Reticulospinal Cells Research Articles

The reticulospinal glutamate synapse in lamprey: plasticity and presynaptic variability

1. The glutamatergic synapses formed between the unbranched giant reticulospinal axons onto spinal neurons in lamprey offer a central vertebrate synapse in which the presynaptic element can be impaled with one or several microelectrodes, which may be used for recording as well as microinjection of different substances. To provide a basis for the use of this synapse in studies of release mechanisms, we have examined the use-dependent modulation of the synaptic response under conditions of conventional cell body stimulation, and during direct stimulation of the presynaptic axon. 2. To examine the stability of the mixed electrotonic and chemical reticulospinal excitatory postsynaptic potential (EPSP) over time, action potentials were evoked at a rate of 1 Hz for 800-1000 trials. In three out of seven synapses the chemical component remained at a similar amplitude, while in four cases a progressive decrease (up to 35%) occurred. The electrotonic component remained at a similar amplitude in all cases. 3. During paired pulse stimulation of the reticulospinal cell body (pulse interval 65 ms) the chemical EPSP component showed a net facilitation in all cases tested [from 0.64 +/- 0.35 to 0.89 +/- 0.48 (SD) mV, n = 13], while the peak amplitude of the electrotonic component was unchanged (1.37 +/- 0.68 and 1.36 +/- 0.66 mV, respectively). Recording of the axonal action potential during paired pulse stimulation showed that the width of the first and second action potential did not differ [1/2 width (2.48 +/- 0.39 ms and 2.48 +/- 0.42 ms, respectively; n = 8)]. 4. The degree of facilitation varied markedly between different synapses, ranging from an increase of a few percent to a two-fold increase (24 +/- 16% mean change of total EPSP amplitude, corresponding to 44 +/- 26% mean change of chemical EPSP amplitude). This type of variability was also observed in synapses made from the same unbranched reticulospinal axon onto different postsynaptic cells. 5. When paired pulse stimulation was applied to the reticulospinal axon in the very vicinity of the synaptic area (0.1-1 mm) a net depression of the chemical component occurred in 11 out of 19 cases, and in the remaining cases the level of net facilitation was lower as compared with cell body stimulation (range between +17 and -23% change of total EPSP amplitude; mean -5%; n = 19). 6. To test if the change of the EPSP plasticity during local stimulation correlated with an increased transmitter release, two microelectrodes were placed in the same reticulospinal axon at different distances from the synaptic area.(ABSTRACT TRUNCATED AT 400 WORDS)

5-HT innervation of reticulospinal neurons and other brainstem structures in lamprey.

In order to determine if reticulospinal neurons involved in the control of locomotion and responsive to exogenously applied 5-hydroxytryptamine (5-HT) are innervated by fibers that contain serotonin, the serotoninergic innervation of reticulospinal neurons, identified by retrograde labeling with fluorescein-conjugated dextran-amine (FDA), was investigated by immunohistochemistry in the lamprey brainstem. A widespread distribution of 5-HT immunoreactive (5-HT-ir) fibers was seen within the basal plate of the brainstem, an area containing reticulospinal somata and dendritic aborizations. Numerous 5-HT varicose fibers were found in close relation to large reticulospinal cell bodies, particularly in the middle and anterior rhombencephalic reticular nuclei (MRRN and ARRN). Some of these reticulospinal somata were surrounded by a very dense pericellular 5-HT innervation. 5-HT-ir fibers were also seen in other brain structures that are known to influence reticulospinal neurons such as the rhombencephalic alar plate containing sensory relay interneurons, cranial nerves (III-X), cerebellum, and tectum. These findings suggest that, as in the spinal cord, motor behavior controlled by reticulospinal neurons may be subject to a serotoninergic modulation.

Giant neurons in the rat reticular formation: a sensorimotor interface in the elementary acoustic startle circuit?

The mammalian acoustic startle response (ASR) is a relatively simple motor response that can be elicited by sudden and loud acoustic stimuli. The ASR shows several forms of plasticity, such as habituation, sensitization, and prepulse inhibition, thereby making it an interesting model for studying the underlying neuronal mechanisms. Among the neurons that compose the elementary startle circuit are giant neurons in the caudal pontine reticular nucleus (PnC), which may be good candidates for analyzing the neuronal basis of mammalian behavior. In a first step of this study, we employed retrograde and anterograde tracing techniques to identify the possible sources of input and the efferent targets of these neurons. In a second step, we performed intracellular recordings in vivo, followed by subsequent injections of HRP for morphological identification, thereby investigating whether characteristic features of the ASR are reflected by physiological properties of giant PnC neurons. Our observations demonstrate convergent, bilateral input from several auditory brainstem nuclei to the PnC, predominantly originating from neurons in the cochlear nuclear complex and the superior olivary complex. Almost no input neurons were found in the nuclei of the lateral lemniscus. As the relatively long neuronal response latencies in several of these auditory nuclei appear to be incompatible with the primary ASR, we conclude that neurons in the cochlear root nuclei most likely provide the auditory input to PnC neurons that is required to elicit the ASR. The giant PnC neurons have a remarkable number of physiological features supporting the hypothesis that they may be a neural correlate of the ASR: (1) they receive short-latency auditory input, (2) they have high firing thresholds and broad frequency tuning, (3) they are sensitive to changes in stimulus rise time and to paired-pulse stimulation, (4) repetitive acoustic stimulation results in habituation of their response, and (5) amygdaloid activity enhances their response to acoustic stimuli. Anterograde tracing showed that most giant PnC neurons are reticulospinal cells. Axon collaterals and terminal arbors were found in the reticular formation as well as in cranial and spinal motoneuron pools. The results of this study indicate that giant PnC neurons form a sensorimotor interface between the cochlear nuclear complex and cranial and spinal motoneurons. This neuronal pathway implies that the elementary acoustic startle circuit is composed of only three central relay stations and thus appears to be organized more simply than assumed in the past.

Differential Responses of Single Reticulospinal Cells to Spatially Localized Stimulation of the Optic Tectum in a Teleost Fish, Salmo trutta

To determine whether the topographically organized retinal input to the optic tectum is subsequently mapped onto the reticular formation, the responses of antidromically identified reticulospinal cells to tectal surface stimulation were investigated in 45 decerebrated, paralysed trout. The tectum was stimulated through a silver ball surface electrode at 24 different locations, and extracellular recordings were made from the rhombencephalic brainstem with glass microelectrodes filled with a 10% solution of horseradish peroxidase (HRP) in Tris buffer (pH 7.4). After recording, the HRP was, in some cases, iontophoretically expelled from the pipette to identify its location and visualized in histological sections by a modified Hanker-Yates method. Individual reticulospinal neurons discharged 1-4 spikes at short latency in response to stimulation of each of the 24 tectal locations. From one tectal location per cell, this initial response was followed by a late, sustained burst. With a short stimulus train (6 pulses, 55 Hz) the burst could last for over a second with discharge rates of up to 500 Hz. Sometimes this burst could be evoked from neighbouring tectal locations, but only by greatly increasing the stimulus strength. We conclude that the reticular formation receives a highly divergent monosynaptic connection from all locations of the tectum and that the longer latency, sustained burst response is due to a mapped connection between the tectum and the reticular formation. Since the late burst could be preceded by a silent period lasting for approximately 32 ms, we cannot rule out a dependence on interneurons situated between the tectum and the reticulospinal cells.

The spino-reticulo-spinal loop can slow down the NMDA-activated spinal locomotor network in lamprey.

Lamprey spino-bulbar neurones modulate the activity of reticulospinal cells during locomotion. The aim of the present study was to investigate the effects of interrupting or increasing this feedback from the spinal cord on the fictive locomotor pattern. Double-bath experiments were performed on in vitro brain stem/spinal cord preparations. Fictive locomotion was induced by perfusing the spinal cord with 150 microM N-methyl-D-aspartate (NMDA). Blocking the synaptic transmission in the brain stem by exposing it to Mn2+ ions increased the locomotor rhythm. Conversely, stimulation of single reticulospinal neurones during the ipsilateral ventral root burst, when they were depolarized, increased the cycle duration by prolonging the ipsilateral motor burst. The spino-reticulo-spinal loop is an integral part of the locomotor network.

Reticulospinal and reticuloreticular pathways for activating the lumbar back muscles in the rat.

These experiments tested hypotheses about the logic of reticulospinal and reticuloreticular controls over deep back muscles by examining descending efferent and contralateral projections of the sites within the medullary reticular formation (MRF) that evoke EMG responses in lumbar axial muscles upon electrical stimulation. In the first series of experiments, retrograde tracers were deposited at gigantocellular reticular nucleus (Gi) sites that excited the back muscles and in the contralateral lumbar spinal cord. The medullary reticular formation contralateral to the Gi stimulation/deposition site was examined for the presence of single- and double-labeled cells from these injections. Tracer depositions into Gi produced labeled cells in the contralateral Gi and Parvocellular reticular nucleus (PCRt) whereas the lumbar injections retrogradely labeled cells only in the ventral MRF, indicating that separate populations of medullary reticular cells project to the opposite MRF and the lumbar cord. In the second series of experiments the precise relationships between the location of neurons retrogradely labeled from lumbar spinal cord depositions of the retrograde trace, Fluoro-Gold (FG) and effective stimulation tracks through the MRF were examined. The results indicate that the Gi sites that are most effective for activation of the back muscles are dorsal to the location of retrogradely labeled lumbar reticulospinal cells. To verify that cell bodies and not fibers of passage were stimulated, crystals of the excitatory amino acid agonist, N-methyl-D-aspartate (NMDA) were deposited at effective stimulation sites in the Gi. NMDA decreased the ability of electrical stimulation to activate back muscles at 5 min postdeposition, indicating a local interaction of NMDA with cell bodies at the stimulation site. In the third series of experiments, electrical thresholds for EMG activation along a track through the MRF were compared to cells retrogradely labeled from FG deposited into the cervical spinal cord. In some experiments, Fast Blue was also deposited into the contralateral lumbar cord. Neurons at low threshold points on the electrode track were labeled following cervical depositions, indicating a direct projection to the cervical spinal cord. The lumbar depositions, again, labeled cells in MRF areas that were ventral to the locations of effective stimulation sites, primarily on the opposite side of the medulla. In addition, the lumbar depositions back-filled cells in the same cervical segments to which the Gi neurons project. These results suggest that one efferent projection from effective stimulation sites for back muscle activation is onto propriospinal neurons in the cervical cord, which in turn project to lumbar cord levels.(ABSTRACT TRUNCATED AT 400 WORDS)

Phasic modulation of transmission from vestibular inputs to reticulospinal neurons during fictive locomotion in lampreys

The aim of this study was to determine whether the transmission from sensory inputs to reticulospinal neurons is modulated during fictive locomotion in lampreys. Reticulospinal neurons play a key role in the control of locomotion; modulation of sensory transmission to these neurons might be of importance for the adaptation of the control they exert during locomotion. In this series experiments, intracellular synaptic responses of reticulospinal neurons of the posterior rhombencephalic reticular nucleus elicited by electrical stimulation of vestibular nerves on each side were studied during fictive locomotion induced by 50 μM N-methyl- d-aspartate (NMDA). Interestingly, shortly after NMDA had reached the bath and much before locomotor discharges were apparent in the recorded ventral roots, there was a significant depression of the synaptic transmission from vestibular nerves. The effect was reversed by washing out the NMDA and persisted in the isolated brainstem after spinal transection at the first segmental level. As locomotor discharges appeared in the ventral roots, synaptic responses elicited by vestibular nerve stimulation showed a clear phasic modulation of their amplitude during the locomotor cycle. Responses to stimulation of the ipsilateral vestibular nerve were smaller during the ipsilateral burst discharge than during the contralateral activity, whilst responses to stimulation of the contralateral vestibular nerve were minimal during contralateral activity and maximal during ipsilateral activity. This opposite pattern of modulation observed in the same reticulospinal neuron suggests that the phasic modulation of vestibular transmission is not due to changes in the membrane properties of the reticulospinal cell but is produced at a pre-reticular level. Although the exact site of modulation in this disynaptic pathway has not yet been established, presynaptic mechanisms are possibly important.

Retinoic acid affects the organization of reticulospinal neurons in developing Xenopus

The effects of all-trans retinoic acid (RA) on the differentiation of the reticulospinal system were studied in Xenopus. RA was applied in concentrations of 10 −5 and 10 −6 M for 30 min at stage 12. When siblings had reached stages 46–48, the spinal cord was transected in anesthetized control and experimental animals and the reticulospinal cells were visualized through retrograde transport of fluorescing dextran amines. The lower concentration of RA led in many animals (22%) to the formation of multiple Mauthner-like cells. Higher concentrations resulted in the formation of two uninterrupted longitudinal columns of rather uniform reticulospinal cells. These data suggest that the normal expression of Hox genes pattern—known to be altered by RA—may be necessary for the differential specification of compartments of the reticulospinal system.

5-Hydroxytryptamine modulates spike frequency regulation in reticulospinal neurons involved in the control of locomotion in lamprey

To investigate the effect of 5-hydroxytryptamine (5-HT) on reticulospinal neurons involved in the initiation and control of locomotion in lamprey, 5-HT (10 mM) was locally pressure ejected on the dorsal surface of the brainstem during intracellular recordings from identified reticulospinal neurons in the in vitro brainstem-spinal cord preparation. 5-HT induced a reduction of the late afterhyperpolarization (AHP) following the spike due to a reduction of a Ca 2+-activated K + current. In addition, 5-HT caused a resting membrane hyperpolarization in a proportion of these cells. Due to the 5-HT induced reduction of the AHP, reticulospinal cells, including those that became hyperpolarized by an application of 5-HT, discharged at a higher rate after 5-HT as a response to the same excitatory drive.

The spinal motor system in early vertebrates and some of its evolutionary changes.

Recent studies of the spinal motor systems of vertebrates allow us to begin to infer the organization of the motor apparatus of primitive vertebrates. This paper attempts to define some of the features of the motor system of early vertebrates based on studies of the motor systems in anamniotes and in Branchiostoma. It also deals with some changes in the primitive motor system during evolution. The primitive motor system consisted of myomeric axial muscles, with a functional subdivision of the musculature into non-spiking slow muscle fibers segregated in the myomeres from spiking fast ones. These fibers were innervated by two major classes of motoneurons in the cord-large motoneurons innervating faster fibers and small motoneurons innervating slow fibers. There was not a simple isomorphic mapping of the position of motoneurons in the motor column onto the location of the muscle fibers they innervated in the myomeres. Early vertebrates used these axial muscles to bend the body, and the different types of muscle fibers and motoneurons reflect the ability to produce slow swimming movements as well as very rapid bending associated with fast swimming or escapes. The premotor network producing bending was most likely a circuit composed of a class of descending interneurons (DIs) that provided excitation of ipsilateral motoneurons and other interneurons, and inhibitory commissural interneurons (CIs) that blocked contralateral activity and played an important role in generating the rhythmic alternating bending during swimming. This DI/CI network was retained in living anamniotes. At least two major descending systems linked the sensory systems in the head to these premotor networks in the spinal cord. The ability to turn on swimming by activation of DI/CI premotor networks in the cord resided at least in part in a midbrain locomotor region (MLR) that influenced spinal networks via projections to the reticular formation. Reticulospinal neurons were important not only for initiation of rhythmic swimming but also in the production of turning movements. The reticulospinal cells involved in turns produced their effects in part via monosynaptic connections with motor neurons and premotor interneurons, including some involved in rhythmic swimming. A prominent and powerful Mauthner cell was most likely present and important for rapid escape or startle movements. Some features of this primitive motor apparatus were conserved during the evolution of vertebrate motor systems, and others changed substantially. Many features of the early motor system were retained in living anamniotes; major changes occur among amniotes.(ABSTRACT TRUNCATED AT 400 WORDS)

Sympatholytic response to stimulation of superior laryngeal nerve in rats

The central pathway mediating a sympatholytic response to stimulation of the superior laryngeal nerve (SLN) was studied in halothane-anesthetized, paralyzed rats. Single-pulse stimulation of SLN inhibited lumbar sympathetic nerve discharge (LSND) with onset latency of 113 +/- 1.7 ms. LSND inhibition was markedly attenuated by bilateral microinjection of kynurenic acid (Kyn, glutamate receptor antagonist, 4.5 nmol/side) into the caudal ventrolateral medulla (CVL) or by bilateral administration of bicuculline methiodide (Bic; gamma-aminobutyric acid-receptor antagonist, 225 pmol/side) into the rostral ventrolateral medulla (RVL). In 13 of 14 cases, the baroreceptor reflex was also severely reduced. Injections of Bic or Kyn elsewhere in the medullary reticular formation were ineffective. Single-pulse stimulation of SLN inhibited 19 of 26 RVL reticulospinal barosensitive cells (onset latency 46 +/- 1.4 ms). This inhibition was attenuated (from 92 +/- 6 to 14 +/- 12%) by iontophoretic application of Bic (n = 7), which also reduced the cells' inhibitory response to aortic coarctation. The remaining seven barosensitive neurons were unaffected by SLN stimulation. In conclusion, the sympathetic baroreflex and the sympathoinhibitory response to SLN stimulation appear to be mediated by similar medullary pathways.

Locomotion evoked by stimulation of the brain stem in the Atlantic stingray, Dasyatis sabina

The primary pathway descending to the spinal cord to initiate locomotion in the stingray is located in the intermediate to ventral portion of the lateral funiculus; a second pathway is located in the dorsolateral funiculus. The goal of this study was to identify the origins of these pathways in the rhombencephalic reticular formation (RF). To do this we used microstimulation of the RF in conjunction with selective lesions of the brain stem and spinal cord. In some animals microinjections of excitatory amino acids were used to avoid stimulating axons of passage. Locomotion in the contralateral pectoral fin was evoked by microstimulation of the dorsal and ventral reticular nuclei, the middle and superior RF, and the ventral portion of the lateral RF. The regions from which locomotion was evoked by chemical stimulation were more restricted and included the rostral dorsal reticular nucleus, the middle RF, and the adjacent ventral lateral RF. This area encompasses the magnocellular RF and coincides with the distribution of numerous reticulospinal cells that project ipsilaterally into the ventral half of the lateral funiculus. Our results indicate, then, that locomotion in the stingray is mediated primarily by a pathway originating in the magnocellular RF that descends ipsilaterally in the ventral half of the lateral funiculus to elicit swimming in the contralateral pectoral fin. We suggest that this primary pathway is specifically associated with the control of locomotion. We also demonstrated that locomotion can be evoked independently from the lateral RF, but is probably mediated by an indirect pathway relaying near the spinomedullary junction or in the rostral spinal cord.

Monosynaptic excitatory amino acid transmission from the posterior rhombencephalic reticular nucleus to spinal neurons involved in the control of locomotion in lamprey

1. The reticulospinal neurons in the lamprey posterior rhombencephalic reticular nucleus (PRRN) and their projections to different types of spinal neurons have been investigated by the use of simultaneous paired intracellular recordings from one pre- and one postsynaptic cell. PRRN is of particular importance for the initiation of locomotion. 2. Intracellular stimulation of single PRRN neurons produced monosynaptic excitatory postsynaptic potentials (EPSPs) in simultaneously recorded motoneurons and spinal premotor interneurons of both the excitatory and inhibitory type. Individual PRRN neurons produced EPSPs in several different types of target cells, as revealed by signal averaging. Each single PRRN neuron had extensive monosynaptic connections to approximately 73% of the motoneuronal population. Conversely, several PRRN neurons converge on individual spinal neurons. The average amplitude of the EPSPs was 0.43 +/- 0.40 (SD) mV. The EPSPs varied in time course (time to peak = 7.5 +/- 2.8 ms; duration at one-half peak amplitude = 21.9 +/- 18.1 ms). 3. The EPSPs produced by reticulospinal cells were composed of either exclusively chemical, exclusively electrical, or mixed chemical and electrical components. The electrical EPSPs remained when the ordinary physiological solution was substituted for one without Ca2+ but with Mn2+. The chemical component of the EPSPs was always depressed when a broad-spectrum excitatory amino acid (EAA) antagonist, such as kynurenic acid, was applied, suggesting that the chemical component was because of EAA transmission. The chemical EPSP could have two components, one late, suppressed by N-methyl-D-aspartate (NMDA) antagonists, and one early because of activation of kainate/quisqualate receptors. 4. Three-dimensional reconstructions of Lucifer yellow-filled PRRN neurons were performed with a confocal laser scanning microscope. PRRN neurons producing monosynaptic excitatory amino acid EPSPs were found to have a fusiform cell body located near the surface of the fourth ventricle and an extensive fanlike dendritic tree extending to the ventral and lateral margin of the brain stem within the basal plate. The axons descend in the lateral funiculi of the spinal cord. 5. PRRN neurons utilizing EAA transmission are active during fictive locomotion. They presumably initiate and reinforce ongoing spinal locomotor activity by monosynaptically increasing the general excitability of the spinal premotor interneurons of the spinal locomotor networks by means of their extensive divergent and convergent monosynaptic connections.

Escape Responses of Herring Larvae to Visual Stimuli

Predation is now considered the main cause of mortality in larval and juvenile fish (Hunter, 1984) and is therefore the most important factor controlling recruitment to the adult population. Marine fish larvae are prey for many different types of predator including medusae, crustaceans and larger fish. When predatory attacks are sensed both adult and larval fish may respond by making a 'C-start', a very fast, simultaneous contraction of the trunk musculature that deforms the body into a C-shape within 20 ms (Eaton & Hackett, 1984). This startle response is mediated by the Mauthner cells, a pair of prominent neurones in the hind brain or by other reticulo-spinal cells located in the same region. As a result of reciprocal inhibition and decussation of the cell axon, stimulation on one side of the body results in contraction of all the muscle on the contralateral side.

Reticulospinal Control of Rapid Escape Turning Maneuvers in Fishes

SYNOPSIS. In teleost fishes, the C-start is a rapid avoidance maneuver that steers the animal away from the strike trajectory of a predator. We review how the axial motor pattern underlying this movement is organized by the reticulospinal system. This system includes the prominent Mauthner cells and other identifiable neurons. Typical of reticulospinal cells, the Mauthner neuron has inputs from auditory, vestibular, and visual areas and has outputs to the trunk motoneurons by means of mono- and disynaptic contacts. Because reticulospinal organization and function is conserved among the vertebrates, the Mauthner network is advantageous as a model for understanding control of movement. We develop three major points. First, despite its seeming anatomical simplicity, the escape network is not a simple hierarchial system with one cell that directs the entire movement. We present evidence that the escape response is mediated by an array of neurons which include the Mauthner cells. Second, though the firing of this cell is adequate to result in a major component of the movement, the Mauthner cell is not indispensible for its execution. The network producing this movement is capable of regulating its output so that the animal produces the normal range of responses despite the absence of one of its major contributors. These findings can be accounted for by one of two models, though the most likely one involves lateral interactions, or corollary discharges, within the array. Third, our recent work also shows that the movement pattern is generated by a ballistic command which does not rely on movement-induced sensory feedback. These system properties allow the animal to produce a highly reliable and orchestrated motor command in only 15 msec from sensory stimulus to EMG.

Anatomical distribution and response patterns of reticular neurons active in relation to acoustic startle

A population of reticulospinal neurons with short latency response to startle-inducing stimuli was identified in the nucleus reticularis 3ontis caudalis (NRPC) and nucleus gigantocellularis (NRGC) of the medial pontomedullary reticular formation. The threshold and magnitude of response to auditory stimuli was correlated in these cells and in the muscles mediating startle. Startle-related neurons were significantly more likely to have high conduction velocity spinal projections than adjacent cells not related to startle. Startle-related cells were not ‘dedicated’ to startle, but were active in relation to spontaneous movements. Both the unit response of the startle-related cells and the startle response recorded in muscles were suppressed by the prior presentation of a weak prepulse. Thus, prepulse inhibition of startle occurs at, or prior to, the medial pontomedullary reticular formation. We conclude that these reticulospinal cells convey the output of the brainstem system modulating and triggering startle.

Reticulospinal pacemaker neurons of the rat rostral ventrolateral medulla with putative sympathoexcitatory function: an intracellular study in vitro

Extra- and intracellular recordings of tonically active neurons were obtained in slices of the rat rostral ventrolateral medulla maintained at 31°C. The predominant type consisted of cells with a regular non-bursting discharge rate of 9 ± 0.3 spikes/s (mean ± S.E.M., n = 84). Intracellular recordings revealed that these neurons ( n = 43) exhibited typical pacemaker potentials reset after a single spike, and an input resistance of 138 ± 10 MΩ (n = 21) . No excitatory postsynaptic potentials were detected even during hyperpolarization (5–10 mV) which invariably resulted in silencing the cells ( n = 28). Eighteen cells were injected intracellularly with Lucifer yellow, and the tissue was subsequently processed for the immunohistochemical detection of the adrenergic marker phenylethanolamine N-methyltransferase (PNMT). None of the 12 dye-marked cells recovered exhibited any PNMT-like immunoreactivity, but all were surrounded by numerous adrenergic neurons. In 7 rats subjected to intraspinal injections (T 3) of rhodamine-tagged microbeads. 4 out of 9 pacemaker cells marked intracellularly with Lucifer yellow were found labeled with the retrograde marker. It is concluded that the rostral ventrolateral medulla contains non-adrenergic reticulospinal cells with intrinsic pacemaker properties. These neurons probably represent a group of sympathoexcitatory cells on which the basal sympathetic tone depends.

Responses of feline medial medullary reticulospinal neurons to cardiac input.

1. Responses of medial medullary reticulospinal (RS) neurons to electrical stimulation of cardiac sympathetic afferents, probing the epicardium and epicardial application of bradykinin, were determined in cats anesthetized with alpha-chloralose. Conduction velocity of RS cells averaged 67 m/s; these neurons were probably part of the RS motor pathway and the bulbospinal pathway that modulates ascending information. Fifty-three RS neurons had unilateral projections and 18 neurons bilateral projections to the thoracic spinal cord. 2. Maximal electrical stimulation of the left stellate ganglion excited 69% of 97 RS neurons with a mean latency of 15 +/- 1.0 ms. Mean spike discharge rate increased from 4 +/- 1.2 to 93 +/- 8.0 spikes/s for responsive neurons. 3. Epicardial bradykinin (0.04 mg) excited 34%, inhibited 2%, and did not affect 64% of 44 RS cells tested. Excited neurons increased their mean discharge rate from 12.3 +/- 3.6 to 18.2 +/- 4.3 spikes/s, with a latency of 24 +/- 3.0 s, in response to bradykinin. Response duration averaged 43 +/- 2.3 s. Neurons responsive to bradykinin had greater spontaneous discharge rates than unresponsive neurons. Thirteen of 16 RS cells excited by bradykinin were located in the gigantocellular tegmental field (FTG); the other three cells were in the paramedian nucleus (PR). 4. Epicardial bradykinin often elicited changes in aortic pressure. Although some neurons responded to altered blood pressure alone, these responses could not account for responses to bradykinin. Furthermore, the percentage of responsive neurons was similar in experiments with intact nerves as well as those with vagotomy and barodenervation. 5. Touching the epicardium with a blunt probe excited 19 of 60 (32%) RS neurons, and visceral receptive fields were mapped for 12 of these cells. Neurons responded with one to three spikes to probing. RS neurons responsive to probing were scattered throughout the medial reticular formation. 6. RS cells were also tested for somatic, auditory, and visual input. Of 63 neurons responsive to sympathetic afferent stimulation, only one did not receive convergent input from at least one of these sources; 51% received input from each tested source. Neurons responsive to bradykinin were more likely to receive visual input than the general population of neurons. RS neurons unresponsive to sympathetic afferent stimulation were less likely to receive convergent inputs from other sensory modalities.(ABSTRACT TRUNCATED AT 400 WORDS)

Substantia nigra influences on the reticulospinal neurons: An electrophysiological and ionophoretic study in cats and rats

The influences exerted by the substantia nigra on reticulospinal neurons and the nature of the synaptic transmitter subserving these projections have been studied in adult cats and rats. Nigral stimulation evokes discharge changes in a significant number of reticulospinal cells (32.4% in cats and 39.1% in rats) on both ipsi- and contralateral sides. The responses were short-latency inhibitions (69.5% in cats and 76.5% in rats), short-latency excitations (22.2% in cats and 23.6% in rats) and in the remaining few cases long-latency excitations. Short-latency excitations, quite similar to the nigra-induced ones, were the predominant response pattern (95.2% in cats and 96.9% in rats) elicited on reticulospinal cells following stimulation of cerebral peduncle. The stimulation of substantia nigra in rats submitted to chronic ablation of sensorimotor cortex elicited only inhibitory responses while stimulation of nigral area in rats with intranigral injection of kainic acid was still capable of evoking short-latency excitations but not short-latency inhibitions. Therefore, the former can be ascribed to activation of corticoreticular fibers running in the cerebral peduncle whereas the latter can be considered as depending on activation of nigral efferents. The nigra-influenced cells were both ‘fast’ and ‘slow’ reticulospinal neurons and resulted mainly located in most rostral regions of reticular formation. Ionophoretic application of GABA suppressed the spontaneous firing of reticulospinal cells while no effect was observed following application of dopamine. The nigra-induced inhibitions were abolished by GABA-antagonist bicuculline and not by dopamine-antagonist fluphenazine. These data suggest that short-latency inhibitions are due to activation of nigral GABAergic fibers that are known to arise in the pars reticulata of substantia nigra. The possible functional significance of such nigro-reticulospinal projection is discussed.

The mesencephalic locomotor region. II. Projections to reticulospinal neurons

Single neurons were recorded extracellularly in an area of the medioventral medulla known to receive descending mesencephalic locomotor region (MLR) input. A large number (47%) of these cells were found to receive short-latency orthodromic input following stimulation of the physiologically identified MLR. Of the medioventral medulla neurons studied, 34% were found to project to the spinal cord (determined by antidromic activation following stimulation of the ventrolateral funiculus). Approximately one-half (17% of the total population) of these reticulospinal cells were found to receive short-latency orthodromic input from the MLR. The regional distribution of this group of reticulospinal cells corresponded with the area found to receive descending projections from the MLR in previous anatomical studies. In addition, electrical and chemical activation of the same region was found to elicit locomotion on a treadmill in a companion study. The present findings demonstrate that descending MLR projections influence a large number of medioventral medulla cells, some of which have direct spinal projections, and suggest that this area is a primary relay in the manifestation of MLR function.