Periaqueductal grey projection to the ventrobasal complex in the cat: An horseradish peroxidase study

Horseradish peroxidase (HRP) was injected within the thalamic ventrobasal complex of 14 cats. The aim was to ascertain whether the periaqueductal grey matter (PAG) sends fibres to this complex. Retrogradely labelled cells were found within the PAG following HRP delivery either in the nucleus ventralis posterolateralis (VPL) or ventralis posteromedialis (VPM). PAG-VPL projection is only ipsilateral and arises mainly from lateral PAG. PAG-VPM projection is bilateral and originates from latero-ventral regions of the central grey. The hypothesis that PAG might control the activity of ventrobasal nociceptive neurones is proposed.     

Afferents to the periaqueductal gray in the rat. A horseradish peroxidase study

Afferent projections to the periaqueductal gray matter in the rat have been studied by use of the retrograde axonal transport of horseradish peroxidase. Iontophoretic injections of horseradish peroxidase were made in dorsal, lateral and medial areas of the periaqueductal gray, primarily at intercollicular levels. The pattern of projections was similar in all of the injections restricted to the periaqueductal gray. Within the brainstem, numerous reticular formation nuclei were labeled, including nucleus reticularis lateralis, nucleus raphe magnus, pallidus and obscurus, the nucleus reticularis pontis oralis and caudalis, the paralemniscal nucleus and the dorsal and ventral parabrachial nuclei. At diencephalic levels, dense projections were seen from the parafascicular nucleus, dorsal premamillary nucleus, zona incerta, dorsomedial and ventromedial nuclei of the hypothalamus and the retrochiasmatic area, in the ventral portion of the anterior hypothalamus. At forebrain levels, occasional cells were seen in the medial preoptic area, lateral septum and the anterior cingulate cortex. Control injections of horseradish peroxidase into structures adjacent to the periaqueductal gray matter included three well localized deposits in the dorsal raphe. Retrogradely-labeled cells were found in lateral reticular nucleus of the medulla, nucleus raphe magnus, nucleus reticularis pontis caudalis, locus ceruleus, dorsal and ventral parabrachial nuclei, substantia nigra and the lateral hypothalamus. No labeled cells were found in the habenular nuclei. It is suggested that many of the descending hypothalamic and forebrain afferents may be relay centers for descending hippocampal formation efferents. Many of the periaqueductal gray afferent systems receive a direct projection from the hippocampal formation and could therefore coordinate influences from this limbic center with information on homeostatic mechanisms controlled by the hypothalamus. The numerous brainstem afferents to the periaqueductal gray could be involved in relay of ascending sensory information important for initiating any of several behavioral responses known to be controlled by the periaqueductal gray. In addition, certain raphe afferents might play a part in a feedback loop of the pain suppression circuit of which the periaqueductal gray is an important component.     

Cortical afferents of the nucleus accumbens in the cat,studied with anterograde and retrograde transport techniques

The cortical afferentation of the nucleus accumbens in the cat was studied with the aid of retrograde tracing techniques. Retrograde experiments were carried out with horseradish peroxidase or one of the fluorescent tracers Bisbenzimid, Nuclear Yellow and Fast Blue. In the anterograde experiments [³H]leucine and [³⁵S]methionine were used as tracers.Following injections in the nucleus accumbens, retrogradely-labelled cells were found in the medial frontal cortex, the anterior olfactory nucleus, the posterior part of the insular cortex, the endopiriform nucleus, the amygdalo-hippocampal area, the entorhinal and perirhinal cortices and the subiculum of the hippocampal formation. In the medial frontal cortex most of the labelled cells were found in layers III and V of the prelimbic area (area 32 of Brodmann), but retrogradely-filled neurons were also present in the infralimbic area and in the caudoventral part of the lateral bank of the proreal gyrus. Retrogradely-labelled cells in the entorhinal and perirhinal cortices were located in the deep cellular layers. Following large injections in the nucleus accumbens, retrograde labelling in the subiculum extended from the most dorsal, septal pole to the most ventral, temporal pole.Injections of anterograde tracers were placed in the frontal cortex, the entorhinal and perirhinal cortices and the hippocampal formation. The prelimbic area was found to project via the internal capsule to mainly the rostral half of the nucleus accumbens, whereas in the caudal half of the nucleus only a lateral region receives frontal cortical fibres. Following injections in the infralimbic area only fibres passing through the nucleus accumbens were labelled. Afferents from the entorhinal and perirhinal cortices reach the nucleus accumbens by way of the external capsule and terminate mainly in a ventral zone of the nucleus accumbens.Afferents from the entorhinal area are distributed to the entire accumbens, whereas the termination field of the perirhinal afferents is largely restricted to the lateral part of the nucleus accumbens. Both the frontal cortex and the entorhinal and perirhinal cortices appear to project also to the nucleus caudatus and the tuberculum olfactorium. These cortical areas also project to the contralateral striatum.Both anterograde and retrograde tracing experiments demonstrated a topographical relationship between the subiculum and the nucleus accumbens. The ventral pole of the subiculum projects via the fornix to the medial part of the caudal half of the nucleus accumbens and to a small dorsomedial area in its rostral half. Successively more dorsal portions in the subiculum project to successively more ventrolateral parts in the rostral nucleus accumbens. The projection from the hippocampus was found to extend also to the tuberculum olfactorium. The results of the present study do not provide unambiguous criteria for the delimitation of the nucleus accumbens in the cat.     

Projections of the pallidal complex: An autoradiographic study in the cat

The efferent projections of the external pallidal segment (‘globus pallidus’), and the internal pallidal segment (entopeduncular nucleus) were studied in separate experiments in the cat by the auto-radiographic tracing method. Injections of tritiated amino acids into the external pallidal segment resulted in labelling of axon systems distributed not only to the subthalamic nucleus but also in sparser density to the nucleus reticularis thalami, the substantia nigra, the caudate nucleus, the putamen, and as yet undefined areas of the cortex. Injections of tritiated amino acids into the internal pallidal segment resulted in labelling of axon systems distributed to the ventrolateral-ventroanterior complex of the thalamus, to the centrum medianum, the lateral habenular nucleus, and the mesencephalic nucleus tegmenti pedunculopontinus, pars compacta. Less prominent termination may also occur in the parafas-cicular nucleus, the nuclei of the fields of Forel, and in the mesencephalic tegmentum rostral to the nucleus tegmenti pedunculopontinus, pars compacta.The fact that this and previous studies show that the projections of the pallidal complex are more widespread than would be expected if it was only involved in motor functions, raises questions about the functional organization of the basal ganglia. These are discussed in the following paper.     

The organization of afferent projections to the midbrain periaqueductal gray of the rat

The retrograde transport technique was utilized in the present study to investigate the afferent projections to the periaqueductal gray of the rat. Iontophoretic injections of horseradish peroxidase were made into the periaqueductal gray of 22 experimental animals and into regions adjacent to the periaqueductal gray in 6 control animals. Utilization of the retrograde transport method permitted a quantitative analysis of the afferent projections not only to the entire periaqueductal gray, but also to each of its four intrinsic subdivisions. The largest cortical input to this midbrain region arises from areas 24 and 32 in the medial prefrontal cortex. The basal forebrain provides a significant input to the periaqueductal gray and this arises predominantly from the ipsilateral lateral and medial preoptic areas and from the horizontal limb of the diagonal band of Broca. The hypothalamus was found to provide the largest descending input to the central gray. Numerous labeled cells occurred in the ventromedial hypothalamic nucleus, the lateral hypothalamic area, the posterior hypothalamic area, the anterior hypothalamic area, the perifornical nucleus and the area of the tuber cinereum. The largest mesencephalic input to the periaqueductal gray arises from the nucleus cuneiformis and the substantia nigra. The periaqueductal gray was found to have numerous intrinsic connections and contained a significant number of labeled cells both above and below the injection site in each case. Other structures containing significant label in the midbrain and isthmus region included the nucleus subcuneiformis, the ventral tegmental area, the locus coeruleus and the parabrachial nuclei. The medullary and pontine reticular formation provide the largest input to the periaqueductal gray from the lower brain stem. The midline raphe magnus and superior central nucleus also supply a significant fiber projection to the central gray. Both the trigeminal complex and the spinal cord provide a minor input to this region of the midbrain.The sources of afferent projections to the periaqueductal gray are extensive and allow this midbrain region to be influenced by motor, sensory and limbic structures. In addition, evidence is provided which indicates that the four subdivisions of the central gray receive differential projections from the brain stem as well as from higher brain structures.     

The lateral reticular nucleus in the cat—I. An experimental anatomical study of its spinal and supraspinal afferent connections

The localization of the neurons from which the main ascending and descending projections to the lateral reticular nucleus originate has been studied in nine cats, using the retrograde axonal transport of horseradish peroxidase injected within that nucleus. The ascending spinoreticular neurons are widely distributed from the cervical to the sacral segments of the spinal cord. These neurons, which are of different sizes, are mainly located within Rexed's laminae VI, VII and VIII, but they spread both dorsally to laminae II–VI as well as ventrally to lamina IX. Labeled neurons of a size similar to motoneurons are particularly found within lamina IX, intermingled with the motoneurons. The spinal projection to the lateral reticular nucleus is crossed and uncrossed, with the ipsilateral spinoreticular neurons being located more dorsally within the grey matter of the spinal cord than the contralateral spinoreticular neurons. Moreover, while neurons with a crossed ascending projection are almost equally distributed along the whole rostro-caudal extension of the spinal cord, those with an uncrossed projection are predominantly located within the cervical segments of the spinal cord. Additional evidence indicates that the spinoreticular projection to the lateral reticular nucleus is somatotopically organized.In addition to the spinoreticular projection, the lateral reticular nucleus receives a crossed rubroreticular projection and a crossed fastigioreticular projection originating from the rostro-ventralmost part of the nucleus. A few neurons in the interposite nuclei also project to the lateral reticular nucleus.     

The synaptic organization of terminals traced from individual labeled retino-geniculate axons in the cat

Individual retino-geniculate axons in the dorsal lateral geniculate nucleus of the cat were filled with horseradish peroxidase and studied with both the light and electron microscope. A procedure was followed which allowed us to identify the size, shape and arrangement of particular terminal swellings by light microscopy and then to study their patterns of synaptic contacts with the electron microscope. Many of the labeled terminals in laminae A and Al have the same fine structural features as retino-geniculate terminals that have been described previously. They are large, contain round synaptic vesicles and pale mitochondria, and are the central processes in glomeruli where they form asymmetric contacts with dendrites and terminals containing pleomorphic synaptic vesicles. Other terminals have the same cytological features but are quite small and are not the central processes in glomeruli. Some of these small terminals form simple axo-dendritec contacts while others participate in very large glomeruli containing several terminals from a single retino-geniculate axon. These different patterns of synaptic contacts made by different terminals can be found on branches of a single axon and correspond to the variations in terminal arbors described in the preceding paper (MASON & ROBSON, 1978).     

The cerebellar projections to the superior colliculus and pretectum in the cat: An autoradiographic and horseradish peroxidase study

Efferent projections from the cerebellar nuclei to the superior colliculus and the pretectum have been studied using both retrograde and orthograde labeling techniques in the cat. In order to identify what parts of the cerebellar nuclei project to the superior colliculus and the pretectum, the retrograde horseradish labeling technique was employed. In another set of experiments, tritiated amino acids were injected into each of the cerebellar regions from which the cerebello-tectal and cerebellopretectal projections arise, and the laminar and spatial distributions of orthograde labeling in the superior colliculus and the pretectum were compared.The results showed that the cerebello-tectal projections arise from two different regions of the cerebellar nuclei: the caudal half of the medial nucleus and the ventrolateral part of the posterior interposed nucleus. Fibers arising from the medial nucleus distribute bilaterally in the superficial zone of the intermediate gray layer in the superior colliculus, while those originating from the posterior interposed nucleus terminate contralaterally in the deeper aspect of the intermediate gray layer and in the deep gray and white layers. Although the lateral nucleus does not contribute to the cerebello-tectal projection, it projects profusely to the pretectum contralaterally. The origin of the cerebello-pretectal projection lies in the parvicellular part of the lateral nucleus. Among several pretectal nuclei, the posterior pretectal, the medial pretectal nucleus and the reticular part of the anterior pretectal nucleus receive the cerebellar afferents.The findings of the differential projections from the cerebellum to the superior colliculus and the pretectum suggest that the cerebellum exerts a regulatory influence on visuo-motor and somato-motor transfer in these midbrain structures by differential circuits.     

Cells of origin of brainstem afferents to lobules I and II of the cerebellar anterior lobe in the cat

Cells of origin of the brainstem afferents to lobules I and II of the cerebellar anterior lobe were identified by means of the retrograde horseradish peroxidase technique. In order to avoid diffusion into other lobules, injections were made under direct visual control through the fourth ventricle, after having removed ventral parts of the posterior lobe.With clearcut localization, the major projections originated from neurons of the following nuclei; the pontine nuclei (dorsal to the lateral nucleus, and the lateral and dorsal part of the peduncular nucleus) and nucleus corporis pontobulbaris; vestibular nuclear complex (the superior, medial and descending vestibular nuclei and group x), nucleus of Martin and interstitial nucleus of the vestibular nerve; the ventrolateral part of the external cuneate nucleus; lateral reticular nucleus (mainly the parvocellular portion); the inferior olivary complex (the caudal and central parts of the medial accessory nucleus and the lateral part of the dorsal accessory nucleus at middle levels). Small projections originated from the paramedian reticular nucleus, prepositus hypoglossi nucleus, nuclei raphe obscurus and pallidus, and the gracile and main cuneate nuclei.It was suggested that lobules I and II function not only as representations of the hindlimb-tail regions but also of the neck region by receiving afferents from the central cervical nucleus and the ventrolateral part of the external cuneate nucleus that receives dorsal root afferents C1 to C4.     

Spinal projections from the periaqueductal grey and dorsal raphe in the rat,cat and monkey

There is considerable evidence that the periaqueductal grey and the dorsal raphe contribute to an endogenous analgesia system and to the regulation of a wide variety of other responses, many of which involve spinal sites of action. To map the areas of the periaqueductal grey and dorsal raphe which contain neurons that project to the spinal cord, wheat germ agglutinin conjugated to horseradish peroxidase was injected into hemisected spinal cords in rat, cat, and monkey. After cervical or lumbar injections labelled neurons were found in the periaqueductal grey and dorsal raphe in all species examined. In the rat, labelling of the dorsal raphe is sparse but numerous labelled neurons are present in the mid and rostral periaqueductal grey. In the cat, the number of retrogradely-labelled neurons in both the dorsal raphe and the periaqueductal grey are considerable. In the monkey, like the rat, the labelling in the dorsal raphe was light but numerous labelled neurons were present in the periaqueductal grey and the adjacent nucleus cuneiformis. Injections into the lumbar spinal cord produced the same pattern of labelling as seen after cervical level injections with approximately 40% fewer labelled cells in all areas. Thus, while each species had a similar pattern of spinal projections from the periaqueductal grey and dorsal raphe, quantitative differences were evident among the species examined.These results suggest that the number of periaqueductal grey and dorsal raphe neurons projecting to the spinal cord in the rat, cat and monkey are considerably more numerous than previously reported and that the effects described during the stimulation of these regions could be, at least partly, due to the involvement of these direct pathways.     

Direct projections of the hypothalamic nuclei to the thalamic mediodorsal nucleus in the cat

Direct projections of the hypothalamic nuclei to the thalamic mediodorsal nucleus (MD) were studied using retrograde and anterograde transport of horseradish peroxidase (HRP) and wheat germ agglutinin (WGA)-HRP. HRP and WGA-HRP were injected into the MD, thalamic paraventricular, lateral habenular and hypothalamic nuclei. The results indicate that the MD, particularly its medial part, receives a moderate amount of hypothalamic afferents, and that most of these afferents originate in the medial part of the lateral hypothalamic nucleus at anterior levels, while a limited number are derived from the dorsal, dorsomedial, ventromedial and anterior hypothalamic and lateral preoptic nuclei.     

Retinal ganglion cells that project to the dorsal lateral geniculate nucleus in the macaque monkey

Horseradish peroxidase was deposited in the optic nerve to retrogradely label and reveal the dendritic form of all classes of ganglion cell, or it was injected into the dorsal lateral geniculate nucleus to reveal only those classes projecting to the thalamus. The results were compared with those of the accompanying paper in which the ganglion cells projecting to the midbrain are selectively revealed. Two major classes of ganglion cells are described, the P alpha and P beta cells. For both classes dendritic field size increases with eccentricity from the fovea and there is no overlap in the two classes at any given eccentricity. Cell body size shows a similar mean difference but with a slight overlap. Both cell bodies and dendritic fields are larger along the temporal horizontal meridian than the nasal horizontal meridian, for P alpha and for P beta cells, but these differences are reduced when naso-temporal differences in ganglion cell density are taken into account, that is, size correlates closely with density. Injections restricted to the parvocellular layers of the lateral geniculate nucleus labelled almost exclusively P beta cells, whereas injections confined to the magnocellular layers labelled almost exclusively P alpha cells. As midbrain injections label no P beta cells and few P alpha cells it can be shown that about 80% of ganglion cells are P beta cells projecting to parvocellular lateral geniculate nucleus, and that about 10% are P alpha cells projecting to magnocellular layers. The coverage factor, that is the number of cells covering each point on the retina, varied from 1.9-2.3 for P beta cells, and from 2-7 for P alpha cells. Comparing the results with those of comparable investigations on cats and rabbits shows a much clearer segregation of the terminal targets of different classes of ganglion cell in monkeys, the greatest difference being the absence in the monkey of a projection to the geniculate from gamma- and epsilon-like cells. Further, axons which branch and innervate both thalamus and midbrain are rare in monkeys but common in other mammals. Comparing the results with those from physiological investigations suggests that the P beta cells correspond to colour-opponent cells, whereas P alpha cells correspond to the achromatic broad-band magnocellular cells.     

Topographical organization of the brainstem afferents to the lateral posterior-pulvinar thalamic complex in the cat

Following stereotaxic injections of horseradish peroxidase in the dorsal thalamus of the cat which were restricted to the lateralis posterior-pulvinar complex, labelled neurons were found in the superficial layers of the superior colliculus and in the brainstem. The retrogradely-filled cells of the brainstem were situated principally in the nucleus tegmenti pedunculopontinus, the locus coeruleus complex, the parabrachial nuclei and the dorsal tegmental nucleus of Gudden; in each case, labelled cells were more numerous on the ipsilateral side. In addition, some scattered neurons were observed in the central grey matter, the mesencephalic reticular formation, the central superior and dorsal raphe nuclei, the cuneiform nucleus, the nucleus reticularis gigantocellularis, the nucleus praepositus hypoglossi and the oculomotor nuclei. A differential organization of these projections was observed.It is concluded that the rostrointermediate subdivision of the lateralis posterior-pulvinar complex receives most of its connections from the nucleus tegmenti pedunculopontinus, from the deep layers of the superior colliculus and from the other brainstem nuclei, while the caudal subdivision (extrageniculate visual subdivision) receives its main projection from the superficial layers of the superior colliculus. The findings may have functional implications for the role of the complex in oculomotor control.     

Physiological and morphological identification of ventrolateral fibers relaying cerebellar information to the cat motor cortex

In the cat motor cortex, recordings were obtained from thalamocortical fibers and then these fibers were intra-axonally injected with horseradish peroxidase. Each fiber was identified by both orthodromic activation from the ventrolateral nucleus of the thalamus and by monosynaptic activation from the brachium conjunctivum. In anesthetized cats, all ventrolateral thalamic fibers tested had bursts of spikes with an intraburst frequency of about 400 Hz. The bursts occurred spontaneously and in response to thalamic or brachium stimulation. Fast pyramidal tract cells had clusters of excitatory postsynaptic potentials with the same internal frequency as the bursting afferents, strongly suggesting that they arise from the ventrolateral afferents. The horseradish peroxidase-injected ventrolateral afferents were distributed to many zones within the motor area and had loose arborizations within cortical layers. Terminals were generally located in layer III and in the upper part of layer VI. None was observed in the more superficial layers.The ventrolateral thalamic afferents in the motor cortex establish a point to zones connectivity. This may provide a morphological basis for the central organization of certain movements that necessitate simultaneous contraction of many muscles.     

Projections of the bed nucleus of the stria terminalis to the mesencephalon,pons, and medulla oblongata in the cat

Summary Injections of HRP in the nucleus raphe magnus and adjoining medial reticular formation in the cat resulted in many labeled neurons in the lateral part of the bed nucleus of the stria terminalis (BNST) but not in the medial part of this nucleus. HRP injections in the nucleus raphe pallidus and in the C2 segment of the spinal cord did not result in labeled neurons in the BNST. Injections of ³H-leucine in the BNST resulted in many labeled fibers in the brain stem. Labeled fiber bundles descended by way of the medial forebrain bundle and the central tegmental field to the lateral tegmental field of pons and medulla. Dense BNST projections could be observed to the substantia nigra pars compacta, the ventral tegmental area, the nucleus of the posterior commissure, the PAG (except its dorsolateral part), the cuneiform nucleus, the nucleus raphe dorsalis, the locus coeruleus, the nucleus subcoeruleus, the medial and lateral parabrachial nuclei, the lateral tegmental field of caudal pons and medulla and the nucleus raphe magnus and adjoining medial reticular formation. Furthermore many labeled fibers were present in the solitary nucleus, and in especially the peripheral parts of the dorsal vagal nucleus. Finally some fibers could be traced in the marginal layer of the rostral part of the caudal spinal trigeminal nucleus. These projections appear to be virtually identical to the ones derived from the medial part of the central nucleus of the amygdala (Hopkins and Holstege 1978). The possibility that the BNST and the medial and central amygdaloid nuclei must be considered as one anatomical entity is discussed.Abbreviations AA anterior amygdaloid nucleus - AC anterior commissure - ACN nucleus of the anterior commissure - ACO cortical amygdaloid nucleus - AL lateral amygdaloid nucleus - AM medial amygdaloid nucleus - APN anterior paraventricular thalamic nucleus - AQ cerebral aqueduct - BC brachium conjunctivum - BIC brachium of the inferior colliculus - BL basolateral amygdaloid nucleus - BNSTL lateral part of the bed nucleus of the stria terminalis - BNSTM medial part of the bed nucleus of the stria terminalis - BP brachium pontis - CA central nucleus of the amygdala - Cd caudate nucleus - CI inferior colliculus - CL claustrum - CN cochlear nucleus - CP posterior commissure - CR corpus restiforme - CSN superior central nucleus - CTF central tegmental field - CU cuneate nucleus - D nucleus of Darkschewitsch - EC external cuneate nucleus - F fornix - G gracile nucleus - GP globus pallidus - HL lateral habenular nucleus - IC interstitial nucleus of Cajal - ICA internal capsule - IO inferior olive - IP interpeduncular nucleus - LC locus coeruleus - LGN lateral geniculate nucleus - LP lateral posterior complex - LRN lateral reticular nucleus - MGN medial geniculate nucleus - MLF medial longitudinal fascicle - NAdg dorsal group of nucleus ambiguus - NPC nucleus of the posterior commissure - nV trigeminal nerve - nVII facial nerve - OC optic chiasm - OR optic radiation - OT optic tract - P pyramidal tract - PAG periaqueductal grey - PC cerebral peduncle - PO posterior complex of the thalamus - POA preoptic area - prV principal trigeminal nucleus - PTA pretectal area - Pu putamen - PUL pulvinar nucleus - R red nucleus - RF reticular formation - RM nucleus raphe magnus - RP nucleus raphe pallidus - RST rubrospinal tract - S solitary nucleus - SC suprachiasmatic nucleus - SCN nucleus subcoeruleus - SI substantia innominata - SM stria medullaris - SN substantia nigra - SO superior olive - SOL solitary nucleus - SON supraoptic nucleus - spV spinal trigeminal nucleus - spVcd spinal trigeminal nucleus pars caudalis - ST stria terminalis - TRF retroflex tract - VC vestibular complex - VTA ventral tegmental area of Tsai - III oculomotor nucleus - Vm motor trigeminal nucleus - VI abducens nucleus - VII facial nucleus - Xd dorsal vagal nucleus - XII hypoglossal nucleus     

Demonstration of a rubrothalamic projection in the cat,with some comments on the origin of the rubrospinal tract

Retrograde intra-axonal transport of horseradish peroxidase was used to classify red nucleus neurons by their efferents. After the injection of horseradish peroxidase in the nucleus ventralis lateralis thalami, labelled neurons were found in the ipsilateral red nucleus, indicating the existence of a rubrothalamic tract in the cat. Rubrothalamic neurons have triangularly shaped somata with an average diameter of27± 9μM (mean± S.E.) and have few dendrites; 80% of them were restricted to the rostral third of the red nucleus (A 5.5–A 7). Horseradish peroxidase injections in the spinal cord showed labelled neurons in the contralateral red nucleus. Rubrospinal neurons have a stellate soma with an average diameter of36±14 μM and have numerous dendrites; 85% of them were found in the caudal two-thirds of the red nucleus (A 3–A 5.5). Rubrothalamic and rubrospinal neurons differ in morphology, in soma dimension and in their distribution within the red nucleus. They thus belong, at least mainly, to two different neuronal populations.Comparison of the morphology of horseradish peroxidase-labelled neurons with Golgi-Cox impregnated neurons indicates that rubrothalamic neurons correspond to Golgi type B and rubrospinal neurons to Golgi type A, neurons of a previous study.It was found that rubrothalamic neurons were most readily labelled by injections of horseradish peroxidase in the dorsal part of the ventrolateral nucleus; this raises the possibility that this part of the ventrolateral nucleus might be involved in the control of axial musculature and/or eye movements.     

Effects of lesion of the interstitial nucleus of Cajal on vestibular horizontal canal neurons in the cat

Effects of procaine infusion into the interstitial nucleus of Cajal (INC) on vestibular nuclear neurons related to the horizontal canal were studied in cats anesthetized with nitrous oxide and paralyzed with gallamine. Neurons that responded to sinusoidal horizontal rotation (at 0.18 Hz) were recorded extracellularly in the medial and descending vestibular nuclei. Spontaneous activity of type I neurons increased, whereas that of type II neurons decreased following procaine infusion into the ipsilateral INC. Gain of the neuronal response to horizontal rotation decreased after the ipsilateral INC infusion, but there was no consistent effect on phase. Infusion into the contralateral INC seemed less effective. Similar effects were obtained with electrolytic lesions that were confined to the ipsilateral INC area. These results suggest that the INC influences type I neurons through inhibitory action of type II neurons and that it eventually controls the gain, but not the phase, of the horizontal vestibular reflexes.     

Trigeminal afferents to the diencephalon in the rat

Trigemino-diencephalic connections were studied in the rat using wheat-germ agglutinin conjugated to horseradish peroxidase as an anterogradely transported axonal tracer. Injection of the tracer into the subnucleus principalis produced two foci of dense labelling: one ventromedial: and one dorsal within the medial part of the ventrobasal complex. Other diencephalic structures containing granules of reaction product were the medial part of the medial geniculate body, the ventral area of the zona incerta and the nucleus lateralis posterior, pars lateralis. Injection of the tracer into the subnucleus interpolaris labelled the same structures, but less densely. After an injection into the subnucleus caudalis, labelling was observed in the same thalamic areas, although projections to the zona incerta or the lateralis posterior were not consistent. Additional labelling was observed in the subfascicular area of the mesodiencephalic junction, the nucleus submedius and the intralaminar nuclei centralis medialis and lateralis. In those cases of injection into the subnuclei principalis and interpolaris, all observed thalamic sites of projection were contralateral to the injection site. Following injection into the subnucleus caudalis, projections toward lateral thalamic structures were contralateral, but the nucleus submedius and the intralaminar nuclei exhibited bilateral labelling. Using high magnification (1250 X) with bright-field illumination, an analysis of the morphology of some terminal arborizations was attempted. Despite some technical limitations, the analysis indicated that in the ventrobasal complex, some terminal ramifications of axons originating from the three trigeminal subnuclei under study arborize so as to encompass a rounded area, the diameter of which could be as large as 100 microns, thereby resembling the classically described "bushy arbors". Such arborizations could not be distinguished in the axons projecting to the medial part of the medial geniculate body. In this latter nucleus, the terminals appeared to arise from a stem fiber as short side branches at approximately right angles to the parent stem axon. In the other areas where afferent terminal labelling was observed, the density of the network of the labelled fibers often complicated the analysis of morphological features. However, arborizations such as those observed in the ventrobasal complex or the medial geniculate nucleus could not be distinguished.(ABSTRACT TRUNCATED AT 400 WORDS)