Ascending Projections of the Superior Colliculus in the Cat

Okajimas Fol. anat. jap., 47: 269-287, 1970

Kahee Niimi, Michihiro Miki and Syosuke Kawamura

The Third Department of Anatomy, Okayama University Medical School, Okayama, Japan

Introduction

In submammals the superior colliculus receives most of its affer- ents from the retina. It also receives fibers from, and transmits fibers to, the brain stem and the spinal cord. The superior colliculus shows a complex laminated structure consisting of alternating layers of fibers and gray matter. Its lamination suggests a high degree of correlative function. Probably the superior colliculus is more than a reflex center ; it is a correlation center for visual with other so- matic and visceral impulses. In response to these impulses adequate behavioral reactions are effected chiefly through the tectofugal pathways. In ascending the mammalian scale, the superior colliculus shows a marked regression, while the cerebral cortex and the dorsal thalamus are progressively evolved. Most of the optic fibers enter the lateral geniculate body to be relayed to the visual cortex which in turn can exert an influence upon the superior colliculus by corticotectal fibers. It is certain that tectothalamic and thalamocortical fibers provide afferent pathways in the neural arcs between the superior colliculus and the cerebral cortex. In this connection, the ascending thalamic projection of the superior colliculus appears to be of great functional significance. The present investigation attempts to clarify the precise distribution of the ascending tectofugal fibers in the cat. Special emphasis is laid upon the tectal projections to the dorsal thalamic nuclei.

Material and Methods

Unilateral electrolytic lesions were placed manually or with the aid of a stereotaxic instrument in the left superior colliculus in six 269 270 K. Niimi, M. Miki and S. Kawamura cats under deep Nembutal anesthesia. Four of these cats showed lesions fairly well localized in the superior colliculus and were used in the present study. In cats 3 and 6 the electrode was introduced manually along the posterior wall of the osseous tentorium until it reached the ventral end of the tentorium. It was then turned somewhat posteriorly, and inserted about 3 mm. anteroventrally to reach the superior colliculus. In cats 2 and 4 the electrode was introduced stereotaxically by a right dorsolateral approach to effect a lesion on the left side. After a survival period of 7 days, the animals were sacrificed and perfused via the carotid artery with normal saline followed by a solution of 10% neutral formalin. The brains were removed from the skull, fixed in 10% neutral formalin for five days, and sectioned about 10 mm. thick transversely. These cut blocks of the brains were stored in 10% neutral formalin for one month, sectioned serially at 40 t on the freezing microtome and impregnated by the N a u t a-G y g a x (C54) silver method. The terminology of the thalamic nuclei employed was based on the comparative cyto-

Text-fig. 1. The left three figures show the site and extent of the lesion of the superior colliculus in transverse sections in each case. The right figure indicates the sagittal extent of the tectal lesion. Ascending Projections of the Superior Colliculus in the Cat 271 architectural study of Niimi ('70).

Results

Cat 3 An electrolytic lesion was placed in the lateral part of the superior colliculus at its middle levels. The lesion was confined to the superior colliculus and did not involve the underlying central gray of the midbrain (Text-fig. 1). Degenerated fibers are found in all layers of the ipsilateral superior colliculus except in the zonal layer. A fair number of degenerated fibers are found to run medially through the commissure of the superior colliculus into the optic and the intermediate gray layers of the superior colliculus and a few into the reticular formation on the opposite side. Descending degenerated fibers from the tectal lesion spread ventrally, ventromedially and ventrolaterally. The ventrally running fibers are distributed in the dorsal half of the reticular formation of the ipsilateral midbrain, but no fibers are seen reaching the red nucleus and the substantia nigra. The ventromedially coursing fibers cross largely through the doral tegmental decussation to descend through the paramedian part of the contra,- lateral pons and medulla oblongata into the spinal cord (the tecto- spinal tract), giving off in course fibers to the bulbar reticular formation (the tectobulbar tract). Some of the ventromedially running fibers spread into the central gray of the ipsilateral midbrain, but there are no fibers entering the nucleus of the oculomotor nerve., At more rostral levels a few fibers are found to run into the nucleus of Dar k s c h e wits c h and the interstitial nucleus of C a j a I. The ventrolaterally and ventrally running fibers descend through the lateral part of the ipsilateral midbrain to end in the dorsolateral part of the pontine nuclei, with some fibers passing into the medial geniculate body, particularly its magnocellular part. Most of the ascending degenerated fibers from the tectal lesion run rostrally directly into the pretectum and the thalamus, but some fibers pass by way of the brachium of the superior colliculus. In the pretectum degenerated fibers are distributed in the pretectal nucleus, the nucleus of the optic tract and the nuclei of the posterior. commissure (Text-fig. 2). Within the former two nuclei a fair number of degenerated fibers are seen. In the pretectal nucleus degenerated fibers are found largely in its dorsal part. Only a few degenerated fibers are encountered in the nuclei of the posterior commissure and the medially adjacent central gray. Within the medial geniculate body degenerated fibers are distributed in the pars magnocellularis, particularly its dorsal portion , 272 K. Niimi, M. Miki and S. Kawamura

Text-fig. 2. Projection drawing of a transverse section passing through the middle portion of the posterior commissure in cat 3. Nauta-Gygax method. In this and subsequent figures, preterminal degeneration is indicated by stippling and degenerated fibers of passage by rows of coarse dots.

and the dorsal and dorsomedial portion of the pars principalis (Text- fig. 2). Proceeding rostrally, they decrease in number and are restricted to the dorsal portion of the pars magnocellularis and the dorsomedial edge of the pars principalis at the most anterior level of the posterior commissure. No degenerated fibers are seen in the anterior extremity of the medial geniculate body. In the most caudal part of the dorsal thalamus degenerated fibers are found in the medial, lateral and inferior pulvinar nuclei, and the suprageniculate nucleus (Text-figs. 2 and 3). The degenerated fibers in the medial pulvinar and suprageniculate nuclei are more numerous than those in the other nuclei. The dorsal parts of medial and lateral pulvinar nuclei are free from degeneration. Passing rostrally, the degenerated fibers gradually decrease in number and are confined to the transition zone among the medial and lateral pulvinar, and suprageniculate nuclei at the most anterior level of the lateral geniculate body. Some degenerated fibers are found also in the most caudal part of the posterior lateral nucleus, corresponding to a rostral continuation of the lateral pulvinar nucleus. A few degenerated fibers are seen running laterally beyond the lateral pulvinar nucleus to pass through the reticular thalamic nucleus into the internal cap- sule. Their farther course, however, cannot be followed. No degenerated fibers are found in the posterior ventral nuclei. At middle levels of the lateral geniculate body some of the degenerated fibers in the dorsal thalamus begin to shift medially to enter the centre median nucleus. At more rostral levels degenerated fibers are found widely in the intralaminar nuclei in a wide sense. In the centre median nucleus degenerated fibers are seen abundantly, Ascending Projections of the Superior Colliculus in the Cat 273

Text-fig. 3. Projection drawing of a transverse section passing through the anterior portion of the posterior commissure in cat 3. Nauta-Gygax method. while in the parafascicular nucleus they are much fewer in number (Text-figs. 4 and 5). Very few fibers are encountered in the lateral part of the dorsomedial nucleus, but their exact termination cannot be determined. Within the lateral central nucleus a fair number of degenerated preterminals are present. In addition, some degenerated beads are seen running dorsomedially through the lateral central nucleus. A few of them reach the region immediately lateral to the lateral habenular nucleus, but no fibers are seen entering this nucleus (Text-fig. 5A). Having passed through the lateral and inferior pulvinar and suprageniculate nuclei, some degenerated fibers run ventrolaterally between the pulvinar nuclei and the dorsal nucleus of the lateral geniculate body. They then pass through the medial interlaminar nucleus and reach lamina B of the dorsal nucleus of the lateral geniculate body. A few of them terminate in lamina B, but others run through this lamina into the ventral nucleus of the lateral geniculate body, where degenerated fibers are found in the internal layer and the ventromedial portion of the external layer (Text-fig. 4). A small number of degenerated fibers are seen running ventrally along the medial margin of the optic tract (Text-fig. 3). They then pass ventromedially to cross dorsal to the optic chiasma (G u d d e n's commissure), but cannot be followed farther. Within the ventral thalamus a fair number of degenerated fibers are seen in the zona incerta and the field of F or el (Text-figs. 3 to 5). Most of the degenerated fibers in the zona incerta appear to be fibers of passage running diagonally into the field of Fore 1, but a 274 K. Niimi, M. Miki and S. Kawamura

Text-fig. 4: Projection drawings of transverse sections passing through the middle (A) and posterior parts (B) of the lateral geniculate body in cat 3. Nauta-Gygax method.

few degenerated fibers are found to terminate there. The field of F o r e l contains abundant amount of degenerated preterminals. Proceeding rostrally, the degenerated fibers in the ventral thalamus decrease in number. Although most of the ascending tectofugal fibers are distributed in the ipsilateral pretectum and thalamus, a small number of ascending fibers cross through the posterior commissure and the commissure of the superior colliculus to be distributed in the contralateral pretectum and thalamus (Text-figs. 2 and 3). Some of them run into the pretectal nucleus and the nuclei of the posterior commissure. In the pretectal nucleus degenerated preterminals are found largely in its dorsal part. The nuclei of the posterior commissure contain only a few preterminals. Most of the crossed degenerated fibers are distributed in the ventral part of the medial pulvinar nucleus, the dorsal part of the suprageniculate nucleus and the medial geniculate body on the side opposite the lesion. Only a few degenerated fibers are discernible in the ventral part of the inferior pulvinar nucleus and the zona incerta. There are no degenerated fibers in the lateral geniculate body. In the medial geniculate body degenerated fibers are found in the pars magnocellularis and the dorsomedial portion of the pars principalis, ajad are most abundant at levels of the caudal edge of the posterior commissue. Cat 2 In this animal the electrolytic lesion was placed in the caudal half of the superior colliculus. In cross section the lesion Ascending Projections of the Superior Colliculusin the Cat 275 was situated in the lateral part of the superior colliculus and extended ventromedially into the dorsolateral part of the central gray of the midbrain, and laterally into the brachium of the inferior colliculus. The pattern of degeneration in the midbrain and diencephalon in this cat is similar to that in the previous case. A marked difference is that a number of degenerated fibers are found in the central gray of the midbrain, particularly in its medial part in cat 2. A small number of degenerated fibers are present in the oculomotor nucleus of both sides. These degenerated fibers in the central gray and the oculomotor nucleus may be attributed to inadvertent damage to the central gray. The degeneration picture of the pretectal region is essentially similar to that in cat 3, but the degenerated fibers are fewer in number. Particularly, they are very scarce in the contralateral pretectum. In the nuclei of the posterior commissure, however, a fair number of degenerated fibers are seen on both sides. At superior collicular levels degenerated fibers are present in the pars magnocellularis and the ventral portion of the pars principalis of the medial geniculate body. As the posterior commissure appears, they are also found in the most dorsal and dorsomedial portion of the pars principalis. At these levels the pattern of degeneration in the posterior group of thalamic nuclei is similar to that in cat 3, though the degenerated fibers are smaller in amount. Proceeding rostrally, the degenerated fibers in the dorsal thalamus decrease in number. In this case also, a few degenerated fibers are found in lamina B of the dorsal nucleus of the lateral geniculate body. A considerable number of degenerated fibers are observed in the ventral nucleus of the lateral geniculate body. Along the medial margin of the optic tract some degenerated fibers are seen running ventrally and then ventromedially to cross dorsal to the optic chiasma (G u d d e n's commissure) and are directed dorsolaterally along the medial edge of the contralateral optic tract. Their farther course, however, cannot be followed. In the zona incerta and the field of Forel more degenerated fibers are found in this case than in cat 3. Within the dorsal thalamus proper the degenerated fibers are distributed in the centre median, parafascicular and lateral central nuclei and a few in the dorsomedial, paracentral and medial central nuclei, in addition to the pulvinar and suprageniculate nuclei. In the intralaminar nuclei in a wide sense the degenerated fibers extend a little more rostrally than in the previous case. A number of degenerated fibers are found dorsomedially through the lateral central nucleus, 276 K. Niimi, M. Miki and S. Kawamura

Text-fig. 5. Projection drawings of transverse sections passing through the anterior (A) and most anterior parts (B) of the lateral geniculate body in cat 3. Nauta-Gygax method. and a few of them are seen to enter the lateral habenular nucleus. The degeneration in the medial central and lateral habenular nuclei may result from the impingement of the lesion on the central gray of the midbrain. In this case axon degeneration is seen in the posteromedial and posterolateral ventral nuclei. This may be attributed to the lesion extending to the brachium of the inferior colliculus. The pattern of degeneration in the contralateral thalamic nuclei in this case is similar to that in cat 3. Fiber degeneration is found in the parafascicular, centre median, lateral central, paracentral, medial central, dorsomedial, posteromedial ventral nuclei, in addition to the pulvinar and suprageniculate nuclei. Within the contralateral medial geniculate body degenerated fibers are found in the pars magnocellularis as well as in the ventral and the most dorsomedial portions of the pars principalis, as on the ipsilateral side, but are fewer in number. Slight degeneration is also seen in the ventral nucleus of the contralateral lateral geniculate body. Cat 4 An electrolytic lesion was made in the caudal half of the superior colliculus and the rostra' part of the inferior colliculus. In cross sections the lesion was found in the middle and lateral part of the superior colliculus, involving the dorsomedial part of the inferior colliculus and a part of the brachium of the inferior colliculus. The tectal lesion extended ventromedially to involve the underlying central gray of the midbrain. AscendingProjections of the SuperiorColliculus in the Cat 277

The pattern of degeneration in this case is similar to that in cat 2. The only notable difference is that in cat 4 degenerated fibers are distributed in both posterolateral ventral nuclei with ipsilateral predominance. This may result from the involvement of the inferior colliculus and its brachium in the lesion. Cat 6 An electrolytic lesion was placed in the posterior half of the superior colliculus. At its anterior and middle levels the lesion was situated in the medial part of the superior colliculus, but caudal- ward it migrated somewhat laterally to lie in the middle part of the superior colliculus. The brachium of the inferior colliculus was not involved in the lesion. The distribution of tectofugal fibers in the midbrain and the diencephalon is essentially similar to that in cat 3, but the degenerated fibers somewhat fewer in number, particularly in the lateral pulvinar nucleus. It should be mentioned that in this case no degenerated fibers are seen in lamina B of the dorsal nucleus of the lateral geniculate body. Unfortunately, the right cerebral hemisphere was not available in this case, so that it was impossible to trace tectofugal fibers to the diencephalon on the side opposite the lesion.

Discussion The ascending projection of the superior colliculus has been studied by many authors (P a p e z and F r e em a n, '30 ; T a s i r o, '39a, '39b, '40a, '40b ; Marburg and Warne r, '47 ; Bucher and Biirgi, '50; Biirgi, '57; Bargi and Bucher, '60; Altman and Carpenter, '61; Myers, '63; Smith and Clarke, '64; Tarlov and Moore, '66; Martin, '69), among whom Tasiro performed the most extensive work in the cat, rabbit, rat and mouse, using the March i method. Tectofugal fibers have been said to distribute in the pars magnocellularis of the medial geniculate body (T a s i r o, '39a, '39b, '40a, '40b; Altman and Carpenter , '61; Smith and Clarke, '64; Marti n, '69). Tasiro found degeneration also in the dorsal portion, of the pars principalis of the medial geniculate body (nucleus b) following lesions of the superior colliculus. In our experiments fibers from the superior colliculus terminate mostly in the pars magnocellularis and partly in the dorsal and dorsomedial portion of the pars principalis of the medial geniculate body in essential agreement with the results of Tasiro. In cases with lesions involving the inferior colliculus and/or the brachium of the inferior colliculus degenerated fibers are found also in the middle and ventral portion 278 K. Niimi, M. Miki and S. Kawamura

of the pars principalis (cats 2 and 4). Moore and Goldberg ('63) stated that degeneration is found in the pars magnocellularis and the ventral portion of the pars principalis of the medial geniculate body following lesions of the inferior colliculus. It is generally accepted that the superior colliculus sends fibers to the ventral nucleus of the lateral geniculate body (T a s i r o, '39a ; Altman and Carpenter, '61; Myers, '63; Smith and Clark e, '64 ; Marti n, '69). Most authors failed to demonstrate the superior collicular projection to the dorsal nucleus of the lateral geniculate body (Altman and Carpenter, '61; Tarlov and Moor e, '66). Some workers, however, found a few degenerated fibers in the dorsal nucleus following lesions of the superipr colliculus (T asir o, '39a, '39b, '40a, '40b ; Smith and Clark e, '64 ; Myer s, '66 ; Marti n , '69), but their descriptions are not sufficiently detailed to enable us to identify the terminal- portion of tectofugal fibers in the dorsal nucleus. Smith and Clarke observed degeneration in the ventral and dorsal nuclei of the lateral geniculate body after destruction of the superior colliculus in the rat, and assumed that the degenerated fibers in the lateral geniculate body may be due to a retrograde process or a tectogeniculate projection described in lower animals. In all our cases, except cat 6 with the lesion in the medial part of the superior colliculus, a small number of degenerated axons and preterminals are found in lamina B of the dorsal nucleus of the lateral geniculate body. This suggests that the lateral (and possibly the middle) part of the superior colliculus sends fibers to lamina B of the dorsal nucleus. In the ventral nucleus of the lateral geniculate body degeneration is found largely in the internal layer in cases with lesions localized in the superior colliculus (cats 3 and 6), while in the other cases a number of degenerated fibers are found in the external layer, in addition to the internal layer. Tarlov and Moore('66) stated that in the rabbit tectogeniculate fibers are distributed in the laterally lying region of fusiform cells (part of the external layer of Niimi et al., '64). Fibers from the superior colliculus are distributed in the posterior group of the dorsal thalamic nuclei. Tasiro ('40a) stated that tectofugal fibers terminate in the pulvinar nucleus (corresponding to our lateral pulvinar nucleus), the posterior nucleus (comparable to our medial pulvinar nucleus) and the suprageniculate nucleus. He observed degeneration also in the region corresponding to the inferior pulvinar nucleus. Later authors also have described such tectal projections to the posterior thalamus (B u c h e r and B U r g i, '50 ; B '57; Biirgi and Bucher, '60; Altman and Carpenter, AscendingProjections of the SuperiorColliculus in the Cat 279

'61 ; Myer s, '63 ; Smith and Clark e, '64). In all our cases degenerated fibers are distributed in the medial, lateral and inferior pulvinar nuclei and the suprageniculate nucleus, in agreement with the above authors. Of these thalamic nuclei the suprageniculate and medial pulvinar nuclei are most severely degenerated. T a s i r o ('40a) found degeneration in the ventral thalamic nuclei (Va and Vb) after lesions in the superior colliculus of the cat. Other authors found Do degeneration in these nuclei. Within the posterior ventral nuclei degeneration is found in cats 2 and 4, where tectal lesions involve the inferior colliculus and/or the brachium of the inferior colliculus, while scarcely any degeneration is found in the ventral nuclei in cats 1 and 4 with lesions localized in the superior colliculus. Some authors reported the projection of the inferior colliculus to the ventral thalamic nuclei (T a s i r o, '40a ; Moore and G o 1 d b e r g, '63). T a s i r o ('39b, '40a) first described the projection of the superior colliculus to the intralaminar thalamic nuclei (in a wide sense). This has been confirmed by later observers. T a s i r o ('40a) found degeneration in the parafascicular and centre median nuclei (nucleus med. c) in the cat following lesions of the superior colliculus. Bucher and B U r g i ('50) stated that the finely myelinated fibers from the superior colliculus disappear in the region of the centre median nucleus. Myers ('63) and Margin ('69) traced tectofugal fibers to the parafascicular nucleus in the monkey and opossum, respectively. Altman and Carpenter ('61), however, found preterminal degeneration in the centre median and parafascicular nuclei in animals with large tectal lesions invading the periaque- ductal gray or dorsal tegmental structures, and observed few if any degenerated fibers in these nuclei in animals with superficial or restricted lesions of the superior colliculus. T a s i r o ('40a) described the tectofugal fibers to the medial central, paracentral and lateral central nuclei (nucleus centralis a, b and c) in the cat. Myers ('63) also demonstrated fibers to the intralaminar nuclei in the monkey. Martin ('69) traced tectal fibers to the paracentral nucleus in the opossum. According to our results, degenerated fibers are found to terminate in the centre median, parafascicular and lateral central nuclei following lesions of the superior colliculus. No degeneration is found in the paracentral and medial central nuclei. However, a few degenerated fibers are observed in these nuclei in cats 2 and 4 with tectal lesions involving the central gray. Tectofugal fibers to the dorsomedial thalamic nucleus have been described by Altman and Carpenter ('61), and Myers ('63). Our results, however, reveal that very few if any degenerated fibers are found in the dorsomedial nucleus in cats 3 and 6, where the 280 K. Niimi, M. Miki and S. Kawamura lesions are localized in the superior colliculus. Tectal projections to the habenular nucleus have been described by Huber and Crosby ('43) in reptiles and mammals. Marburg and Warner ('47) demonstrated such fibers in the cat by means of the Mar chi method. Later workers, however, denied the ex- istence of tectohabenular fibers. In our cats 3 and 6 with lesions localized in the superior colliculus, degenerated fibers can be traced to the region just lateral to the lateral habenular nucleus, but no fibers are seen entering this nucleus. In cats with tectal lesions involving the central gray, a few degenerated fibers appear to terminate in the lateral habenular nucleus. Tectofugal fibers have been said to distribute to the thalamic reticular nucleus, the zona incerta and the field of F o r e l (B u c h e r and Bilrgi, '50 ; B U r g 1, '57 ; BUrgi and Bucher, '60). Parti- cularly, tectal projections to the last two nuclei have been demonstrated by many authors (T a s i r o, '40a ; Myer s, '63 ; Marti n, '69) . Altman and Carpenter ('61), however, stated that the degeneration in the field 1-1, of F o r e 1 and the reticular nucleus was found in cases with large lesions involving the central gray or the dorsal tegmental region. In all our cases degeneration is found in the zona incerta and the field of Fore 1, but no degenerated preterminals are seen in the reticular nucleus. The projection of the superior colliculus to the pretectal region has been described by a number of authors. However, a precise projection to each element of the pretectal region is not well known. In all our cases tectofugal fibers are distributed in the pretectal nucleus, the nucleus of the optic tract and the nuclei of the posterior commissure. T a s i r o ('39 a, '39b, '40a, '40b) described the crossed ascending projection of the superior colliculus. In the cat ('40a) he found degeneration in the posterior thalamic nucleus (corresponding to our medial pulvinar nucleus) and nucleus a and b of the medial geniculate body on the side contralateral to superior collicular lesions. A I t- m a n and Carpenter ('61) stated that diencephalic degeneration following tectal lesions is almost exclusively ipsilateral, and that some degenerated fibers are observed in a few cases in the posterior commissure, as well as in the nuclei of the posterior commissure, in the contralateral nuclei of the posterior commissure, in the contralateral pretectum and centre median nucleus. According to Marti n's ('69) findings, a few fine degenerated fibers cross within the nucleus centralis and the nucleus commissuralis interanterodorsalis only in cases with large tectal lesions extending into the central gray. Our results reveal that the crossed ascending tectofugal fibers are dis. Ascending Projections of the Superior Colliculus in the Cat 281 tributed in the pars magnocellularis and the dorsal and dorsomedial portion of the pars principalis of the medial geniculate body, the medial, lateral and inferior pulvinar nuclei, the suprageniculate nucleus, the zona incerta, the pretectal nucleus and the nuclei of the posterior commissure. The degeneration in the intralaminar nuclei (in a wide sense) and the ventral nucleus of the lateral geniculate body on the contralateral side may result from inadvertent damage to the central gray (cats 2 and 4). T a s i r o ('40b) compared the tectal projection to the thalamus in the cat, rabbit, rat and mouse, and concluded that the tectothalamic fibers in the cat are more abundant than those in lower forms. Our observations also demonstrated a large number of tectothalamic fibers in the cat. It should be noted that these fibers are distributed mainly in the posterior group of thalamic nuclei which are strongly connected to the association cortex. In this connection the tectothalamic projection provides an afferent pathway in the neural arcs between the superior colliculus and the cerebral cortex, which are concerned with the higher correlative mechanism of vision. Recent experimental studies revealed that the superior colliculus contributes fibers to the commissure of G u d d e n and these fibers are distributed in the ventral nucleus of the contralateral lateral geniculate body (M y e r s, '63 in the monkey ; T a r 1 o v and Moor e, '66 in the rabbit ; Marti n , '69 in the opossum). Altman and Carpenter described a similar pathway in the cat, but could not determine its termination on the side opposite the lesion, in accord- ance with our results.

Summary Unilateral electrolytic lesions were placed manually or stero- taxically in the superior colliculus, and the ensuing degeneration in the diencephalon was studied using the N a u t a-G y g a x ('54) silver method. 1. The superior colliculus sends ascending fibers directly or by way of the brachium of the superior colliculus to the pretectal region, the medial and lateral geniculate bodies, the dorsal thalamic nuclei and the ventral thalamus on the ipsilateral side. It contributes some fibers to the commissure of G u d d e n. 2. Within the pretectum tectofugal fibers terminate in the pretectal nucleus, the nucleus of the optic tract and the nuclei of the posterior commissure. 3. The superior colliculus projects fibers to the pars magnocellularis and the most dorsal and dorsomedial portion of the pars 282 K. Niimi, M. Miki and S. Kawamura

principalis of the medial geniculate body. In the lateral geniculate body tectofugal fibers terminate largely in the ventral nucleus, but a few fibers are found to end in lamina B of the dorsal nucleus in cases with lesions in the lateral and middle part of the superior colliculus. 4. A number of ascending tectofugal fibers are distributed in the medial, lateral and inferior pulvinar nuclei and the suprageniculate nucleus as well as the intralaminar nuclei in a wide sense, which include the centre median, parafascicular and lateral central nuclei. A few tectal fibers are scattered in the most posterior part of the posterior lateral nucleus, but no fibers are found in the ventral nuclei. 5. A few tectofugal fibers reach the region just lateral to the lateral habenular nucleus, but no fibers are seen entering this nucleus. Very few if any tectal fibers are found in the dorsomedial nucleus. 6. In the ventral thalamus tectofugal fibers terminate in the zona incerta and the field of F ore 1. The reticular thalamic nucleus receives no fibers from the superior colliculus. 7. A fair number of tectofug al fibers cross the posterior corn- missure and andthe commissure of the superior colliculus and end in the pars magnocellularis and the dorsomedial portion of the pars principalis of the medial geniculate body, the medial and inferior pulvinar nuclei, the suprageniculate nucleus, the zona incerta, the pretectal nucleus and the nuclei of the posterior commissure on the opposite side.

Abbreviations

A, Lamina A of dorsal nucleus of Glv, Ventral nucleus of lateral geni- lateral geniculate body culate body Al, Lamina Al of dorsal nucleus of Gmm, Pars magnocellularis of medial lateral geniculate body geniculate body B, Lamina B of dorsal nucleus of Gmp, Pars principalis of medial geni- lateral geniculate body culate body Bci, Brachium of inferior colliculus HI, Lateral habenular nucleus Bcs, Brachium of superior colliculus Ld, Dorsal lateral nucleus Cem, Centre median nucleus Li, Nucleus limitans Ci, Inferior colliculus Md, Dorsomedial nucleus Cl, Lateral central nucleus Mt, Mamillothalamic tract Cm, Medial central nucleus Ncp, Nuclei of posterior commis- Cp, Posterior commissure sure Cs, Superior colliculus Nic, Central interlaminar nucleus of F, Fornix dorsal nucleus of lateral geniculate FF, Field of Forel body Gld, Dorsal nucleus of lateral geni- Nim, Medial interlaminar nucleus of culate body dorsal nucleus of lateral genicu- Ascending Projections of the Superior Colliculus in the Cat 283

late body Rt, Thalamic reticular nucleus Nto, Nucleus of optic tract Sg, Suprageniculate nucleus Pc, Paracentral nucleus Sgc, Central gray substance Pf, Parafascicular nucleus To, Optic tract Pi, Inferior pulvinar nucleus Vpl, Posterolateral ventral nucleus P1, Lateral pulvinar nucleus Vpm, Posteromedial ventral nucleus Pm, Medial pulvinar nucleus Zi, Zona incerta Pt, Pretectal nucleus

References

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Explanation of Plate

All figures are photomicrographs from transverse sections through the diencephalon of cat 3. Nauta-Gygax technique. x 400 1. Preterminal degeneration in lamina B of the dorsal nucleus of the lateral geniculate body on the operated side. 2. Preterminal degeneration in the lateral pulvinar nucleus on the operated side. 3. Preterminal degeneration in the centre median nucleus on the operated side. 4. Preterminal degeneration in the principal part of the medial geniculate body on the operated side. 5. Preterminal degeneration in the magnocellular part of the medial geniculate body on the side contralateral to the lesion. 6. Preterminal degeneration in the suprageniculate nucleus on the side contralateral to the lesion. 287

Plate

K.Niimi, M.Miki, and S .Kawamura