Symposium on the Role of the Vestibular Organs in Space Exploration (1970)

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Anatomical Aspects on the Functional Organization of the Vestibulospinal Projection, With Special Reference to the Sites of Termination ROLF NYBERG-HANSEN University of Oslo, Norway SUMMARY A review is given of the functional organization of the vestibulospinal projection in the cat. Special emphasis is placed on the sites of termination of vestibulospinal fibers within the spinal gray matter. The spinal projection from the vestibular nuclei can be separated into two different fiber systems: the classical vestibulospinal tract and the fibers descending in the medial longitudinal fasciculus. Because of their origin from the medial vestibular nucleus and their medial course in the brainstem and the spinal cord, the latter is called the medial vestibulospinal tract, and the more laterally coursing classical vestibulospinal fibers is called the lateral vestibulospinal tract. The lateral tract comes from the lateral vestibular nucleus and descends in the ventrolateral funiculus organized in a somatotopical manner throughout the whole cord. The fibers terminate ipsi- laterally in the entire lamina VIII and the neighboring parts of lamina VII of the spinal gray matter. No terminations are found among the perikarya of the motoneurons in lamina IX. The medial tract descends bilaterally in the dorsomedial part of the ventral funiculus to the rostral half of the cord only. There is no evidence of a somatotopical organization within this tract. The fibers terminate bilaterally in the dorsal half of lamina VIII and the adjacent part of lamina VII. The fibers on the ipsilateral side outnumber those on the contralateral side. Although the lateral and medial vestibulospinal tracts exhibit some striking mutual resemblances, they differ in other anatomical respects. This makes it likely that the two pathways also, at least to some extent, are functionally dissimilar. The lateral tract exerts a tonic facilitatory effect on postural tonus and spinal extensor mechanisms. This influence is mediated both on a- and y-motoneurons, chiefly by way of spinal interneurons in laminae VII and VIII. There is some physiological evidence of minor direct monosynaptical connections to the motoneurons. The most likely anatomical explana- tion of this is the terminating of vestibulospinal fibers on dendrites of motoneurons extending beyond the confines of lamina IX, into laminae VII and VIII. The medial vestibulospinal tract is concerned with movements of the head and neck and, according to recent physiological observations, also with presynaptic inhibition of primary afferents in the spinal cord. Impulses of vestibular origin are also conveyed to the spinal cord indirectly by way of reticulo- spinal pathways. Pontine reticulospinal and lateral vestibulospinal pathways exhibit striking mutual resemblances anatomically as well as physiologically. The anatomical observations on the vestibulospinal pathways presented in this review are in general agreement with conclusions reached in recent physiological investigations. 167

168 THE ROLE OF THE VESTIBULAR ORGANS IN SPACE EXPLORATION INTRODUCTION The anatomical organization of the vestibular nuclear complex is rather intricate. This is already apparent when subdivision of the nuclear complex into specific nuclei on a cytoarchitectonic basis is performed, but the complexity is even more outstanding when the various afferent and efferent fiber connections of the vestibular nuclei are taken into account. Recent advances in the neurophysiology of the vestibular nuclei and their connections with other regions of the central nervous system have increased the demand for a detailed knowledge of their minute anatomy, among other things of the anatomical organiza- tion of the vestibulospinal projection, including the sites of termination within the spinal gray matter. Anatomical data of these kinds are indispensable for functional interpretations, and for the analysis of the vestibular influences on spinal mechanisms. In the present account, anatomical aspects of the organization of the vestibular projection to the spinal cord will be presented with special emphasis on the sites of termination, which will be referred to the laminar organization of the spinal gray matter described by Rexed (refs. 1 and 2). Rexed's observations that the spinal gray matter may be subdivided on a cytoarchi- tectonic basis into 10 different laminae, repre- senting at least in part functionally different regions, have in recent years provided a fruitful common basis of reference for the sites of termi- nation of various afferent fiber systems to the cord. This has allowed more precise correlation to be made between anatomical and physiological observations and their functional interpretations, and thus unveiled functional aspects of the in- trinsic organization of the spinal cord as well (see, for example, refs. 3 to 6). In this presentation, attempts will likewise be made to correlate the anatomical observations with relevant physiological data in order to reveal aspects of the functional organization of the vestibulospinal projection. The methods used for tracing the course and determining the sites of termination of vestibulo- spinal fibers following lesions of the various ves- tibular nuclei have been the silver impregnation methods of Nauta (ref. 7) and Glees (ref. 8). The descending vestibulospinal fibers have been studied in transverse as well as in longitudinal sections of the cord, a procedure which allows much more detailed observations to be made than can be obtained in transverse sections alone, which are most commonly used. THE VESTIBULOSPINAL PROJECTION The fiber connections from the vestibular nuclei to the spinal cord can be separated into two different fiber systems: the classical ves- tibulospinal tract and the fibers descending in the medial longitudinal fasciculus. These two pathways should be called the lateral and the medial vestibulospinal tract, respectively, as proposed by Nyberg-Hansen (ref. 6). The Lateral Vestibulospinal Tract The experimental study in the cat by Pom- peiano and Brodal (ref. 9), using the modified Gudden method (ref. 10), confirmed the results of some previous investigators that the lateral vestibulospinal tract originates exclusively from the lateral nucleus of Deiters. The lateral ves- tibular nucleus is defined as that part of the ves- tibular nuclear complex which contains the giant cells of Deiters (ref. 11). However, this nucleus also contains a considerable number of medium- sized and small neurons as well, and all types of cells project to the spinal cord. This was empha- sized by Pompeiano and Brodal (ref. 9), and later confirmed physiologically (refs. 12 and 13). The anatomically demonstrated somatotopical organization within the lateral vestibulospinal tract has likewise been verified physiologically (refs. 12, 14, and 15). Figure 1 diagrammatically summarizes the findings of Pompeiano and Brodal (ref. 9) and shows how the rostroventral part of the lateral vestibular nucleus projects to the cervical cord, while the fibers to the lumbosacral cord take origin from the dorsocaudal part. Fibers to the thoracic cord come from intermedi- ate regions. Abbreviations used in figures 1, 2, 3, 7, 8, and 11 are listed below. (Note that roman numerals are used in reference to the laminar organization of the spinal gray matter, while the numerals VI and VII are also used to indi-

FUNCTIONAL ORGANIZATION OF THE VESTIBULOSPINAL PROJECTION 169 cate the positions of the cranial nerves of those numbers. However, there should be no con- fusion as the former are used in conjunction with sectional views of the spinal cord, and the latter label the indicated nerves seen in cross section at the level of the fourth ventricle.) B.c brachium conjunctivum C. with subscript cervical Cc column of Clarke C.r corpus restiform C.t corpus trapezoideum D descending (inferior) vestibular nucleus il intermediolateral cell column I left I., within drawing lateral vestibular nucleus 1., with subscript lumbar \t medial vestibular nucleus N.c.t nucleus of the trapezoid body IS.cu.e nucleus cuneatus externus N.tr.sp.V nucleus of spinal tripeminal tract N.n.VI, N.n.VII nuclei of cranial nerves VI and VII N.VI, N.VII. N.VIII, cranial nerves VI. VII Ol.i inferior olive Ol.s superior olive p.h nucleus praepositus hypoglossi R right S superior vestibular nucleus Th, with subscript thoracic Tr.sp.V spinal trigeminal tract x small-celled group x, lateral to the descending vestibular nucleus In an experimental study with silver-impreg- nation methods, Nyberg-Hansen and Mascitti (ref. 16) confirmed the observation of Pompeiano and Brodal (ref. 9) that the lateral vestibulo- spinal tract takes its origin solely from the lateral vestibular nucleus. Following lesions of the superior, medial, and descending ves- tibular nuclei, they found no degeneration in the lateral vestibulospinaj tract. Figure 2 shows the finding in one of their cases (cat B. St. L. 307, killed after 8 days) where the lesion is restricted to the lateral nucleus, which is completely destroyed. The ensuing degenerating fibers of the lateral vestibulospinal tract descend purely ipsilaterally along the periphery of the ventrolateral funiculus. medial • fibers to cervical cord o —"— thoracic —"— + "— lumbosocral—-— FIGURE 1. — Diagram showing the somatotopical pattern with- in the lateral vestibulospinai tract in the cat as demon- strated experimentally by Pompeiano and Brodal (ref. 9). To the left (1-6) a series of transverse sections through the lateral nucleus; to the right (1-6) a longitudinal reconstruc- tion of the nucleus. (See text for explanation of abbrevia- tions. I The tract gradually decreases in size as more caudal levels are reached, but it can be followed throughout the whole cord. Its position changes during its descent in the cord. In the cervical enlargement the tract is located peripherally in the ventrolateral funiculus extending laterally to the most laterad emerging ventral root fibers, and does not extend into the dorsal three-fourths of the ventral funiculus where the medial longi- tudinal fasciculus is located along the anterior median fissure. During its descent in the thoracic cord, the lateral vestibulospinal tract is gradually displaced in a dorsomedial direction, and in the lumbosacral enlargement it is found

170 THE ROLE OF THE VESTIBULAR ORGANS IN SPACE EXPLORATION medially in the ventral funiculus along the anterior median fissure (fig. 2). This dorso- medial shift has previously been noticed by some authors (refs. 9 and 17 to 19) and is a feature of rcT~~~^ >• i FIGURE 2. —Diagrammatic representation of the distribution of degenerating coarser (wavy lines) and terminal fibers (dots) within the spinal cord in a case with a total lesion (hatchings) of the lateral vestibular nucleus involving the origin of the lateral vestibulospinal tract. From the tho- racic cord only transverse and sagittal sections are shown, while horizontal sections as well are shown from the cervi- cal and lumbar enlargements. The positions of the longi- tudinal sections are indicated in the drawings of the trans- verse sections. The roman numerals refer to the laminae of Rexed. (From ref. 16.) (See text for explanation of abbre- viations.) practical importance in physiological studies of the lateral vestibulospinal tract. As will be further commented upon in the section on the medial vestibulospinal tract, it follows from this arrangement that the only conclusive way to identify fibers from the vestibular nuclei descend- ing in the medial longitudinal fasciculus in the spinal cord is to observe them in the area allotted to the fascicle in the cervical cord. From a quantitative point of view, it appears that the cervical and lumbosacral enlarge- ments receive equal numbers of lateral vestib- ulospinal tract fibers, while a smaller number go to the thoracic cord. Although the precise diameter of degenerating fibers cannot be deter- mined, it seems likely that the lateral vestibulo- spinal tract is composed of fibers of different size, but the majority of them are rather thick. FIGURE 3. — Diagrammatic representation of the distribution of degenerating fibers of the lateral vestibulospinal tract within the spinal cord as seen in transverse and sagittal sections from the cervical, thoracic, and lumbar cord in a case with a lesion (hatchings) of the dorsocaudal, "hind- limb" region of the lateral vestibular nucleus (above\. (From ref. 16.1 (See text for explanation of abbreviations.\

FUNCTIONAL ORGANIZATION OF THE VESTIBULOSPINAL PROJECTION 171 Recently the conduction velocity of the fibers of the lateral vestibulospinal tract has been found to range from 20 to 140 m/sec (refs. 12 and 13), the majority having values between 50 and 120 m/sec (ref. 13). Observations made by Nyberg-Hansen and Mascitti (ref. 16) in another case (cat B. St. L. 265, killed after 11 days) shown in figure 3 confirmed the somatotopic origin of the lateral vestibulo- spinal tract first demonstrated by Pompeiano and Brodal (ref. 9). In this case the lesion as con- cerns the lateral vestibular nucleus is restricted to the caudal, "hindlimb" region, and the en- suing degeneration in the cord is almost exclu- sively restricted to its lower half (fig. 3). As to the sites of termination, fibers leave the lateral vestibulospinal tract and enter the spinal gray matter of the ventral horn corresponding to the medial aspect of lamina VIII and the extreme ventral aspect of lamina VII (figs. 2, 3, and 4/4). They then radiate in a dorsolateral direction to terminate in the entire lamina VIII and the neighboring medial and central parts of lamina VII (figs. 2, 3, and 5). Occasionally a few fibers are seen in the ventral part of lamina VI and in lamina IX, but they are never seen in contact with nerve cells in these laminae. Lateral vestibulospinal fibers bypass the ven- tromedial group of motoneurons. In the photo- micrographs they are never seen to establish contact with the soma and proximal dendrites of these neurons, while a few contacts occur in the thoracic cord. No fibers terminate in the inter- mediolateral cell column or in the column of Clarke. Within the laminae of termination, rows of minute black dots indicating the finest degen- erating fibers, and fine isolated black fragments, in part presumably representing degenerating boutons (refs. 20 to 22) are found in the neuropil, but also in close contact with somata (fig. 4fi) and particularly with dendrites (figs. 4C, 6/4, and 6B). Since, however, thin glial sheets, which can be seen only in the electron microscope, may be interposed between boutons and postsynaptic structures, all close contacts between terminal structures and somata and dendrites in silver- impregnated sections may not represent true synaptic contacts. However, since isolated y. ,c > FIGURE 4. — Photomicrographs showing degeneration with the Nauta method. A: Degenerating lateral vestibulo- spinal fibers entering the spinal gray matter (to the left) along the dendrites of nerve cells medially in laminae VIII. B: Degeneration on the perikaryon and especially along a proximal dendrite of a small nerve cell in lamina VII. (.: Degeneration on a proximal dendrite (arrow) of a nerve cell in lamina VIII in a case with a lesion of the medial vestibular nucleus involving the origin of the medial vestibu- lospinal tract. (From refs. 16 and 46.) i. .: , /.x±&' . f * ,. v », -. • *-' .,-./. FIGURE 5. — A photomicrograph of degenerating lateral ves- tibulospinal fibers in the neuropil in lamina VllL Nauta method. (From ref. 16.)

172 THE ROLE OF THE VESTIBULAR ORGANS IN SPACE EXPLORATION FIGURE 6.— Photomicrographs showing degeneration in silver sections. A: Degeneration along a proximal dendrite of a nerve cell in lamina VIII, Nauta method. B: Degenerating bouton with a terminal fiber leading up to it (arrow) in lamina VIII, Glees method. (From ref. 16.) fragments tend to be more frequently observed around dendrites and in the neuropil than around the somata (figs. 4B, 6/4, and 6B), the suggestion may be ventured that a relatively large number of the fibers of the lateral vestibulospinal tract terminate on dendrites. The final answer on this important point, however, can be obtained only in electron-miroscopic studies. The area of termination of the lateral vestibulo- spinal fibers reported by Schimert (ref. 23) ap- pers to be in accordance with the findings out- lined above. However, his statement that terminations are almost exclusively on medial motoneurons is scarcely tenable, since the majority of neurons in lamina VIII, interpreted by Schimert as medial motoneurons, most prob- ably are spinal interneurons (refs. 1 and 24 to 29). The almost complete lack of terminations on motoneurons appears to be in accordance with the findings of some previous investigators (refs. 30 to 32), although their lesions have not been restricted to the lateral vestibular nucleus. Re- cently Petras (ref. 33) has made observations similar to those of Nyberg-Hansen and Mascitti (ref. 16), although he does not correlate his ob- servations with Rexed's laminae. The considerations made above concerning the sites of termination, and particularly the lack of direct termination on the somata of moto- neurons in lamina IX, are subject to the following qualification: It is well known that dendrites of spinal motoneurons extend for considerable distances from the perikarya beyond the con- fines of lamina IX, into the territories of laminae VII and VIII (refs. 4, 24, 27, 34, and 35). Some of the terminations in these laminae may thus actually be on dendrites of motoneurons extend- ing into them. The demonstration by Lund and Pompeiano (ref. 36) of monosynaptic ex- citatory postsynaptic potentials (EPSP's) in spinal extensor motoneurons following stimula- tion of the lateral vestibular nucleus, indeed indicates that this really may be the case. How- ever, in accordance with the anatomical observa- tions outlined above, physiological studies as well leave no doubt that a large number, if not the majority of lateral vestibulospinal tract fibers terminate on interneurons in laminae VII and VIII, and not directly on motoneurons (ref. 37). The heavy termination in lamina VIII deserves additional comment. Rexed (ref. 1) drew atten- tion to the fact that several earlier authors had described the neurons in this region as com- missural cells sending their axons across the midline in the anterior commissure (see refs. 1 and 16). In recent years this observation has repeatedly been confirmed (refs. 27 to 29 and 38). Since the lateral vestibulospinal tract is purely ipsilateral, the commissural nature of the neurons in lamina VIII may account for at least some of the contralateral effects on spinal mechanisms obtained on vestibular stimulation (refs. 14, 37, and 39 to 42). However, transmission of vestibular impulses to the brainstem reticular formation via vestibuloreticular fibers from the vestibular nuclei (ref. 43) and further relayed to the cord by way of the bilateral reticulospinal projection (refs. 44 and 45) may also account for some of the contralateral effects.

FUNCTIONAL ORGANIZATION OF THE VESTIBULOSPINAL PROJECTION 173 The Medial Vestibulospinal Tract Using the modified Gudden method (ref. 10), Pompeiano and Brodal (ref. 9) found retrograde cellular changes only in the lateral vestibular nucleus giving origin to the lateral vestibulo- spinal tract. However, from a critical analysis of the pertinent literature, they concluded that fibers descending in the medial longitudinal fasciculus most probably are derived from the medial nucleus. Experimental evidence for this suggestion has later been given by this author (ref. 46). Following lesions of the medial vestibular nucleus, this author found fibers in the dorsal three-fourths of the ventral funiculus of the cervical cord, within the area along the anterior median fissure generally allotted to the medial longitudinal fasciculus in the cord. Because of this position it is often called fascicu- lus sulcomarginalis (fig. 7). The degenerating fibers do not extend as far ventrolaterad as the most dorsomedially localized lateral vestibulo- spinal fibers. On account of their origin in the medial vestibular nucleus and their more medial course in the brainstem and in the spinal cord, Nyberg-Hansen (ref. 6) proposed that these fibers should be called the medial vestibulospinal tract. Figure 7 shows one case from Nyberg-Hansen's study (ref. 46) (cat B. St. L. 311, killed after 7 days) with a lesion localized to the medial nucleus. As can be seen, the medial vestib- ulospinal tract is confined to the upper half of the cord. Its fibers can be traced bilaterally to midthoracic levels in the dorsal part of the ventral funiculus along the anterior median fissure. The fibers on the ipsilateral side outnumber those on the contralateral one. These findings appear to be in accordance with some older investigations (refs. 47 to 49). The number of medial vestibulospinal fibers is very modest when compared with those in the lateral vestibulospinal tract, which is the major pathway from the vestibular nuclei to the spinal cord, as recently confirmed physiologically by Wilson, Wylie, and Marco (ref. 50). The fibers in the medial tract also have a smaller diameter than those in the lateral tract (ref. 46). In cases with lesions in the superior, lateral, CoIBSILSi I : it ' •' 1 ' ' • R VI VH ._• vi V-IL: FIGURE 1. —Diagrammatic representation of the distribution of degenerating coarser (wavy lines) and terminal fibers (dots) within the spinal cord in a case with a lesion of the medial vestibular nucleus (above) involving the origin of the medial vestibulospinal tract. From the thoracic cord only transverse and sagittal sections are shown, while horizontal sections as well are shown from the cervical cord. (From ref. 46.) (See text for explanation of abbreviations.) and descending vestibular nuclei, no degenera- tion has been found in the medial vestibulo- spinal tract. Figure 8 shows two cases (from Nyberg-Hansen's study, ref. 46) where there were lesions of the descending nucleus, in which no degeneration could be observed in the spinal cord. However, as is shown, the rostral and caudal parts of the descending nucleus are not destroyed. The observation that the descending nucleus does not project to the cord agrees with the anatomical findings of Carpenter (ref. 51) and of Carpenter, Alling, and Bard (ref. 52). Re- cently, however, Wilson, Wylie, and Marco (refs. 50 and 53) have presented physiological evidence for very modest contribution to the medial vestib- ulospinal tract from the medial as well as from the descending nucleus.

174 THE ROLE OF THE VESTIBULAR ORGANS IN SPACE EXPLORATION FIGURES. — Drawings of the lesions (hatchings) of the descend- ing vestibular nucleus in two cases in which no degeneration is present in the spinal cord. The rostral and caudal parts of the descending nucleus are not destroyed. (From ref. 46.) (See text for explanation of abbreviations.) Concerning the sites of termination of the fibers of the medial vestibulospinal tract, figure 7 shows that the majority of them terminate in the cervical cord. They enter the gray matter of the ventral horn corresponding to the dorso- medial aspect of lamina VIII, and terminate in the dorsal half of this lamina and the neighboring medial parts of lamina VII (figs. 4C and 7). No fibers terminate among the soma of the moto- neurons in lamina IX, neither in the intermedio- lateral cell column nor in the column of Clarke. The area of termination is much less extensive than is that of the lateral vestibulospinal tract. FUNCTIONAL CONSIDERATIONS The lateral vestibular nucleus and the lateral vestibulospinal tract are known to exert a tonic facilitatory influence on postural tonus and spinal extensor mechanisms (for references, see ref. 54). In accordance with this, EPSP's have been re- corded in spinal extensor a-motoneurons follow- ing stimulation of the lateral vestibular nucleus (refs. 36 and 55), some of them being the result of monosynaptic vestibulospinal connections to the motoneurons (ref. 36). However, in accord- ance with the anatomical observations outlined above, there is in addition physiological evidence for ample vestibulospinal influence on spinal interneurons (refs. 37, 41, and 56). This arrange- ment provides a basis for a variable interplay at the interneuronal level among afferent impulses from other sources as well. Interaction and con- vergence of vestibulospinal, corticospinal, and propriospinal impulses at an interneuronal level in the cord have been reported (ref. 57), and re- cently a more precise localization of these particu- lar interneurons has been attempted by Erulkar, Sprague, Whitsel, Dogan, and Janetta (ref. 37). Interneurons activated by vestibular nerve stimu- lation alone were located chiefly in lamina VIII. those responding in addition to dorsal root stimu- lation mainly centromedially in lamina VII, and finally interneurons activated by vestibular nerve and motor cortex stimulation appear to be situ- ated laterally in laminae VI and VII (ref. 37). Furthermore, Grillner, Hongo, and Lund (ref. 56) reported that vestibulospinal impulses mono- synaptically activated interneurons mediating the antagonistic extensor la inhibition to flexor a-motoneurons, and recently these particular interneurons have been localized to the ventral part of lamina VII, just mediad to lamina IX (ref. 58). Vestibulospinal influences on the y-moto- neurons innervating the intrafusal muscle fibers are also well known. Thus, stimulation of the vestibular receptors (refs. 59 and 60) and the vestibular nerve (refs. 41, 61, and 62) as well as the lateral vestibular nucleus (refs. 62 to 64) have all been shown to activate the y-motoneurons, especially to extensor muscles. Some evidence of a monosynaptic connection to extensor y-motoneurons through the lateral vestibulospinal tract has also been reported (ref. 64), but there appears to be no doubt that the polysynaptic route is by far the largest (refs. 41 and 64). Since the y-motoneurons supplying a particular muscle are located within the group of a-motoneurons innervating the same muscle (refs. 65 and 66), the importance of the polysynaptic route is in accordance with the anatomical observations reported above that the fibers of the lateral vestibulospinal tract chiefly terminate on interneurons in laminae VII and VIII (figs. 2 and 3).

FUNCTIONAL ORGANIZATION OF THE VESTIBULOSPINAL PROJECTION 175 Following stimulation of the vestibular nerve, the y-motoneurons discharge at lower threshold and with higher frequencies than do the a- motoneurons (refs. 41 and 62). Furthermore, stimulation of the vestibular nerve is much more effective for y-motoneuron activation than is stimulation of the lateral vestibular nucleus itself (ref. 62). The well-known activation of the reticular formation of the lower brainstem following vestibular stimulation (refs. 41 and 67), and the participation of reticulospinal pathways, in addition to vestibulospinal ones in conduction of vestibular impulses to the cord (refs. 41, 62, and 67), may be the possible explanation for these differences. Since there is no evidence of primary vestibular fibers to the reticular forma- tion (ref. 68), the recently demonstrated heavy vestibuloreticular projection from the vestibular nuclei (ref. 43) may account for the transmission of vestibular impulses to the reticular formation. In this connection it is of great interest to notice that reticulospinal pathways, thus being the final path for at least some vestibular impulses to the spinal cord, have been shown in part to course together with and terminate within the same laminae of the spinal gray matter as do the vestibulospinal pathways (ref. 45). Figure 9 shows the course and sites of termination of medullary and pontine reticulospinal fibers (ref. 45), and, as further shown in figure 10, which is from the study of Nyberg-Hansen (ref. 6), the anatomical similarities are most conspicuous as concern the pontine reticulo- spinal and lateral vestibulospinal fiber systems (refs. 6 and 45). These mutual anatomical resemblances presumably reflect common functional features as well, a suggestion indeed supported by physiological studies which show that pontine reticulospinal and lateral vestib- ulospinal pathways both facilitate spinal ex- tensor motoneurons (see refs. 69 and 54, re- spectively). Both pathways appear, furthermore, to be engaged with monosynaptical activation of neurons of the ventral spinoreticular tract (bVFRT) (refs. 70 and 71) polysynaptically in- fluenced by flexor reflex afferent (FRA) impulses (ref. 72). The neurons of the ventral spino- Medullary ret.sp. fibre Medullary ret sp fibres ftxitine retsp fibres » Sites of termination of pontine ret sp fibres 0 Sites of termination of medullary ret sp fibres FIGURE 9. —Diagram of a transverse section of the spinal cord showing the location in the ventral and lateral funiculi and the sites of termination within the spinal gray matter of pontine and medullary reticulospinal filiers arising in the nucleus reticularis pontis caudalis and nucleus reticular gigantocellularis, respectively. Note the different lo- cation in the white matter, and the partially dissimilar areas of termination within the /fray matter of the two contingents of reticulospinal fibers. Note, furthermore, the anatomical similarities between the pontine reticulo- spinal and vestibulospinal fibers, (From ref. 45.) C8 Lot. vest, sp! fibres Pontine ret. sp. fibres a Sites of termination of lot. vest sp fibres A Sites of termination of pontine ret. sp.fibres FIGURE 10. — Diagram of a transverse section of the spinal cord showing the location in the ventrolateral funiculus and the sites of termination within the spinal gray matter of pontinr reticulospinal and lateral vestibulospinal fibers. Note the similarities between the two pathways with regard to the course and sites of termination, (l-'rom ref. 6.)

176 THE ROLE OF THE VESTIBULAR ORGANS IN SPACE EXPLORATION reticular tract are usually claimed to be located ventromedially in the spinal gray matter (ref. 73), where pontine reticulospinal as well as lateral vestibulospinal fibers terminate, as shown in figure 10. There appears thus to be a satis- factory anatomical substratum for the physio- logically demonstrated monosynaptical activation. Because of its restriction to the upper half of the spinal cord, and its origin from the medial vestibular nucleus, the medial vestibulospinal tract is generally assumed to be concerned with the adjustments of the tonus in the muscles of the head and neck and with movements of the head occurring simultaneously with conjugate deviation of the eyes, particularly in the hori- zontal plane (see refs. 46 and 54). Some recent physiological observations indicate that the medial vestibulospinal tract also may be engaged in the control of incoming sensory impulses in the cord. Thus, the phasic inhibition of mono- synaptic and polysynaptic spinal reflexes which takes place during REM sleep (ref. 74) has been shown to be due to presynaptic inhibition of group la afferent fibers (ref. 75). Since bilateral lesions of the medial and descending vestibular nucleus abolished the rapid eye movements as well as the accompanying phasic inhibition of monosynaptic reflexes during REM sleep (ref. 76), Pompeiano (ref. 77) concluded that the medial and descending nuclei by way of the medial vestibulospinal tract exert a pre- synaptic inhibition of primary afferent fibers in the spinal cord. The observation by Car- penter, Engberg, and Lundberg (ref. 78) that presynaptic inhibition of group la, Ib, and FRA afferents occurs on stimulation of the region of the medial longitudinal fasciculus in the medulla oblongata and lower pons fits in with this line of reasoning. It should, however, be emphasized that in addition to medial vestibulospinal fibers, pontine reticulospinal (ref. 45) and interstitio- spinal fibers (ref. 79) as well course within the area of the medial longitudinal fasciculus at this level of the brainstem. Furthermore, since the medial vestibulospinal tract only can be traced to midthoracic levels, it follows that the effect on the lower half of the cord must be relayed farther down by way of descending propriospinal fibers. CONCLUSION It will appear from the foregoing account that the vestibular nuclear complex has at its disposal two different direct routes by which vestibulo- spinal impulses may be mediated to the spinal cord: the lateral and medial vestibulospinal tracts. In addition, impulses of vestibular origin are also conveyed to the spinal cord by Med vest sp. fibres Lot. vtst. sp fibres FIGURE 11.—/4 diagram showing the sites of origin, course, and sites of termination of the lateral and medial vestibulo- spinal tract as determined by !\'yberg-Hansen und Musdtti (ref. 16) and Nyberg-Hansen (ref. 46). The medial tract arises from the medial vestibulur nucleus and courses chiefly ipsilaterally within the descending medial longitudinal fasciculus to midthoracic levels of the cord. The lateral tract originates from the lateral nucleus und descends purely ipsilaterad to sacral levels, organized in a somatotupical manner. Both tracts terminate in laminae Vll and H1I. The lateral vestibulospinal tract is the major spinal projec- tion from the vestibular nuclei. (See text for explanation of abbreviations.)

FUNCTIONAL ORGANIZATION OF THE VEST1BULOSPINAL PROJECTION 177 way of reticulospinal fibers. There appear anatomically and physiologically to be mutual resemblances between the lateral vestibulo- spinal tract and the pontine reticulospinal fiber system. The two direct vestibulospinal path- ways differ in some anatomical aspects, which make it likely that they, in part, are functionally dissimilar, also. The lateral vestibulospinal tract originates solely from the lateral nucleus and descends ipsilaterally, organized in a somatotopical manner throughout the whole cord. The medial tract, on the other hand, comes from the medial (and possibly descending) vestibular nucleus and descends bilaterally to the rostral half of the cord only. Fibers of both pathways terminate in laminae VII and VIII of the spinal gray matter, chiefly on interneurons, those of the medial tract in more dorsal regions (fig. 11). The medial vestibulospinal tract is, furthermore, very modest when compared with the lateral one, which, by far, is the major pathway mediat- ing vestibulospinal impulses to the cord. The differences between the two pathways are further emphasized when the origin of peripheral vestibular impulses to the lateral and medial vestibular nucleus is considered. While the utricular macula appears to be the main source of origin of primary vestibular fibers to the lateral nucleus, those to the medial nucleus are primarily derived from the cristae of the circular ducts (refs. 54 and 80 to 82). It follows from this arrangement that the lateral nucleus and the lateral vestibulospinal tract are well qualified to exert the well-known facilita- tory influence on postural tonus and spinal extensor mechanisms. This influence is exerted on a- as well as on y-motoneurons, and available anatomical and physiological data point to the indirect route via interneurons in laminae VII and VIII as the most important one. There is, however, in addition, some physiological evidence in favor of a direct monosynaptic vestibulospinal route to both a- and y-extensor motoneurons. This anatomical arrangement, with the majority of lateral vestibulospinal tract fibers terminating on spinal interneurons, and not directly on the motoneurons, is well suited to permit a large degree of plasticity and freedom of the motoneurons. It provides, furthermore, a basis for a variable interplay and integration of afferent impulses from several sources at an interneuronal level before the impulses finally converge upon the motoneurons. In contrast to the lateral tract which may act on all spinal levels, the medial vestibulospinal tract is restricted to the rostral half of the cord. In concert with this, the medial vestibular nucleus and the medial tract are primarily assumed to be concerned with the control of proprioceptive mechanisms of the cervical cord and with move- ments of the head which may occur simul- taneously with eye movements. However, recent physiological studies indicate that the medial and descending vestibular nucleus by way of the medial vestibulospinal tract exert a pre- synaptic inhibition of primary afferents in the spinal cord, and that this is the likely basis for the phasic inhibition of spinal reflexes occurring during REM sleep. REFERENCES 1. REXED, B.: The Cytoarchitectonic Organization of the Spinal Cord in the Cat. J. Comp. Neurol., vol. 96, 1952, pp. 415-496. 2. REXED, B.: A Cytoarchitectonic Atlas of the Spinal Cord in the Cat. J. Comp. Neurol., vol. 100, 1954, pp. 297-379. 3. REXED, B.: Some Aspects of the Cytoarchitectonics and Synaptology of the Spinal Cord. Progr. Brain Res., vol. 11, 1964, pp. 58-90. 4. Sl'RAGUE, J. M.; AND HA, H.: The Terminal Fields of Dorsal Root Fibers in the Lumbosacral Spinal Cord of the Cat, and the Dendritic Organization of the Motor Nuclei. Progr. Brain Res., vol. 11, 1964, pp. 120-152. 5. LUNDBERG, A.: Integration in the Reflex Pathway. Muscular Afferents and Motor Control, R. Granit, ed., Almqvist & Wiksell, 1966. pp. 275-305. 6. NYBERG-HANSEN, R.: Functional Organization of De- scending Supraspinal Fibre Systems to the Spinal Cord. Anatomical Observations and Physiological Correlations. Er'iebn. Anat. Entwickl.-Gesch., voL 39, 1966, pp. 1-48.

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180 THE ROLE OF THE VESTIBULAR ORGANS IN SPACE EXPLORATION 72. LUNDBERG. A.; AND OsCARSSON, O.: Two Ascending Spinal Pathways in the Ventral Part of the Cord. Acta Physiol. Scand.. vol. 54, 1962. pp. 270-286. 73. OSCARSSON. O.: Differential Course and Organization of Uncrossed and Crossed Long Ascending Spinal Tracts. Progr. Brain Res., vol. 12, 1964, pp. 164-176. 74. GASSEL. M. M.: MARCHIAFAVA, P. L.; AND POMPEIANO. O.: Tonic and Phasic Inhibition of Spinal Reflexes During Deep, Desynchronized Sleep in Unrestrained Cats. Arch. Ital. Biol., vol. 102, 1964, pp. 471-499. 75. MORRISON, A. R.: AND POMPEIANO, O.: Central Depolari- zation of Group la Afferent Fibers During Desynchro- nized Sleep. Arch. Ital. Biol., vol. 103. 1965, pp. 517- 537. 76. POMPEIANO, O.; AND MORRISON, A. R.: Vestibular In- fluences During Sleep. III. Dissociation of the Tonic and Phasic Inhibition of Spinal Reflexes During De- synchronized Sleep Following Vestibular Lesions. Arch. Ital. Biol., vol. 104, 1966. pp. 213-246. 77. POMPEIANO, O.: Sensory Inhibition During Motor Ac- tivity in Sleep. Neurophysiological Basis of Normal DISCUSSION Tang: How did you place the lesions and how do you determine the extent of your lesions? Nyberg-Hansen: The lesions were made stereotaxically by an electrode going through the cerebellum in an oblique angle in order to avoid tentorium cerebelli. The lesions were checked histologically in serial sections at 15-micron stained thionirie. Every fifth section was mounted and examined. Tang: What electrode are you using, and are you using alternating or direct current to destroy the tissue? Nyberg-Hansen: We used steel electrodes, and the lesions were made with direct current. Tang: I asked because I found by using the Prussian blue staining method that the lesion is much more extensive than histologically shown because the ion diffusion goes way beyond the histologically identifiable big hole. The ion can diffuse way beyond the site of the lesion. Nyberg-Hansen: I can say only that in our sections we got only a very small hole, and the lesion as depicted in my illustrations is not the hole which is made, but includes the glia reaction as well. I think that with weak current and small lesions, examined closely in thionine sections, you can outline the lesion very well. Pompeiano: I should like to mention the results of some unpublished experiments made recently by Cook, Cangiano, and myself, indicating that the effects of Vestibular nerve stimulation on the primary afferents in the lumbar cord are not mediated by the medial vestibulospinal tract nor by propriospinal descending pathways, but are transmitted to the spinal cord by collaterals from the vestibular nuclei to brainstem structures whose descending pathways course along the ventral quadrants. 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W.: Tonic and Kinetic Re- sponses of Cat's Vestibular Neurons to Horizontal Angular Acceleration. J. Neurophysiol., vol. 28. 1965. pp. 991-1013. a-motoneurons is also relevant to your anatomical findings. This problem needs further experimental data, but a few observations are available on this subject. It has been found recently (S. Lund and O. Pompeiano: Monosynaptic Excitation of Alpha Motoneurones From Supraspinal Structures in the Cat. Ada Physiol. Scand., vol. 73. 1968, pp. 1-21) that the values for the "time to peak" and the decay of the mono- synaptic EPSP induced in extensor motoneurons by single- shock stimulation of the Deiters' nucleus are well in accord- ance with the corresponding values for the homonymims group la EPSP's. This suggests that, in the cat. there is no difference in the average distance from the soma of the postsynaptic membrane for the two kinds of synapses, as has been found in the frog (E. Fadiga and J. M. Brookhart: Monosynaptic Activation of Different Portions of the Motor Neuron Membrane. Am. J. Physiol., vol. 198, 1960, pp. 963-703). Nyberg-Hansen: Yes. I want to stress that we have seen some very few fibers in lamina IX among the moto- neurons. but it has been impossible to tell whether they have any relationship to these neurons. With present anatomical methods, it is only safe to conclude that there are no terminations on the soma. If there are any termina- tions on the motoneurons, as the physiologists have shown, it must be on the dendrites. I may add that we have recently started doing electron- microscopic studies with degeneration techniques on this fiber system. So far our results are only preliminary. Such studies may turn out to show synapses on the dendrites of motoneurons. But I think that with electronmicroscopic studies, this will not come out so much concerning the soma. because it you sec few fibers among the soma in silver studies, it shall be very difficult to see them in electronmicroscopic studies.

FUNCTIONAL ORGANIZATION OF THE VESTIBULOSPINAL PROJECTION 181 Wilson: As far as the location of the terminals in the vestibulospinal tract is concerned, we found that the rise time of the EPSP's in the neck is quite slow. So there I would certainly look on the dendrites. The position of the people in your laboratory used to be that most of the cells or all of the cells in Deiters' nucleus project to the spinal cord. Does this still remain your viewpoint or do you think there might also be interneurons in Deiters' nucleus? Do you still think all the cells project to the spinal cord or would you consider there might be interneurons in there also? Nyberg-Hansen: I have not personally done retrograde studies on this, but from the studies of Pompeianoand Brodal, it appears like all large, medium-sized cells and small neurons of the lateral nucleus project to the cord. But there may very well be axon collaterals going in a rostral direction, and as recently shown by Ladpli and Brodal. there is a heavy inter- connection between the vestibular nuclei as well as a pro- jection from the vestibular nuclei to the reticular formation. This may, of course, be collaterals of vestibulospinal tract fibers.