Pontis Caudalis Research Articles

Opposing roles of the amygdala and dorsolateral periaqueductal gray in fear-potentiated startle

The whole-body acoustic startle response is a short-latency reflex mediated by a relatively simple neural circuit in the lower brainstem and spinal cord. The amplitude of this reflex is markedly enhanced by moderate fear levels, and less effectively increased by higher fear levels. Extensive evidence indicates that the amygdala plays a key role in the potentiation of startle by moderate fear. More recent evidence suggests that the periaqueductal gray is involved in the loss of potentiated startle at higher levels of fear. The influence of both structures may be mediated by anatomical connections with the acoustic startle circuit, perhaps at the level of the nucleus reticularis pontis caudalis. The mediated by anatomical connections with the acoustic startle circuit, perhaps at the level of the nucleus reticularis pontis caudalis. The present chapter reviews these data.

A Primary Acoustic Startle Pathway: Obligatory Role of Cochlear Root Neurons and the Nucleus Reticularis Pontis Caudalis

Davis et al. (1982) proposed a primary acoustic startle circuit in rats consisting of the auditory nerve, posteroventral cochlear nucleus, an area near the ventrolateral lemniscus (VLL), nucleus reticularis pontis caudalis (PnC), and spinal motoneurons. Using fiber-sparing lesions, the present study reevaluated these and other structures together with the role of neurons embedded in the auditory nerve [cochlear root neurons (CRNs)], recently hypothesized to be involved in acoustic startle. Small electrolytic lesions of the VLL of ventrolateral tegmental nucleus (VLTg) failed to eliminate startle. Large electrolytic lesions including the rostral ventral nucleus of the trapezoid body (rVNTB) and ventrolateral parts of PnC or lesions of the entire PnC blocked startle. However, small NMDA-induced lesions of the rVNTB failed to block startle, making it unlikely that the rVNTB itself is part of the startle pathway. In contrast, NMDA lesions of the full extension of the ventrolateral part of the PnC blocked startle completely, suggesting that the ventrolateral part of the PnC is critically involved. Bilateral kainic acid lesions of CRNs also blocked the startle reflex completely, providing the first direct evidence for an involvement of CRNs in startle. This blockade probably was not caused by damage to the auditory nerve, because the lesioned animals showed intact compound action potentials recorded from the ventral cochlear nucleus. Hence, a primary acoustic startle pathway may involve three synapses onto (1) CRNs, (2) neurons in PnC, and (3) spinal motoneurons.

Distribution of serotonin-immunoreactivity in the brain of the pigeon (Columba livia).

The distribution of serotonin (5-HT)-containing perikarya, fibers and terminals in the brain of the pigeon (Columba livia) was investigated, using immunohistochemical and immunofluorescence methods combined with retrograde axonal transport. Twenty-one different groups of 5-HT immunoreactive (IR) cells were identified, 2 of which were localized at the hypothalamic level (periventricular organ, infundibular recess) and 19 at the tegmental-mesencephalic and rhombencephalic levels. Ten of the cell groups were situated within the region of the midline from the isthmic to the posterior rhombencephalic level and constituted the raphe system (nucleus annularis, decussatio brachium conjunctivum, area ventralis, external border of the nucleus interpeduncularis, zona peri-nervus oculomotorius, zona perifasciculus longitudinalis medialis, zona inter-flm, nucleus linearis caudalis, nucleus raphe superior pars ventralis, nucleus raphe inferior). The 9 other cell populations belonged to the lateral group and extended from the posterior mesencephalic tegmentum to the caudal rhombencephalon [formatio reticularis mesencephali, nucleus ventrolateralis tegmenti, ectopic area (Ec) of the nucleus isthmo-opticus (NIO), nucleus subceruleus, nucleus ceruleus, nucleus reticularis pontis caudalis, nucleus vestibularis medialis, nucleus reticularis parvocellularis and nucleus reticularis magnocellularis]. Combining the retrograde axonal transport of rhodamine beta-isothiocyanate (RITC) after intraocular injection and immunohistofluorescence (fluoresceine isothiocyanate: FITC/5-HT) showed the centrifugal neurons (NIO, Ec) to be immunonegative. Serotonin-IR fibers and terminals were found to be very broadly distributed within the brain and were particularly prominent in several structures of the telencephalon (archistriatum pars dorsalis, nucleus taeniae, area parahippocampalis, septum), diencephalon (nuclei preopticus medianus, magnocellularis, nucleus geniculatus lateralis pars ventralis, nucleus triangularis, nucleus pretectalis), mesencephalon-rhombencephalon (superficial layers of the optic tectum, nucleus ectomamillaris, nucleus isthmo-opticus and in most of the cranial nerve nuclei). Comparing the present results with those of previous studies in birds suggests some major serotonin-containing pathways in the avian brain and clarifies the possible origin of the serotonin innervation of some parts of the brain. Moreover, comparing our results in birds with those obtained in other vertebrate species shows that the organization of the serotoninergic system in many regions of the avian brain is much like that found in reptiles and mammals.

Saccadic premotor neurons in the brainstem: functional neuroanatomy and clinical implications

Saccadic eye movements are generated by immediate premotor excitatory (EBN) and inhibitory (IBN) medium-lead burst neurons, long-lead burst neurons (LBN), and omnipause neurons (OPN) in the brainstem. The histological identification of the functional neuron populations is essential for performing any neuropathological analysis of clinical cases with saccadic disorders. This paper describes the localization of some premotor saccadic neurons in human brain and their neurochemistry, based on monkey data. In humans, the EBNs for horizontal saccades (EBNH) lie in the paramedian pontine reticular formation in the nucleus reticularis pontis caudalis (nrpc) and form a compact group of medium-sized neurons. The medium-sized IBNs lie caudal to the nucleus abducens in the nucleus paragigantocellularis dorsalis (pgd). The EBNs for vertical saccades (EBNV) are medium-sized neurons in the rostral interstitial nucleus of the medial longitudinal fasciculus (riMLF) in the rostral mesencephalon. The OPNs are located at the...

Prearcuate cortex in the cebus monkey has cortical and subcortical connections like the macaque frontal eye field and projects to fastigial-recipient oculomotor-related brainstem nuclei

The cortical and subcortical connections of the prearcuate cortex were studied in capuchin monkeys ( Cebus apella, albifrons) using the anterograde and retrograde transport capabilities of the horseradish peroxidase technique. The findings demonstrate remarkable similarities to those of the macaque frontal eye field and strongly support their homology. The report then focuses on specific prearcuate projections to oculomotor-related brainstem nuclei that were shown in a companion experiment to entertain connections with the caudal oculomotor portion of the cerebellar fastigial nucleus. The principal corticorticol connections of the cebus prearcuate cortex were with dorsomedial prefrontal cortex, lateral intraparietal sulcal cortex, posterior medial parietal cortex, and superior temporal sulcal cortex, which were for the most part reciprocal and columnar in organization. The connections of the dorsal prearcuate region were heavier to the dorsomedial prefrontal and posterior medial parietal cortices, and those of the ventral region were heavier to the superior temporal sulcal cortex. The prearcuate cortex projects to several brainstem areas which also receive projections from the caudal fastigial nucleus, including the supraoculomotor periaqueductal gray matter, superior colliculus, medial nucleus reticularis tegmenti pontis, dorsomedial basilar pontine nucleus, dorsolateral basilar pontine nucleus, nucleus reticularis pontis caudalis, pontine raphe, and nucleus prepositus hypoglossi. The findings define a neuroanatomical framework within which convergence of prearcuate (putative frontal eye field) and caudal fastigial nucleus connections might occur, facilitating their potential interaction in saccadic and smooth pursuit eye movement.

Involvement of cyclic AMP at the level of the nucleus reticularis pontis caudalis in the acoustic startle response

Rats were implanted with cannulas in the nucleus reticularis pontis caudalis (PnC), an obligatory part of the neural pathway that mediates the acoustic startle reflex. Following at least 1 week of recovery, rats were tested for acoustic startle amplitude before or after infusion of compounds known to alter the second messenger, adenosine cyclic 3′, 5′-monophosphate (cAMP). Local infusion into the PnC of the cAMP analog, 8-bromo cAMP (0.125–1.0 μg), increased the amplitude of the acoustic startle response in a dose-dependent manner. In addition, local infusion of a phosphodiesterase inhibitor, rolipram (10 μg) or the water soluble adenylate cyclase activator, forskolin-DHA (2.5 μg), produced a significant enhancement of startle amplitude. These effects probably resulted from intracellular actions because cAMP itself, which does not readily penetrate lipid membranes, had no effect. Moreover, the effects seemed somewhat specific because the precursor of cAMP, ATP or 8-bromo cGMP, also failed to alter startle at doses where 8 bromo-cAMP did. The fact that a phosphodiesterase inhibitor elevated startle suggests that cAMP serves to tonically elevate startle at this level of the pathway. Hence, treatments that either increase (fear, sensitization) or decrease (habituation, pre-pulse inhibition) startle at the level of the PnC may do so via release of neurotransmitters either positively or negatively coupled to cAMP, which in turn may alter either sound evoked transmitter release, excitability of PnC neurons or both.

Histological identification of premotor neurons for horizontal saccades in monkey and man by parvalbumin immunostaining.

The premotor excitatory and inhibitory burst neurons are essential for horizontal saccades. In the monkey, excitatory burst neurons lie in the ipsilateral paramedian pontine reticular formation, and the inhibitory burst neurons lie more caudally in the contralateral nucleus paragigantocellularis dorsalis. For a neuropathological analysis of degenerative changes in saccadic disorders of patients, the histological identification of the burst neuron areas in man is important. Here, we show that this is possible with parvalbumin immunostaining as a histological marker. First, in monkeys, the premotor burst neurons were backlabeled by injections of wheat germ agglutinin-horseradish peroxidase or cholera toxin subunit B into the abducens nucleus or tetanus toxin fragment C into the lateral rectus muscle and shown by double labeling to contain parvalbumin. Then, human brainstem sections were immunoreacted for parvalbumin, and, by comparing the resulting staining pattern to that in the monkey, the homologous burst neuron areas were defined in man. In the monkey, excitatory burst neurons were confirmed to the nucleus reticularis pontis caudalis and did not extend farther rostrally into the nucleus reticularis pontis oralis. All retrogradely labeled cells in both burst neuron areas were parvalbumin positive, and approximately 70% of the parvalbumin-positive cells were retrogradely labeled. Both burst neuron areas were highlighted by their parvalbumin staining pattern and could be outlined in man as well. The putative excitatory burst neuron area in man is in the medial part of the nucleus reticularis pontis caudalis (extending 2.5 mm mediolaterally), immediately rostral (250 microns) to the omnipause neurons and extending 2.2 mm rostrally, and the putative inhibitory burst neuron area lies in the medial part of the paragigantocellular nucleus caudal to the abducens nucleus, extending 1.8 mm caudally. The location of the burst neuron areas, including the burst neurons themselves, via parvalbumin immunostaining will help in the analysis of clinical cases with slow saccades.

Activity of neurons in the medial pontomedullary reticular formation during orienting movements in alert head-free cats

1. Single unit activities of 236 neurons were recorded in the medial pontomedullary reticular formation during visually triggered orienting gaze shifts in 10 alert cats under head-free conditions using movable tungsten-needle electrodes attached to the skull. The activities were analyzed mainly in relation to the head movement that was triggered by presentation of a light-emitting diode (LED) in one of eight directions separated radially by 45 deg after fixation of the center LED. Of these, 120 neurons were recorded in the pontine reticular formation, chiefly in the nucleus reticularis pontis caudalis, and the remaining 116 were in the medullary reticular formation, chiefly in the nucleus reticularis gigantocellularis. Activities of 65 pontine and 65 medullary neurons were modulated in relation to the dynamic phase of orienting movements ("orienting-related neurons"). Activities of the remaining neurons were modulated either irregularly or not at all during orienting movement ("irregular or no-response neurons"). Input from the contralateral superior colliculus and cerebral cortex and projections to the spinal cord were also investigated. 2. Among the orienting-related neurons, 62 pontine and 55 medullary neurons showed increases in activity preceding the onset of eye and head movement by 0-155 ms ("pretype"). Three pontine and 10 medullary neurons showed increases in activity only after the onset of movement ("posttype"). Of the pretype neurons, 61 pontine and 51 medullary neurons showed directional preference of activity ("directional" neurons). One pontine and four medullary neurons were classified as "omnidirectional" because these neurons increased activity preceding movements in all directions tested, and no directional preference was apparent. 3. In the pretype-directional cells, the average firing frequency during bursts was correlated with amplitude and angular velocity of head movements. Activities of the directional neurons during movements in the eight different directions could be well fitted with cosine functions in the majority of cases. The preferred directions of most pontine neurons and of about half the medullary neurons, as determined by first-degree sinusoidal regression analysis, were distributed around the ipsiversive horizontal axis. However, there were also a considerable number of neurons whose preferred directions were upward, downward, contraversive, or oblique in the medulla. 4. Among the directional cells preferring ipsiversive horizontal movements, 11 pontine neurons showed activity, the onset of which was locked to visual stimuli with latencies of 40-70 ms, in addition to phasic discharges locked to the onset of movement. This "stimulus-locked activity" was sometimes modulated depending on the attentional state of the animal.(ABSTRACT TRUNCATED AT 400 WORDS)

Subcortical pathways related to optokinetic nystagmus in the cat

In recent experiments, it has been disclosed that the nucleus of the optic tract (NOT) is the visut-motor relay between the retina and preoculomotor structures in the pathway mediating optokinetic nystagmus (OKN). In the present study, how OKN signals are transmitted from the NOT in the brainstem was investigated using biocytin and triticum vulgaris (Wheat Germ) peroxidase labeled (WGA-HRP) as anterograde and retrograde tracers, respectively. Following biocytin injections into the NOT, labeled fibers were observed in each of the following efferent pathways: 1) those that project to the contralateral NOT via the posterior commissure; 2) those that course through the nucleus pontis orklis to terminate in the Edinger-Westphal complex and nucleus reticularis tegmenti pontis; and 3) those that descend via the medical lemniscus to the level of the medulla to terminate in the dorsal cap of the inferior olive, during which their axons branch to the dorsolateral pontine nucleus, nucleus prepositus hypoglossi, and the nucleus pontis caudalis, superior and lateral. Furthermore, differences in the distribution of labeled cells in the NOT were observed following WGA-HRP injections into the nucleus prepositus hypoglossi and medial vestibular nucleus. The retrograde labeled cells in the NOT were distributed to the medial area at the rostral level following the tracer injections into the medial vestibular nucleus. On the other hand, labeled cells were recognized in the part of the caudal NOT following the tracer injections into the nucleus prepositus hypoglossi. A recent neurophysiological study demonstrated that areas adjacent to the medial vestibular nucleus apparently to participate in the production of OKN, because both the slow component and after-nystagmus similar to OKN and optokinetic after nystagmus were elicited by stimulation of the vestibular nuclei. The present study shows that this direct projection from the NOT to the vestibular nuclei may serve to drive velocity storage in the vestibular nuclei.

Unrestrained walking in cats with medial pontine reticular lesions

The early postoperative effects of lesions, aimed to destroy the caudal pole of the nucleus reticularis pontis oralis (NRPO) and the rostral pole of the nucleus reticularis pontis caudalis (NRPC), were tested in freely moving cats, walking at moderate speed (0.4–1.0 m/s). In cats in which these structures were partly or completely destroyed, the main effect of lesions was an impairment of fore-hindlimb coordination, as shown by a change in the relationships between the lateral and diagonal time shift durations and the step cycle duration. In the second week after the surgery the values of the slopes of linear regressions relating these variables were markedly changed as compared to the preoperative data. The results suggest that the NRPO and NRPC are involved in maintaining the proper forehindlimb coordination during unrestrained locomotion in cats.

Afferents to the nucleus reticularis parvicellularis of the cat medulla oblongata: a tract-tracing study with cholera toxin B subunit.

The aim of this study was to examine anatomical evidence in cats of whether the nucleus reticularis parvicellularis (Pc) is part of the circuit responsible for the inhibition of brainstem motoneurons during paradoxical sleep. For this purpose, we made iontophoretic injections of the retrograde and anterograde tracer cholera toxin B subunit (CTb) in the Pc. After CTb injections in the Pc, a large number of retrogradely labeled neurons were seen in the central nucleus of the amygdala, the lateral part of the bed nucleus of the stria terminalis, the posterior hypothalamic areas, the mesencephalic reticular formation, the nucleus locus subcoeruleus, the nucleus pontis caudalis, other portions of the Pc, the nucleus reticularis dorsalis, the trigeminal sensory complex, and the nucleus of the solitary tract. We further found that the Pc receives 1) serotoninergic afferents from the raphe dorsalis, magnus, and obscurus nuclei; 2) noradrenergic inputs from the dorsolateral pontine tegmentum; 3) cholinergic afferents from the lateral medullary reticular formation; 4) substance P-like afferents from the central nucleus of the amygdala, bed nucleus of the stria terminalis, periaqueductal gray, and nucleus of the solitary tract; and 5) methionine-enkephalin-like projections from the periaqueductal gray, the nucleus of the solitary tract, the lateral pontine and medullary reticular formation, and the spinal trigeminal nucleus. We further found that the Pc do not receive afferents from brainstem structures responsible for muscle atonia, such as the ventromedial medulla and the dorsomedial pontine tegmentum, and therefore may not be part of the circuit inhibiting the brainstem motoneurons during paradoxical sleep.

Projections of the dorsal raphe nucleus to the brainstem: PHA‐L analysis in the rat

Early studies that used older tracing techniques reported exceedingly few projections from the dorsal raphe nucleus (DR) to the brainstem. The present report examined DR projections to the brainstem by use of the anterograde anatomical tracer Phaseolus vulgaris leucoagglutinin (PHA-L). DR fibers were found to terminate relatively substantially in several structures of the midbrain, pons, and medulla. The following pontine and midbrain nuclei receive moderate to dense projections from the DR: pontomesencephalic central gray, mesencephalic reticular formation, pedunculopontine tegmental nucleus, medial and lateral parabrachial nuclei, nucleus pontis oralis, nucleus pontis caudalis, locus coeruleus, laterodorsal tegmental nucleus, and raphe nuclei, including the central linear nucleus, median raphe nucleus, and raphe pontis. The following nuclei of the medulla receive moderately dense projections from the DR: nucleus gigantocellularis, nucleus raphe magnus, nucleus raphe obscurus, facial nucleus, nucleus gigantocellularis-pars alpha, and the rostral ventrolateral medullary area. DR fibers project lightly to nucleus cuneiformis, nucleus prepositus hypoglossi, nucleus paragigantocellularis, nucleus reticularis ventralis, and hypoglossal nucleus. Some differences were observed in projections from rostral and caudal parts of the DR. The major difference was that fibers from the rostral DR distribute more widely and heavily than do those from the caudal DR to structures of the medulla, including raphe magnus and obscurus, nucleus gigantocellularis-pars alpha, nucleus paragigantocellularis, facial nucleus, and the rostral ventrolateral medullary area. A role for the dorsal raphe nucleus in several brainstem controlled functions is discussed, including REM sleep and its events, nociception, and sensory motor control.

Brainstem sites for the carbachol elicitation of the hippocampal theta rhythm in the rat.

The effects of brainstem microinjections of carbachol on the hippocampal theta rhythm were examined in urethane anesthetized rats. The two most effective theta-eliciting sites with carbachol were the nucleus pontis oralis (RPO) and the acetylcholine-containing pedunculopontine tegmental nucleus (PPT) of the dorsolateral pontine tegmentum. RPO injections generated theta at mean latencies of 38.5 +/- 70.8 s and for mean durations of 12.9 +/- 5.1 min. Five of seven RPO injections gave rise to theta virtually instantaneously, i.e., before the completion of the injection. PPT injections generated theta at mean latencies of 1.7 +/- 1.1 min and for mean durations of 11.9 +/- 6.0 min. Injections rostral or caudal to RPO in the caudal midbrain reticular formation (RF) or the caudal pontine RF (nucleus pontis caudalis) generated theta at considerably longer latencies (generally greater than 5 min) or were without effect. Medullary RF injections essentially failed to alter the hippocampal EEG. The finding that theta was produced at very short latencies at RPO suggests that RPO, the putative brainstem source for the generation of theta, is modulated by a cholinergic input. The further demonstration that theta was also very effectively elicited with PPT injections suggests this acetylcholine-containing nucleus of the dorsolateral pons may be a primary source of cholinergic input to RPO in the generation of theta. The hippocampal theta rhythm is a major event of REM sleep. The present results are consistent with earlier work showing that each of the other major events of REM sleep, as well as the REM state, are cholinergically activated at the level of the pontine tegmentum.

NMDA and non-NMDA antagonists infused into the nucleus reticularis pontis caudalis depress the acoustic startle reflex

The neural pathway that mediates the acoustic startle reflex has been proposed; however, the pharmacology underlying this reflex is less well known. The present study examined the role of excitatory amino acid receptors at the level of the nucleus reticularis pontis caudalis, a brainstem nucleus obligatory for the whole body startle reflex and implicated as the locus where extrinsic systems such as the amygdala may act to modulate acoustic startle. Twenty-nine rats, chronically implanted with bilateral cannulae aimed at the nucleus reticularis pontis caudalis, were tested to assess the effects of γ- d-glutamylglycine (DGG), dl-2-amino-5-phosphonopentanoic acid (AP5), and 6-cyano-7-nitroquinoxaline-2, 3-dione (CNQX) on the amplitude of the acoustic startle reflex. Local infusion of each of the 3 compounds significantly reduced startle amplitude by as much as 70–80%. AP5 and CNQX attenuated startle over a dose range which indicated that the reticularis pontis caudalis may be much more sensitive to these compounds than other nuclei along the primary startle pathway. These results suggest that, at the level of the nucleus reticularis pontis caudalis, an excitatory amino acid neurotransmitter may mediate acoustic startle, and that both NMDA and non-NMDA receptor subtypes may be important for the expression of the acoustic startle reflex.

Pontine microinjection of carbachol and critical zone for inducing postural atonia in reflexively standing decerebrate cats

Pontine carbachol injection sites critically related to the induction of postural atonia were explored in reflexively standing acute decerebrate cats. Carbachol (4.0 μg/0.25 μl) and atropine (2.0 μg/0.25 μl) were focally injected into the pontomedullary reticular formation, and their effects on hindlimb extensor muscle tone were studied. The effective carbachol injection sites were concentrated in the region between P 1.5 and P 3.5, H −3.0 and H −5.0, and LR 1.2 and LR 2.5 (Horsley-Clarke coordinates). The effective sites corresponded to the dorsomedial part of both the nucleus reticularis pontis oralis (NRPo) and the rostral portion of the nucleus reticularis pontis caudalis (NRPc). The mean latency to the beginning of carbachol-induced postural atonia was about 90 s (92 ± 28 s; n = 24).

The audiospinal reaction in parkinsonian patients reflects functional changes in reticular nuclei

Audiospinal facilitation using the soleus H-reflex as a test was compared in 16 control subjects and 23 parkinsonian patients. In the patients, facilitation was significantly reduced during the 75 to 150 msec after the conditioning stimulation. This reduction was seen bilaterally even in patients with a hemisyndrome. It was corrected by L-dopa but not by anticholinergic agents. Facilitation at the 75-msec delay showed an inverse linear correlation with the bradykinesia intensity (r = -0.7, p < 0.01). The results argue in favor of a reduced excitability of the nucleus reticularis pontis caudalis from which a reticulospinal tract emanates as effector of the audiospinal facilitation. This would represent another example of dysfunction of reticular nuclei in Parkinson's disease.

Axonal trajectories of single Forel's field H neurones in the mesencephalon, pons and medulla oblongata in the cat.

We studied axonal trajectories of single Forel's field H (FFH) neurones (n = 19) in the mesencephalon, pons and medulla by systematic antidromic threshold mapping in cats and differentiated them into two major types. Type I neurones were characterized by projections to the oculomotor nucleus (IIIn) and type II neurones by lack of projections to the IIIn. 2. Type I neurones (11/19) were further classified into three subtypes by the lowest level of projections; type Ic (n = 3) which projected to the cervical cord and type Ib (n = 7) which terminated at the ponto-medullary level and type Ia (n = 1) at more rostral level. In the mesencephalon, stem axons passed just lateral to the IIIn and projected collaterals to the IIIn and the ventral part of the periaqueductal gray matter. In the lower brain stem, stem axons of type Ib and Ic neurones passed in the dorsal part of the reticular formation or in the medial longitudinal fasciculus and projected collaterals to the dorsal part of the nucleus reticularis pontis caudalis (NRPC) and the nucleus reticularis gigantocellularis (NRG) and the reticular formation underlying the nucleus prepositus hypoglossi (PH) and the raphe region. Projections to the superior colliculus were observed in two cases. 3. Type II neurones (8/19) were classified into 2 type IIb projecting to the ponto-medullary reticular formation and 6 type IIc projecting to the cervical spinal cord. In the mesencephalon, stem axons passed through a more lateral region than those of type I and projected collaterals to the mesencephalic reticular formation and the red nucleus. In the lower brain stem, the stem axons passed in the ventral part of the reticular formation corresponding to the central tegmental tract and projected collaterals to the ventral part of the NRPC and NRG. Projections to the interstitial nucleus of Cajal, the inferior olive and the reticular formation underlying the PH were also observed. 4. The dorsal and ventral location of, respectively, stem axons of type I and type II neurones in the lower brain stem was confirmed in a larger number of neurones in experiments with restricted mapping. 5. There was not much difference in location of cell bodies of type I (totally n = 50) and type II (n = 46) neurones. The proportion of spinal-projecting neurones were larger in type II (21/46, 46%) than in type I (7/50, 14%) neurones.

Brainstem Reticular Nuclei that Project to the Cerebellum in Rats A Retrograde Tracer Study; pp. 39–53

The nuclear origins of projections from the brainstem reticular formation to the cerebellum were examined using four retrograde tracer substances: horseradish peroxidase, wheat germ agglutinin-horseradish peroxidase conjugate, Fluoro-Gold, and rhodamine beads. Tracer injections were made into each of the three major longitudinal zones of the cerebellar cortex (vermis, paravermal hemisphere, and lateral hemisphere) as well as into the various deep cerebellar nuclei. Counts of retrogradely labeled cells were done on a large sample of select cases. The data generated by these cell counts indicate that the strongest reticulocerebellar projections arise from the three specialized pre-cerebellar reticular nuclei: the lateral reticular nucleus, the medullary paramedian reticular nucleus, and the reticulotegmental nucleus. The presumed noradrenergic locus coeruleus (A6 cell group) was also densely packed with retrogradely labeled neurons. However, strong reticulocerebellar projections also arose from other presumed catecholamine cell groups such as those in the ventrolateral medulla (the A1/C1 complex) and the caudal pons (A5). Substantial cerebellar projections originated from most of the various presumed serotonergic brainstem raphe cell groups (particularly raphe obscurus in the medulla), as well as from the presumed cholinergic Ch5 cell group (the pedunculopontine pars compact us nucleus). Labeled cells were also seen in several nonaminergic isodendritic reticular nuclei thought to be involved in visuomotor activity (e.g. paragigantocellularis dorsalis, raphe interpositus, and the pontine dorsomedial tegmental area), as well as in the lateral reticular zone of the medulla and lower pons (reticularis dorsalis and parvocellularis). Tracer injections into the deep nuclei produced relatively greater numbers of labeled neurons in large-celled medial reticular nuclei associated with skeletomotor activity, such as gigantocellularis. magnocellularis, and pontis caudalis. Reticular nuclei conspicuous in their lack of projections to the cerebellum included reticularis ventralis in the medulla, pontis oralis, and both subdivisions of the midbrain reticular formation (cuneiformis and subcuneiformis). As a whole, the various isodendritic reticular nuclei project most strongly to midline cerebellar structures (vermal cortex or fastigial nuclei), less strongly to the paravermal cortex or interposed nuclei, and least strongly to the lateral cortex or dentate nucleus. Within individual reticular nuclei, the morphology of labeled neurons is identical to that reported previously by this laboratory subsequent to spinal or cortical HRP injections, thus strengthening this laboratory's hypothesis that the various brainstem reticular nuclei can be distinguished on the basis of neuronal morphology.

Mono- and disynaptic pathways from Forel's field H to dorsal neck motoneurones in the cat.

1. We analysed the synaptic actions produced by Forel's field H (FFH) neurones on dorsal neck motoneurones and the pathways mediating the effects. 2. Stimulation of ipsilateral FFH induced negative field potentials of several hundred microvolts with the latency of about 1.1 ms in the medial ponto-medullary reticular formation, being largest in the ventral part of the nucleus reticularis pontis caudalis (NRPC), and in the dorsal part of the nucleus reticularis gigantocellularis (NRG). 3. Stimulation of ipsilateral FFH induced excitatory postsynaptic potentials (EPSPs) in 90% (47/52) and inhibitory postsynaptic potentials (IPSPs) in 19% (10/52) of the reticulospinal neurones (RSNs) in the NRPC and the NRG. Latencies of the EPSPs and IPSPs were 0.7-3.0 ms, the majority of which were in the monosynaptic range. The monosynaptic connexions were confirmed by spike triggered averaging technique both in excitatory (n = 4) and inhibitory (n = 2) pathways. 4. Single stimulation of FFH induced EPSPs at the segmental latencies of 0.3-1.0 ms in neck motoneurones, which were clearly in the monosynaptic range. Repetitive stimulation of FFH produced marked temporal facilitation of EPSPs in neck motoneurones. The facilitated components of the EPSPs had a little longer latencies and their amplitude reached several times as large as that evoked by single stimulation in all the tested motoneurones. These facilitated excitations are assumed to be mediated by RSNs in the NRPC and NRG, since RSNs were mono- and polysynaptically fired by stimulation of FFH and they were previously shown to directly project to neck motoneurones. 5. EPSPs were induced in 91% (82/91) of motoneurones supplying m. biventer cervicis and complexus (BCC; head elevator), 10% (3/29) of motoneurones supplying m. splenius (SPL; lateral head flexor). Likewise, stimulation of FFH produced EMG responses in BCC muscles, while not in SPL muscle. Thus FFH neurones produce excitations preferentially in BCC motoneurones. 6. Systematic tracking in and around FFH revealed that the effective sites for evoking above effects were in FFH and extended caudally along their efferent axonal course. 7. These results suggested that FFH neurones connect with neck motoneurones (chiefly BCC, head elevator) mono-, di- and/or polysynaptically and are mainly concerned with the control of vertical head movements.

Descending projections of Forel's field H neurones to the brain stem and the upper cervical spinal cord in the cat.

1. Descending projections from Forel's field H (FFH) to the brain stem and upper cervical spinal cord were studied in cats. 2. Following implantation of HRP pellets into the spinal gray matter (C1-C3) or in the ponto-medullary reticular formation, the nucleus reticularis pontis caudalis (NRPC) or in the nucleus reticularis gigantocellularis (NRG), numerous neurones were retrogradely labelled in FFH on the ipsilateral side. In the former cases, the sizes of labelled neurones were medium-large (20-40 microns in diameter) while both small and medium-large neurones were labelled in the latter cases. 3. The lowest levels of spinal projection of single FFH neurones (n = 70) were assessed by antidromic spikes elicited by stimulating electrodes placed in C1, C3 and C7. The majority (59%) projected to C1 (but not to C3), about 27% to C3 (but not to C7), and only 14% to C7. 4. Axonal trajectories of single FFH neurones in C1-C3 segments were investigated by antidromic threshold mapping methods. The stem axons of spinal-projecting FFH neurones descended in the ventral or in the ventrolateral funiculi and the collaterals were projected to neck motor nuclei (lamina IX, Rexed 1954) and laminae V-VIII. The conduction velocities were estimated as 8-37 m/s from the antidromic latencies. 5. Axonal trajectories of 7 FFH neurones were investigated in the ponto-medullary reticular formation. All were antidromically activated from C1. In six neurones, the stem axons were located in the ventral part of the central tegmental tract and collaterals were projected to the NRPC and/or the NRG. Some of them projected to the inferior olive and the nucleus prepositus hypoglossi as well. The stem axon, in the remaining cell, was in the most dorso-medial part of the medial longitudinal fasciculus and collaterals were projected mainly to the dorsal part of the NRPC and the NRG, and also to the medial vestibular nucleus. 6. Anterograde transport of WGA-HRP injected into FFH revealed that in the upper cervical spinal cord, stem axons were found in the ventral funiculus and ventral part of the lateral funiculus. Collateral projections and presumed bouton-like deposits were observed in the laminae VI-IX, especially in their medial part. In the brain stem, dense bundles of the descending fibres were found in the central and the medial tegmental tracts and in the medial longitudinal fasciculus. FFH neurones projected densely to the caudal half of the NRPC and to the rostral half of the NRG.(ABSTRACT TRUNCATED AT 400 WORDS)