Cortical, tectal and medullary descending pathways to the cervical spinal cord

Visual information increases the indirect corticospinal excitation via cervical interneurons in humans

Modulatory actions of inputs from the visual system to cervical interneurons (IN) for arm muscle control are poorly understood in humans. In the present study, we examined whether visual stimulation modulates the excitation of cervical IN systems mediating corticospinal tract (CST) inputs to biceps brachii (BB). Twenty-eight healthy volunteers were seated, and electromyogram recordings from the BB were performed across six experiments, each with discrete objectives. A flash stimulator for visual stimulation (50-μs duration) was placed 60 cm from the participant's eye. The CST was stimulated with transcranial magnetic/electrical stimulation (TMS/TES, respectively) contralateral to the recording site. Visual stimulation with TMS/TES was randomly delivered during weak tonic BB contractions. Single TMS/TES-induced motor-evoked potentials (MEPs) were markedly enhanced from 60-100 ms after visual stimulation compared with the control condition. The MEPs were significantly increased by combining the electrical stimulation of the ulnar nerve at the wrist [7.5-12 ms of nerve stimulation (NERVE)/TMS interval] with and without visual stimulation compared with the algebraic summation of responses obtained with either TMS or NERVE. Interestingly, the combined stimulation-induced MEP facilitation was significantly increased after visual stimulation compared with the control. Single motor unit (MU) recording also revealed the further enhancement of combined stimulation effects on the firing probabilities of MU during visual stimulation, which was observed in the peaks of the peristimulus time histogram, 1-2 ms later than the onset latency. The present findings suggest that visual stimulation facilitates the oligosynaptic CST excitation of arm motoneurons mediated by the cervical IN system.NEW & NOTEWORTHY To date, little is known about how visual information modulates the human cervical motor systems, including the presumed interneuron (IN) circuitry. This study demonstrates that photic visual stimulation influences presumed oligosynaptic corticospinal transmission to arm motoneurons, which are mediated by cervical INs. In animals, these systems are known to be crucial for visually guided switching movements, and similar visual input systems to INs may exist in humans.

Integration of Descending Command Systems for the Generation of Context-Specific Locomotor Behaviors

Over the past decade there has been a renaissance in our understanding of spinal cord circuits; new technologies are beginning to provide key insights into descending circuits which project onto spinal cord central pattern generators. By integrating work from both the locomotor and animal behavioral fields, we can now examine context-specific control of locomotion, with an emphasis on descending modulation arising from various regions of the brainstem. Here we examine approach and avoidance behaviors and the circuits that lead to the production and arrest of locomotion.

Transitional Nerve: A New and Original Classification of a Peripheral Nerve Supported by the Nature of the Accessory Nerve (CN XI)

Classically, the accessory nerve is described as having a cranial and a spinal root. Textbooks are inconsistent with regard to the modality of the spinal root of the accessory nerve. Some authors report the spinal root as general somatic efferent (GSE), while others list a special visceral efferent (SVE) modality. We investigated the comparative, anatomical, embryological, and molecular literature to determine which modality of the accessory nerve was accurate and why a discrepancy exists. We traced the origin of the incongruity to the writings of early comparative anatomists who believed the accessory nerve was either branchial or somatic depending on the origin of its target musculature. Both theories were supported entirely by empirical observations of anatomical and embryological dissections. We find ample evidence including very recent molecular experiments to show the cranial and spinal root are separate entities. Furthermore, we determined the modality of the spinal root is neither GSE or SVE, but a unique peripheral nerve with a distinct modality. We propose a new classification of the accessory nerve as a transitional nerve, which demonstrates characteristics of both spinal and cranial nerves.

Topographic organization of excitatory and inhibitory commissural connections in the superior colliculi and their functional roles in saccade generation

Our electrophysiological study showed that there are topographic connections between excitatory and inhibitory commissural neurons (CNs) in one superior colliculus (SC) and tectoreticular neurons (TRNs) in the opposite SC. To obtain morphological evidence for these topographic commissural connections between the SCs, tracers were injected into various parts of the SC, the inhibitory burst neuron (IBN) area and Forel's field H (FFH), in the cat. Retrogradely labeled CNs were classified into three types according to their somatic areas and identified as GABA-positive or -negative immunohistochemically. Caudal SC injections labeled small GABA-positive CNs (<200 μm(2)) in the deep layers of the opposite rostral SC. Rostral SC injections mainly labeled medium-sized GABA-negative CNs (200-700 μm(2)) in the upper intermediate layer of the opposite rostral SC and small GABA-positive CNs in its deeper layers. Lateral SC injections labeled small GABA-positive CNs in the opposite medial SC and mainly medium-sized GABA-negative CNs in its lateral part. Medial SC injections labeled small GABA-positive CNs in the lateral SC and medium-sized GABA-negative CNs in the medial SC. In comparison, TRNs projecting to the FFH or IBN region were large (>700 μm(2)) and medium-sized. Many of the medium-sized GABA-negative CNs were TRNs projecting to the FFH. These results indicate that mirror-symmetric excitatory pathways link medial to medial (upper field) and lateral to lateral (lower field) parts of the SCs, whereas upper and lower field representations are linked by reciprocal inhibitory pathways in the tectal commissure. These connections presumably play important roles in conjugate upward and downward vertical saccades.

Brainstem control of head movements during orienting; organization of the premotor circuits

When an object appears in the visual field, animals orient their head, eyes, and body toward it in a well-coordinated manner (orienting movement). The head movement is a major portion of the orienting movement. Interest in the neural control of head movements in the monkey and human have increased in the 1990's, however, fundamental knowledge about the neural circuits controlling the orienting head movement continues to be based on a large number of experimental studies performed in the cat. Thus, it is crucial now to summarize information that has been clarified in the cat for further advancement in understanding the neural control of head movements in different animal species. The superior colliculus (SC) has been identified as the primary brainstem center controlling the orienting. Its output signal is transmitted to neck motoneurons via two major separate pathways: one through the reticulospinal neurons (RSNs) in the pons and medulla and the other through neurons in Forel's field H (FFH) in the mesodiencephalic junction. The tecto-reticulo-spinal pathway controls orienting chiefly in the horizontal direction, while the tecto-FFH-spinal pathway controls orienting in the vertical direction. In each pathway, a subgroup of neurons functions as premotor neurons for both extraocular and neck motoneurons, while others are specified for each, which allows both coordinated and separate control of eye and head movements. Head movements almost always produce shifts in the center of gravity that might cause postural disturbances. The postural equilibrium may be maintained by transmitting the orienting command to the limb segments via descending axons of the reticulospinal and long propriospinal neurons. The SC and brainstem relay neurons receive descending inputs from higher order structures such as the cerebral cortex, cerebellum, and basal ganglia. These inputs may serve context-dependent control of orienting by modulating the activities of the primary brainstem pathways.

Electrophysiological and neuropharmacological study of tectoreticular pathways in lampreys

Tectoreticular (TR) cells along the diencephalic-mesencephalic border are the origin of prominent crossed and uncrossed pathways that project to the middle (MRRN) and posterior (PRRN) rhombencephalic reticular nuclei in juvenile and adult lampreys [I.C. Zompa, R. Dubuc, Diencephalic and mesencephalic projections to rhombencephalic reticular nuclei in lampreys, Brain Res. (1998) in press.]. This study investigated the synaptic contacts between TR axons and the reticular cells. Intracellular recordings were carried out in reticular neurones (n=124) while microstimulating the TR regions. Tectoreticular inputs were recorded in all reticular cells studied (248 PSPs); although stronger responses were evoked in the MRRN neurones. The majority of responses were excitatory, but increasingly mixed and inhibitory when recorded in the middle and caudal part of the reticular nuclei. The excitation had the shortest onset latencies and sharpest slopes measured in both reticular nuclei, while the inhibition was longer and smoother. The characteristics of TR inputs to different reticular cell types is also presented. The transmission of evoked responses was isolated to the crossed and uncrossed TR pathways by studying the effects of 1% Xylocaine ejections and surgical lesions. The TR inputs were transmitted to reticular cells through monosynaptic and polysynaptic contacts. The synaptic transmission involved excitatory amino acids, acting through AMPA and NMDA receptors, while the inhibition was glycinergic. Comparisons with other sensory systems in lampreys are discussed.

Efferents from the lateral frontal cortex to spinomedullary target areas, trigeminal nuclei, and spinally projecting brainstem regions in the hedgehog tenrec

This study was done in the Madagascan lesser hedgehog tenrec, an insectivore with a very poorly differentiated neocortex. The cortical region, known to give rise to spinal projections, was injected with tracer, and the cortical efferents to brainstem and spinal cord were analyzed. Bulbar reticular fields, in addition, were identified according to their cells of origin and the laterality of their spinal projections after injection of tracer. Only few cortical fibers could be traced from the bulbar pyramid into the ipsilateral spinal cord, particularly to the lateral funiculus. The projections to the dorsal column nuclei and the classical spinally projecting brainstem regions were also weak. Faint projections were demonstrated to the nucleus of the posterior commissure and the nucleus of Darkschewitsch. In comparison to other mammals, there was no evidence that the contralateral cortico-bulbo-spinal pathway was strengthened, substituting for the almost non-existent contralateral corticospinal projection. Unlike the sensorimotor apparatus controlling limb and body movements, the brainstem regions controlling the head and neck received prominent cortical projections. Direct corticotrigeminal projections and indirect pathways were well represented. The projections to the trigeminal nuclei and the lateral reticular fields were clearly bilateral; those to the superior colliculus were predominantly ipsilateral. The corticobulbar fibers left the pyramid along its entire extent; the principal trigeminal nucleus and the dorsolateral pontine tegmentum were supplied by additional fibers of the corticotegmental tract. The lateral frontal cortex also projected densely to the dorsolateral hypothalamus, the periaqueductal gray, and the adjacent mesencephalic tegmentum, components of the emotional motor system.

Morphology of single axons of tectospinal neurons in the upper cervical spinal cord

Morphology of single axons of tectospinal (TS) neurons was investigated by intraaxonal injection of horseradish peroxidase (HRP) at the upper cervical spinal cord of the cat. TS axons were electrophysiologically identified by their direct responses to stimulation of the contralateral superior colliculus (SC). None of these axons responded to thoracic stimulation at Th2. Three-dimensional reconstructions of the axonal trajectories were made from 20 well-stained TS axons at C1-C3. Cell bodies of these axons were located in the intermediate or deep layers of the caudal two-thirds of the SC. Usually, TS axons had multiple axon collaterals, and up to seven collaterals were given off per stem axon [2.7 +/- 1.6 (mean +/- S.D.); n = 20]. Collaterals had simple structures and ramified a few times mainly in the transverse plane. The number of terminals for each collateral was small. These collaterals terminated in the lateral parts of laminae V-IX, mainly in laminae VI, VII, and VIII. There were usually gaps free from terminal arborizations between adjacent collaterals, because the rostrocaudal spread of each collateral (mean = 700 microns) was narrower than the intercollateral interval (mean = 2,500 microns). Seven of the 19 TS axons had terminals in the lateral parts of laminae V-VIII, with little projection to lamina IX, and the other 12 axons had terminals in lamina IX besides the projection to the lateral parts of laminae V-VIII. Axon terminals in lamina IX did not appear to make contacts with the somata or proximal dendrites of retrogradely labeled motoneurons, but contacts were found with the somata of counterstained interneurons in the lateral parts of laminae V-VIII. Three spinal interneurons (two in lamina VIII and one in lamina V at C1) that received monosynaptic excitation from the SC were stained, and their axonal trajectories were reconstructed. They had multiple axon collaterals at C1-C2 and mainly projected to laminae VIII and IX, with smaller projections to lamina VII. Many axon terminals of the interneurons were found in multiple neck motor nuclei, where some of them made contacts with retrogradely labeled motoneurons. The present finding provides evidence that the direct TS projection to the spinal cord may influence activities of multiple neck muscles, mainly via spinal interneurons, and may play an important role in control of head movement in parallel with the tectoreticulospinal system.

Coordination of gaze shifts in primates: brainstem inputs to neck and extraocular motoneuron pools

To determine whether there are brainstem regions that provide common input to the motoneurons that move both the head and the eyes, we injected wheat germ agglutinin-horseradish peroxidase complex (WGA-HRP) into neck motoneuron pools at spinal level C2 (N = 3) and extraocular motoneuron pools in the abducens (N = 1) and oculomotor/trochlear (N = 1) nuclei of rhesus and fascicularis macaques. We also injected WGA-HRP into spinal level C5-7 (N = 1) of a fascicularis macaque for comparison. After injections into C2, we observed retrogradely labeled cells in the ventral reticular formation (NRV), the gigantocellular reticular formation (NRG), and both the oral (NRPO) and the caudal (NRPC) divisions of the paramedian pontine reticular formation (PPRF). There was also a column of labeled cells in the cuneate reticular nucleus (NCUN) just lateral to the ipsilateral periaqueductal gray (PAG). This column extended rostrally into the central mesencephalic reticular formation (CMRF). In addition, there were labeled cells in the region ventral and caudal to the rostral interstitial nucleus of the MLF (riMLF), the area lateral to the interstitial nucleus of Cajal (INC), and the ventral part of the lateral vestibular nucleus (LVN) and lateral part of the medial vestibular nucleus (MVN). There were also a few labeled cells in the fastigial (FN) and interposed (IN) nuclei of the cerebellum but very few in the superior colliculus (SC). In contrast, the injection into C5-7 labeled many cells in the lateral vestibular nucleus (LVN) and very few in FN or IN. Injecting WGA-HRP into the abducens nucleus and the surrounding tissue labeled many cells in SC, PPRF, MVN, FN, and nucleus prepositus hypoglossi (NPH). Injecting into the oculomotor/trochlear nuclei and nearby tissue labeled cells in SC, INC, riMLF, FN, IN, MVN, and superior vestibular nucleus (SVN). Structures that project to both neck and eye motoneuron pools, and therefore probably participate in both head and eye movements, include the lateral part of the MVN and both NRPO and NRPC in the PPRF. Those that project primarily to neck motoneurons in C2 include the NRV, the NRG, and the NCUN-CMRF column. Those projecting exclusively to extraocular nuclei include the NPH, INC, riMLF, NRPD, and SC. We use these data to propose a scheme for control of combined eye-head movements in monkeys.

Immunocytochemical mapping of noradrenergic projections to the rat spinal cord with an antiserum against noradrenaline

The mapping of noradrenergic innervation was performed in transverse and longitudinal sections of the adult rat spinal cord using noradrenaline immunocytochemistry. Noradrenergic fibres and terminals were distributed in the dorsal horn (mainly in the superficial part), in the vicinity of the different groups of motoneurons, and concentrated in the intermediolateral cell column and around the central canal. The ultrastructural study showed principally axodendritic synapses in the ventral horn and in the intermediolateral cell column. Fewer axosomatic synapses were detected. In the dorsal horn, noradrenaline-innervation was predominantly non-synaptic. It is hypothesized that the noradrenergic modulation of nociception is not mediated through classical synapses. The concept of 'volume transmission' can explain such an influence. Conversely, noradrenaline may be involved in the control of locomotion and automatic functions through conventional synapses.

Eye-head coupling in humans. II. Phasic components

A tonic coupling between the horizontal component of eye position and dorsal neck muscle activity has been demonstrated in animals and humans. In addition, a transient saccade related coupling has been found in animals. In order to investigate such a phasic component of the eye-head synergy in humans, we have recorded the activity of isolated motor units in the splenius muscle during large horizontal eye movements in head fixed subjects. Eye movement recording was achieved by conventional binocular electro-oculography and the activity of the right splenius muscle was recorded with Bronks coaxial electrodes inserted manually at the C4-C5 intervertebral level. We found two main types of motor unit discharge patterns in the splenius (SPMU), the first type (type A, 14 SPMUs) shows a phasic modulation of firing rate during saccades with a triphasic profile composed of a pre-saccadic suppression, a per-saccadic burst and a post saccadic tonic discharge proportional to eye position. The second type (type B, 6 SPMUs) exhibits little, if any, modulation of firing rate with either fixation or saccades. These results suggest that eye-head coupling is present not only during the fixation period but also during saccades and that a phasic activity or suppression related to saccadic eye velocity is present in dorsal neck muscle EMG.

Direct catecholaminergic innervation of primate spinothalamic tract neurons

Catecholaminergic axonal varicosities identified by immunocytochemical staining for dopamine-beta-hydroxylase were observed at the light microscopic level apposing the somata of retrogradely labeled spinothalamic tract neurons in the monkey spinal cord. Three retrogradely labeled and two intracellularly labeled spinothalamic neurons were serially sectioned and examined at selected intervals at the electron microscopic level. Electron microscopic study revealed that axonal boutons directly contacted the somata and/or dendrites of lamina I, IV, and V spinothalamic tract neurons. All of the profiles apposing one of the retrogradely labeled lamina I spinothalamic tract neurons were categorized from eight planes of section spaced at 1-micron intervals. Of the 305 profiles counted that were adjacent to this soma, 17 (5.6%) stained positively for dopamine-beta-hydroxylase. Of these 17 appositions, three were followed in serial sections to confirm that they had synaptic thickenings and alignment of vesicles along the membrane contacting the spinothalamic tract soma. Catecholaminergic boutons were observed apposing the somata and dendrites of intracellularly filled STT cells characterized as high threshold and wide dynamic range neurons. These observations clearly indicate a direct innervation of spinothalamic tract neurons by catecholaminergic neurons, providing anatomical data to support previous physiological findings demonstrating that catecholamines modulate nociceptive transmission.

Brainstem afferents to the oculomotor omnipause neurons in monkey

To determine how saccade-related areas in the brainstem address the saccade generator, we examined the afferents to the nucleus raphe interpositus. This region contains the omnipause neurons, which are pivotal in the generation of saccades. Horseradish peroxidase injected iontophoretically into the nucleus raphe interpositus retrogradely labeled a variety of brainstem nuclei. The greatest numbers of labeled neurons were in the paramedian pontomedullary reticular formation, in the nuclei reticularis gigantocellularis, and paragigantocellularis lateralis. Labeling was more modest but consistent in the interstitial nucleus of Cajal and the adjacent mesencephalic reticular formation, the middle gray of the superior colliculi, the region dorsolateral to the nucleus reticularis tegmenti pontis, and the medial vestibular nucleus. A few neurons were labeled around the habenulopeduncular tract and in the medial portion of the nucleus of the fields of Forel, in the nucleus reticularis medullaris ventralis, and in the spinal nucleus of the trigeminal nerve, the cochlear nucleus, and the superior olivary complex. The distribution and density of labeling suggest that omnipause neurons in the monkey are more intimately connected with other oculomotor structures than those in the cat. In addition, the rhombencephalic reticular afferents to the monkey omnipause neurons are more concentrated in their immediate vicinity than in the cat. The label consistently found dorsolateral to the nucleus reticularis tegmenti pontis may be a newly discovered link in saccade generation.

Descending pathways to the spinal cord: II. Quantitative study of the tectospinal tract in 23 mammals

To study the early evolution of the mammalian motor systems, we have collected quantitative data on the nuclear origins of tracts descending into the spinal cord in 99 individuals representing 23 species of mammals and one species of reptile. In each individual, the spinal cord was hemisected at the C1-C2 junction and raw HRP immediately applied to the cut fibers. After a 3-day survival period, brain and spinal cord sections were treated with conventional tetramethylbenzidine procedures. In every case, this procedure resulted in heavy retrograde labeling of neural somata throughout the neuraxis from coccygeal cord to cerebral neocortex. Many thousands of supraspinal neurons were vividly labeled within at least 27 discrete cell groups in every mammal (Nudo and Masterton, '88). Despite the vast number and wide diversity of heavily labeled neurons, however, relatively few labeled somata were found in the superior colliculus. The total number of labeled cells in the tectum contralateral to the hemisection was highest in the cat (909) and second highest in the raccoon (628). In the remaining animals, the number was considerably less--averaging only 243 in the 23 mammalian species, 193 in the 21 noncarnivores, and 95 in the iguana. In 7 species of primates the average was 220, and in 3 species of Old World monkeys the average was 142. This wide variation in the number of tectospinal neurons is not related to body size, brain size, or absolute and relative tectum size. Arranging the animals in order of their kinship or recency-of-last-common-ancestor with Man, the average number of labeled tectal cells tends to decrease slightly, whereas arranging the same animals in order of their kinship with the cat or raccoon shows a marked and statistically reliable increase. Neither the evolutionary increase in the tectospinal tract along the Carnivora lineage nor the slight decrease along Man's lineage is altered by mathematical corrections for allometric or scaling factors. Of an array of morphological, visual, motor, and ecological traits tested statistically as a possible source of the variation in size of the tectospinal tract, only a primarily carnivorous feeding preference was found to be reliably related. The relatively small number of tectospinal fibers in most mammals in our sample, including the primates, suggests that the tectospinal tract in Man may be quite small, perhaps far too small to warrant continuing description as a "major descending tract."

Projections from the rostral mesencephalic reticular formation to the spinal cord. An HRP and autoradiographical tracing study in the cat

Eye and head movements are strongly interconnected, because they both play an important role in accurately determining the direction of the visual field. The rostral brainstem includes two areas which contain neurons that participate in the control of both movement and position of the head and eyes. These regions are the caudal third of Field H of Forel, including the rostral interstitial nucleus of the medial longitudinal fasciculus (riMLF) and the interstitial nucleus of Cajal with adjacent reticular formation (INC-RF). Lesions in the caudal Field H of Forel in monkey and man result in vertical gaze paralysis. Head tilt to the opposite side and inability to maintain vertical eye position follow lesions in the INC-RF in cat and monkey. Projections from these areas to extraocular motoneurons has previously been observed. We reported a study of the location of neurons in Field H of Forel and INC-RF that project to spinal cord in cat. The distribution of these fiber projections to the spinal cord are described. The results indicate that: 1. Unlike the neurons projecting to the extra-ocular muscle motoneurons, the major portion of the spinally projecting neurons are not located in the riMLF or INC proper but in adjacent areas, i.e. the ventral and lateral parts of the caudal third of the Field H of Forel and in the INC-RF. A few neurons were also found in the nucleus of the posterior commissure and ventrally adjoining reticular formation. 2. Neurons in caudal Field H of Forel project, via the ventral part of the ventral funiculus, to the lateral part of the upper cervical ventral horn. This area includes the laterally located motoneuronal cell groups, innervating cleidomastoid, clavotrapezius and splenius motoneurons. At lower cervical levels labeled fibers are distributed to the medial part of the ventral horn. Projections from the caudal Field H of Forel to thoracic or more caudal spinal levels are sparse. 3. Neurons in the INC-RF, together with a few neurons in the area of the nucleus of the posterior commissure, project bilaterally to the medial part of the upper cervical ventral horn, via the dorsal part of the ventral funiculus. This area includes motoneurons innervating prevertebral flexor muscles and some of the motoneurons of the biventer cervicis and complexus muscles. Further caudally, labeled fibers are distributed to the medial part of the ventral horn (laminae VIII and adjoining VII) similar to the projections of Field H of Forel. A few INC-RF projections were observed to low thoracic and lumbosacral levels.(ABSTRACT TRUNCATED AT 400 WORDS)

Descending pathways to the spinal cord: a comparative study of 22 mammals

In order to estimate the qualitative commonalities and range of variation among major descending spinal pathways relevant to mankind's ancestral lineage, the supraspinal cell groups originating fibers descending directly to the spinal cord were examined in 22 mammalian species. In a standardized retrograde tract-tracing procedure, flakes of raw HRP were applied directly to the freshly cut fibers of the spinal cord after it had been hemisected at the C1-C2 junction. After a 72-hour survival period, brain and spinal cord tissues were processed by conventional HRP-processing techniques. This procedure was performed on 94 individual animals. Of this total, 41 individual cases were eliminated by a rigorous culling procedure. The results are based on 53 individuals representing 15 species selected for their successive kinship with mankind and seven species in two other lineages selected for the convergence of their visual or sensorimotor systems with anthropoids. The 22 species represent 19 genera, 14 families, eight orders, and two subclasses of Mammalia. The results show that at least 27 supraspinal cell groups, each containing intensely labeled cells, can be readily identified in each of the species. Despite vast quantitative differences in cell number and cell size, this qualitative uniformity among the relatively large number of diverse taxa suggests that the same pathways were probably present in the extinct ancestors throughout mankind's mammalian lineage and are probably still present in extant viviparous mammals as well. If so, these pathways are as old in phylogenetic history as the last common ancestor of marsupial and placental mammals--dating from the late Jurassic to early Cretaceous, perhaps 145-120 million years ago. Further comparison of the results with similar experimental findings in members of other vertebrate classes supports the notion that several of these same pathways can be traced to even more remote ancestry, with some possibly as old as the entire vertebrate subphylum--dating from the early Devonian or before, perhaps 430 million years ago. Within mankind's ancestral lineage, from the appearance of vertebrates to the appearance of mammals, there seems to have been an irregular stepwise augmentation of the set of descending pathways until the full mammalian complement was finally attained with the appearance of the corticospinal tract.

Ultrastructural study of large efferent neurons in the superior colliculus of the cat after retrograde labeling with horseradish peroxidase

The ultrastructure of large neurons in the stratum griseum intermedium of the cat superior colliculus was examined following injections of horseradish peroxidase (HRP) into the dorsal tegmental decussation. Four HRP-labeled cells were selected, and the synaptology of their cell bodies and selected regions of proximal and distal dendrites was examined. The four neurons represent four morphologically distinct cell types: multipolar radiating, tufted, large vertical, and medium-sized trapezoid radiating. These four neurons correspond with cell types X1, X2, X3, and T1 respectively, according to the recent classification of neurons in the superior colliculus of the cat by Moschovakis and Karabelas (J. Comp Neurol. 239:276-308, '85). The three X type neurons are similar in having 83% of their somata and over 74% of their proximal dendrites contacted by synaptic profiles. Distal dendrites of the X type neurons, however, receive fewer synaptic contacts. In contrast, in the T1 cell, only 69% of the soma membrane is contacted by synaptic profiles, and the synaptic coverage on proximal and distal dendrites does not vary much from this. Of the eight types of synaptic terminals described in the stratum griseum intermedium of the cat superior colliculus by Norita (J. Comp. Neurol. 190:29-48, '80), only five are found in contact with the X and T type efferent neurons described here. There are some regional differences in terminal distribution, although each terminal is represented on each cell. Type III terminals (small, contain mostly pleomorphic vesicles, and make symmetrical contacts) are the most abundant on cell bodies and dendrites of all four cell types. Terminal types II (medium-sized, containing round and flattened vesicles, and making asymmetrical contacts), and IV (medium to large in size, containing flattened vesicles, and making symmetrical contacts) are well represented. In general, terminal types I (small, containing densely packed round vesicles, and making asymmetrical contacts) and VI (small and irregular in shape, containing flattened vesicles and making symmetrical contacts) are found infrequently. The identity of different types of synaptic terminal is discussed.

Comparative topography of projections from the mesodiencephalic junction to the inferior olive, vestibular nuclei, and upper cervical cord in the cat

Distributions of neurons located in the central rostral mesencephalon and caudal diencephalon that project to the upper cervical spinal cord, vestibular nuclei, or inferior olive were studied in the cat by using retrograde axonal transport of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP). Afferent sources to all of these targets were observed in the interstitial nucleus of Cajal (INC), the region surrounding the fasciculus retroflexus (PF), and the nucleus of the fields of Forel (NFF). Three-dimensional reconstruction revealed differences in densities of cells projecting from these common areas. Spinal projecting cells were present in slightly greater numbers in the caudal two-thirds of the INC, whereas those projecting to the vestibular complex were more numerous in the rostral two-thirds of this nucleus. A relatively smaller number of olivary projecting cells were dispersed throughout the INC. Olivary afferent sources outnumber those with spinally directed or vestibularly directed axons in the PF region. In the fields of Forel, cells projecting to the vestibular nuclei or inferior olive were concentrated medially, whereas cells projecting to the spinal cord appeared both medially and laterally. Each type of afferent source was also seen in the nucleus of the posterior commissure and the posterior ventral lateral hypothalamic area. Unique sources of afferents to the inferior olive were observed in the parvicellular red nucleus (ipsilateral to the injections) and the anterior and posterior pretectal nuclei. A large number of labeled neurons was seen in the nucleus of Darkschewitsch after injections of tracer into the inferior olive, but this projection did not appear to be unique, as small numbers of labeled cells were also seen after injections into the cervical spinal cord. The Edinger-Westphal nucleus and the adjacent somatic oculomotor nucleus contained cells which projected separately to the spinal cord or the vestibular complex, and the superior colliculus contained cells which projected separately to the contralateral spinal cord or the contralateral inferior olive. In this study, it was also noted that neurons in the medial terminal nucleus of the accessory optic tract projected to the ipsilateral inferior olive or to the contralateral vestibular complex. These differences in locations and densities of cells projecting to the cervical spinal cord, vestibular complex, and inferior olive may underlie functional specializations in these areas in relation to vertical eye and head movement control and to neural systems controlling postural adjustments accompanying limb movements.

Somatosensory projections to the superior colliculus of the anaesthetized cat

1. Experiments in anaesthetized cats have shown that the superior colliculus receives deep afferent input from the forelimb and hindlimb, but not from the large superficial neck muscles. 2. Neuronal activity in the superior colliculus is readily elicited by electrical stimulation of C2 and C3 cutaneous nerves. A significant proportion of neurones so activated have multiple receptive fields and some with no identifiable receptive fields in regions innervated by C2 and C3 nerves have receptive fields elsewhere on the body surface. Many collicular neurones activated by C2 and C3 stimulation had no identifiable receptive fields. 3. Natural stimuli to the limbs, hitherto believed to activate only cutaneous receptors, are sufficient to activate deep receptors which contribute to the neuronal responses in the superior colliculus elicited by the natural stimulus. These same natural stimuli set up transmitted vibration adequate to excite receptors some distance from the applied stimulus. 4. No evidence was found for a rigorous somatotopy in the superior colliculus. The great majority of neurones received trigeminal input which is widely distributed throughout the superior colliculus. 5. Tactile stimuli to the face are most effective in eliciting unit activity in the superior colliculus and many neurones activated by these stimuli were shown to be tectospinal neurones. In particular, the specialized receptors of the face, including the glabrous skin of the snout (the planum nasale) and the vibrissae, are major sources of input to collicular neurones including tectospinal neurones. 6. It is suggested that a major role of the superior colliculus is in the organization of head movements associated with the use of the specialized receptor organs of the face in exploratory behaviours. The superior colliculus may also be involved in the organization of aversion movements of the head.

Limb specific connections of the cat magnocellular red nucleus

Afferent and efferent connections of the limb specific divisions of the cat magnocellular red nucleus (RNm) were traced using the bidirectional transport of wheatgerm agglutinin-horseradish peroxidase complex (WGA-HRP). Injection sites within forelimb or hindlimb RNm regions were identified by microelectrode recording and confirmed by the position of labeled rubrospinal terminals. Additional injections into structures that project to, or receive input from, RNm confirmed the somatotopic organization of these pathways. The forelimb region of RNm receives input from the posteriolateral part of the anterior interpositus nucleus (NIA) and the intermediate part of the posterior interpositus nucleus (NIP). The hindlimb region of RNm receives input from anteriomedial NIA and medial NIP. Terminals of NIA cells densely fill all of RNm, but terminals of NIP cells form a half shell on the medial, ventral, and posterior borders of RNm without encroaching on RNm's lateral edge or central core. Forelimb and hindlimb RNm are reciprocally connected with the caudal cuneate and gracile nuclei respectively. There is little or no input to RNm from the medial or lateral cerebellar nuclei. Forelimb RNm, which also contains a face representation, projects to the lateral reticular nucleus, cell group f of the inferior vestibular nucleus, the facial nucleus, the main sensory nucleus of the trigeminal nerve, the caudal cuneate nucleus, the parvicellular reticular formation, and cervical segments of the spinal cord. A few fibers from forelimb RNm project directly to motor neurons in the lower cervical cord. Hindlimb RNm projects to only the lateral reticular nucleus, gracile nucleus, and lower spinal segments. Forelimb and hindlimb RNm project to different regions of the lateral reticular nucleus with some overlap.

Anatomical organization of cortico-mesencephalo-olivary pathways in the cat as demonstrated by axonal transport techniques

Cortical projections arising from areas 4 and 6 and terminating in midbrain cell groups known to project to the inferior olive (IO) have been studied in the cat. Injections of the bidirectional tracers horseradish peroxidase (HRP) and wheat germ agglutinin (WGA) conjugated to HRP were made into the midbrain. All cases of lateralized midbrain injections resulted in virtually ipsilateral labelling of lamina V cortical neurons. Retrogradely labelled neurons in cortical areas 4 and 6 were found after injections located in the interstitial nucleus of Cajal (INC), nucleus of Darkschewitsch (ND), and in the caudal parafascicular (Pf) and subparafascicular (sPf) nuclei (perifascicular region, PF). Injections that were more caudal and within the parvi- and magnocellular red nucleus (RNp and RNm) labelled cells not only in areas 4 and 6 but also in portions of adjacent areas 3a, 3b, 5a, and 7. These midbrain injections also resulted in the anterograde labelling of projections observed to terminate in the ipsilateral IO. The distribution in the midbrain of projections arising from cortical areas 4 and 6, and portions of areas 3a and 3b, was studied with autoradiographic methods. After injections of tritium-labelled amino acids in those cortical areas, a pattern of largely ipsilateral terminations was revealed. Whereas all cortical areas studied labelled the PF, differential grain distributions in central mesencephalic nuclei were apparent after injections in various portions of the motor and adjacent somatosensory cortex. Injections involving the frontal eye fields (FEF) labelled the INC bilaterally, but ipsilateral terminations were densest. These cases also labelled the region of the fields of Forel. When the neck region of the cortex was involved in the injections, the more caudal aspects of the INC (INCc) and the RN were labelled. The cortical areas related to the upper limb gave rise to terminations in the ND and the RNp. Contributions to both ND and RNp inputs from injections in the FEF and neck regions were also occasionally but not consistently noted. A relatively discrete injection in the vibrissae field weakly labelled ND. Additional components of the motor cortical projections to the superior colliculus (SC) and pretectal nuclei were also analysed since those regions also project to the IO. Cortical regions involving the representation of the neck musculature were shown to project principally ipsilaterally to lamina IV of the SC as well as to the anterior pretectal nucleus.(ABSTRACT TRUNCATED AT 400 WORDS)

A chronic unit study of the sensory properties of neurons in the forelimb areas of rat sensorimotor cortex

The sensory properties of neurons in the several forelimb areas of rat sensorimotor cortex were examined using the technique of extracellular single-unit recording in the awake, head-restrained rat. Cells with peripheral receptive fields were tested for the amount and modality of sensory input during joint manipulation and brushing and tapping of limbs, face and trunk. Input-output correlations were made on the basis of the results of receptive field mapping and intracortical microstimulation in the same electrode penetration. It was found that neurons (n = 117) in the rostral forelimb area receive virtually no sensory input while 30% of neurons (n = 114) in the caudal forelimb primary motor area do receive such input. The inputs to caudal forelimb motor area neurons were primarily (83%) from single joints; along perpendicular electrode penetrations the same joint that activated a cortical cell also moved when microstimulation was delivered along the same electrode penetration. In the granular and dysgranular zones of somatic sensory forelimb cortex, 70% of neurons (n = 82) were responsive to peripheral sensory inputs, with most of the cells in the granular cortex responsive to cutaneous inputs while cells in the dysgranular cortex were more responsive to deep inputs. The lack of sensory inputs to the rostral forelimb motor area is consistent with the proposal that this region may be a part of the supplementary motor area of the rat.

Bulbospinal projections in the primate: a light and electron microscopic study of a pain modulating system

The projections of the nucleus raphe magnus (NRM) and the immediately adjacent reticular formation were studied in the macaque monkey following injections of the rostroventral medulla with 3H-leucine and examination of the resultant labeled axons and terminals by light and electron microscopic autoradiography. Five monkeys had accurately placed injections, which resulted in fiber pathway labeling that coursed caudally, laterally, and dorsally to project to laminae I, II, and V of subnucleus caudalis of the trigeminal and then traveled in the dorsolateral funiculus of the cord and terminated in similar laminae of the spinal dorsal horn at cervical levels. The pathway was only lightly labeled caudal to the cervical enlargement and could not be readily discerned above background in the thoracic or lumbar cord. Electron microscopy revealed that axons and terminals serving this system constitute a heterogeneous population. Large-diameter myelinated axons (3-6-micron diameter), small myelinated axons (0.75-3-micron diameter), and clusters of nonmyelinated axons were labeled. Terminals in laminae I, II, and V contained mixtures of clear round and granular vesicles or clear pleomorphic and granular vesicles or formed the central element in synaptic glomeruli. The labeled profiles formed asymmetrical or symmetrical synapses on medium and small dendrites; labeled axosomatic synapses were not observed. In rare instances there were contacts between labeled profiles and vesicle-containing structures, which were probably dendritic, but whether the NRM axon was pre- or postsynaptic to such structures could not be determined. It was concluded that the NRM in the monkey is organized in a manner quite similar to that previously described in the cat. The wide variety of fiber types and synaptic terminals serving this system suggests that different classes of neurons participate in it, probably using several transmitter substances that result in varying postsynaptic effects on neurons located in the trigeminal complex and dorsal horn.

Retrograde labeling of neurons in the brain stem following injections of [3H]choline into the rat spinal cord

In an attempt to identify cholinergic neurons in the brain stem which project to the spinal cord, [3H]choline (100, 20, 10, 5 or 1 microCi) was injected into the upper cervical spinal cord in 55 rats. The animals were killed 20 h later and the brains processed for autoradiography of diffusible substances. At all doses of [3H]choline, cells were consistently, retrogradely labeled in the medical medullary reticular formation, the lateral vestibular nucleus, the dorsolateral pontine tegmentum and the red nucleus. The retrogradely labeled cells were found to be moderately to darkly stained for acetylcholinesterase. Injection of [3H]noradrenaline (50 microCi) into the upper cervical spinal cord resulted in retrograde labeling of cells in the locus coeruleus, subcoeruleus and the ventrolateral pontine tegmentum, that correspond in position to the neurons of the A6, A7 and A5 catecholamine cell groups, respectively. Injection of [3H]serotonin (20 microCi) into the upper cervical spinal cord was associated with retrograde labeling of cells in the raphe pallidus, obscurus and magnus nuclei that correspond in position to those of the B1, B2 and B3 serotonin cell groups, respectively. Injection of True Blue into the upper cervical spinal cord was followed by retrograde labeling of a large number of cells located in the areas where cells were retrogradely labeled by [3H]choline, [3H]noradrenaline and [3H]serotonin, and additionally, in the solitary tract nucleus, the lateral, parvicellular medullary reticular formation, the caudal and oral pontine reticular formation, the mesencephalic reticular formation and the superior colliculus. These results indicate that from the cervical spinal cord, [3H]choline selectively retrogradely labels a certain population of non-monaminergic, acetylcholinesterase-positive cells localized in the medial medullary, and secondarily the dorsolateral pontine, reticular formation, the lateral vestibular nucleus, and the red nucleus.

Complementary and non-matching afferent compartments in the cat's superior colliculus: innervation of the acetylcholinesterase-poor domain of the intermediate gray layer

Three tectal afferent-fiber systems were experimentally labeled in the cat to learn how their distributions within the superior colliculus were related to the prominent compartments of high acetylcholinesterase activity found in the intermediate gray layer. Presumptive somatic sensory afferents were labeled by injections of horseradish peroxidase-wheatgerm agglutinin conjugate placed at the bulbospinal junction and in the ventral anterior ectosylvian cortex corresponding to somatic sensory area SIV. Vision-related afferents were labeled by injections of the same tracer substance into the lateral suprasylvian visual area. In each animal, a single type of injection was made and a detailed study was carried out to compare the patterns of anterograde labeling and acetylcholinesterase staining in serially adjoining sections through the superior colliculus. Fibers labeled by the three types of injection were distributed in clusters that resembled the acetylcholinesterase-positive patches in the intermediate gray layer. In no case, however, were the afferent-fiber clusters in register with the histochemically defined patches. Instead, the innervations derived from the bulbospinal junction, anterior estosylvian sulcus and lateral suprasylvian visual area all formed patchworks within the acetylcholinesterase-poor domain of the intermediate gray layer. In some instances, the afferent-fiber clusters and enzyme-positive patches appeared to have complementary distributions. In other instances, the afferent-fiber clusters seemed to be arranged in the acetylcholinesterase-poor parts of the intermediate layer in a fashion independent of, but not significantly overlapping, the acetylcholinesterase-positive patches. Not all of the space between the acetylcholinesterase-positive patches was taken up by any one of the afferent-fiber systems labeled. The complementary and non-matching distribution of these afferent systems in relation to the acetylcholinesterase-rich patches of the intermediate gray layer stands in contrast to the spatial registration of two other tectal afferent systems with the zones of high acetylcholinesterase activity. Both nigrotectal and frontotectal afferents converge on the acetylcholinesterase-positive patches. We conclude that afferent systems projecting to the intermediate gray layer can be divided into at least two groups: those innervating the acetylcholinesterase-rich compartments and those avoiding them.(ABSTRACT TRUNCATED AT 400 WORDS)

Laminar distribution and patchiness of cytochrome oxidase in mouse superior colliculus

The cytochrome oxidase (CO), acetylcholinesterase (AChE), myelin, and Nissl stains were studied and compared to develop an anatomical system identifying the laminar architecture of the mouse superior colliculus. The CO and myelin stains are shown to define collicular laminae more distinctly than does the Nissl stain. The layer of large rostrocaudally coursing fiber bundles that has formerly been referred to in the rodent literature as stratum album intermediale (SAI; layer V) is renamed as a sublayer of the stratum griseum intermediale (SGI; layer IV) to conform with the nomenclature for the cat superior colliculus of Kaneseki and Sprague ('74, J. Comp. Neurol. 158:319-338). Patches of CO activity in layer IV (SGI) are shown that contain intensely stained, large, multipolar cell bodies. The CO patches do not correspond to those previously reported for AChE. The CO, myelin, and AChE stains all indicate the presence of a large lateral extension termed the flank of layer IV (SGI). In contrast to the classical lamination pattern of the superior colliculus, the flank has no overlying layer II (stratum griseum superficiale, SGS) or layer III (stratum opticum, SO).

Hypothalamic and ventral thalamic projections to the superior colliculus in the cat

The present report describes the organization of collicular afferents that arise within either the hypothalamus or the ventral thalamus. Following the placement of large injections of WGA-HRP into the superior colliculus of the cat, retrogradely labeled neurons are located within the reticular nucleus of the thalamus, the zona incerta, the fields of Forel, and throughout the hypothalamus. Although the dorsal hypothalamic area contains the largest number of labeled hypothalamic neurons, labeled cells are also found within the periventricular, paraventricular, dorsomedial, ventromedial, posterior, lateral, and anterior hypothalamic nuclei. A strikingly similar pattern of distribution of labeled neurons is also observed following placement of small injections of WGA-HRP that are restricted within the stratum griseum intermedium (SGI). In contrast, hypothalamic and ventral thalamic labeling is not seen after placement of injections within the stratum griseum superficiale. Following the placement of injections of tritiated anterograde tracers within the dorsal hypothalamic area, transported label is organized in two bands of clusters over the SGI. When injections of tritiated tracers are placed within the zona incerta, terminal label is also located over the SGI; however, the distribution of silver grains does not appear as clusters or distinct puffs. On the basis of the comparison of the cellular types that give rise to these projections and the differences in terminal distribution, we suggest that the hypothalamic and ventral thalamic projections to the superior colliculus are totally separate and unrelated pathways. The functional implications of the hypothalamotectal pathway are also discussed.

The cerebellar and vestibular nuclear complexes in the turtle. I. Projections to mesencephalon, rhombencephalon, and spinal cord

Cerebellar and vestibular projections were investigated in the turtle Pseudemys scripta elegans following injection of 35S-methionine into the cerebellar and vestibular nuclear complexes at various locations. Fibers arising from the cerebellar nuclei were traced via the cerebellar commissure to the contralateral vestibular nuclear complex (particularly the n. vestibularis inferior and n. vestibularis ventrolateralis) and caudal rhombencephalic tegmentum. Ascending projections crossing the midline in the ventral isthmomesencephalic tegmentum terminated in the contralateral red nucleus and nuclei of the fasciculus longitudinalis medialis (f lm). Vestibular projections ascending mainly via the f lm terminated in the nuclei of the f lm, the nuclei of the posterior commissure, and particularly the extraocular motor nuclei. Vestibulo-ocular projections arising from the rostral vestibular nuclear complex were almost exclusively ipsilateral; those from the caudal vestibular nuclear complex were bilateral. Evidence for a topographic organization of the projections to the trochlear and oculomotor nuclei was also obtained. There were some vestibular projections to the contralateral rhombencephalic tegmentum and n. vestibularis inferior. Spinal projections coursing within the ipsilateral ventral descending tract and the ipsilateral fasciculus longitudinalis medialis were found to arise from both rostral and caudal vestibular regions. The caudal vestibular nuclear complex in addition gave rise to fibers descending in the contralateral fasciculus longitudinalis medialis. Evidence for the existence of labeled fibers crossing at spinal levels was also obtained. Vestibulospinal terminations appeared restricted to the ventral horn.

Observations on the somatodendritic morphology and axonal trajectory of intracellularly HRP-labeled efferent neurons located in the deeper layers of the superior colliculus of the cat

Efferent neurons of the deeper layers of the cat's superior colliculus were stained with horseradish peroxidase (HRP) to demonstrate patterns of somatodendritic morphology and axonal trajectory. A combination of somatodendritic and axonal features of the HRP-labeled cells revealed the existence of three major groups of tectal efferent neurons (X, T, and I). X neurons are mostly large and multipolar and participate in the crossed descending and ipsilateral ventral ascending projections of the superior colliculus. The X group includes multipolar radiating (X1), tufted (X2), large vertical (X3), medium-sized vertical (X4), and medium-sized horizontal (X5) neurons. T neurons participate in one or two of the major tectofugal bundles (medial descending ipsilateral, lateral descending ipsilateral, medial dorsal ascending, crossed descending) besides providing a commissural branch. They also issue recurrent collaterals distributed within a more or less restricted area of the deeper layers. The T group includes medium-sized, trapezoid, radiating (T1) and small or medium-sized, ovoid, vertical (T2) neurons. I neurons participate in the ipsilateral descending projection of the superior colliculus. They are small, triangular or ovoid, sparsely ramified cells that provide long, varicose collaterals irregularly distributed within the deeper layers. The majority of T neurons are located in the ventral stratum opticum or dorsal stratum griseum intermediale; X3 and X5 neurons are situated immediately below in the dorsal stratum griseum intermediale, while X1, X2, X4, and I neurons are indiscriminately distributed within the deeper layers. The polythetic classification presented here provides a conceptual framework for the description of tectal efferent neurons. It is open-ended and can thereby accommodate new cells types as indicated by the disclosure of a small horizontal (A) and a small radiating (unclassified) neuron. Moreover, it does not preclude the construction of alternate taxonomies. A dendro-architectonic classification into four groups [vertical (X3, X4, T2, I), horizontal (X5, A), radiating (X1, T1, I), and tufted (X2)] can be made and would relate to the mode of integration of various tectopetal inputs. A classification based on the dorsoventral location of tectal efferent neurons is also possible and would relate to the dorsoventral distribution of neurons with specific response properties.

Anatomical connections of the nucleus prepositus of the cat

The afferent and efferent connections of the nucleus prepositus hypoglossi with brainstem nuclei were studied using anterograde and retrograde axonal transport techniques, and by intracellular recordings and injections of horseradish peroxidase into prepositus hypoglossi neurons. The results of experiments in which horseradish peroxidase was injected into the prepositus hypoglossi suggest that the major inputs to the prepositus hypoglossi arise from the ipsi- and contralateral perihypoglossal nuclei (particularly the prepositus hypoglossi and intercalatus), vestibular nuclei (particularly the medial, inferior, and ventrolateral nuclei), the paramedian medullary and pontine reticular formation, and from the cerebellar cortex (flocculus, paraflocculus, and crus I; the nodulus was not available for study). Regions containing fewer labeled cells included the interstitial n. of Cajal, the rostral interstitial n. of the medial longitudinal fasciculus, the n. of the posterior commissure, the superior colliculus, the n. of the optic tract, the extraocular motor nuclei, the spinal trigeminal n., and the central cervical n. The efferent connections of the prepositus hypoglossi were studied by injecting 3H-leucine into the prepositus hypoglossi, and by following the axons of intracellularly injected prepositus hypoglossi neurons. The results suggest that in addition to the cerebellar cortex, the most important extrinsic targets of prepositus hypoglossi efferents are the vestibular nuclei (particularly the medial, inferior, and ventrolateral nuclei, and the area X), the inferior olive (contralateral dorsal cap of Kooy and ipsilateral subnucleus b of the medial accessory olive), the paramedian medullary and pontine reticular formation, the reticular formation surrounding the parabigeminal n., the contralateral superior colliculus and pretectum, the extraocular motor nuclei (particularly the contralateral abducens nucleus and the ipsilateral medial rectus subdivision of the oculomotor nucleus), the ventral lateral geniculate n., and the central lateral thalamic nucleus. Other areas which were lightly labeled in the autoradiographic experiments were the contralateral spinal trigeminal n., the n. raphe pontis, the Edinger Westphal n., the zona incerta, and the paracentral thalamic n. Many of the efferent connections of the prepositus hypoglossi appear to arise from principal prepositus hypoglossi neurons whose axons collateralize extensively in the brainstem. On the other hand, small prepositus hypoglossi neurons project to the inferior olive, and multidendritic neurons project to the cerebellar flocculus, apparently without collateralizing in the brainstem.(ABSTRACT TRUNCATED AT 400 WORDS)

Pyramidal effects in dorsal neck motoneurones of the cat

The effects of contralateral pyramidal stimulation have been investigated with intracellular recording from cat alpha-motoneurones that innervate the dorsal neck musculature. A short train of stimuli evoked three types of synaptic effects: predominant excitation or inhibition and mixed effects characterized chiefly by early excitation followed by inhibition. Latency measurements indicated a minimal disynaptic linkage for excitation and for inhibition. Splenius motoneurones received primarily excitation whereas biventer cervicis-complexus motoneurones received a more varied input characterized by mixed effects or inhibition. Following transection of the pyramid just rostral to the decussation (lower pyramidal lesion) pyramidal stimulation above the lesion still produced disynaptic excitation and longer latency (possibly trisynaptic) inhibition. Pyramidal stimulation just caudal to this transection evoked inhibition with a minimal disynaptic latency, as well as longer latency excitation. The incidence of longer latency excitation was found to be reduced in cats with corticospinal tract transections at the level of the second cervical spinal segment. No post-synaptic potentials were evoked by pyramidal stimulation rostral to a pyramidal transection at the level of the trapezoid body. It is suggested that disynaptic excitation evoked by pyramidal stimulation above the lower pyramidal lesion is mediated by medullary reticulospinal neurones possessing monosynaptic excitatory connexions with neck motoneurones. Longer latency excitation appears to be mediated by neurones that receive corticospinal tract input and are located in the spinal segments containing the neck motoneurones. Disynaptic inhibition is mediated by neurones likely to be situated between the second cervical spinal segment and the level of the lower pyramidal lesion. The results also suggest that the first neurone in the chain mediating longer latency inhibition is located in the brain stem. The differences in pyramidal synaptic input between splenius and biventer cervicis-complexus motoneurones are considered in relation to the roles these muscles may serve in head position control.

An EM-autoradiographic and EM-HRP study of the commissural projection of the superior colliculus in the cat

Terminals of the commissural projection in the cat were characterized ultrastructurally by autoradiographic and horseradish peroxidase methods. The results of the two studies are complementary. Terminals of commissural cells are present in the intermediate and deep layers of the cat superior colliculus. Two distinct populations of terminals are present: one containing mostly round vesicles and forming asymmetric specializations, and a second containing mostly pleomorphic vesicles and forming symmetric specializations. Both populations contact small dendrites or dendritic appendages. The two populations, mostly round and mostly pleomorphic, are present in the ratio of 2:1. Terminals measure approximately 1.1 micron in mean diameter and contact profiles ranging in size from 0.2 to 4.6 micron. There is no significant difference between the two populations in either pre- or postsynaptic profile size. The colocalization of terminals of commissural neurons with other afferent and efferent projections of the intermediate layers of the superior colliculus is discussed.

Relationships between the nigrotectal pathway and the cells of origin of the predorsal bundle

The goal of this study was to define the anatomical relationships between the terminal field of the nigrotectal pathway and the tectal neurons which project to contralateral brainstem gaze centers by way of the predorsal bundle. The distribution and morphology of the cells of origin for the predorsal bundle were determined by using a modification of the retrograde horseradish peroxidase technique which homogeneously filled their somas and dendrites. The terminal distribution of the nigrotectal tract was determined using both anterograde horseradish peroxidase and autoradiographic procedures. The results indicate that, in the grey squirrel (Sciurus carolinensis), the predorsal bundle cells are a heterogeneous population whose dendritic fields form a well-defined band confined to the inner half of stratum griseum intermediate. This inner sublamina also can be identified in Nissl and myelin stains. The same sublamina is the major target of the nigrotectal tract. The striking anatomical correspondence between the distribution of nigrotectal terminals and the cells projecting in the predorsal bundle supports a proposal, based on recent physiological investigations, that the nigrotectal tract plays an important role in the initiation of the saccade-related activity of the deep tectal cells (Chevalier et al., '81; Hikosaka and Wurtz, '83a-d).

Reticulospinal and vestibulospinal pathways in the snake Python regius

In the present HRP study extensive reticulospinal projections and more modestly developed vestibulospinal pathways have been demonstrated in the snake Python regius. The funicular trajectories of the main reticulospinal pathways have been shown: via the lateral funiculus pass spinal projections of the nucleus reticularis superior pars lateralis, the nucleus reticularis inferior and nucleus raphes inferior; via the ventral funiculus fibers arising in the nucleus reticularis superior and nucleus reticularis medius. Spinal projections of the locus coeruleus and subcoeruleus area reach their targets via both the lateral and ventral funiculi. Two vestibulospinal pathways have been demonstrated: an ipsilateral tractus vestibulospinalis lateralis arising in the ventrolateral vestibular nucleus, and a contralateral tractus vestibulospinalis medialis from the descending and ventromedial vestibular nuclei. After HRP gel implants into the vestibular nuclear complex direct vestibulocollic projections to motoneurons in the rostral spinal cord were observed. Spinal projections from the ventral part of the nucleus reticularis inferior and the descending and ventromedial vestibular nuclei are mainly aimed at the thin "neck area" (approximately the first 50 spinal segments). This area is extensively used in such acts as orientation and prey-catching, requiring a rather delicate brain stem control.

Studies of the principal sensory and spinal trigeminal nuclei of the rat: projections to the superior colliculus, inferior olive, and cerebellum

We have analyzed the connections between the sensory trigeminal nuclei and two major sensorimotor areas (i.e., the superior colliculus and crura I and II of the cerebellar cortex) in which tactile input from peri-oral and other facial regions is a prominent feature. Following injections of horseradish peroxidase into the superior colliculus, retrogradely labeled cells occupy the ventral one-third of the contralateral principal sensory and spinal trigeminal nucleus; trigeminocollicular neurons are especially numerous within the subnucleus interpolaris (Svi). Injections of either 3H-proline or horseradish peroxidase (HRP) into the Svi reveal that trigeminocollicular axons reach the rostral two-thirds to three-quarters of the contralateral superior colliculus, where they distribute in a nonuniform, patchy manner within layers IV-VI. In addition to demonstrating the trigeminocollicular projection, anterograde and retrograde transport studies of the Svi also reveal a trigeminoolivary projection which terminates primarily within the contralateral rostral dorsal accessory (DAO) and adjacent principal (PO) olives; some of the Svi neurons innervate both the superior colliculus and the DAO-PO via axon collaterals. Data from a final set of retrograde tracing experiments show that the trigeminorecipient zone of the DAO-PO contains neurons which project to crura I and/or II of the cerebellar cortex. Of the various submodalities conveyed by the trigeminal system, it is likely that the trigeminal connections we have demonstrated are carrying tactile information. This is indicated by the fact that responses to tactile stimulation of the face have been reported for cells in (1) the deeper collicular layers, (2) the trigeminorecipient zone of the DAO-PO, and (3) cerebellar targets of this zone, crura I and II. All data are discussed in the context of the anatomical and physiological literature.

Relation between cell size and response characteristics of medullary reticulospinal neurons to labyrinth and neck inputs

The activity of presumably inhibitory reticulospinal neurons with cell bodies located in the medial aspects of the medullary reticular formation and axons projecting to lumbosacral cord has been recorded in decerebrate cats and their response characteristics to sinusoidal stimulation of labyrinth receptors (134 neurons) and neck receptors (110 neurons) have been related to cell size inferred from the conduction velocity of the corresponding axons. No significant correlation was found between resting discharge and conduction velocity of the axons. Among the recorded reticulospinal neurons, 64/134 (i.e. 47.8%) units responded to roll tilt, while 66/110 (i.e. 60.0%) units responded to neck rotation (0.026 Hz, +/- 10 degrees). A positive correlation was found between gain (imp./s/deg) of the labyrinth and neck responses and conduction velocity of the axons. Thus, due to absence of correlation between resting discharge and conduction velocity of the axons, larger neurons exhibited a greater percentage modulation (sensitivity) to the labyrinth and the neck input than smaller neurons. These findings are attributed to an overall increase in density or efficacy of the synaptic contacts made by the vestibular and neck afferent pathways on reticulospinal neurons of increasing size. Units receiving neck-macular vestibular convergence showed on the average an higher gain of the neck (GN) response with respect to the labyrinth (GL) response (GN/GL: 1.95 +/- 1.49, S.D.; n = 43); however, due to a parallel increase in gain of the reticulospinal neurons to both neck and labyrinth inputs, the relative effectiveness of the two inputs did not vary in different units as a function of cell size. The reticulospinal neurons were mainly excited by the direction of animal orientation and/or neck displacement. In particular, most of these positional sensitive units were excited by side-up animal tilt (37/58, i.e. 63.8%) and by side-down neck rotation (47/60, i.e. 78.3%). These predominant response patterns were particularly found between large size neurons, whereas small size neurons tended to show also other response patterns. The evidence indicates that in addition to intrinsic neuronal properties related to cell size, the quantitative and qualitative organization of synaptic inputs represents the critical factor controlling the responsiveness of reticulospinal neurons to vestibular and neck stimulation.

Direct projections from the cerebellar nuclei to the superior colliculus in the rabbit: an HRP study

Cerebellar projections to the superior colliculus in the rabbit were studied by the anterograde and retrograde HRP methods. Cerebellotectal fibers arise mainly from the anterior and posterior interpositus nuclei and terminate contralaterally in layer VII, layer VI, layer V, and the deep tier of layer IV of the superior colliculus. Cerebellotectal fibers from the posterior interpositus nucleus originate from the lateral part of the nucleus and end chiefly in the caudal part of the superior colliculus. Cerebellotectal fibers from the anterior interpositus nucleus arise from the ventral part of the nucleus and terminate mainly in the rostromedial part of the superior colliculus. Some neurons in the lateral cerebellar nucleus also send fibers contralaterally to the intermediate and deep layers of the superior colliculus, especially to its rostral and lateral parts. Few, if any, cerebellotectal fibers arise from the medial cerebellar nucleus.

The efferent connections of the nucleus of the optic tract and the superior colliculus in the rabbit

3H-leucine injections were made in tectal and pretectal areas in the rabbit. After injections in the nucleus of the optic tract (NOT) labeled fibers were distributed bilaterally to the superior colliculus, the dorsal part of the medial geniculate nucleus (MGd), and the pulvinar nucleus, and ipsilaterally to the external layer of the ventral lateral geniculate nucleus (LGv), the dorsal geniculate nucleus (LGd) pars beta, the reticular thalamic nucleus, and the lateral and medial terminal nucleus (LTN, MTN). Many labeled fibers were distributed to the lateral and some to the medial parts of the pontine nuclei. more caudally, coarse labeled fiber bundles descended ipsilaterally, distributing fibers to the prepositus hypoglossi and abducens nucleus and to the caudally adjoining medial reticular formation. Many labeled fibers were also present in the inferior olive, especially ipsilaterally in the dorsal cap and the ventrally adjoining pars beta, and a few in the contralateral dorsal cap area. Contralaterally, some descending fibers terminated in the dorsal part of the facial nucleus, in which motoneurons are located innervating the orbicularis oculi muscle. The superficial layers of superior colliculus distributed fibers bilaterally to the internal layer in the ventral lateral geniculate nucleus (LGv), the LGd alpha (lateral part), the MGd, the pulvinar, and more caudally to the ipsilateral parabigeminal and lateral pontine nuclei. The deep collicular layers distributed fibers ipsilaterally to MG (internal division), pulvinar, and the internal layer of LGv. Furthermore, ascending connections were found to the suprageniculate nucleus, the zona incerta, the mediodorsal nucleus, and some intralaminar and midline nuclei. Descending fibers terminated in the mesencephalic lateral tegmentum, pontine nuclei, and ventrally in the pontine and high medullary reticular formation. Contralaterally fibers were distributed to the nucleus reticularis tegmenti pontis (NRTP), the medial reticular formation, and the inferior olive just lateral to the nucleus beta. In one case fibers were also distributed to the lateral part of the contralateral facial nucleus in which motoneurons are located innervating the upper lip muscles.