Rubrobulbar projections in the rabbit. A light and electron microscopic study

Anatomical organization of descending cortical projections orchestrating the patterns of cortically induced rhythmical jaw muscle activity in guinea pigs

Repetitive electrical microstimulation to the cortical masticatory area (CMA) evokes distinct patterns of rhythmical jaw muscle activities (RJMAs) in animals. This study aimed to investigate the characteristics of the descending projections from the CMA, associated with distinct patterns of RJMAs, to the thalamus, midbrain, pons and medulla in guinea pigs. RJMAs with continuous masseter and digastric bursts (CB-RJMAs) and stimulus-locked digastric sub-bursts (SLB-RJMAs) were induced from the anterior and posterior areas of the rostral region of the lateral agranular cortex, and chewing-like RJMAs from the rostral region of the granular cortex. Anterograde tracer, biotinylated dextran amine, was injected into the three cortical areas. The cortical area inducing CB-RJMAs had strong ipsilateral projections to the motor thalamus, red nucleus, midbrain reticular formation, superior colliculus, parabrachial nucleus, and supratrigeminal region, and contralateral projections mainly to the lateral reticular formation around the trigeminal motor nucleus (Vmo). The cortical area inducing SLB-RJMAs had moderate projections to the motor thalamus and lateral reticular formation around the Vmo, but few projections to the midbrain nuclei. The cortical area inducing chewing-like RJMAs had strong projections to the ipsilateral sensory thalamus and contralateral trigeminal sensory nuclei, and moderate projections to the lateral reticular formation. The three cortical areas consistently had few projections to the ventromedial reticular formation. The present study demonstrates that multiple direct and indirect descending projections from the CMA onto the premotor systems connecting the trigeminal motoneurons represent the neuroanatomical repertoires for generating RJMAs during the distinct phases of natural ingestive behavior.

Regional (14C) 2-deoxyglucose uptake during forelimb movements evoked by rat motor cortex stimulation: pons, cerebellum, medulla, spinal cord, muscle

Electrical stimulation of the right forelimb motor (MI) sensory (SI) cortex in normal, adult rats produced repetitive left forelimb movements. Regions of increased (14C) 2-deoxyglucose (2DG) uptake were mapped auto-radiographically during these movements. MI stimulation activated the ipsilateral reticular tegmental pontine nucleus (RTP) and the middle (rostral-caudal) third of the pontine nuclei including pyramidal (P), medial (POM), ventral (POV), and lateral (POL) pontine nuclei. The ipsilateral inferior olivary complex was activated including dorsal accessory olive (DAO), principal olive (PO), and medial accessory olive (MAO). The contralateral lateral reticular (LR) nucleus and nucleus cuneatus (CU) were activated. Lateral vermal, paravermal, and hemispheric portions of the contralateral cerebellum were also activated. Parts of vermian lobules IV, V, VI, VII, and VIII, and lobulus simplex, crus I, crus II, paramedian lobule, and copula pyramidis were activated. Granule cell layers were activated much more than molecular layers. Discrete microzones of high granule cell 2DG uptake alternated with zones of low uptake in left paramedian lobule and copula pyramidis and may correlate with the fractured cerebellar somatotopy described physiologically by Welker and his associates. Portions of the left lateral and interpositus nuclei were metabolically activated. Medial portions of laminae I-VI were activated in the dorsal horn of cervical spinal cord. The 2DG uptake was either unchanged or decreased in the ventral horn. Thoracic and lumbar spinal cord were not activated. Monsynaptic MI and SI connections to P, POM, POV, POL, RTP, DAO, PO, MAO, LR, CU, and spinal cord could account for activation of those structures. However, there are no direct MI or SI connections to the deep cerebellar nuclei, the cerebellar hemisphere, or the muscles. Activation of these structures must be due to activation of polysynaptic pathways, sensory feedback from the moving forelimb, or both. The present experiments cannot distinguish these possibilities. Comparison of the regions activated during forelimb MI stimulation (FLMIS) to those activated during vibrissae MI stimulation (VMIS) suggests that the pontine nuclei, cerebellar hemisphere, and possibly the deep cerebellar nuclei are somatotopically organized. RTP, LR, CU, and spinal cord were activated during FLMIS but were not activated during VMIS. The failure to activate the ventral horn of cervical spinal cord may be due to known inhibition of alpha-motor neurons during motor cortex stimulation.

Projection of the magnocellular red nucleus to the region of the accessory abducens nucleus in the rabbit

The projection of the magnocellular red nucleus (RNm) to the region of the accessory abducens nucleus (AABD) was traced in rabbit using the bidirectional tracer wheat germ agglutinin-horseradish peroxidase (WGA-HRP). In one set of animals, recordings of antidromic responses from RNm neurons elicited by electrical stimulation of the rubrospinal tract were used to localize injections of WGA-HRP for orthograde labeling of RNm terminals. In a different set of animals, horseradish peroxidase was injected into the retractor bulbi muscle to retrogradely label motoneurons of the AABD. The positions of RNm fibers and terminals were examined and compared to the locations and distribution of AABD cell bodies and labeled dendrites. Analyses revealed that along the entire rostrocaudal extent of the AABD, RNm efferents terminate primarily lateral to, or in the lateral aspects of, labeled motoneurons. For the rostral AABD, RNm efferents terminate only lateral to the nucleus. Although the terminals are not positioned to contact cell bodies of the AABD, they could overlap with dendrites that extend in the lateral direction. RNm efferents terminate more extensively within the posterior AABD, overlapping within both dendritic and cell body regions of the nucleus. Even in this posterior region, however, RNm efferents were distributed primarily over the lateral half of the nucleus. These data show that RNm can monosynaptically influence the AABD, through primarily its lateral and posterior aspects. Our findings also show that a major target of RNm efferents is the reticular cell population located lateral to the AABD, suggesting that the RNm also may affect AABD motoneuronal output indirectly through its projection to reticular cells.

Distribution of cholinergic neurons in cell group K of the rabbit brainstem

The cell bodies of efferent neurons supplying the masseter and digastric muscles of the rabbit are located in two brainstem nuclei: the trigeminal motor nucleus and cell group k. The latter also contains neurons innervating muscles of the middle ear and Eustachian tube, as well as neurons that project to the cerebellum and the oculomotor complex. As part of an attempt to identify the functional subpopulations within the three cell divisions (kl-k3) that make up cell group k, we have investigated the distribution of neurons containing choline acetyltransferase, because these are likely to be motoneurons. Five rabbits anaesthetized with sodium pentobarbital (90 mg/kg, i.v.) were used in this study. They were perfused with 4% paraformaldehyde and 0.1% glutaraldehyde in phosphate buffer (0.1 M, pH 7.4). Two animals were used for preliminary studies. In the other three cases, serial Vibratome coronal sections of the brainstem were cut at 50 microm and two series of alternating sections were collected. The first was stained with a monoclonal antibody (code AB8, Incstar) directed against choline acetyltransferase, using the avidin-biotin-peroxidase method. The other was stained with Cresyl Violet. Cell counts and three-dimensional reconstructions were made for both series to determine positions and ratios of cholinergic and non-cholinergic neurons within the trigeminal motor nucleus and the subdivisions of cell group k. The results showed that the numbers of choline acetyltransferase- and Nissl-stained neurons within the trigeminal motor nucleus were almost identical. In cell group k, significantly fewer choline acetyltransferase-stained cells were counted in all three animals (ratios of choline acetyltransferase/Nissl=0.53-0.71). In addition, the distribution of cholinergic neurons was not uniform throughout cell group k. Subdivisions kl and k3 contained proportionately fewer choline acetyltransferase-positive cells (ratios of choline acetyltransferase/Nissl=0.23-0.64) than did k2 (ratios choline acetyltransferase/ Nissl=0.75-0.88). Within each subdivision, there were significant differences in the spatial coordinates of Nissl- and choline acetyltransferase-positive neurons. We conclude that cell group k contains at least two populations of neurons which are unevenly distributed between and within the three subdivisions. While the majority of neurons in subgroup k2 contain choline acetyltransferase and thus are likely to be motoneurons, more than half of the neurons in subgroups k1 and k3 are not cholinergic. It remains to be determined whether these are the neurons that project to the cerebellum and to other CNS regions.

The projections of the lateral reticular nucleus to the deep cerebellar nuclei. An experimental analysis in the rat

The projections of the lateral reticular nucleus (LRN) to the cerebellar nuclei were studied using the retrograde axonal transport of tetramethyl rhodamine dextran amine (10% solution in 0.01 M neutral phosphate buffer) in 19 adult Wistar strain rats. The cerebellar nuclei receive topographically organized projections from the LRN. The projections are bilateral with an ipsilateral predominance and they are symmetrical. The contralateral component is progressively larger for projections to the nuclei interpositalis, to the nucleus lateralis and to the nucleus medialis. The projections to the various cerebellar nuclei arise from rostrocaudally oriented columns of neurons located in different (partly overlapping) areas of the magnocellular division of the LRN. The nucleus lateralis receives terminals from the dorsomedial area (mainly from the rostral level of the LRN), the nuclei interpositalis from the dorsolateral area (mainly from the central level) and the nucleus medialis from the intermedioventral area (mainly from the caudal level). Afferent fibres from the small subtrigeminal division were traced to the three cerebellar nuclei and from the parvocellular division to the nuclei interpositalis and medialis. The density of the projections from the LRN to the nuclei interpositalis increases progressively with the shift of the terminal field from the rostrolateral to the caudomedial part of the nucleus. The projections to the nucleus lateralis reach principally the dorsolateral hump, whereas only a few neurons project to the other divisions (parvo- and magnocellular). The projections to the various regions of the nucleus medialis show different densities. The highest density was found for projections to the caudal part, in particular to the dorsolateral protuberance and to the ventrolateral area of the middle division. Conversely, a low density of projections was found for the other areas of the middle division. The regions of the magnocellular division of the LRN which project to the nuclei lateralis (and are thus related to the cerebral cortex), interpositalis (related to the red nucleus) and medialis (related to the spinal cord) also receive afferent terminals from the cerebral cortex, the red nucleus and the spinal cord respectively, in addition to various afferent inputs. Thus, each of these areas is apparently concerned with integrating some spinal and supraspinal information in reverberating circuits.

Afferent organization of the lateral reticular nucleus in the rat: an anterograde tracing study

The organization of the afferent projections to the lateral reticular nucleus of the rat was investigated following placement of horseradish peroxidase-conjugated wheatgerm agglutinin into the red nucleus, fastigial nucleus, various levels of the spinal cord or the sensorimotor area of the cerebral cortex. The pattern of distribution of anterogradely labelled profiles visualized with tetramethylbenzidine revealed that the caudal three-fourths of the lateral reticular nucleus received a large, topographically organized projection from the entire length of the contralateral spinal cord. The lateral part of the rostral half of the lateral reticular nucleus received a small projection from the contralateral red nucleus, the dorsal part of the middle third of the nucleus received a diffuse projection from the contralateral fastigial nucleus, and the extreme rostromedial part of the nucleus received a sparse projection from the contralateral cerebral cortex. The dorsal part of the middle third of the lateral reticular nucleus also received a small projection from the ipsilateral cervical spinal cord. The distribution of afferent fibres from different levels of the spinal cord, red nucleus, and fastigial nucleus overlapped substantially in the middle third of the lateral reticular nucleus, whereas the cerebral cortical receiving area was separate. These data suggest that the middle third of the lateral reticular nucleus integrates spinal and supraspinal impulses to the cerebellum, while the rostral part of the nucleus is involved in a separate cerebral cortico-cerebellar pathway.

Single-unit activity in red nucleus during the classically conditioned rabbit nictitating membrane response

Previous investigations have suggested that the cerebellum and associated brainstem structures, including the red nucleus, are essential for the expression of the classically conditioned nictitating membrane (NM) response. The present study examined the firing patterns of extracellularly-recorded single units in the red nucleus of the awake rabbit during differential conditioning. Tones were used as conditioned stimulus (CS+ and CS-) and periocular electrostimulation was used as the unconditioned stimulus (US). Most units exhibited one or more changes in firing rate during the presentation of the CS, and increases in firing were much more common than decreases. The onset of some of these changes appeared to be time-locked to the onset of the CS ('CS-locked' responses), while other changes were time-locked to the onset of the CR ('CR-locked' responses). About one-third of all CS-locked changes were CR-dependent, meaning that the neuronal response was reduced when the CR did not occur. About two-thirds of all CR-locked responses preceded the onset of the CR, and lead times varied considerably across units. Many CR-locked units were located in what has been described as a dorsal face region of the red nucleus. Most units responded to the US, and some of the US responses were CR-dependent: i.e., a smaller US response was evoked when a CR preceded the US than when the CR was absent. Our results support the notion that cerebellum-brainstem circuits are involved in generating NM CRs.

Monoaminergic, peptidergic, and cholinergic afferents to the cat facial nucleus as evidenced by a double immunostaining method with unconjugated cholera toxin as a retrograde tracer

Using a sensitive double immunostaining technique with unconjugated cholera-toxin B subunit as a retrograde tracer, the authors determined the nuclei of origin of monoaminergic, peptidergic, and cholinergic afferent projections to the cat facial nucleus (FN). The FN as a whole receives substantial afferent projections, with relative subnuclear differences, from the following areas: 1) the perioculomotor areas, the contralateral paralemniscal region, and the mesencephalic reticular formation dorsal to the red nucleus; 2) the ipsilateral parabrachial region and the nucleus reticularis pontis, pars ventralis; and 3) the nuclei reticularis parvicellularis, magnocellularis, ventralis, and dorsalis of the medulla. In addition, the present study demonstrated that the lateral portion of the FN receives specific projections from the contralateral medial and olivary pretectal nuclei and the ipsilateral reticular formation of the pons. It was also found that the FN receives: 1) serotoninergic inputs mainly from the nuclei raphe obscurus, pallidus, magnus, and the caudal ventrolateral bulbar reticular formation; 2) catecholaminergic afferent projections from the A7 noradrenaline cell group located in the Kölliker-Fuse, parabrachialis lateralis, and locus subcoeruleus nuclei; 3) methionin-enkephalin-like inputs originating in the pretectal complex, the nucleus paragigantocellularis lateralis and the caudal raphe nuclei; 4) substance P-like afferent projections mainly from the Edinger-Westphal complex and the caudal raphe nuclei; and 5) cholinergic afferents from an area located ventral to the nucleus of the solitary tract at the level of the obex. In the light of these anatomical data, the present report discusses the physiological significance of FN inputs relevant to tonic and phasic events occurring at the level of the facial musculature during the period of paradoxical sleep in the cat.

Pharmacological analysis of the magnocellular red nucleus during classical conditioning of the rabbit nictitating membrane response

Previous experiments have suggested that the red nucleus is an essential structure in the neural pathways subserving the conditioned responses (CRs) elicited in several simple associative learning paradigms. The present investigation confirms the involvement of the magnocellular red nucleus in production of the classically conditioned nictitating membrane response in the rabbit and suggests that gamma-aminobutyric acid (GABA) processes within this structure are involved in expression of the CR. Specifically, these studies demonstrate that microinfusion of a GABA antagonist (either picrotoxin or bicuculline methiodide) into the magnocellular red nucleus can selectively and reversibly reduce or abolish retention of the CR, without altering the unconditioned reflex response. Furthermore, these pharmacological manipulations that disrupt the CR are both anatomically and pharmacologically specific, and demonstrate a predictable dose-dependent function. These findings suggest that GABAergic processes within the magnocellular red nucleus are part of the critical circuitry subserving the CR.

Projections from the red nucleus and surrounding areas to the brainstem and spinal cord in the cat. An HRP and autoradiographical tracing study

HRP injections at the C2, T1 and S1 spinal levels and in the medullary lateral tegmental field revealed that the contralaterally projecting rubro-bulbospinal neurons are located not only in the caudal but also to a certain extent in the rostral red nucleus (RN). These RN projections are somatotopically organized. Neurons projecting to the sacral cord are located in the ventrolateral RN, those projecting to the upper part of the spinal cord lie in the dorsomedial RN and those projecting to the medullary lateral tegmentum were found in the dorsal portions of the RN. These last neurons are smaller than many of the other RN neurons. The HRP results also revealed that the RN does not project to the caudal raphe nuclei. The autoradiographical results confirmed the HRP findings. They further indicated that the contralateral RN projections to the caudal brainstem precerebellar nuclei (nucleus corporis pontobulbaris, lateral reticular nucleus, lateral cuneate nucleus) and the dorsal column nuclei are also somatotopically organized. This was also true for the RN projections to the dorsomedial and intermediate facial subnuclei and the caudal pontine and medullary lateral tegmental field. These areas receive afferents from mainly the dorsal portions of the RN. Regarding the RN projections to the spinal cord, the autoradiographical tracing results revealed somatotopically organized contralateral RN projections to laminae V, VI and VII. Moreover, a small but distinct RN projection to a dorsolaterally located group of motoneurons at the C8-T1 level was demonstrated. Ipsilaterally a minor projection to the cervical and upper thoracic lateral intermediate zone was observed. Finally, strong ipsilateral projections from the rostral mesencephalon to the inferior olive were seen. These projections were derived from various rostral mesencephalic areas, including the nucleus of Darkschewitsch, the nucleus accessorius medialis of Bechterew, the interstitial nucleus of Cajal and the area of the rostral interstitial nucleus of the medial longitudinal fasciculus. In the cat it was difficult to define which of the mesencephalic areas projecting to the inferior olive represented the parvocellular RN. A new subdivision of the RN is proposed based on its projections and not on the size of its cells. In this concept the first group is formed by the RN neurons projecting contralaterally to the caudal brainstem and spinal cord. The second group consists of RN neurons projecting to the inferior olive.(ABSTRACT TRUNCATED AT 400 WORDS)

Afferents to the lateral reticular nucleus from the oculomotor region. II. The oculomotor nucleus, the interstitial nucleus of Cajal and the nucleus of the posterior commissure

By means of retrograde transport of the wheat germ agglutinin-horseradish peroxidase complex, afferent fibres to the lateral reticular nucleus from the oculomotor and accessory oculomotor nuclei were demonstrated in the cat. Small iontophoretic ejections were made into the main part of the lateral reticular nucleus from a ventral approach. Significant numbers of retrogradely labelled neurons were found bilaterally in all parts of the oculomotor nucleus. The majority was of small size and distributed along the dorsal and lateral boundaries of the nucleus. Some labelled neurons were located just outside these boundaries, in the periaqueductal gray and the adjacent mesencephalic reticular formation. Retrogradely labelled neurons were also found in the accessory oculomotor nuclei: The interstitial nucleus of Cajal featured a substantial number of labelled neurons. Some labelled neurons were consistently found also in the nucleus of the posterior commissure, but no labelled neurons were found in the nucleus of Darkschewitch. The labelled neurons in the interstitial nucleus of Cajal were of different sizes and located bilaterally, mainly in its rostral part. Caudal as well as rostral parts of the main lateral reticular nucleus appear to receive the descending afferents from the oculomotor region, but higher numbers of labelled neurons were found subsequent to ejections in the rostral part. The findings are discussed and some comments are made concerning the lateral reticular nucleus as a possible relay nucleus for oculomotor input to the cerebellum.

Afferents to the lateral reticular nucleus from the oculomotor region. I. The Edinger-Westphal nucleus

By means of retrograde axonal transport of the wheat germ agglutinin-horseradish peroxidase complex, a projection from the Edinger-Westphal nucleus to the lateral reticular nucleus was demonstrated in the cat. Following small tracer ejections in the main part of the lateral reticular nucleus, a significant number of labelled neurons were found bilaterally throughout the Edinger-Westphal nucleus. Most of the labelled cells were located on the ipsilateral side. The projecting neurons are spindle-shaped to round with a maximum diameter of the cell body between 15 and 30 microns. The findings are discussed in relation to other Edinger-Westphal efferent projections, and some comments are made concerning the cytoarchitecture and delineation of the feline Edinger-Westphal nucleus.

A horseradish peroxidase study of the rubral and cortical afferents to the lateral reticular nucleus in the rat

The origin and organization of supraspinal afferents to the lateral reticular nucleus (LRN) in the rat were studied by means of the retrograde axonal transport of horseradish peroxidase (HRP). HRP was deposited into the LRN via both dorsal (stereotaxic) and ventral (microsurgical) routes. The entire cerebrum, brainstem, and cerebellum were surveyed for retrogradely labelled neurons. Significant projections arose from the contralateral red nucleus and the contralateral frontoparietal cortex. The rubral projection arose from neurons in the caudal two-thirds of the red nucleus. Ventrally and ventrolaterally located neurons projected to rostrolateral LRN, while dorsal and dorsomedial neurons projected to rostromedial LRN. The projection from the cerebral cortex arose from neurons located in layer V of the frontoparietal region. Rubral and cerebrocortical projections overlap in the rostral LRN, making this region of the nucleus a site of integration of descending inputs with ascending spinal signals.

Afferent projections to the orbicularis oculi motoneuronal cell group. An autoradiographical tracing study in the cat

The motoneurons innervating the orbicularis oculi muscle from a subgroup within the facial nucleus, called the intermediate facial subnucleus. This makes it possible to study afferents to these motoneurons by means of autoradiographical tracing techniques. Many different injections were made in the brainstem and diencephalon and the afferent projections to the intermediate facial subnucleus were studied. The results indicated that these afferents were derived from the following brainstem areas: the dorsal red nucleus and the mesencephalic tegmentum dorsal to it; the olivary pretectal nucleus and/or the nucleus of the optic tract; the dorsolateral pontine tegmentum (parabrachial nuclei and nucleus of Kölliker-Fuse) and principal trigeminal nucleus; the ventrolateral pontine tegmentum at the level of the motor trigeminal nucleus; the caudal medullary medial tegmentum; the lateral tegmentum at the level of the rostral pole of the hypoglossal nucleus and the ventral part of the trigeminal nucleus and the nucleus raphe pallidus and caudal raphe magnus including the adjoining medullary tegmentum. These latter projections probably belong to a general motoneuronal control system. The mesencephalic projections are mainly contralateral, the caudal pontine and upper medullary lateral tegmental projections are mainly ipsilateral and the caudal medullary projections are bilateral. It is suggested that the different afferent pathways subserve different functions of the orbicularis oculi motoneurons. Interneurons in the dorsolateral pontine and lateral medullary tegmentum may serve as relay for cortical and limbic influences on the orbicularis oculi musculature, while interneurons in the ventrolateral pontine and caudal medullary tegmentum may take part in the neuronal organization of the blink reflex.

Ultrastructure of spinal efferents to the lateral reticular nucleus: an EM study using anterograde transport of WGA-HRP complex

Anterograde transport of lectin-conjugated horseradish peroxidase and subsequent incubation with tetramethylbenzidine were employed to label the spinal terminals within the feline lateral reticular nucleus (NRL) for ultrastructural identification. Quantitative studies demonstrated that compared to the unlabelled terminals the spinal boutons were more than twice as large and contained fewer synaptic vesicles. Most of the synaptic contacts of the labelled terminals were located on dendritic shafts but contacts on dendritic spines as well as perikarya were also present. In four cases (all with cervical injections), the postsynaptic cells could be studied in the transversal sections of the medulla in the nuclear plane. The neurons were large and elongated with longest and shortest diameters of about 60 X 30 microns, belonging to the largest category of cells within the NRL. The observations were discussed and related to the findings made in other studies of the NRL.

The projection from the superior colliculus to the lateral reticular nucleus in the cat as studied with retrograde transport of WGA-HRP

Medium-sized and large superior collicular neurons were retrogradely labelled after small ejections of the wheat germ agglutinin-horseradish peroxidase complex in the lateral reticular nucleus of the feline medulla. The projection from the superior colliculus to the lateral reticular nucleus is bilateral with a contralateral predominance. It originates mainly from the intermediate, but also from the deep gray layer of the superior colliculus. Our observations provide evidence that the lateral reticular nucleus is an important target of tectal efferents. The findings are discussed in relation to the organization of other fiber connections of the superior colliculus.

A projection from the periaqueductal grey to the lateral reticular nucleus in the cat

A projection from the periaqueductal grey (PAG) to the lateral reticular nucleus (NRL) in the cat was demonstrated by means of retrograde transport of the wheat germ agglutinin-horseradish peroxidase complex. The connection has its main origin ipsilaterally in the ventral part of the caudal PAG, but scanty projections from other parts of the PAG were also found. The neurons projecting to the NRL are of varying shapes and sizes, but most cells have a maximum diameter of less than 20 micron. The findings are discussed in relation to the other afferent and efferent connections of the NRL.

Brachium conjuntivum and rubrobulbar tract: brain stem projections of red nucleus essential for the conditioned nictitating membrane response

The trajectory of the rubrobulbar tract in rabbit to the level of the accessory abducens nucleus is described: orthograde labeling of fibers of the rubrobulbar tract following horseradish peroxidase implants into red nucleus of 8 animals permitted ad hoc analysis of the effects of brain stem lesions on the rabbit's conditioned nictitating membrane response. Twenty-four rabbits, trained to give conditioned NM responses from both eyes, received unilateral lesions of the right pontine brain stem. Six of the 7 cases of post-lesion disruption of ipsilateral conditioned responding involved either ipsilateral brachium conjunctivum or the rubrobulbar tract. This finding, together with a reexamination of data from two related studies from this laboratory strongly support the conclusion that an essential premotor component of the conditioned NM response is a doubly decussating circuit from the interpositus nucleus of the cerebellum to magnocellular red nucleus, via brachium conjunctivum and its decussation, and from red nucleus caudally, via the ventral tegmental bundle and the rubrobulbar tract, to the accessory abducens nucleus, where motoneurons involved in the NM response are located. These findings are consistent with published reports on the essential role of interpositus nucleus of the cerebellum in NM conditioning. The possible role of the supratrigeminal reticular formation in this learned behavior is also discussed.

An ipsilateral projection from the red nucleus to the lateral reticular nucleus in the cat

Injections of the wheat germ agglutinin--horseradish peroxidase complex into the lateral reticular nucleus reveal that in addition to the well known contralateral rubroreticular connection, there is also a small but clear cut ipsilateral projection. Cells of various sizes participate in this ipsilateral pathway, and the retrogradely labelled neurons lie dispersed throughout the entire red nucleus.

Mesencephalic projections to the facial nucleus in the cat. An autoradiographical tracing study

In 33 cats the projections of different parts of the mesencephalon to the facial nucleus were studied with the aid of the autoradiographical tracing method. The results indicate the existence of many different mesencephalo-facial pathways. The dorsomedial facial subnucleus, containing motoneurons innervating ear muscles, receives afferents from 4 different mesencephalic areas: a, the most rostral mesencephalic reticular formation; b, the nucleus of Darkschewitsch and/or the ventral part of the rostral PAG; c, the interstitial nucleus of Cajal and/or the mesencephalic tegmentum dorsomedial to the red nucleus. These areas project bilaterally by way of an ipsilateral medial tegmental pathway. The medial part of the deep tectum. This area projects bilaterally by way of the tecto-spinal tract. The lateral mesencephalic tegmentum close to the parabigeminal nucleus. This area projects mainly contralaterally by way of a separate contralateral lateral tegmental fiber bundle. The mesencephalic tegmentum just dorsolateral to the red nucleus and perhaps from the dorsolateral red nucleus itself. This area projects contralaterally by way of the rubrospinal tract. The intermediate facial subnucleus containing motoneurons innervating the muscle around the eye, receives afferents from two different mesencephalic areas: The dorsal part of the rostral as well as caudal red nucleus (but not from its caudal pole) and from the dorsally adjoining mesencephalic tegmentum including the area of the nucleus of Darkschewitsch and the interstitial nucleus of Cajal. These areas project contralaterally by way of the contralateral rubrospinal tract. The nucleus of the optic tract and/or the olivary pretectal nucleus. This area projects contralaterally by way of a contralateral medial tegmental pathway. The lateral and ventrolateral facial subnuclei containing motoneurons innervating the muscles around the mouth receive afferents from two different mesencephalic areas: The lateral part of the deep tectal layers. This area projects contralaterally by way of the tecto-spinal tract. The nucleus raphe dorsalis and perhaps the nucleus centralis superior. This area projects by way of the lateral tegmentum of caudal pons and medulla.

Autoradiographic studies of the projections of the midbrain reticular formation: descending projections of nucleus cuneiformis

The descending projections of nucleus cuneiformis in the cat were traced by autoradiography in the transverse and sagittal planes following stereotaxically placed injections of 3H-leucine. Many descending axons are organized into distinct fiber systems, of which the largest and most well-defined crosses directly in the midbrain and descends through the ventromedial tegmentum of the brain stem. This fiber system first terminates profusely in n. reticularis tegmenti pontis and then proceeds through the rhombencephalic tegmentum emitting transversely oriented branches to n. reticularis pontis caudalis and gigantocellularis, the raphe magnus and the facial nucleus...

Red nucleus lesions disrupt the classically conditioned nictitating membrane response in rabbits

Sixteen rabbits received classical conditioning of the right nictitating membrane response using a tone CS and electrostimulation of the right eye as the US. Single electrocoagulating RF lesions of the medial portion of the left magnocellular red nucleus eliminated or severely reduced the previously acquired conditioned response. This finding is consistent with the idea that an essential anatomical substrate of the conditioned response includes a circuit from the cerebellum to the contralateral red nucleus which projects contralaterally in turn to pontine motoneurons mediating the defensive nictitating membrane response.

A crossed rubrobulbar projection in the snake Python regius

In the present study a distinct crossed rubrobulbar projection has been demonstrated in the snake Python regius, a limbless vertebrate which lacks a rubrospinal tract. This rubrobulbar projection is presumably involved in the neural control of mastication. The red nucleus may relay cerebellar influence to the trigeminal and facial nuclei.

A pretectofacial projection in the cat: a possible link in the visually-triggered blink reflex pathways

A direct projection from the pretectum to the facial motor nucleus was shown to exist in the cat by the anterograde and retrograde horseradish peroxidase (HRP) methods. Pretectofacial fibers arise from the olivary pretectal nucleus and end mainly in the dorsal division of the facial motor nucleus, bilaterally, with a contralateral predominance. It is known that the olivary pretectal nucleus receives retinal fibers, and that the dorsal division of the facial motor nucleus contains orbicularis oculi motoneurons. Thus, the pretectofacial fibers are assumed to cause protective lid closure with certain visual stimuli.

An HRP study of the brainstem afferents to the accessory abducens region and dorsolateral pons in rabbit: implications for the conditioned nictitating membrane response

Brain projections to the accessory abducens region and dorsolateral pons were investigated in rabbit using implants of crystalline horseradish peroxidase (HRP). Following implantation of HRP in the accessory abducens region (N = 3), labeled cells were observed in the sensory trigeminal nuclei and other regions implicated in the reflex pathway of the defensive nictitating membrane (NM) response. Neurons in the supratrigeminal zone were also labeled, as were portions of the contralateral red nucleus. Implantation of HRP into the dorsolateral pons (N = 5) revealed ipsilateral projections from deep-cerebellar nuclei in some cases. In addition, the parvocellular reticular formation displayed bilateral labeling of cells and an ipsilateral network of fibers and apparent terminations. Many cells of the contralateral supratrigeminal zone were labeled in these cases. Results were discussed in relation to lesioning and electrophysiological studies implicating the supratrigeminal region and other structures in the control of the classically conditioned NM response. Specifically, the possibility that supratrigeminal neurons are premotor elements responsible for the conditioned response is considered. Alternative hypotheses are discussed, including pathways by which cerebellar nuclei could control conditioned responding.

A supratrigeminal region implicated in the classically conditioned nictitating membrane response

The dorsolateral pontine brain stem was investigated as a possible locus of neural elements mediating the classically conditioned nictitating membrane (NM) response in rabbit. Recording and brain stimulation were employed for this purpose. Low-impedance tungsten monopolar microelectrodes were chronically implanted into the pontine brain stem. Multiple-unit recording during classical conditioning revealed a conditioned increase in multiple-unit activity (MUA) which developed and extinguished concurrently with the acquisition and extinction of the behavioral conditioned response. Pseudoconditioning and conditioned inhibition controls indicated that the increase in MUA was an associative learning phenomenon. Histology indicated that electrode tips recording the CR-associated electrical activity were located mostly adjacent or dorsal to the motor trigeminal nucleus. Periocular shock pulses elicited short latency evoked responses throughout most of the dorsolateral pons, suggesting that information concerning the unconditioned stimulus is relayed to this region. Furthermore, electrical stimulation of this region produced a robust ipsilateral nictitating membrane response in a number of cases, suggesting that neural elements of the dorsolateral pons project to the motoneurons that produce membrane extension. A consideration of several criteria based on these experiments implicates a supratrigeminal zone [22] as containing the neural elements of dorsolateral pons most intimately associated with the conditioned NM response. Other interpretations, concerning fibers of passage through this region and

Midbrain nuclei projecting to the medial medulla oblongata in the monkey

To identify the midbrain nuclei that project to the medial part of the lower brainstem in the monkey, labeled cells were mapped in the midbrain following the injection of horseradish peroxidase into the medial medulla oblongata. After the general distribution of labeled cells was observed in three animals with large injections, more discrete injections of HRP were made in different locations in six additional animals. The small injections were centered in the nucleus raphe magnus, nucleus reticularis gigantocellularis, or nucleus medullae oblongatae centralis. The five labeled midbrain nuclei were the periaqueductal gray, nucleus cuneiformis, deep layers of the superior colliculus, nucleus of Darkschewitsch, and the interstitial nucleus of Cajal. In addition, the parvocellular division of the red nucleus and the posterior pretectal nucleus contained large numbers of cells when the injection spread into the inferior olive. No major differences in the distribution of labeled cells between different injection sites were found with the exception that the superior colliculus did not contain any labeled cells when the injection was restricted to midline structures. The functional implications of these anatomical findings are discussed in relation to the descending control of pain.

An electron microscopic study of the afferent connections of the lateral reticular nucleus of the cat

The mode and pattern of termination of the afferents to the lateral reticular nucleus (LRN) of the cat were examined at the cellular level through the ultrastructural localization of induced degeneration. Examination of the LRN following hemicordotomy at the fifth and sixth cervical levels revealed that most of the degenerating terminals were in contact with intermediate and distal dendrites, and that most of these degenerating terminals were small and contained round vesicles. Fewer degenerating terminals were observed on the somata and proximal dendrites after spinal hemisection, and most of these terminals were large and contained round vesicles. Following lesions of the pericruciate cortex, small degenerating terminals were occasionally observed making contact onto intermediate and distal dendrites. Degenerating rubral terminals were observed synapsing on somata, somatic and dendritic spines, proximal dendrites and most commonly on intermediate and distal dendrites following lesioning of the red nucleus. The degenerating axosomatic rubro-LRN terminals belonged to the large, round-vesicle terminal population, while those degenerating terminals contacting intermediate and distal dendrites belonged to the small, round-vesicle population. Small, degenerating terminals were occasionally seen following lesions of the fastigial nucleus, and they made synaptic contact mainly onto intermediate and distal dendrites and dendritic spines. The present ultrastructural observations taken together with the convergence pattern of LRN afferents and the available electrophysiological data on inputs to the LRN suggest an extensive integration of converging impulses from two or more afferent sources to the rostral LRN neurons. The results of this study therefore support the veiw that the rostral LRN functions as a comparator of command signals from the motor cortex and red nucleus and feedback signals from the spinal cord and cerebellum during ongoing movement.

Anatomical evidence for direct fiber projections from the cerebellar nucleus interpositus to rubrospinal neurons. A quantitative EM study in the rat combining anterograde and retrograde intra-axonal tracing methods

A quantitative electron microscopic (EM) study combining the anterograde intra-axonal transport of radioactive amino acids and the retrograde intra-axonal transport of the enzyme horseradish peroxidase (HRP) was performed in the magnocellular red nucleus of the rat to obtain anatomical evidence as to whether there is a direct projection from the cerebellar nucleus interpositus to the cells in the red nucleus that give rise to the rubrospinal tract. Large asymmetrical synaptic terminals were radioactively labeled in the magnocellular red nucleus following injections of [3H]leucine into the cerebellar nucleus interpositus. In these same animals, the postsynaptic target neurons were labeled with HRP granules after injection of this substance in the rubrospinal tract. A quantitative analysis showed that more than 85% of the large and giant neurons in the magnocellular red nucleus were labeled with HRP granules and also received synaptic contacts from radioactively-labeled terminals. Thus, it can be concluded that in the rat, afferents from the cerebellar nucleus interpositus establish asymmetrical synaptic contacts with large and giant rubrospinal neurons, thus confirming and extending the previous physiological evidence of such direct monosynaptic connections.

Origin, course and terminations of the rubrospinal tract in the pigeon (Columba livia)

The red nucleus and its spinal projections in the pigeon (Columba livia) have been studied using both normal and experimental material. The cytoarchitecture of the nucleus is described on the basis of Nissl-stained sections and reveals an organization generally similar to that of mammals. The large neurons (40-50 mum) tend to be located dorsomedially and ventrolaterally at more caudal nuclear levels, while the small- and medium-sized neurons (15-35 mum) predominate at rostral levels. However, neurons of all sizes are present throughout the nucleus. Following lesions of the nucleus, the course of degenerating axons stained with the Fink-Heimer method has been traced throughout the brainstem and spinal cord. The rubrospinal tract crosses the midline, courses past the ventrocaudal aspect of the contralateral nucleus ruber, and then descends rostro-ventral and lateral to the nucleus tegmenti pontinus. In its caudal continuation the tract lies ventral to the brachium conjunctivum and the entering radix of the trigeminal nerve. It then assumes a ventrolateral position in the caudal brainstem before shifting to a dorsolateral position in the lateral funiculus of the spinal cord. Within the spinal grey the rubrospinal tract terminates in laminae V, VI and to a lesser extent VII. The possibility of a topographical organization of the nucleus was investigated with injections of horseradish peroxidase into brachial, thoracic and lumbar spinal cord. Regardless of the level of injection, labelled neurons of all sizes were present throughout the contralateral nucleus ruber, indicating the absence of an obvious topography.

The cerebellar projection from the lateral reticular nucleus as studied with retrograde transport of horseradish peroxidase

The cerebellar projection from the lateral reticular nucleus (NRL) was studied in cats by means of retrograde axonal transport of horseradish peroxidase (the projection to the paramedian lobule was not included, see Brodal, 1975, for afferents to this cortical region). The entire cerebellar cortex and all cerebellar nuclei receive fibres from the NRL. The strongest connection is with the anterior lobe and lobulus VIIIB of the posterior lobe vermis. As concerns the anterior lobe the observations confirm the previous finding by Brodal (1975) that there is a clearcut topical pattern in the nuclear projection to this part of the cerebellum. The observations furthermore show that crus II is the only cerebellar region devoid of fibres from the subtrigeminal part of the NRL. The cerebellar projection from the NRL is bilateral with a heavy ipsilateral preponderance. The large majority of the labeled cells within the NRL are of the small category (less than 25 micrometer in size). This and the other findings are discussed in relation to previous studies on the efferent and afferent connections of the nucleus.

The nucleus corporis pontobulbaris of the North American opossum

The nucleus of the pontobulbar body (PBu) in the North American opossum is located, for the most part, adjacent to the motor root of the trigeminal nerve. Material prepared by degeneration and autoradiographic methods shows that the PBu receives projections from the facial motor-sensory cortex, red nucleus, spinal cord and cerebellum. The latter fibers probably take origin within the fastigial nucleus. Each of the afferent connections ends in a restricuted part of the PBu, but there is considerable overlap. Use of the horseradish peroxidase technique reveals that the PBu projects to the spinal cerebellum (anterior lobe, pyramis and paramedian lobules), to visual-auditory areas of the vermis and to the lobus simplex as well as to crus I and II of the hemispheres. Although there is some topography to such projections, it is not sharply defined and many regions of the PBu contain labelled neurons after injections of horseradish peroxidase into widely separate areas of the cerebellar cortex. Because of its embryogenesis and position, the PBu is often considered part of the dorsolateral basilar pons. It appears from our material, however, that the organization of PBu afferent and efferent connections is different from that of the adjacent basilar pons, and arguments for considering the PBu a separate precerebellar nucleus are presented.

Demonstration of a somatotopically organized projection onto the paramedian lobule and the anterior lobe from the lateral reticular nucleus: an experimental study with the horseradish peroxidase method

Using the retrograde axonal transport of horseradish peroxidase, the projection from the lateral reticular nucleus (NRL) to the cerebellar anterior lobe and paramedian lobule has been studied in 13 cats. Both the anterior lobe and the paramedian lobule receive a somatotopically organized projection from the NRL. The projection to the paramedian lobule is nearly exclusively ipsilateral and originates mainly in the dorsal part of NRL, while the projection to the anterior lobe is bilateral (with ipsilateral predominance) and takes origin from all parts of the NRL. The lateral part of the NRL (closely coinciding with the parvocellular nucleus) projects to the rostral part of the anterior lobe and caudal parts of the paramedian lobule (both representing the hindlimb), while the medial part of the NRL (magnocellular nucleus) projects to the caudal parts of the anterior lobe and rostral parts of the paramedian lobule (representing the forelimb). The small subtrigeminal nucleus projects onto the anterior lobe as well as the paramedian lobule, but apparently mainly to their forelimb areas.

An electron microscope study of synaptic organization in the lateral reticular nucleus of the medulla oblongata in the cat

Synaptic organization in the lateral reticular nucleus (LRN) was investigated electron microscopically in the cat. The number of synaptic knobs encountered in a survey of 69 somatic profiles cut through the nucleolar plane varied from 0 to 20 per profile. In 5564 mum of cell perimeters analyzed the number of synaptic knobs per 100 mu was 5.2, ranging from 0 to 22 in each somatic profile. About 46% of the axosomatic synaptic knobs were filled with round vesicles, and 54% with pleomorphic ones. Out of 1424 axodendritic synaptic knobs, about 63% were filled with round vesicles, and 37% with pleomorphic ones. After placing lesions in the spinal cord, frontal cortices or red nuclear areas, electron-dense degenerated synaptic knobs were observed in the LRN. In the cats with spinal lesions degenerated knobs with the active zones of the synapses were found both on somatic and dendritic profiles. On the other hand, in the cats with cortical or rubral lesions degenerated knobs with the active zones were encountered only upon dendritic profiles. These degenerated knobs found in the present study never exceeded more than 3% of the total synaptic population in the areas examined. Thus, the axon terminals of fibers arising from the spinal cord, frontal cortices and red nuclear areas constituted only a small fraction of the total axonal endings in the LRN.