Brain Research, 95 (1975) 309-322 © Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
309
THE P O N T I N E P R O J E C T I O N F R O M T H E I N F E R I O R C O L L I C U L U S IN THE CAT. AN E X P E R I M E N T A L A N A T O M I C A L STUDY
KOKI KAWAMURA
Department of Anatomy, School of Medicine, Iwate Medical University, Morioka 020 (Japan) and Anatomical Institute, University of Oslo, Oslo (Norway)
SUMMARY
The present study was carried out in order to clarify the organization of the pontine projection from the inferior colliculus in the cat with the methods of Nauta 27 and Fink and Heimer 16 following lesions of different sizes and locations in the inferior colliculus. The inferior colliculus projects exclusively to the anterior two-thirds of the ipsilateral dorsolateral pontine nucleus. The majority of fibers originates from cells in the central nucleus and a small amount from cells in the external and the pericentral nuclei as well. Areas of the present projection largely overlap with areas of projection from the auditory cortex, and also fairly well with those from the superior colliculus. Possible pathways of the acoustic inputs to the cerebellum involving the pontine nuclei are discussed based upon the current findings, as well as upon those obtained by preliminary experimental studies of anterograde and retrograde axonal flow.
INTRODUCTION
Fibers from the inferior colliculus to the pontine nuclei have been described in various animal species, for example in the cata2,~s,ao, 39, the monkey 25, the opossum24, 31, the rabbit a7, the rat 38, the mouse 4°, the kangaroo rat la and the guinea pig4L Most authors who studied the termination with some precision indicate this to be the dorsolateral pontine nucleus. In the published studies, all o f which were carried out with axonal degeneration methods, the lesions often involved a part of the superior colliculus as well as the inferior colliculus, and some authors were not sure whether fibers coming from the superior colliculus might have been transected, even if the lesions appeared to be confined to the inferior colliculus. Kawamura and Brodal 2~ recently traced the course
310 of the tectopontine fibers from the superior colliculus in sagittally cut sections (see Fig. 3 of ref. 22), and found that these fibers at the level of the inferior colliculus pass so far ventrally that there is no risk of transecting them even with fairly deep lesions of the inferior colliculus. It thus appears that such lesions may be used in studies of the efferent fiber projections of the nuclei of the inferior colliculus. With this knowledge it may be possible to arrive at more definite answers than before to the following questions. (I) Are the fibers from the inferior colliculus to the pontine nuclei distributed unilaterally or bilaterally? (2) In what part of the pontine nuclei do they terminate'? (3) Do the fields of termination of the inferior collicular projection overlap with those of other pontine afferents, and if so, with what afferents ? MATERIAL A N D M E T H O D S
Fifteen adult cats were used in this investigation. In 7 cats lesions of various sizes and locations were made in the inferior colliculus under Nembutal anesthesia, and another 8 cats with lesions of the superior colliculus (see ref. 22) were included for purposes of comparison. In two of the 7 cats (cat KA 12R, used in another study by Kawamura and Brodal 2z, and cat KA 18L) an electrolytic lesion was made with an electrode 1.0 mm in diameter. This was inserted from the anterior direction at 40 ° from the vertical plane, and passed beneath the cerebellar tentorium so that its tip reached the central nucleus of the inferior colliculus. For making large lesions of the left inferior colliculus it was preferable to work under direct vision. Therefore, in 5 other cases (cats KA 17L, KA 19L, D 16L, D 17L, and D 22L), after opening the skull and the dura mater, the visual cortex and the underlying white matter were removed by suction until the bony tentorium cerebelli and the anterolateral portion of the inferior colliculus were visible. The part of the tentorium covering the inferior colliculus was carefully removed, and multiple lesions were made in the inferior colliculus by means of a dental drill and/or a heated metal rod. Six to 7 days after the operation, the animals were perfused intravitally under Nembutal anesthesia with 0 . 9 ~ saline followed by 10~ formalin. The brains were dissected free and transferred to 10~ formalin for further fixation. The pons was cut transversely (cats KA 17L and D 17L) or horizontally (cats KA 12R, KA 18I_, KA 19L, D 16L, D 22L) at 20 ,urn on the freezing microtome. The tectum containing the lesions was cut transversely in all cases. Every 10th section was impregnated according to the methods of Nauta z7 and Fink and Heimer ~6. In addition, some sections containing the lesions were stained with thionin. Drawings of the sections through the colliculus and the pons were made in a projection apparatus and the extent of the lesions and the distribution of the ensuing degeneration, as checked in the microscope, were entered in the drawings. In these, coarse degenerating fibers are indicated by wavy lines and fine granules by dots. If every 10th section (transversely cut, 20/~m thick) is selected, and these are
311
Fig. 1. Photographs and photomicrographs showing examples of lesions and degeneration in the pontine nuclei. A: the lesion in the left inferior colliculus in cat KA 17L, seen from above (cfi Fig. 3). B: photomicrograph of transversely cut, thionin stained section through the inferior colliculus in cat KA 19L, to show the depth and extent of the lesion ( x 8). Arrows point to ventral border of the central nucleus of the inferior coUiculus. C : photomicrograph (Fink and Heimer method, x 260) of a horizontal section through the pons in cat KA 19L, showing the distribution of degeneration in the dorsolateral pontine nucleus (see inset) following a lesion of the ipsilateral inferior colliculus (cfi Fig. 3). Abbreviations used in all figures: B.p., brachium pontis; C.i.c., commissure of inferior colliculus; CN, central nucleus of inferior colliculus; EN, external nucleus of inferior colliculus; I.c., inferior colliculus; L.I., lemniscus lateralis; L.m., lemniscus medialis; N.c.s., nucleus centralis superior (raphe); N.dl., nucleus dorsolateralis pontis; N.I., nucleus lateralis pontis; N.p., nucleus peduncularis pontis; N.pm., nucleus paramedianus pontis; N.r.t., nucleus reticularis tegmenti pontis; N.v., nucleus ventralis pontis; N.V, root fibers of trigeminal nerve; Pc, pericentral nucleus of inferior colliculus; Ped, pedunculus cerebri (corticospinal and corticopontine fibers); P.g., periaqueductal gray matter; S.c., superior colliculus; T, trapezoid body.
m o u n t e d in a series, there will be a b o u t 30 m o u n t e d sections passing t h r o u g h the pons. In all series the sections p a s s i n g t h r o u g h the c a u d a l m o s t a n d r o s t r a l level o f the p o n tine nuclei were n u m b e r e d as 1 a n d 30, respectively, in o r d e r to facilitate the c o m p a r i son o f d i s t r i b u t i o n o f d e g e n e r a t i o n a m o n g different cases. RESULTS
(1) Delimitation and nomenclature of the inferior colliculus and the pontine nuclei Cajal i2 first described the inferior colliculus as being c o m p o s e d o f 3 nuclei, a n d these have c o m e to be called the central nucleus (CN), the external nucleus ( E N ) a n d the pericentral nucleus (Pc), the largest being the central nucleus. This consists
312 of densely packed small and medium sized cells. The EN forms a rather broad band of cells covering the CN rostrally and laterally, and the Pc, a narrower band of cells, borders the CN caudally and dorsally (Figs. 2, 3 and 4). In the pontine nuclei, a subdivision into the minor parts (nuclei) distinguished by Brodal and Jansen 4, was attempted even if the borders between them are arbitrary. (2) Degeneration in the pontine nuclei in silver impregnated sections Following lesions of the inferior colliculus, coarse degenerating fibers, obviously representing passing fibers, enter the dorsolateral pontine nucleus from the rostrolateral aspect (Fig. l) and proceed caudomedially. Interspersed between them are seen fairly fine degenerating fibers as well as isolated argyrophilic fragments of different shapes. Some of them are situated close to the perikarya, but the majority is found in the neuropil (Fig. 1C). An area containing abundant fragments of the types described above, is here considered as a 'terminal field'. In the present study the terminal fields determined with the methods of Nauta 27 and of Fink and Heimer 16 are identical. However, more isolated fine granules are present within the terminal fields in Fink and Heimer sections than in Nauta sections. The same is the case for the corticopontineS, 6 and for the superior colliculopontine projections 22. (3) Presentation of cases As described in Material and Methods, the damage of the inferior colliculus is in most cases complicated by lesions of the visual cortex. To facilitate the following presentation and discussion, we start by considering one case (cat KA 18L) with a pure lesion of the inferior colliculus, and another one (cat KA 12R) in which there is a lesion of the left superior colliculus as well as the right inferior coIliculus (Fig. 2). The latter case has been described previously (see Fig. 9 of ref. 22). In cat KA 18L (brain stem cut horizontally, inferior colliculus cut transversely, killed after 7 days, Fig. 2), only a fairly superficial lesion of the inferior colliculus was made, partially involving the EN and the Pc. The CN is free from damage. Degeneration found only in a restricted area of the ipsilateral dorsolateral nucleus in the pons is sparse. In cat KA 12R (pons cut horizontally, colliculus cut transversely, killed after 6 days, Fig. 2), a fairly large lesion involved the central portion of the CN together with the EN and the Pc. There is more degeneration in the dorsolateral nucleus, chiefly confined to the dorsolateral part. As shown above, in cats KA 18L and KA 12R the lesions of the inferior colliculus are not complete. Therefore, the possibility remains that there are more extensive areas of termination than those found in the above cases. In the next presentation, however, some cases of larger collicular lesions provide a more complete indication of the total extent of the area of termination. In these cases the visual cortex has been removed, but this is not a source of ambiguity since the visual cortex has been shown to project ipsilaterally to a different part of the pons (see refs. 8, 9 and 22). Large lesions of the inferior colliculus with additional visual cortex invoh'ement From the 5 cases (cats KA 19L, KA 17L, D 16L, D 17L, and D 22L), cats
313
KA 18 L
KAI2R
KA 18 L
KAI2 R
''''
KA I IR
30~ ~ KA 18L
KA 12R
)
A0orrom,CORTEX
VENTRA~ KA II R
(,,m_
0
('
~? / / / /
? ////
ii-XLml
~ t
""\ \~
,,
ii
li
.
~
/~
IA
IA , ,/
i
!
Fig. 2. Diagrams illustrating the findings in two cases of inferior collicular lesions (cats K A 18L and K A 12R). The lesions (black) are shown (above) as seen on the surface of the inferior colliculus and (below) as appearing in sections at the levels indicated in diagram above. Degeneration in the pontine nuclei is shown (dots) in drawings of horizontal sections at 4 equally spaced levels of the pons. The stippled lines in this and the following figures indicate the borders between the different nuclei in the pontine gray and in the inferior colliculus. For comparison, the findings in a case with a lesion of the auditory cortex (cat K A 1 IR) are shown (to the right). Levels through the pons correspond to those used in the two series to the left.
314 KA 19 L ond R
VENTRAL
r~
z:,
O l
o
18
2O
22
4
Fig. 3. Diagram showing the findings in cat KA 19 with a large lesion (black) of the left inferior colliculus as well as lesions of the visual cortex of both sides. Above to the right: lesions as seen in 4 transverse sections through the inferior colliculus at levels indicated in a diagram of the colliculus are shown (partial destruction shown hatched). Below: a series of drawings of 8 horizontal sections through the pons taken with equal intervals. The stippled lines indicate the borders between the various nuclei in the pontine gray and in the inferior colliculus. Note the differences in distribution of degeneration on the two sides of the pons. The degeneration in the dorsolateral nucleus on the left (dots) is due to the lesion of the left inferior colliculus, while the corticopontine projection from the visual cortex (crosses) is bilateral (cf. text). Microphotograph of degeneration in drawing 22 shown in Fig. 1C.
315 KA 19L and KA 17L will be presented below as representative; the findings obtained in the pons are approximately the same in the other 3 cases. Cat KA 19L and R (brain stem cut horizontally, inferior colliculus cut transversely, killed after 7 days, Fig. 3). The left inferior coIliculus as well as both visual cortices are largely destroyed. The CN of the left inferior colliculus is heavily damaged except for its rostral portion, and the EN and the Pc are considerably involved as well. The right inferior colliculus is intact. The corticopontine projection from the visual cortex has been shown to be ipsilateralS,9,22, as is also the projection from the inferior colliculus to the ports. The degeneration found in the right pontine nuclei, therefore, is due to the lesion of the right visual cortex only. This is marked with crosses on a diagram of Fig. 3. In the left pons, however, the degeneration results from damage of both the inferior colliculus and the cortex. Most of the degeneration, which is distributed symmetrical to that seen on the other side, is considered to be due to the lesion of the left visual cortex (marked also with crosses), and the additional degeneration found in the dorsolateral nucleus must, therefore, be due to the lesion in the colliculus (marked with dots in the same figure). Compared with the previous cases (cats KA 18L and KA 12R), degeneration found in the dorsolateral nucleus of this case is a little more extensive. The present case gives a more complete impression of the total inferior collicular projection than do the previous cases, even if the lesion does not totally involve the inferior colliculus. Although the slight possibility still remains that degeneration may be found elsewhere with total removal of the inferior colliculus, the area delimited probably represents almost the entire terminal field in the pons of fibers descending from the inferior colliculus. Cat KA 17L (both brain stem and inferior colliculus cut transversely, killed after 7 days, left side of Fig. 4). A large lesion is confined within the left inferior coIliculus and to a large extent involves the CN, except for its rostral part, as well as the two accessory nuclei (EN and Pc). In addition, the visual cortex is affected on the same side. The distribution of degenerating fibers corresponds well to that of the previous case (cat KA 19L, cf. Fig. 3), and the degeneration in the left pontine nuclei is considered to have two components. Using the same convention indicated above, degeneration due to the visual cortex lesion is plotted on a diagram as crosses and that due to the involvement of the inferior colliculus as dots. Another guide in differentiating degenerating fibers from the two different sources is that the corticopontine fibers enter the nucleus from the cerebral peduncle, whereas the colliculopontine fibers pass from the dorsolateral aspect of the pontine gray. The corticopontine group projects to fairly extensive areas in the ventral, paramedian, peduncular and lateral nuclei (here confirming the findings of P. Brodal s,9 and Kawamura and Broda122); but the colliculopontine group projects to the approximately rostral two-thirds of the ipsilateral dorsolateral nucleus, more specifically to its dorsolateral part. On the right side of Fig. 4, a map showing the total pontine projection from the superior colliculus is presented for comparison with the current results. This summarizes the findings of 7 cases which have been described before (see ref. 22) together with another new case (cat KA 16L). For a detailed description of the superior collic-
316
Sc
/ i
, Pg J ,, '
",
,
0
Cc
",!
.4' ~ KA 17 L
ROSTRAL
,'I
/
' ~'~ 1~ I / ~ ~ / "~ %,, ~_C -.,,.\
~ .,~ / ' "
-'h".: / (
"
23
\ • %'; ~
~'~
i
~':"
-'.,-.. C."' -'-. '-':'"
& \
....,'~
'
\
'"",% ',' / ;.':hi
/,iO' /
\ -. ,,,'~,,..~. ~,
/ /'
', ~ ~.. CAUDAL
Fig. 4. Drawing showing the distribution of degeneration in the pontine nuclei (on the left) following lesions (black) of the left inferior colliculus and the left visual cortex in cat KA 17L, marked as dots and crosses respectively. Above to the right the collicular lesion is shown as appearing in transverse sections. In the diagram of the colliculi and visual cortex (above to the left) the hatched area indicates outlines of the lesions of the superior colliculus in altogether 8 cases. The total projecting area of the superior colliculus is shown in the right pontine half (dots). Principles of presentation as in Figs. 2 and 3.
317 ular projection, readers are referred to the previous paper 22. As demonstrated in the diagram, fields of termination in the dorsolateral pontine nucleus of the two collicular projections overlap considerably. Detailed observations, however, reveal that the fibers from the superior colliculus, compared with those from the inferior, distribute more abundantly, particularly caudally. Since the auditory cortex likewise projects to the dorsolateral nucleus10,~2, it is of some interest to present the case of cat KA 11R (pons cut horizontally, killed after 6 days, see Fig. 2, right), where the right auditory cortex was severely damaged. Another large lesion was made in the left superior colliculus, but this does not interfere with the interpretation of the findings of the auditory cortex projection since each projection is ipsilateral. This case was described previously by Kawamura and Broda122, but is illustrated here for comparison, showing horizontal sections of approximately the same levels as the cases of collicular lesions (cats KA 18L and KA 12R in Fig. 2). The corticopontine fibers enter the nucleus from the cerebral peduncle and end exclusively in the dorsolateral nucleus, showing an extensive area of overlap with the collicular projections. DISCUSSION
Origin within the inferior colliculus of fibers to the pontine nuclei It is of interest to know if all parts of the inferior colliculus give off fibers to the pontine nuclei, because the structure of the inferior colliculus is not homogeneous, and the afferent fiber systems are differentially distributed within it (for details see refs. 32-34). According to the works of Rockel and Jones fields of termination of ascending and descending fibers in the colliculus scarcely overlap with each other. Fibers from the auditory cortex terminate only in the dorsomedial division of the CN, which is composed of large cells (30 #m × 40 #m) 32, as well as in the Pc and EN, whereas those from the lateral lemniscus are distributed mainly to the ventrolateral two-thirds to three-quarters portion of the CN 34. From the present study it is difficult to arrive at any definite conclusion as to the probable identity of cells of origin of the tectopontine pathway, but it seems that most of the neurons which give off fibers to the pons are located in the CN, with only a few in the Pc and EN (see cat KA 18L of Fig. 2). The chief aim of the present study was to determine the total distribution of the inferior collicular projection in the pontine nuclei, and further experimental study is needed to decide the exact location of cells in the colliculus sending fibers to the pons. This is being pursued with the autoradiographical and the horseradish peroxidase methods.
Patterns of organization of projection from the inferior colliculus to the pons The area in the pons to which the descending fibers from the inferior colliculus are distributed has been proved in the present study to be coextensive with the dorsolateral nucleus, except probably its most caudal part. Considering that the lesions of the inferior colliculus were subtotal, it is possible that the degeneration observed in the pons was less than it would have been if the whole colliculus had been involved.
318 Even at the survival time of 6 or 7 days, the parts of the colliculus unaffected by the primary lesion usually displayed a considerable degree of gliosis and contained 'morbid' neurons, but it is uncertain to what extent this secondary involvement caused degeneration in the ports. Since, however, the lesions produced in the present study together covered almost the total area of the inferior colliculus, they can give a good impression of the total degeneration in the pons. Furthermore, the results of the present study generally confirm the descriptions of previous authors22,zs,30, 39, who, however, did not indicate the pontine areas of termination as precisely as was done here. Thus, the pontine projection from the inferior colliculus is now shown to be confined almost exclusively to the ipsilateral dorsolateral nucleus. As stated above, the majority of the fibers appear to originate from cells in the CN, but a small portion of the Pc and EN may also contribute. There are certain other structures which give off fibers to the dorsolateral pontine nucleus. The superior colliculusl, z2 and the auditory cortex 1°,2z are the major sources, and these will be discussed below. A small restricted medial portion of the dorsolateral nucleus, together with its immediate neigborhood, receives a small number of fibers from the second somatosensory cortex 6, from the anterior part of the orbital gyrus 7, and also from the cerebellar nuclei 3. It appears from the diagrams published by P. BrodaI6, 7 and A. Brodal et al. 3, that there is only a little overlap of the areas of termination of these 3 sources with those of the two colliculi.
The tecto-ponto-cerebellar projection and the pathways for acoustic inputs to the cerebellum The pontine projection from the inferior colliculus and that from the superior colliculus have almost similar areas of termination in the dorsolateral nucleus (see Fig. 4). The fibers from the inferior colliculus, however, do not appear to terminate in the caudal third of the nucleus and do not end in the lateral and the peduncular nuclei which receive a small number of fibers from the superior colliculus (see also ref. 22). Before discussing possible routes for acoustic and optic inputs to the cerebellum, it is necessary to consider some data on the topography of the pontocerebellar projection. Even if this is not yet known in sufficient detail, some data are available concerning those pontine subdivisions which are of particular relevance for the problems to be discussed here. In their study of 1946, Brodal and Jansen 4, using the modified Gudden Method 2, could elucidate only the major features of the pontocerebellar projection in the cat and rabbit. It is of interest, however, that the lateral pontine nucleus appears to send a considerable portion of its fibers to the paraflocculus (their experiment cat 0.75). Our autoradiographical studies confirm this projection (see ref. 22a). Furthermore, Brodal and Jansen 4 found that pontine fibers pass from the dorsolateral nucleus as well as from the paramedian nucleus to the vermis, and more particularly to its middle part, including lobule VII (see their experiments cat 0.41 and rabbit 0.27). This has recently been confirmed with more refined methods. Graybiel is, following injection of horseradish peroxidase (HRP) in the midvermal region of the rat, found HRP-positive neurons in the dorsolateral and paramedian
319 pontine nuclei, but extremely few in other pontine subnuclei. In our recent z2~ experiments with HRP, largely similar findings have been obtained in the cat. Thus, the optic and acoustic areas of the cerebellar vermis (largely lobule VII of Larsell, Cz of Bolk) receive massive fibers from the dorsolateral and the paramedian nucleus (particularly from the part of the latter lying close to the midline according to our own observations). According to our preliminary studies, lobule VIIb receives a somewhat larger amount of fibers from the dorsolateral nucleus than does lobule VIIa. A [3H]leucine study in our laboratory appears to confirm this correlation and furthermore indicates that a projection from the lateral pontine nucleus to lobule VII is of a lesser degree g2a. It may be concluded from this and from the findings made in the present study, that acoustic impulses mediated via the inferior colliculus may be transmitted via the dorsolateral pontine nucleus to the acoustic area of the cerebellar vermis (lobule VII). Since the projection from the acoustic cerebral cortex to the pons likewise terminates in the dorsolateral nucleus (cat KA l l R in Fig. 2, see also ref. 10), it appears that impulses from the acoustic cortex mediated via the pons, will reach the same cerebellar area. However, although the termination of the two different pathways whereby acoustic impulses may reach the pons overlap almost completely in the dorsolateral nucleus, closer observation of the distribution of degenerating fibers (cf. Fig. 2) following lesions of the two structures, as undertaken in the present study, reveals that the projection from the cerebral cortex extends a little more ventrally, close to the lateral nucleus. This suggests that the acoustic impulses arriving directly from the inferior colliculus and those coming via the cerebral cortex to some extent terminate on different groups of pontine cells, even if there is considerable overlapping, and it may be asked whether the pathway from the acoustic cortex via the pons to the cerebellum differs from that from the inferior colliculus. Physiologically responses t o acoustic stimuli have been recorded outside the classical acoustic area 36 of the cerebellar cortex; thus by Buser and Franche111 in what is referred to as the crus I! (but according to their figure is actually the paraftocculus), and by Fadiga and Pupilli 15 in extensive parts of the cerebellar hemisphere. Most recently Mortimer 2n has confirmed this in the monkey and indicates as maximal sites of responses the intermediate zone of lobule VI (lobulus simplex), the posterior portion of the paramedian lobule and the adjacent dorsal paraflocculus. While some of these responses may be mediated via the olive and its climbing fibers (see ref. 26), some are likely to be mediated via the pons. This would probably require that pathways for acoustic impulses reach parts of the pons which project onto the hemispherical cerebellar areas mentioned above. As shown here, fibers from the inferior colliculus as well as from the acoustic cortex appear to end exclusively in the dorsolateral pontine nucleus. Future studies by means of the peroxidase method may disclose that some cells of the dorsolateral nucleus in fact send their axons to cerebellar areas other than the classical acoustic area, since it appears that a particular small cerebellar region in general receives pontine afferents from more than one pontine cell group, as recently shown for the paramedian lobule 19. However, the fibers from the acoustic cortex ending in the dorsolateral pontine nucleus
320 extend very close to the lateral nucleus. Since this appears to project onto the paraflocculus (see above), it may be hypothesized that dendrites from its cells extend into the dorsolateral nucleus and are contacted by cortical afferents. This might explain the transmission of acoustic impulses to the paraflocculus.
The routes of visual inputs to the cerebellum compared with those of acoustic inputs As stated above, there is a better correspondence between the pontine projections of the auditory cortex and the inferior colliculus than between the projection zones of the inferior colliculus and the superior colliculus. Considering that the inferior colliculus is dominated by acoustic inputs, it makes sense that the projection of the inferior colliculus overlaps more closely with that of the auditory cortex than with that of the superior colliculus which is primarily visual. The teleceptive pathways for visual and acoustic impulses seem to be relatively independent before they reach the cerebral cortex. Moreover, the convergence of the two pathways at the cortical level appears to be minimaP 4,2°,21. In the superior colliculus, however, there is extensive anatomical overlap. Superficial layers of the superior colliculus are known to receive fibers primarily from the 3 visual sources (the retina, the visual cortex and the ventral lateral geniculate nucleus), while deeper layers receive fibers from the auditory cortex17, 29 and possibly from the sensorimotor cortex 17 and from other sources as well. Thus it is quite probable that the first station for integration of optic and acoustic impulses is in the superior colliculus, and that they are further 'integrated' in the rostral two-thirds of the dorsolateral nucleus in the pons before they reach the cerebellum. The main cerebellar cortical areas which receive fibers from this nucleus are located in the tuber and folium vermis~,35, 36. A slight difference in location of these visual and acoustic cerebellar areas, brought out by careful examination of the experimental physiological data, may be related to the slight anatomical differences in the projections from the superior and inferior colliculi to the pons and the cerebellar projection from the latter. With regard to cortico-ponto-cerebellar pathways, there is a marked difference between those for acoustic and visual impulses. Responses to visual stimuli have been recorded from the same cerebellar areas as mentioned above for acoustic stimuli 15,~6, and in addition from the floeculus 23. The visual corticopontine fibers terminate in far more extensive and different parts of the ports than do the acoustic (see Figs. 3 and 4 here, and refs. 8, 9 and 22), many of them in the ventral and the paramedian nuclei. While the latter projection is probably concerned in the further propagation of impulses to the lobule VII ('see above), the cerebellar projections of the ventral nucleus are not clear, but they may well turn out to be to the hemispheres (see ref. 4). Although decisive morphological evidence is still lacking, it can, nevertheless, be assumed that the optically evoked responses obtained in the cerebellar vermis and in the ansoparamedian lobule are attributable to two different pontocerebellar pathways.
321 REFERENCES 1 ALTMAN,J., AND CARPENTER,M. B., Fiber projections of the superior colliculus in the cat, J. comp. Neurol., 116 (1961) 157-177. 2 BRODAL,A., Modification of Gudden method for study of cerebral localization, Arch. Neurol. Psychiat. (Chic.), 43 (1940) 46-58. 3 BRODAL,A., DESTOMBES,J., LACERDA,A. M., AND ANGAUT,P., A cerebellar projection onto the pontine nuclei. An experimental anatomical study in the cat, Exp. Brain Res., 16 (1972) 115-139. 4 BRODAL, A., AND JANSEN, J., The ponto-cerebellar projection in the rabbit and cat, J. comp. Neurol., 84 (1946) 31-118. 5 BRODAL, P., The corticopontine projection in the cat. I. Demonstration of a somatotopically organized projection from the primary sensorimotor cortex, Exp. Brain Res., 5 (1968) 210-234. 6 BROOAL,P., The corticopontine projection in the cat. Demonstration of a somatotopically organized projection from the second somatosensory cortex, Arch. ital. Biol., 106 (1968) 310-332. 7 BRODAL,P., The corticopontine projection in the cat. II. The projection from the orbital gyrus, J. comp. Neurol., 142 (1971) 141-152. 8 BRODAL,P., The corticopontine projection from the visual cortex in the cat. I. The total projection and the projection from area 17, Brain Research, 39 (1972) 297-317. 9 BRODAL,P., The corticopontine projection from the visual cortex in the cat. II. The projection from areas 18 and 19, Brain Research, 39 (1972) 319-335. 10 BRODAL,P., The corticopontine projection in the cat. The projection from the auditory cortex, Arch. ital. Biol., 110 (1972) 119-144. 11 BOSER, P., ET FRANCHEL,H., Neurophysiologie. Existence d'un foyer de projection sensorielle acoustique au niveau du lobe ansiforme du cervelet cbez le chat, C. R. Acad. Sci. (Paris), 251 (1960) 791-793. 12 CAJAL, S. RAM6N Y, Histologie du Syst~me Nerveux de l'Homme et des Vertdbrds, Maloine, Paris, 1909-1911. 13 CAREY,C. L., AND WEBSTER,D. B., Ascending and descending projections of the inferior colliculus in the kangaroo rat (Dipodomys merriami), Brain Behav. Evol., 4 (1971) 401-412. 14 DIAMOND,I. T., JONES,E. G., AND POWELL,T. P. S., The association connections of the auditory cortex of the cat, Brain Research, l l 0968) 560-579, 15 FADIGA,E., ANDPUPILLI,G. C., Teleceptive components of the cerebellar function, Physiol. Rev., 44 (1964) 432-486. 16 FINK, R. P., AND HEIMER, L., Two methods for selective silver impregnation of degenerating axons and their synaptic endings in the central nervous system, Brain Research, 4 (1967) 369-374. 17 GAREY,L. J., JONES, E. G., AND POWELL, T. P. S., Interrelationships of striate and extrastriate cortex with the primary sites of the visual pathway, J. Neurol. Neurosurg. Psychiat., 31 (1968) 135-157, 18 GRAYBIEL,A. M., Visuo-cerebellar and cerebello-visual connections involving the ventral lateral geniculate nucleus, Exp. Brain Res., 20 (1974) 303-306. 19 HODDEVIK, G. H., The pontocerebellar projection onto the paramedian lobule in the cat. An experimental study with the use of horseradish peroxidase as a tracer, Brain Research, 95 (1975) 291-307. 20 KAWAMURA,K., Corticocortical fiber connections of the cat cerebrum. I. The temporal region, Brain Research, 51 (1973) 1-21. 21 KAWAMURA,K., Corticocortical fiber connections of the cat cerebrum. IlI. The occipital region, Brain Research, 51 (1973) 41-60. 22 KAWAMURA,K., AND BRODAL,A., The tectopontine projection in the cat: an experimental anatomical study with comments on pathways for teleceptive impulses to the cerebellum, J. comp. Neurol., 149 (1973) 371-390. 22a KAWAMURA,K., AND HASHIKAWA,T., Pontocerebellar projections of cats with comments on teleceptive impulses to the cerebellar cortex. An experimental anatomical study by means of ortho- and retrograde axonal flow techniques. Abstr. Papers presented at lOth Int. Congr. Anatomists, Tokyo, 1975. 23 MAEKAWA,K., AND SIMPSON,J. I., Climbing fiber responses evoked in vestibulocerebellum of rabbit from visual system, J. Neurophysiol., 36 (1973) 649-666. 24 MARTIN,G., Efferent tectal pathways of the opossum (Didelphis virginiana), J. comp. Neurol., 135 (1969) 209-224.
322 25 MOORE, R. Y., AND GOLDBERG,J. M., Projections of the colliculus in the monkey, Exp. Neurol., 14 (1966) 429-438. 26 MORTIMER,J. A., Cerebellar responses to teleceptive stimuli in alert monkeys, Brain Research, 83 (1975) 369-390. 27 NAUTA, W. J. H., Silver impregnation of degenerating axons. In W. F. WINDLE (Ed.), New Research Techniques of Neuroanatomy, Thomas, Springfield, 111., 1957, pp. 17--26. 28 NOORT, J. VAN, The Structure and Connections of the Inferior Colliculus. An Investigation of the Lower Auditory System, Thesis, Van Gorcum, Assen, 1969, 118 pp. 29 PAULA-BARBOSA,M. M., AND SOUSA-PXNTO,A., Auditory cortical projections to the superior colliculus in the cat, Brain Research, 50 (1973) 47-61. 30 POWELL,E. W., AND HATTON,J. B., Projections of the inferior colliculus in cat, J. comp. Neurol., 136 (1969) 183-192. 31 RAFOLS,J. A., AND MATZKE,H. A., Efferent projections of the superior colliculus in the opossum, J. comp. Neurol., 138 (1970) 147-160. 32 ROCKEL, A. J., AND JONES, E. G., The neuronal organization of the inferior colliculus of the adult cat. I. The central nucleus, J. comp. Neurol., 147 (1973) 11-60. 33 ROCKEL, A. J., AND JONES, E. G., Observations on the fine structure of the central nucleus of the inferior colliculus of the cat, J. comp. NeuroL, 147 (1973) 61-92. 34 ROCKEL,A. J., AND JONES, E. G., The neuronal organization of the inferior colliculus of the adult cat. II. The pericentral nucleus, J. comp. Neurol., 149 (1973) 301-334. 35 SN1DER, R.~ AND ELDRED,E., Electro-anatomical studies on cerebrocerebellar connections in the cat, J. comp. Neurol., 95 (1951) 1-16. 36 SNIDER, R., AND STOWELL,A., Receiving areas of the tactile, auditory, and visual systems in the cerebellum, J. Neurophysiol., 7 (1944) 331-358. 37 TASlRO, S., Experimentell-anatomische Untersuchung fiber die efferenten Bahnen aus den Vierhfigeln beim Kaninchen, Z. mikr.-anat. Forsch., 45 (1939) 321-375. 38 TASIRO, S., Experimentell-anatomische Untersuchung fiber die efferenten Bahnen aus den Vierhfigeln der weissen Ratte, Niigata Igakkai Zasshi, 54 (1939) 1402-1421 (in Japanese). 39 TASIRO,S., Experimentell-anatomische Untersuchung fiber die efferenten Bahnen aus den Vierhfigeln der Katze, Z. mikr.-anat. Forsch., 47 (1940) 1-32. 40 TASIRO,S., Experimentell-anatomische Untersuchungen fiber die efferenten Bahnen aus den Vierh/igeln der Maus, Niigata Igakkai Zasshi, 55 (1940) 75-93 (in Japanese). 41 WOOLLARD,H. H., AND HARPMAN, J. A., The connections of the inferior colliculus and of the dorsal nucleus of the lateral lemniscus, J. Anat. (Lond.), 74 (1940) 441-458.
309
THE P O N T I N E P R O J E C T I O N F R O M T H E I N F E R I O R C O L L I C U L U S IN THE CAT. AN E X P E R I M E N T A L A N A T O M I C A L STUDY
KOKI KAWAMURA
Department of Anatomy, School of Medicine, Iwate Medical University, Morioka 020 (Japan) and Anatomical Institute, University of Oslo, Oslo (Norway)
SUMMARY
The present study was carried out in order to clarify the organization of the pontine projection from the inferior colliculus in the cat with the methods of Nauta 27 and Fink and Heimer 16 following lesions of different sizes and locations in the inferior colliculus. The inferior colliculus projects exclusively to the anterior two-thirds of the ipsilateral dorsolateral pontine nucleus. The majority of fibers originates from cells in the central nucleus and a small amount from cells in the external and the pericentral nuclei as well. Areas of the present projection largely overlap with areas of projection from the auditory cortex, and also fairly well with those from the superior colliculus. Possible pathways of the acoustic inputs to the cerebellum involving the pontine nuclei are discussed based upon the current findings, as well as upon those obtained by preliminary experimental studies of anterograde and retrograde axonal flow.
INTRODUCTION
Fibers from the inferior colliculus to the pontine nuclei have been described in various animal species, for example in the cata2,~s,ao, 39, the monkey 25, the opossum24, 31, the rabbit a7, the rat 38, the mouse 4°, the kangaroo rat la and the guinea pig4L Most authors who studied the termination with some precision indicate this to be the dorsolateral pontine nucleus. In the published studies, all o f which were carried out with axonal degeneration methods, the lesions often involved a part of the superior colliculus as well as the inferior colliculus, and some authors were not sure whether fibers coming from the superior colliculus might have been transected, even if the lesions appeared to be confined to the inferior colliculus. Kawamura and Brodal 2~ recently traced the course
310 of the tectopontine fibers from the superior colliculus in sagittally cut sections (see Fig. 3 of ref. 22), and found that these fibers at the level of the inferior colliculus pass so far ventrally that there is no risk of transecting them even with fairly deep lesions of the inferior colliculus. It thus appears that such lesions may be used in studies of the efferent fiber projections of the nuclei of the inferior colliculus. With this knowledge it may be possible to arrive at more definite answers than before to the following questions. (I) Are the fibers from the inferior colliculus to the pontine nuclei distributed unilaterally or bilaterally? (2) In what part of the pontine nuclei do they terminate'? (3) Do the fields of termination of the inferior collicular projection overlap with those of other pontine afferents, and if so, with what afferents ? MATERIAL A N D M E T H O D S
Fifteen adult cats were used in this investigation. In 7 cats lesions of various sizes and locations were made in the inferior colliculus under Nembutal anesthesia, and another 8 cats with lesions of the superior colliculus (see ref. 22) were included for purposes of comparison. In two of the 7 cats (cat KA 12R, used in another study by Kawamura and Brodal 2z, and cat KA 18L) an electrolytic lesion was made with an electrode 1.0 mm in diameter. This was inserted from the anterior direction at 40 ° from the vertical plane, and passed beneath the cerebellar tentorium so that its tip reached the central nucleus of the inferior colliculus. For making large lesions of the left inferior colliculus it was preferable to work under direct vision. Therefore, in 5 other cases (cats KA 17L, KA 19L, D 16L, D 17L, and D 22L), after opening the skull and the dura mater, the visual cortex and the underlying white matter were removed by suction until the bony tentorium cerebelli and the anterolateral portion of the inferior colliculus were visible. The part of the tentorium covering the inferior colliculus was carefully removed, and multiple lesions were made in the inferior colliculus by means of a dental drill and/or a heated metal rod. Six to 7 days after the operation, the animals were perfused intravitally under Nembutal anesthesia with 0 . 9 ~ saline followed by 10~ formalin. The brains were dissected free and transferred to 10~ formalin for further fixation. The pons was cut transversely (cats KA 17L and D 17L) or horizontally (cats KA 12R, KA 18I_, KA 19L, D 16L, D 22L) at 20 ,urn on the freezing microtome. The tectum containing the lesions was cut transversely in all cases. Every 10th section was impregnated according to the methods of Nauta z7 and Fink and Heimer ~6. In addition, some sections containing the lesions were stained with thionin. Drawings of the sections through the colliculus and the pons were made in a projection apparatus and the extent of the lesions and the distribution of the ensuing degeneration, as checked in the microscope, were entered in the drawings. In these, coarse degenerating fibers are indicated by wavy lines and fine granules by dots. If every 10th section (transversely cut, 20/~m thick) is selected, and these are
311
Fig. 1. Photographs and photomicrographs showing examples of lesions and degeneration in the pontine nuclei. A: the lesion in the left inferior colliculus in cat KA 17L, seen from above (cfi Fig. 3). B: photomicrograph of transversely cut, thionin stained section through the inferior colliculus in cat KA 19L, to show the depth and extent of the lesion ( x 8). Arrows point to ventral border of the central nucleus of the inferior coUiculus. C : photomicrograph (Fink and Heimer method, x 260) of a horizontal section through the pons in cat KA 19L, showing the distribution of degeneration in the dorsolateral pontine nucleus (see inset) following a lesion of the ipsilateral inferior colliculus (cfi Fig. 3). Abbreviations used in all figures: B.p., brachium pontis; C.i.c., commissure of inferior colliculus; CN, central nucleus of inferior colliculus; EN, external nucleus of inferior colliculus; I.c., inferior colliculus; L.I., lemniscus lateralis; L.m., lemniscus medialis; N.c.s., nucleus centralis superior (raphe); N.dl., nucleus dorsolateralis pontis; N.I., nucleus lateralis pontis; N.p., nucleus peduncularis pontis; N.pm., nucleus paramedianus pontis; N.r.t., nucleus reticularis tegmenti pontis; N.v., nucleus ventralis pontis; N.V, root fibers of trigeminal nerve; Pc, pericentral nucleus of inferior colliculus; Ped, pedunculus cerebri (corticospinal and corticopontine fibers); P.g., periaqueductal gray matter; S.c., superior colliculus; T, trapezoid body.
m o u n t e d in a series, there will be a b o u t 30 m o u n t e d sections passing t h r o u g h the pons. In all series the sections p a s s i n g t h r o u g h the c a u d a l m o s t a n d r o s t r a l level o f the p o n tine nuclei were n u m b e r e d as 1 a n d 30, respectively, in o r d e r to facilitate the c o m p a r i son o f d i s t r i b u t i o n o f d e g e n e r a t i o n a m o n g different cases. RESULTS
(1) Delimitation and nomenclature of the inferior colliculus and the pontine nuclei Cajal i2 first described the inferior colliculus as being c o m p o s e d o f 3 nuclei, a n d these have c o m e to be called the central nucleus (CN), the external nucleus ( E N ) a n d the pericentral nucleus (Pc), the largest being the central nucleus. This consists
312 of densely packed small and medium sized cells. The EN forms a rather broad band of cells covering the CN rostrally and laterally, and the Pc, a narrower band of cells, borders the CN caudally and dorsally (Figs. 2, 3 and 4). In the pontine nuclei, a subdivision into the minor parts (nuclei) distinguished by Brodal and Jansen 4, was attempted even if the borders between them are arbitrary. (2) Degeneration in the pontine nuclei in silver impregnated sections Following lesions of the inferior colliculus, coarse degenerating fibers, obviously representing passing fibers, enter the dorsolateral pontine nucleus from the rostrolateral aspect (Fig. l) and proceed caudomedially. Interspersed between them are seen fairly fine degenerating fibers as well as isolated argyrophilic fragments of different shapes. Some of them are situated close to the perikarya, but the majority is found in the neuropil (Fig. 1C). An area containing abundant fragments of the types described above, is here considered as a 'terminal field'. In the present study the terminal fields determined with the methods of Nauta 27 and of Fink and Heimer 16 are identical. However, more isolated fine granules are present within the terminal fields in Fink and Heimer sections than in Nauta sections. The same is the case for the corticopontineS, 6 and for the superior colliculopontine projections 22. (3) Presentation of cases As described in Material and Methods, the damage of the inferior colliculus is in most cases complicated by lesions of the visual cortex. To facilitate the following presentation and discussion, we start by considering one case (cat KA 18L) with a pure lesion of the inferior colliculus, and another one (cat KA 12R) in which there is a lesion of the left superior colliculus as well as the right inferior coIliculus (Fig. 2). The latter case has been described previously (see Fig. 9 of ref. 22). In cat KA 18L (brain stem cut horizontally, inferior colliculus cut transversely, killed after 7 days, Fig. 2), only a fairly superficial lesion of the inferior colliculus was made, partially involving the EN and the Pc. The CN is free from damage. Degeneration found only in a restricted area of the ipsilateral dorsolateral nucleus in the pons is sparse. In cat KA 12R (pons cut horizontally, colliculus cut transversely, killed after 6 days, Fig. 2), a fairly large lesion involved the central portion of the CN together with the EN and the Pc. There is more degeneration in the dorsolateral nucleus, chiefly confined to the dorsolateral part. As shown above, in cats KA 18L and KA 12R the lesions of the inferior colliculus are not complete. Therefore, the possibility remains that there are more extensive areas of termination than those found in the above cases. In the next presentation, however, some cases of larger collicular lesions provide a more complete indication of the total extent of the area of termination. In these cases the visual cortex has been removed, but this is not a source of ambiguity since the visual cortex has been shown to project ipsilaterally to a different part of the pons (see refs. 8, 9 and 22). Large lesions of the inferior colliculus with additional visual cortex invoh'ement From the 5 cases (cats KA 19L, KA 17L, D 16L, D 17L, and D 22L), cats
313
KA 18 L
KAI2R
KA 18 L
KAI2 R
''''
KA I IR
30~ ~ KA 18L
KA 12R
)
A0orrom,CORTEX
VENTRA~ KA II R
(,,m_
0
('
~? / / / /
? ////
ii-XLml
~ t
""\ \~
,,
ii
li
.
~
/~
IA
IA , ,/
i
!
Fig. 2. Diagrams illustrating the findings in two cases of inferior collicular lesions (cats K A 18L and K A 12R). The lesions (black) are shown (above) as seen on the surface of the inferior colliculus and (below) as appearing in sections at the levels indicated in diagram above. Degeneration in the pontine nuclei is shown (dots) in drawings of horizontal sections at 4 equally spaced levels of the pons. The stippled lines in this and the following figures indicate the borders between the different nuclei in the pontine gray and in the inferior colliculus. For comparison, the findings in a case with a lesion of the auditory cortex (cat K A 1 IR) are shown (to the right). Levels through the pons correspond to those used in the two series to the left.
314 KA 19 L ond R
VENTRAL
r~
z:,
O l
o
18
2O
22
4
Fig. 3. Diagram showing the findings in cat KA 19 with a large lesion (black) of the left inferior colliculus as well as lesions of the visual cortex of both sides. Above to the right: lesions as seen in 4 transverse sections through the inferior colliculus at levels indicated in a diagram of the colliculus are shown (partial destruction shown hatched). Below: a series of drawings of 8 horizontal sections through the pons taken with equal intervals. The stippled lines indicate the borders between the various nuclei in the pontine gray and in the inferior colliculus. Note the differences in distribution of degeneration on the two sides of the pons. The degeneration in the dorsolateral nucleus on the left (dots) is due to the lesion of the left inferior colliculus, while the corticopontine projection from the visual cortex (crosses) is bilateral (cf. text). Microphotograph of degeneration in drawing 22 shown in Fig. 1C.
315 KA 19L and KA 17L will be presented below as representative; the findings obtained in the pons are approximately the same in the other 3 cases. Cat KA 19L and R (brain stem cut horizontally, inferior colliculus cut transversely, killed after 7 days, Fig. 3). The left inferior coIliculus as well as both visual cortices are largely destroyed. The CN of the left inferior colliculus is heavily damaged except for its rostral portion, and the EN and the Pc are considerably involved as well. The right inferior colliculus is intact. The corticopontine projection from the visual cortex has been shown to be ipsilateralS,9,22, as is also the projection from the inferior colliculus to the ports. The degeneration found in the right pontine nuclei, therefore, is due to the lesion of the right visual cortex only. This is marked with crosses on a diagram of Fig. 3. In the left pons, however, the degeneration results from damage of both the inferior colliculus and the cortex. Most of the degeneration, which is distributed symmetrical to that seen on the other side, is considered to be due to the lesion of the left visual cortex (marked also with crosses), and the additional degeneration found in the dorsolateral nucleus must, therefore, be due to the lesion in the colliculus (marked with dots in the same figure). Compared with the previous cases (cats KA 18L and KA 12R), degeneration found in the dorsolateral nucleus of this case is a little more extensive. The present case gives a more complete impression of the total inferior collicular projection than do the previous cases, even if the lesion does not totally involve the inferior colliculus. Although the slight possibility still remains that degeneration may be found elsewhere with total removal of the inferior colliculus, the area delimited probably represents almost the entire terminal field in the pons of fibers descending from the inferior colliculus. Cat KA 17L (both brain stem and inferior colliculus cut transversely, killed after 7 days, left side of Fig. 4). A large lesion is confined within the left inferior coIliculus and to a large extent involves the CN, except for its rostral part, as well as the two accessory nuclei (EN and Pc). In addition, the visual cortex is affected on the same side. The distribution of degenerating fibers corresponds well to that of the previous case (cat KA 19L, cf. Fig. 3), and the degeneration in the left pontine nuclei is considered to have two components. Using the same convention indicated above, degeneration due to the visual cortex lesion is plotted on a diagram as crosses and that due to the involvement of the inferior colliculus as dots. Another guide in differentiating degenerating fibers from the two different sources is that the corticopontine fibers enter the nucleus from the cerebral peduncle, whereas the colliculopontine fibers pass from the dorsolateral aspect of the pontine gray. The corticopontine group projects to fairly extensive areas in the ventral, paramedian, peduncular and lateral nuclei (here confirming the findings of P. Brodal s,9 and Kawamura and Broda122); but the colliculopontine group projects to the approximately rostral two-thirds of the ipsilateral dorsolateral nucleus, more specifically to its dorsolateral part. On the right side of Fig. 4, a map showing the total pontine projection from the superior colliculus is presented for comparison with the current results. This summarizes the findings of 7 cases which have been described before (see ref. 22) together with another new case (cat KA 16L). For a detailed description of the superior collic-
316
Sc
/ i
, Pg J ,, '
",
,
0
Cc
",!
.4' ~ KA 17 L
ROSTRAL
,'I
/
' ~'~ 1~ I / ~ ~ / "~ %,, ~_C -.,,.\
~ .,~ / ' "
-'h".: / (
"
23
\ • %'; ~
~'~
i
~':"
-'.,-.. C."' -'-. '-':'"
& \
....,'~
'
\
'"",% ',' / ;.':hi
/,iO' /
\ -. ,,,'~,,..~. ~,
/ /'
', ~ ~.. CAUDAL
Fig. 4. Drawing showing the distribution of degeneration in the pontine nuclei (on the left) following lesions (black) of the left inferior colliculus and the left visual cortex in cat KA 17L, marked as dots and crosses respectively. Above to the right the collicular lesion is shown as appearing in transverse sections. In the diagram of the colliculi and visual cortex (above to the left) the hatched area indicates outlines of the lesions of the superior colliculus in altogether 8 cases. The total projecting area of the superior colliculus is shown in the right pontine half (dots). Principles of presentation as in Figs. 2 and 3.
317 ular projection, readers are referred to the previous paper 22. As demonstrated in the diagram, fields of termination in the dorsolateral pontine nucleus of the two collicular projections overlap considerably. Detailed observations, however, reveal that the fibers from the superior colliculus, compared with those from the inferior, distribute more abundantly, particularly caudally. Since the auditory cortex likewise projects to the dorsolateral nucleus10,~2, it is of some interest to present the case of cat KA 11R (pons cut horizontally, killed after 6 days, see Fig. 2, right), where the right auditory cortex was severely damaged. Another large lesion was made in the left superior colliculus, but this does not interfere with the interpretation of the findings of the auditory cortex projection since each projection is ipsilateral. This case was described previously by Kawamura and Broda122, but is illustrated here for comparison, showing horizontal sections of approximately the same levels as the cases of collicular lesions (cats KA 18L and KA 12R in Fig. 2). The corticopontine fibers enter the nucleus from the cerebral peduncle and end exclusively in the dorsolateral nucleus, showing an extensive area of overlap with the collicular projections. DISCUSSION
Origin within the inferior colliculus of fibers to the pontine nuclei It is of interest to know if all parts of the inferior colliculus give off fibers to the pontine nuclei, because the structure of the inferior colliculus is not homogeneous, and the afferent fiber systems are differentially distributed within it (for details see refs. 32-34). According to the works of Rockel and Jones fields of termination of ascending and descending fibers in the colliculus scarcely overlap with each other. Fibers from the auditory cortex terminate only in the dorsomedial division of the CN, which is composed of large cells (30 #m × 40 #m) 32, as well as in the Pc and EN, whereas those from the lateral lemniscus are distributed mainly to the ventrolateral two-thirds to three-quarters portion of the CN 34. From the present study it is difficult to arrive at any definite conclusion as to the probable identity of cells of origin of the tectopontine pathway, but it seems that most of the neurons which give off fibers to the pons are located in the CN, with only a few in the Pc and EN (see cat KA 18L of Fig. 2). The chief aim of the present study was to determine the total distribution of the inferior collicular projection in the pontine nuclei, and further experimental study is needed to decide the exact location of cells in the colliculus sending fibers to the pons. This is being pursued with the autoradiographical and the horseradish peroxidase methods.
Patterns of organization of projection from the inferior colliculus to the pons The area in the pons to which the descending fibers from the inferior colliculus are distributed has been proved in the present study to be coextensive with the dorsolateral nucleus, except probably its most caudal part. Considering that the lesions of the inferior colliculus were subtotal, it is possible that the degeneration observed in the pons was less than it would have been if the whole colliculus had been involved.
318 Even at the survival time of 6 or 7 days, the parts of the colliculus unaffected by the primary lesion usually displayed a considerable degree of gliosis and contained 'morbid' neurons, but it is uncertain to what extent this secondary involvement caused degeneration in the ports. Since, however, the lesions produced in the present study together covered almost the total area of the inferior colliculus, they can give a good impression of the total degeneration in the pons. Furthermore, the results of the present study generally confirm the descriptions of previous authors22,zs,30, 39, who, however, did not indicate the pontine areas of termination as precisely as was done here. Thus, the pontine projection from the inferior colliculus is now shown to be confined almost exclusively to the ipsilateral dorsolateral nucleus. As stated above, the majority of the fibers appear to originate from cells in the CN, but a small portion of the Pc and EN may also contribute. There are certain other structures which give off fibers to the dorsolateral pontine nucleus. The superior colliculusl, z2 and the auditory cortex 1°,2z are the major sources, and these will be discussed below. A small restricted medial portion of the dorsolateral nucleus, together with its immediate neigborhood, receives a small number of fibers from the second somatosensory cortex 6, from the anterior part of the orbital gyrus 7, and also from the cerebellar nuclei 3. It appears from the diagrams published by P. BrodaI6, 7 and A. Brodal et al. 3, that there is only a little overlap of the areas of termination of these 3 sources with those of the two colliculi.
The tecto-ponto-cerebellar projection and the pathways for acoustic inputs to the cerebellum The pontine projection from the inferior colliculus and that from the superior colliculus have almost similar areas of termination in the dorsolateral nucleus (see Fig. 4). The fibers from the inferior colliculus, however, do not appear to terminate in the caudal third of the nucleus and do not end in the lateral and the peduncular nuclei which receive a small number of fibers from the superior colliculus (see also ref. 22). Before discussing possible routes for acoustic and optic inputs to the cerebellum, it is necessary to consider some data on the topography of the pontocerebellar projection. Even if this is not yet known in sufficient detail, some data are available concerning those pontine subdivisions which are of particular relevance for the problems to be discussed here. In their study of 1946, Brodal and Jansen 4, using the modified Gudden Method 2, could elucidate only the major features of the pontocerebellar projection in the cat and rabbit. It is of interest, however, that the lateral pontine nucleus appears to send a considerable portion of its fibers to the paraflocculus (their experiment cat 0.75). Our autoradiographical studies confirm this projection (see ref. 22a). Furthermore, Brodal and Jansen 4 found that pontine fibers pass from the dorsolateral nucleus as well as from the paramedian nucleus to the vermis, and more particularly to its middle part, including lobule VII (see their experiments cat 0.41 and rabbit 0.27). This has recently been confirmed with more refined methods. Graybiel is, following injection of horseradish peroxidase (HRP) in the midvermal region of the rat, found HRP-positive neurons in the dorsolateral and paramedian
319 pontine nuclei, but extremely few in other pontine subnuclei. In our recent z2~ experiments with HRP, largely similar findings have been obtained in the cat. Thus, the optic and acoustic areas of the cerebellar vermis (largely lobule VII of Larsell, Cz of Bolk) receive massive fibers from the dorsolateral and the paramedian nucleus (particularly from the part of the latter lying close to the midline according to our own observations). According to our preliminary studies, lobule VIIb receives a somewhat larger amount of fibers from the dorsolateral nucleus than does lobule VIIa. A [3H]leucine study in our laboratory appears to confirm this correlation and furthermore indicates that a projection from the lateral pontine nucleus to lobule VII is of a lesser degree g2a. It may be concluded from this and from the findings made in the present study, that acoustic impulses mediated via the inferior colliculus may be transmitted via the dorsolateral pontine nucleus to the acoustic area of the cerebellar vermis (lobule VII). Since the projection from the acoustic cerebral cortex to the pons likewise terminates in the dorsolateral nucleus (cat KA l l R in Fig. 2, see also ref. 10), it appears that impulses from the acoustic cortex mediated via the pons, will reach the same cerebellar area. However, although the termination of the two different pathways whereby acoustic impulses may reach the pons overlap almost completely in the dorsolateral nucleus, closer observation of the distribution of degenerating fibers (cf. Fig. 2) following lesions of the two structures, as undertaken in the present study, reveals that the projection from the cerebral cortex extends a little more ventrally, close to the lateral nucleus. This suggests that the acoustic impulses arriving directly from the inferior colliculus and those coming via the cerebral cortex to some extent terminate on different groups of pontine cells, even if there is considerable overlapping, and it may be asked whether the pathway from the acoustic cortex via the pons to the cerebellum differs from that from the inferior colliculus. Physiologically responses t o acoustic stimuli have been recorded outside the classical acoustic area 36 of the cerebellar cortex; thus by Buser and Franche111 in what is referred to as the crus I! (but according to their figure is actually the paraftocculus), and by Fadiga and Pupilli 15 in extensive parts of the cerebellar hemisphere. Most recently Mortimer 2n has confirmed this in the monkey and indicates as maximal sites of responses the intermediate zone of lobule VI (lobulus simplex), the posterior portion of the paramedian lobule and the adjacent dorsal paraflocculus. While some of these responses may be mediated via the olive and its climbing fibers (see ref. 26), some are likely to be mediated via the pons. This would probably require that pathways for acoustic impulses reach parts of the pons which project onto the hemispherical cerebellar areas mentioned above. As shown here, fibers from the inferior colliculus as well as from the acoustic cortex appear to end exclusively in the dorsolateral pontine nucleus. Future studies by means of the peroxidase method may disclose that some cells of the dorsolateral nucleus in fact send their axons to cerebellar areas other than the classical acoustic area, since it appears that a particular small cerebellar region in general receives pontine afferents from more than one pontine cell group, as recently shown for the paramedian lobule 19. However, the fibers from the acoustic cortex ending in the dorsolateral pontine nucleus
320 extend very close to the lateral nucleus. Since this appears to project onto the paraflocculus (see above), it may be hypothesized that dendrites from its cells extend into the dorsolateral nucleus and are contacted by cortical afferents. This might explain the transmission of acoustic impulses to the paraflocculus.
The routes of visual inputs to the cerebellum compared with those of acoustic inputs As stated above, there is a better correspondence between the pontine projections of the auditory cortex and the inferior colliculus than between the projection zones of the inferior colliculus and the superior colliculus. Considering that the inferior colliculus is dominated by acoustic inputs, it makes sense that the projection of the inferior colliculus overlaps more closely with that of the auditory cortex than with that of the superior colliculus which is primarily visual. The teleceptive pathways for visual and acoustic impulses seem to be relatively independent before they reach the cerebral cortex. Moreover, the convergence of the two pathways at the cortical level appears to be minimaP 4,2°,21. In the superior colliculus, however, there is extensive anatomical overlap. Superficial layers of the superior colliculus are known to receive fibers primarily from the 3 visual sources (the retina, the visual cortex and the ventral lateral geniculate nucleus), while deeper layers receive fibers from the auditory cortex17, 29 and possibly from the sensorimotor cortex 17 and from other sources as well. Thus it is quite probable that the first station for integration of optic and acoustic impulses is in the superior colliculus, and that they are further 'integrated' in the rostral two-thirds of the dorsolateral nucleus in the pons before they reach the cerebellum. The main cerebellar cortical areas which receive fibers from this nucleus are located in the tuber and folium vermis~,35, 36. A slight difference in location of these visual and acoustic cerebellar areas, brought out by careful examination of the experimental physiological data, may be related to the slight anatomical differences in the projections from the superior and inferior colliculi to the pons and the cerebellar projection from the latter. With regard to cortico-ponto-cerebellar pathways, there is a marked difference between those for acoustic and visual impulses. Responses to visual stimuli have been recorded from the same cerebellar areas as mentioned above for acoustic stimuli 15,~6, and in addition from the floeculus 23. The visual corticopontine fibers terminate in far more extensive and different parts of the ports than do the acoustic (see Figs. 3 and 4 here, and refs. 8, 9 and 22), many of them in the ventral and the paramedian nuclei. While the latter projection is probably concerned in the further propagation of impulses to the lobule VII ('see above), the cerebellar projections of the ventral nucleus are not clear, but they may well turn out to be to the hemispheres (see ref. 4). Although decisive morphological evidence is still lacking, it can, nevertheless, be assumed that the optically evoked responses obtained in the cerebellar vermis and in the ansoparamedian lobule are attributable to two different pontocerebellar pathways.
321 REFERENCES 1 ALTMAN,J., AND CARPENTER,M. B., Fiber projections of the superior colliculus in the cat, J. comp. Neurol., 116 (1961) 157-177. 2 BRODAL,A., Modification of Gudden method for study of cerebral localization, Arch. Neurol. Psychiat. (Chic.), 43 (1940) 46-58. 3 BRODAL,A., DESTOMBES,J., LACERDA,A. M., AND ANGAUT,P., A cerebellar projection onto the pontine nuclei. An experimental anatomical study in the cat, Exp. Brain Res., 16 (1972) 115-139. 4 BRODAL, A., AND JANSEN, J., The ponto-cerebellar projection in the rabbit and cat, J. comp. Neurol., 84 (1946) 31-118. 5 BRODAL, P., The corticopontine projection in the cat. I. Demonstration of a somatotopically organized projection from the primary sensorimotor cortex, Exp. Brain Res., 5 (1968) 210-234. 6 BROOAL,P., The corticopontine projection in the cat. Demonstration of a somatotopically organized projection from the second somatosensory cortex, Arch. ital. Biol., 106 (1968) 310-332. 7 BRODAL,P., The corticopontine projection in the cat. II. The projection from the orbital gyrus, J. comp. Neurol., 142 (1971) 141-152. 8 BRODAL,P., The corticopontine projection from the visual cortex in the cat. I. The total projection and the projection from area 17, Brain Research, 39 (1972) 297-317. 9 BRODAL,P., The corticopontine projection from the visual cortex in the cat. II. The projection from areas 18 and 19, Brain Research, 39 (1972) 319-335. 10 BRODAL,P., The corticopontine projection in the cat. The projection from the auditory cortex, Arch. ital. Biol., 110 (1972) 119-144. 11 BOSER, P., ET FRANCHEL,H., Neurophysiologie. Existence d'un foyer de projection sensorielle acoustique au niveau du lobe ansiforme du cervelet cbez le chat, C. R. Acad. Sci. (Paris), 251 (1960) 791-793. 12 CAJAL, S. RAM6N Y, Histologie du Syst~me Nerveux de l'Homme et des Vertdbrds, Maloine, Paris, 1909-1911. 13 CAREY,C. L., AND WEBSTER,D. B., Ascending and descending projections of the inferior colliculus in the kangaroo rat (Dipodomys merriami), Brain Behav. Evol., 4 (1971) 401-412. 14 DIAMOND,I. T., JONES,E. G., AND POWELL,T. P. S., The association connections of the auditory cortex of the cat, Brain Research, l l 0968) 560-579, 15 FADIGA,E., ANDPUPILLI,G. C., Teleceptive components of the cerebellar function, Physiol. Rev., 44 (1964) 432-486. 16 FINK, R. P., AND HEIMER, L., Two methods for selective silver impregnation of degenerating axons and their synaptic endings in the central nervous system, Brain Research, 4 (1967) 369-374. 17 GAREY,L. J., JONES, E. G., AND POWELL, T. P. S., Interrelationships of striate and extrastriate cortex with the primary sites of the visual pathway, J. Neurol. Neurosurg. Psychiat., 31 (1968) 135-157, 18 GRAYBIEL,A. M., Visuo-cerebellar and cerebello-visual connections involving the ventral lateral geniculate nucleus, Exp. Brain Res., 20 (1974) 303-306. 19 HODDEVIK, G. H., The pontocerebellar projection onto the paramedian lobule in the cat. An experimental study with the use of horseradish peroxidase as a tracer, Brain Research, 95 (1975) 291-307. 20 KAWAMURA,K., Corticocortical fiber connections of the cat cerebrum. I. The temporal region, Brain Research, 51 (1973) 1-21. 21 KAWAMURA,K., Corticocortical fiber connections of the cat cerebrum. IlI. The occipital region, Brain Research, 51 (1973) 41-60. 22 KAWAMURA,K., AND BRODAL,A., The tectopontine projection in the cat: an experimental anatomical study with comments on pathways for teleceptive impulses to the cerebellum, J. comp. Neurol., 149 (1973) 371-390. 22a KAWAMURA,K., AND HASHIKAWA,T., Pontocerebellar projections of cats with comments on teleceptive impulses to the cerebellar cortex. An experimental anatomical study by means of ortho- and retrograde axonal flow techniques. Abstr. Papers presented at lOth Int. Congr. Anatomists, Tokyo, 1975. 23 MAEKAWA,K., AND SIMPSON,J. I., Climbing fiber responses evoked in vestibulocerebellum of rabbit from visual system, J. Neurophysiol., 36 (1973) 649-666. 24 MARTIN,G., Efferent tectal pathways of the opossum (Didelphis virginiana), J. comp. Neurol., 135 (1969) 209-224.
322 25 MOORE, R. Y., AND GOLDBERG,J. M., Projections of the colliculus in the monkey, Exp. Neurol., 14 (1966) 429-438. 26 MORTIMER,J. A., Cerebellar responses to teleceptive stimuli in alert monkeys, Brain Research, 83 (1975) 369-390. 27 NAUTA, W. J. H., Silver impregnation of degenerating axons. In W. F. WINDLE (Ed.), New Research Techniques of Neuroanatomy, Thomas, Springfield, 111., 1957, pp. 17--26. 28 NOORT, J. VAN, The Structure and Connections of the Inferior Colliculus. An Investigation of the Lower Auditory System, Thesis, Van Gorcum, Assen, 1969, 118 pp. 29 PAULA-BARBOSA,M. M., AND SOUSA-PXNTO,A., Auditory cortical projections to the superior colliculus in the cat, Brain Research, 50 (1973) 47-61. 30 POWELL,E. W., AND HATTON,J. B., Projections of the inferior colliculus in cat, J. comp. Neurol., 136 (1969) 183-192. 31 RAFOLS,J. A., AND MATZKE,H. A., Efferent projections of the superior colliculus in the opossum, J. comp. Neurol., 138 (1970) 147-160. 32 ROCKEL, A. J., AND JONES, E. G., The neuronal organization of the inferior colliculus of the adult cat. I. The central nucleus, J. comp. Neurol., 147 (1973) 11-60. 33 ROCKEL, A. J., AND JONES, E. G., Observations on the fine structure of the central nucleus of the inferior colliculus of the cat, J. comp. NeuroL, 147 (1973) 61-92. 34 ROCKEL,A. J., AND JONES, E. G., The neuronal organization of the inferior colliculus of the adult cat. II. The pericentral nucleus, J. comp. Neurol., 149 (1973) 301-334. 35 SN1DER, R.~ AND ELDRED,E., Electro-anatomical studies on cerebrocerebellar connections in the cat, J. comp. Neurol., 95 (1951) 1-16. 36 SNIDER, R., AND STOWELL,A., Receiving areas of the tactile, auditory, and visual systems in the cerebellum, J. Neurophysiol., 7 (1944) 331-358. 37 TASlRO, S., Experimentell-anatomische Untersuchung fiber die efferenten Bahnen aus den Vierhfigeln beim Kaninchen, Z. mikr.-anat. Forsch., 45 (1939) 321-375. 38 TASIRO, S., Experimentell-anatomische Untersuchung fiber die efferenten Bahnen aus den Vierhfigeln der weissen Ratte, Niigata Igakkai Zasshi, 54 (1939) 1402-1421 (in Japanese). 39 TASIRO,S., Experimentell-anatomische Untersuchung fiber die efferenten Bahnen aus den Vierhfigeln der Katze, Z. mikr.-anat. Forsch., 47 (1940) 1-32. 40 TASIRO,S., Experimentell-anatomische Untersuchungen fiber die efferenten Bahnen aus den Vierh/igeln der Maus, Niigata Igakkai Zasshi, 55 (1940) 75-93 (in Japanese). 41 WOOLLARD,H. H., AND HARPMAN, J. A., The connections of the inferior colliculus and of the dorsal nucleus of the lateral lemniscus, J. Anat. (Lond.), 74 (1940) 441-458.
