Kennedy and Towe (11) to be the sum of antidromic activity in individual fibers. The distribution of antidromic field potentials in the thalamus was similar to the distribution of sites thought to represent thalamocortical projection regions from the anatomical studies in the present investigation. We conclude, consequently, that projections to both the AS and PC regions of the cortex originated from similar regions of the thalamus. No evidence was found for a short-latency response representing a projection from the MGmc to either the AS or PC regions of the cortex. These data consist of three separate features that help to define the role of the thalamus in the processing of vestibular information en route to the cortex. These features are the identification of the thalamic nuclei that (i) send axons to the vestibular projection regions of the cerebral cortex, (ii) receive ascending vestibular activity at short latency, and (iii) have antidromic responses to stimulation of the vestibular projection sites in the cerebral cortex. The nuclei that have all these features in common are limited to the VPL, VPM, and the most rostra1 portion of the PO. These findings are not compatible with the idea that the MGmc serves as the principal thalamic relay for rapid mediation of vestibular activity to the
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cerebral cortex. This idea stemmed from the observation that responses to vestibular stimulation could be recorded at short latency from this nucleus and from the results of lesion experiments (5, 17). Blum et al. (3), however, did not find units in this region showing a short-latency response to vestibular stimulation and concluded that the vestibular relay must occur elsewhere in the thalamus. Evidence that other components of the thalamus may be involved came from the work of Sans er al. (20) who identified the VPL as a site important for ascending vestibular activity. Subsequently, Deecke et al. (7) identified the nucleus ventral posterior inferior in the monkey as the important relay nucleus for the cortical projection of vestibular information. These investigators and associates (14) later revised this conclusion, reporting evidence on the basis of single-unit data in the monkey for a vestibular relay also in the.VPL and PO. The present findings support those conclusions through the combined use of anatomical and physiological techniques to identify the regions of the thalamus that relay vestibular information at short latency to the cerebral cortex. We have identified the VPL, VPM, and the most rostra1 portion of the PO as the thalamic regions where the highest-amplitude and shortest-latency responses occur after vestibular nerve stimulation and, in addition, where neurons are present that project to vestibular-activated regions of the cerebral cortex. It is concluded, therefore, that neurons in VPL, VPM, and rostra1 PO relay vestibular information to the cerebral cortex at short latency. REFERENCES 1. ABRAHAM, L., P. COPACK, AND S. GILMAN. 1977. Brainstem pathways for vestibular projections to cerebral cortex in the cat. Exp. Neurol. 55: 436-448. 2. ANDERSSON, S., AND B. E. GERNANDT. 1954. Cortical projections ofthe vestibular nerve in the cat. Acta Otolaryngol. Suppl. 116: lo- 18. 3. BLUM, P. S., L. ABRAHAM, AND S. GILMAN. 1979. Vestibular, auditory, and somatic input to posterior thalamus. Exp. Brain Res. 34: l-9. 4. BLUM, P. S., AND S. GILMAN. 1979. Vestibular, somatosensory, and auditory input to the thalamus of the cat. Exp. Neurof. (in press). 5. COPACK, P., N. DAFNY, AND S. GILMAN. 1972. Neurophysiological evidence of vestibular projection to thalamus, basal ganglia, and cerebral cortex. Pages 309-339 in T. L. FRIGYESI, E. RINVIK, AND M. D. YAHR, Eds., Corticothalamic Projections and Sensorimotor Activities. Raven Press, New York. 6. DAY, M., P. BLUM, M. B. CARPENTER, AND S. GILMAN. 1976. Thalamic components of ascending vestibular projections. Sot. Neurosci. Abstr. 2: 1058. 7. DEECKE, L., D. W. F. SCHWARZ, AND .I. M. FREDRICKSON. 1974. Nucleus ventroposterior inferior (VPI) as the vestibular thalamic relay in the rhesus monkey. Exp. Brain Res. 20: 88- 100. 8. FREDRICKSON,J. M., U. FIGGE, P. SCHEID, AND H. H. KORNHUBER. 1966. Vestibular nerve projection to the cerebral cortex of the rhesus monkey. Exp. Brain Res. 2: 318-327.
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9. HAND, P. J., AND A. R. MORRISON. 1970. Thalamocortical projections from the ventrobasal complex to somatic sensory areas I and II. Exp. Neural. 26: 291-308. 10. JASPER, H. H., AND C. AJMONE-MARSAN. 1954.A Stereotaxic Atlasof the Diencephalon of the Cat. The National Research Council of Canada, Ottawa. 11. KENNEDY, T. T., AND A. L. TOWE. 1962. Identification of a fast lemnisco-cortical system in the cat. J. Physiol. (London) 160: 535-547. 12. LANDGREN, A., H. SILVENIUS, AND D. WOLSK. 1967. Vestibular, cochlear and trigeminal projections to the cortex in the anterior suprasylvian nucleus of the cat. J. Physiol. (London) 191: 561-573. 13. LIEDGREN, S. R. C., K. KRISTENSSON, B. LARSBY, AND L. M. ODKVIST. 1976. Projections of thalamic neurons to cat primary vestibular cortical fields studied by means of retrograde axonal transport of horseradish peroxidase. Exp. Brain Res. 24: 237-243. 14. LIEDGREN, S. R. C., A. C. MILNE, A. M. RUBIN, D. W. F. SCHWARZ, AND R. D. TOMLINSON. 1976. Representation of vestibular afferents in somatosensory thalamic nuclei of the squirrel monkey (Sarmiri scuireus). J. Neurophysiol. 39: 601-612. 15. MICKLE, W. A., AND H. W. ADES. 1954. Rostra1 projection pathway of the vestibular system. Am. J. Physiol. 176: 243-246. 16. ODKVIST, L. M., A. M. RUBIN, AND D. W. F. SCHWARZ. 1973. Vestibular cortical projection in the rabbit. J. Comp. Neural. 149: 117- 120. 17. POTEGAL, M., P. COPACK, J. M. B. V. DEJONG, G. KRALJTHAMER, AND S. GILMAN. 1971. Vestibular input to the caudate nucleus. Exp. Neural. 32: 448-465. 18. RINVIK, E. 1968. A re-evaluation of the cytoarchitecture of the ventral nucleus complex of the cats thalamus on the basis of corticothalamic connections. Brain Res. 8: 237-254. 19. ROUCOUX-HANUS, M., AND N. BOISACQ-SCHEPENS. 1977. Ascending vestibular projections: further results at cortical and thalamic levels in the cat. Exp. Brain Res. 29: 283-292. 20. SANS, A., J. RAYMOND, AND R. MARTY. 1970. Responses thalamique et corticales ti la stimulation electrique du nerf vestibulaire chez la chat. Exp. Brain RPS. 10: 265-275. 21. SNIDER, R. S., AND W. T. NIEMER. 1961. A Stereotaxic Atlas of the Cat Brain. Univ. of Chicago Press, Chicago. 22. SPIEGEL, E. A., E. G. SZEKELY, AND P. L. GILDENBERG. 1965. Vestibular responses in midbrain, thalamus, and basal ganglia. Arch. Nemo/. 12: 258-269. 23. WONG-RILEY, M. T. 1976. Endogenous peroxidase activity in brain stem neurons as demonstrated by their staining with diaminobenzidine in normal squirrel monkeys. Brain Res. 108: 257-277.
NEUROLOGY
Thalamic P. S. BLUM, Departments
64,587-603
Components
(1979)
of the Ascending System
M. J. DAY, M. B. CARPENTER,
of Neurology
and Anatomy, and Surgeons, New Received
Vestibular
AND S. GILMAN’
Columbia University, College York, New York 10032
January
of Physicians
22, 1979
A combined anatomical and physiological approach was used to identify the tha!amic nuclei that relay vestibular activity to the cerebral cortex at short latency in the cat. For the anatomical experiments, electrical stimulation was applied to the vestibular nerve, the cortical sites showing maximal amplitude responses were defined, and horseradish peroxidase was injected in these sites. Two days later, the animals were killed and brain sections were processed to localize enzyme reaction products in thalamic neurons. After either anterior suprasylvian injection or posterior cruciate region injection, most labeled neurons were in the ventral posterolateral nucleus. A few labeled neurons were found in the intralaminar and posterior groups of nuclei. In separate physiological experiments, responses to vestibular nerve stimulation and cerebral cortical stimulation were recorded from the thalamus. Short-latency (~3.5 ms), large-amplitude evoked potentials from vestibular nerve stimulation and antidromic field potentials from cortical stimulation were recorded within the ventral basal complex and the most rostra1 portions of the posterior group of thalamic nuclei. These data indicate that neurons in the ventral basal complex and the region between the ventral basal complex and the posterior group relay vestibular activity to both the anterior suprasylvian and posterior cruciate regions of the cerebral cortex. Abbreviations: AS-anterior suprasylvian, PC-posterior cruciate, VPM-ventral posteromedial nucleus, VPL-ventral posterolateral nucleus, MGmc-magnocellular portion of the medial geniculate nucleus, HRP-horseradish peroxidase, PO-posterior nucleus. r This work was supported in part by U.S. Public Health Service grants NS11307 and 01538. The present address of Dr. Blum is Department of Physiology, Thomas Jefferson University, Philadelphia, PA 19107; of Dr. Day, the Department of Physical Therapy, University of Pennsylvania, Philadelphia, PA 19174; of Dr. Carpenter, the Department of Anatomy, University of the Uniformed Services Health Sciences, Bethesda, MD 90014; and of Dr. Gilman, the Department of Neurology, University of Michigan, Ann Arbor, MI 48109. 587
0014-4886/79/060587-17$02.00/O Copyright 0 1979 by Academic Press, Inc. All rights of reproduction in any form reserved.
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INTRODUCTION Activity originating in the vestibular nerve of the cat reaches two small sites in the cerebral cortex after a latency of 4 to 5 ms. These sites are in the anterior fold of the anterior suprasylvian (AS) gyrus and in the posterior sigmoid gyrus immediately rostra1 to the posterior cruciate (PC) dimple (1, 5, 12, 15, 19). Equivalent sites have been described for other animals (8, 16). Although the vestibular projections to the cerebral cortex have been delineated, the thalamic nuclei that relay vestibular activity to these sites in the cortex at short latency have not been identified definitively. Mickle and Ades (15) presented evidence that the thalamic relay to cortex is in a small region between the medial geniculate nucleus (MG) and ventral posterolateral nucleus (VPL). Subsequently, the magnocellular (mc) portion of the MG was implicated because it contained potentials evoked by vestibular stimulation at short latency (1, 5, 17, 22). In addition, lesions destroying MGmc and other portions of the posterior nucleus (PO) of the thalamus eliminated vestibular responses recorded in the cortex (17, 22). Other thalamic nuclei have been implicated as well. Short-latency potentials evoked by vestibular nerve stimulation were recorded in the ventral posterior inferior, ventral posteromedial (VPM), ventral lateral, and intralamellar nuclei (5,7, 20, 22). In a recent anatomical study, the retrograde transport of the enzyme horseradish peroxidase (HRP) was used to examine the sites of thalamic neurons projecting to the vestibular receptive area of the cortex (13). It was concluded that the MGmc projects to the AS and that the VPL projects to the PC regions of the cerebral cortex. Characterization of the thalamic nuclei that relay vestibular activity to the cerebral cortex has been hampered by many problems. Those problems include (i) the inadvertent stimulation of ascending systems other than vestibular (5), (ii) the small and variable amplitude of thalamic potentials evoked by vestibular stimulation (3, 5, 19), and (iii) the difficulty in interpreting function from anatomic findings alone. The present study was designed to delineate the thalamic components of the ascending vestibular projections with a combination of anatomic and physiologic techniques. Initially, HRP was injected into regions of cerebral cortex showing electrophysiologic responses to vestibular nerve stimulation. The retrograde transport of this enzyme made it possible to identify the regions of thalamus projecting to the areas of vestibular representation in the cortex. In separate experiments, potentials evoked by electrical stimulation of the vestibular nerve were recorded in each experiment from the entire region of thalamus identified in anatomical studies as potentially vestibular relay sites. As a result, it was possible to
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determine the distribution of amplitudes and latencies of the responses recorded in the thalamus after vestibular nerve stimulation. In the same animals, thalamic regions were explored for an antidromic field potential after stimulation of the vestibular projection areas of the cerebral cortex. Data from these experiments suggest that neurons responsible for relaying vestibular information to the cortex are found in the VPL, VPM, and the most rostra1 portion of the PO. A preliminary account of these data was published (6). Data on the sensory properties of neurons in these thalamic nuclei are presented separately (4). METHODS Anatomicaf Studies. Adult cats anesthetized with sodium pentobarbital (35 mg/kg, i.p.) were mounted in a head holder. The utricular and superior canal branches of the vestibular nerve (2) were exposed by microdissection through the right bulla. A stainless-steel bipolar electrode was positioned on the vestibular nerve, the bulla was filled with petroleum jelly, and the electrode was cemented to the cut edge of the bulla. Two areas of cerebral cortex were exposed contralateral to the implanted electrodes, the auditory projection area in the middle ectosylvian gyrus and the vestibular projection area in either the AS or PC gyri. To monitor current spread during electrical stimulation of the vestibular nerve, one monopolar ball electrode was positioned on the cortex of the auditory projection area and another on the cortex of the vestibular projection area. The vestibular nerve was stimulated at 0.3 Hz initially with sufficient intensity to record short-latency (~5 ms) responses in both areas of cortex. Because there is no known vestibular projection to the auditory receptive area of the cortex (5, 17), it was assumed that the response recorded in the auditory projection area resulted from current spread within the bulla from the vestibular to the nearby cochlear nerve. The stimulus intensity then was lowered progressively until a response could be detected in the vestibular projection area but not in the auditory projection area. At this stimulus intensity there was no longer stimulus spread to the cochlear nerve. The electrode on the vestibular projection area of the cortex was repositioned until a response with maximum amplitude was found. In experiments involving injections into both cerebral hemispheres, the procedure described above was followed separately for each side (Fig. 1A). Horseradish peroxidase (Sigma Type VI) was injected in volumes of 0.2 ~1 (30% solution) into the cerebral cortex at a depth of approximately 1 mm at the site of the maximum amplitude response to vestibular nerve stimulation. The total volume of enzyme was injected during 25 min, and the needle remained in place an additional 30 min. After the needle was
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removed, the muscles and skin were closed. After 2 days, the animals were anesthetized deeply, and the brain was perfused with an intracardiac infusion of 6% dextran in saline followed by a fixative consisting of 2.5% glutaraldehyde and 0.5% paraformaldehyde in phosphate buffer atpH 7.4. The brain was removed and cut in the coronal plane into blocks 5 to 10 mm thick. These blocks were placed in fixative plus 30% sucrose for 1 day and then transferred to phosphate buffer (pH 7.4) containing 30% sucrose for 1 to 2 days before frozen sections were cut at 40 pm. Sections were made through the injection site, basal ganglia, and thalamus. Two sections in five were treated with diaminobenzidine in the presence of hydrogen peroxide to visualize HRP within neurons. Sections stained with cresyl violet and unstained sections were used to identify neurons labeled with enzyme. One control brain was examined to determine if endogenous peroxidase activity was present in thalamic neurons (23). All procedures described above were carried out except for the substitution of saline solution for HRP. No evidence was found for the presence of endogenous peroxidase
A
FIG. 1. A-representation of the injection sites in the cerebral cortex. Dotted lines in the anterior suprasylvian (AS) and posterior cruciate (PC) regions of the cerebral cortex show the approximate region where horseradish peroxidase was injected. Typical averaged evoked potentials are shown that were recorded in AS and PC cortex in response to vestibular nerve stimulation. B-distribution of labeled neurons in the thalamus after injection of the AS (left) and PC (right) regions of the cerebral cortex. Abbreviations in text and (10) for this and succeeding figures.
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activity in the neurons of the thalamus. The positions of labeled neurons were determined using the atlases of Jasper and Ajmone-Marsan (lo), Snider and Niemer (21), and Rinvik (18). Electrophysiological Studies. Adult cats were anesthetized with (Ychloralose (60 mg/kg, i.v.), and cannulas were placed in the trachea, a femoral vein, and a femoral artery. Arterial blood pressure was monitored, and experiments were terminated if it decreased to less than 80 mm Hg. Body temperature was monitored rectally and maintained above 37°C with a thermostatically controlled heating pad. Microdissection of the right bulla exposed the cochlear nerve (12) and the utricular and superior canal branches of the vestibular nerve. A stainless-steel bipolar electrode was positioned on the vestibular nerve and an identical electrode on the cochlear nerve. The bulla was filled with petroleum jelly, and the electrodes were cemented to the cut edge of the bulla. To monitor current spread during electrical stimulation of those nerves, bipolar concentric electrodes were inserted stereotaxically in the ipsilateral lateral vestibular and dorsal cochlear nuclei. The following procedure was used to determine the appropriate stimulus intensity to stimulate the vestibular nerve without current spread to the cochlear nerve. Vestibular nerve stimulation was applied with sufficient intensity to yield an evoked response detected with a signal averager in both vestibular and cochlear nuclei. The stimulus voltage then was lowered progressively until a response persisted in the vestibular nucleus with no response recorded in the cochlear nucleus. An analogous procedure was followed to determine the stimulus intensity required to stimulate the cochlear nerve without current spread to the vestibular nerves. Recordings were taken from the cochlear and vestibular nuclei at regular intervals throughout each experiment, and stimulus intensities were adjusted as necessary to prevent spread of current. Square wave (0. 1-ms duration, 1 to 10 V) electrical stimuli were applied to sites on the AS and PC regions of cerebral cortex from which maximum amplitude responses to vestibular nerve stimulation had been recorded in the same experiment. These sites were identified by exploration of the surface of the cortex for a maximum amplitude response to stimulation of the contralateral vestibular nerve. Recordings were taken from the thalamus with bipolar concentric electrodes. To facilitate the exploration of a large region of thalamus in each experiment, an array of four electrodes was used, with the electrodes spaced 2.0 mm apart, and recordings were taken simultaneously from the four sites. The signal from each electrode was fed into a preamplifier with a bandpass of 0.3 to 1000 Hz, viewed with an oscilloscope, and recorded on FM magnetic tape. In each recording site the responses to 16 vestibular nerve stimuli at 0.3 Hz were averaged.
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The electrode array was oriented in the frontal plane contralateral to the vestibular nerve stimulated, with the most medial electrode 3 mm and the most lateral electrode 9 mm from the midline. Recordings were taken from sites in the frontal plane at 1.5-mm intervals between anterior +3 and + 11. In half the experiments the recording array was introduced first into the most anterior site and subsequent sites were sampled by proceeding sequentially in a caudal direction. In the remaining experiments the order of recording was reversed, with the electrode array progressing in a rostra1 direction from the most caudal site. In each frontal plane, recordings were made at preselected horizontal positions from +5 to -3. This recording procedure resulted in a three-dimensional matrix of 120 to MO recording sites per animal distributed throughout the ventral, intralaminar, and posterior groups of nuclei. At each recording site the response was obtained to 0.3-Hz vestibular stimulation and l- to IOOO-Hz stimulation of the AS and PC cortex. The cortical response to vestibular nerve stimulation was recorded both before and after the thalamic recordings to ascertain that the pathway from vestibular nerve to cortex remained functional throughout the experiment. Experiments were included for analysis only if the potentials evoked by vestibular nerve stimulation could be obtained from the cortex both before and after thalamic recordings. After recordings were completed from the most ventral row of sites in the last frontal position, a small electrolytic lesion was made with each thalamic electrode and with the electrodes in the vestibular and cochlear nuclei by passing 10 to 20pA of current for 10 to 20 s. The animal then was perfused intracardially with saline followed by 10% buffered formalin. Frozen sections were cut at 40 to 50pm and stained with cresyl violet to reconstruct the position of the electrode tracts in the thalamus and brain stem. RESULTS Anatomical Experiments. Forty-eight hours after HRP had been injected into either the AS or PC site, the enzyme spread in the cortical mantle varied in diameter from 0.2 to 2.0 mm (Figs. 2A, B). Within this region thin strands of fibers were stained and many of the neurons contained HRP granules. No neurons were labeled at the AS site after injection in the PC region or in the PC region following injection at the AS site. Subcortically, labeled neurons were found only within the ipsilateral thalamus. The majority of these labeled neurons was assembled in a cluster running throughout the rostral-caudal extent of the VPL. Injection in the anterior suprasylvian cortex. Injections were made into the vestibular projection area of the AS region of six cerebral hemispheres. Labeled neurons were located in clusters in the dorsolateral region of the
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FIG. 2. A-photomicrographs of the site of injection in the posterior cruciate cortex of 0.2 ~130% horseradish peroxidase in saline. B-photomicrograph of the site of injection in the anterior suprasylvian (AS) cortex. C- dark-field photomicrograph of a cluster of labeled neurons in the ipsilateral ventral posterolateral nucleus after injection of the anterior suprasylvian region. Calibration bar is 2 mm in A and B.
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VPL (Figs. lB, 2C). These clusters could be found throughout the anterior posterior extent of this nucleus. A few single, labeled neurons also were found in the VPM, lateral posterior, ventral lateral, and central medial nuclei. A few scattered neurons were found in various regions of the PO, including the MGmc. There was great consistency between animals in the distribution of labeled neurons within the VPL after injection in the AS region. Injection in the posterior cruciate cortex. Injections were made into the vestibular projection region of the PC gyrus of seven cerebral hemispheres. Again, the majority of labeled neurons was found in the VPL; however, these neuronal clusters were situated more rostrally, ventrally, and medially than those observed after AS injection. Labeled neurons also were found in the VPM, ventral lateral, lateral posterior, central medial, and posterior nuclei (Fig. 1B). There was greater variability in the distribution of labeled neurons in the VPL nucleus after PC injection than after AS injection. Znj’ection in the anterior suprasylvian andposterior cruciate cortex in the same experiment. In three animals, injections were made in the AS region
of one hemisphere
and in the PC region of the opposite hemisphere.
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permitted a comparison in the same animal of the sites of labeled neurons in corresponding thalamic nuclei on either side of the midline. The sites found in these experiments were consistent with the results obtained after unilateral injection. In Fig. 3 for example, neurons in the left thalamus labeled by an injection in the left AS region are in an analogous area with neurons in the right thalamus labeled by an injection in the right PC region. Electrophysiological Experiments. Potentials evoked by vestibular nerve stimulation varied considerably in latency and amplitude. The latencies to onset of responses were 1.5 to 25 ms and the amplitudes 5 to 200 PV (Fig. 4). During several electrode penetrations there was a reversal in polarity of the evoked potential as the recording electrode was advanced vertically between two recording sites (Fig. 4, traces A, B). Vestibular responses could be recorded throughout the region of thalamus sampled (Fig. 5). Short- (~3.5 ms) and long-latency evoked potentials were found in similar thalamic sites, including the ventral, intralaminar, and posterior groups of nuclei. Within each experiment the amplitudes of the short- and long-latency evoked potentials were placed in two separate
FIG. 3. Photomicrograph of a frontal section through the thalamus. Two injections of horseradish peroxidase were made in a single preparation, one in the left anterior suprasylvian region of the cortex and the other in the right posterior cruciate region of the cortex. Labeled neurons can be seen within an analogus region of the ventral posterolateral nucleus on each side of midline. Calibration bar is 2 mm.
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5 mscc FIG. 4. Potentials recorded within the thalamus after vestibular nerve stimulation. A and B-potentials recorded within the ventral basal complex. A was recorded 1 mm dorsal to site of recording of potential in B. C-potential recorded within the posterior nucleus.
categories and ranked from highest to lowest. Sites containing evoked potentials in the 50th percentile or greater in amplitude are enclosed in dotted lines in Fig. 5. Although short-latency evoked potentials were located throughout the regions of thalamus investigated, the largestamplitude potentials were found in a much smaller region. Large-amplitude, short-latency potentials evoked by vestibular nerve stimulation were found in the VPL, VPM, and the most rostra1 portions of the PO. Stimulation of the AS and PC regions of the cerebral cortex evoked antidromic field potentials in the thalamus. These potentials were recognized by their short (0.5 to 0.7 ms) latencies and their persistence during highfrequency (500 to 1000 Hz) stimulation. The potentials were found most frequently in the VPL and VPM and less frequently in the PO nucleus (Fig. 6). Figure 7 contains a representation of the sites showing potentials evoked at short latency by vestibular nerve stimulation and also shows an antidromic field potential evoked by stimulation of either the AS or PC region of the cortex. These sites include the ventral basal portion of the thalamus and the region between the ventral basal part of the thalamus and the most anterior portion of the posterior group of nuclei. Thirty-five percent of these sites showed a large-amplitude response as defined above. No sites were found in the MGmc, the lateral posterior, ventral lateral, or mtra-
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FIG. 5. Sites of recording of potentials evoked by vestibular nerve stimulation in five experiments. A line encloses sites in which the recorded potentials were in the upper 50th percentile of amplitude in each experiment. Section A is at the most rostra1 site, section D is at the most caudal site.
laminar nuclei with both short-latency antidromic field potentials.
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DISCUSSION
This investigation represents a combined anatomical and physiological approach to the identification of the thalamic nuclei that relay vestibular
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FIG. 6. Sites of recording of antidromic field potentials after stimulation of anterior suprasylvian (AS, A) and posterior crucidate (PC, B) regions of the cerebral cortex in five experiments.
information to the cerebral cortex at short latency. Anatomical procedures were used to identify thalamic neurons with axons coursing to the vestibular projection regions in the cerebral cortex. Electrophysiological procedures were used to identify the thalamic regions showing responses evoked by stimulation of the vestibular nerve and antidromic field potentials evoked by stimulation of the vestibular projection regions in the cerebral cortex.
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FIG. 7. All individual sites showing both short-latency potentials evoked by vestibular nerve stimulation and an antidromic field potential from cortical stimulation.
The anatomical studies demonstrated that the vestibular projection regions in the cerebral cortex receive inputs from a number of thalamic nuclei, including the VPL, VPM, PO, and intralaminar nuclei. Projections to the anterior suprasylvian region were found to originate from more dorsolateral and caudal regions within the VPL than the projections to the PC region. Nevertheless, projections to both cortical vestibular projection regions appeared to originate in adjacent and probably overlapping sites in the VPL. There was no evidence to indicate that projections to the AS region originate in a site separate from projections to the PC, as reported by
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Liedgren et al. (13). Data by Hand and Morrison (9) support our conclusion that neurons in the common region of the ventral thalamus may send axons to both vestibular projection regions of the cortex because they found degenerating fiber terminals in both the AS and PC regions of the cerebral cortex after making small lesions within the VPL. The anatomical studies also demonstrated that there is no direct interconnection between the two vestibular-responsive regions of the same cerebral hemisphere, as labeled neurons were not found in the AS region after injection of the PC or in the PC region after injection of the AS region. This information supports the notion that there are two direct and possibly parallel pathways mediating vestibular information from thalamus to the cortex without direct connections between the two cortical sites. The electrophysiological studies demonstrated that potentials evoked by vestibular nerve stimulation can be recorded at short latency from numerous thalamus nuclei, including VPL, VPM, PO, and intralaminar nuclei. These findings are compatible with those reported previously (5, 19, 20, 22) and indicate that many thalamic nuclei receive vestibular information. The sites having short-latency evoked potentials with the greatest amplitude, however, are located in the VPL and VPM and in the region between the VPL and PO. Attempts to identify the thalamic nuclei that transmit vestibular information to the two cortical projection regions at short latency were carried out by stimulating the two cortical projection sites and recording antidromic field potentials in these nuclei. Similar potentials, recorded at short latency and persisting at high stimulus frequencies, were interpreted by Deecke et al. (7) and by Kennedy and Towe (11) to be the sum of antidromic activity in individual fibers. The distribution of antidromic field potentials in the thalamus was similar to the distribution of sites thought to represent thalamocortical projection regions from the anatomical studies in the present investigation. We conclude, consequently, that projections to both the AS and PC regions of the cortex originated from similar regions of the thalamus. No evidence was found for a short-latency response representing a projection from the MGmc to either the AS or PC regions of the cortex. These data consist of three separate features that help to define the role of the thalamus in the processing of vestibular information en route to the cortex. These features are the identification of the thalamic nuclei that (i) send axons to the vestibular projection regions of the cerebral cortex, (ii) receive ascending vestibular activity at short latency, and (iii) have antidromic responses to stimulation of the vestibular projection sites in the cerebral cortex. The nuclei that have all these features in common are limited to the VPL, VPM, and the most rostra1 portion of the PO. These findings are not compatible with the idea that the MGmc serves as the principal thalamic relay for rapid mediation of vestibular activity to the
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cerebral cortex. This idea stemmed from the observation that responses to vestibular stimulation could be recorded at short latency from this nucleus and from the results of lesion experiments (5, 17). Blum et al. (3), however, did not find units in this region showing a short-latency response to vestibular stimulation and concluded that the vestibular relay must occur elsewhere in the thalamus. Evidence that other components of the thalamus may be involved came from the work of Sans er al. (20) who identified the VPL as a site important for ascending vestibular activity. Subsequently, Deecke et al. (7) identified the nucleus ventral posterior inferior in the monkey as the important relay nucleus for the cortical projection of vestibular information. These investigators and associates (14) later revised this conclusion, reporting evidence on the basis of single-unit data in the monkey for a vestibular relay also in the.VPL and PO. The present findings support those conclusions through the combined use of anatomical and physiological techniques to identify the regions of the thalamus that relay vestibular information at short latency to the cerebral cortex. We have identified the VPL, VPM, and the most rostra1 portion of the PO as the thalamic regions where the highest-amplitude and shortest-latency responses occur after vestibular nerve stimulation and, in addition, where neurons are present that project to vestibular-activated regions of the cerebral cortex. It is concluded, therefore, that neurons in VPL, VPM, and rostra1 PO relay vestibular information to the cerebral cortex at short latency. REFERENCES 1. ABRAHAM, L., P. COPACK, AND S. GILMAN. 1977. Brainstem pathways for vestibular projections to cerebral cortex in the cat. Exp. Neurol. 55: 436-448. 2. ANDERSSON, S., AND B. E. GERNANDT. 1954. Cortical projections ofthe vestibular nerve in the cat. Acta Otolaryngol. Suppl. 116: lo- 18. 3. BLUM, P. S., L. ABRAHAM, AND S. GILMAN. 1979. Vestibular, auditory, and somatic input to posterior thalamus. Exp. Brain Res. 34: l-9. 4. BLUM, P. S., AND S. GILMAN. 1979. Vestibular, somatosensory, and auditory input to the thalamus of the cat. Exp. Neurof. (in press). 5. COPACK, P., N. DAFNY, AND S. GILMAN. 1972. Neurophysiological evidence of vestibular projection to thalamus, basal ganglia, and cerebral cortex. Pages 309-339 in T. L. FRIGYESI, E. RINVIK, AND M. D. YAHR, Eds., Corticothalamic Projections and Sensorimotor Activities. Raven Press, New York. 6. DAY, M., P. BLUM, M. B. CARPENTER, AND S. GILMAN. 1976. Thalamic components of ascending vestibular projections. Sot. Neurosci. Abstr. 2: 1058. 7. DEECKE, L., D. W. F. SCHWARZ, AND .I. M. FREDRICKSON. 1974. Nucleus ventroposterior inferior (VPI) as the vestibular thalamic relay in the rhesus monkey. Exp. Brain Res. 20: 88- 100. 8. FREDRICKSON,J. M., U. FIGGE, P. SCHEID, AND H. H. KORNHUBER. 1966. Vestibular nerve projection to the cerebral cortex of the rhesus monkey. Exp. Brain Res. 2: 318-327.
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9. HAND, P. J., AND A. R. MORRISON. 1970. Thalamocortical projections from the ventrobasal complex to somatic sensory areas I and II. Exp. Neural. 26: 291-308. 10. JASPER, H. H., AND C. AJMONE-MARSAN. 1954.A Stereotaxic Atlasof the Diencephalon of the Cat. The National Research Council of Canada, Ottawa. 11. KENNEDY, T. T., AND A. L. TOWE. 1962. Identification of a fast lemnisco-cortical system in the cat. J. Physiol. (London) 160: 535-547. 12. LANDGREN, A., H. SILVENIUS, AND D. WOLSK. 1967. Vestibular, cochlear and trigeminal projections to the cortex in the anterior suprasylvian nucleus of the cat. J. Physiol. (London) 191: 561-573. 13. LIEDGREN, S. R. C., K. KRISTENSSON, B. LARSBY, AND L. M. ODKVIST. 1976. Projections of thalamic neurons to cat primary vestibular cortical fields studied by means of retrograde axonal transport of horseradish peroxidase. Exp. Brain Res. 24: 237-243. 14. LIEDGREN, S. R. C., A. C. MILNE, A. M. RUBIN, D. W. F. SCHWARZ, AND R. D. TOMLINSON. 1976. Representation of vestibular afferents in somatosensory thalamic nuclei of the squirrel monkey (Sarmiri scuireus). J. Neurophysiol. 39: 601-612. 15. MICKLE, W. A., AND H. W. ADES. 1954. Rostra1 projection pathway of the vestibular system. Am. J. Physiol. 176: 243-246. 16. ODKVIST, L. M., A. M. RUBIN, AND D. W. F. SCHWARZ. 1973. Vestibular cortical projection in the rabbit. J. Comp. Neural. 149: 117- 120. 17. POTEGAL, M., P. COPACK, J. M. B. V. DEJONG, G. KRALJTHAMER, AND S. GILMAN. 1971. Vestibular input to the caudate nucleus. Exp. Neural. 32: 448-465. 18. RINVIK, E. 1968. A re-evaluation of the cytoarchitecture of the ventral nucleus complex of the cats thalamus on the basis of corticothalamic connections. Brain Res. 8: 237-254. 19. ROUCOUX-HANUS, M., AND N. BOISACQ-SCHEPENS. 1977. Ascending vestibular projections: further results at cortical and thalamic levels in the cat. Exp. Brain Res. 29: 283-292. 20. SANS, A., J. RAYMOND, AND R. MARTY. 1970. Responses thalamique et corticales ti la stimulation electrique du nerf vestibulaire chez la chat. Exp. Brain RPS. 10: 265-275. 21. SNIDER, R. S., AND W. T. NIEMER. 1961. A Stereotaxic Atlas of the Cat Brain. Univ. of Chicago Press, Chicago. 22. SPIEGEL, E. A., E. G. SZEKELY, AND P. L. GILDENBERG. 1965. Vestibular responses in midbrain, thalamus, and basal ganglia. Arch. Nemo/. 12: 258-269. 23. WONG-RILEY, M. T. 1976. Endogenous peroxidase activity in brain stem neurons as demonstrated by their staining with diaminobenzidine in normal squirrel monkeys. Brain Res. 108: 257-277.