Lateral turns in the Lamprey. I. Patterns of motoneuron activity.

The activity of motoneurons during lateral turns was studied in a lower vertebrate, the lamprey, to investigate how a supraspinal command for the change of direction during locomotion is transmitted from the brain stem and integrated with the activity of the spinal locomotor pattern generator. Three types of experiments were performed. 1) The muscular activity during lateral turns in freely swimming adult lampreys was recorded by electromyography (EMG). It was characterized by increased cycle duration and increased duration, intensity, and cycle proportion of the bursts on the side toward which the animal turns. 2) Electrical stimulation of the skin on one side of the head in a head-spinal cord preparation of the lamprey during fictive locomotion elicited asymmetric ventral root burst activity with similar characteristics as observed in the EMG of intact lampreys during lateral turns. The cycle duration and ventral root burst intensity, duration, and cycle proportion on the side of the spinal cord contralateral to the stimulus were increased; hence a fictive lateral turn away from the stimulus could be produced. The fictive turn propagated caudally with decreasing amplitude. The increase in burst duration during the turn correlated well with the increase in cycle duration, while changes in contralateral burst intensity and burst duration did not co-vary. Turning responses varied depending on the timing (phase) of the skin stimulation: stimuli in the first two-thirds of a cycle evoked a turn in the same cycle, whereas stimuli in the last third gave a turn in the following cycle. The largest turns were evoked by stimuli in the first third of a cycle. 3) Fictive turns were abolished after transection of the trigeminal nerve or a rhombencephalic midline split, but not in a rhombencephalic preparation with transected cerebellar commissure. High spinal hemisection was sufficient to block turning toward the lesioned side, while turns toward the intact side remained. Taken together these findings suggest that the reticulospinal turn command is essentially unilateral and generated in the rhombencephalon.     

Differential contribution of reticulospinal cells to the control of locomotion induced by the mesencephalic locomotor region

In lampreys as in other vertebrates, the reticulospinal (RS) system relays inputs from the mesencephalic locomotor region (MLR) to the spinal locomotor networks. Semi-intact preparations of larval sea lamprey were used to determine the relative contribution of the middle (MRRN) and the posterior (PRRN) rhombencephalic reticular nuclei to swimming controlled by the MLR. Intracellular recordings were performed to examine the inputs from the MLR to RS neurons. Stimulation of the MLR elicited monosynaptic excitatory responses of a higher magnitude in the MRRN than in the PRRN. This differential effect was not attributed to intrinsic properties of RS neurons. Paired recordings showed that at threshold intensity for swimming, spiking activity was primarily elicited in RS cells of the MRRN. Interestingly, cells of the PRRN began to discharge at higher stimulation intensities only when MRRN cells had reached their maximal discharge rate. Glutamate antagonists were ejected in either nucleus to reduce their activity. Ejections over the MRRN increased the stimulation threshold for evoking locomotion and resulted in a marked decrease in the swimming frequency and the strength of the muscle contractions. Ejections over the PRRN decreased the frequency of swimming. This study provides support for the concept that RS cells show a specific recruitment pattern during MLR-induced locomotion. RS cells in the MRRN are primarily involved in initiation and maintenance of low-intensity swimming. At higher frequency locomotor rhythm, RS cells in both the MRRN and the PRRN are recruited.     

Identification of interneurons with contralateral, caudal axons in the lamprey spinal cord: synaptic interactions and morphology

1. As part of a continuing investigation of the organization of the spinal cord of the lamprey, propriospinal interneurons with axons projecting contralaterally and caudally (CC interneurons) were surveyed with intracellular recordings. 2. CC interneurons were identified by recording their axon spikes extracellularly in the spinal cord during intracellular stimulation of the cell body. The axon projections of Cc interneurons were confirmed after intracellular injection and development of horseradish peroxidase. 3. Intracellular stimulation of CC interneurons produced synaptic potentials in myotomal motoneurons, lateral interneurons and other CC interneurons that lay caudally on the opposite side of the spinal cord. Most CC interneurons were inhibitory, but some were excitatory. 4. CC interneurons were divided into three classes on the basis of reticulospinal Müller cell inputs. CC1 interneurons were excited by the ipsilateral Müller cell B1 and the contralateral Mauthner cell. CC1 interneurons were inhibitory. They were excited polysynaptically by ipsilateral sensory dorsal cells and were inhibited by contralateral dorsal cells. They were distinguished morphologically by having no rostral axon branch and no contralateral dendrites. CC1 interneurons were phasically active during fictive swimming with their peak depolarizations preceding those of myotomal motoneurons by about 0.15 cycle. 5. CC2 interneurons were also inhibitory, but they were distinguished from CC1 interneurons by their excitation from the ipsilateral Müller cells B2-4 nd by their thin rostral and thicker caudal axonal branches on the contralateral side of the spinal cord. 6. CC3 interneurons were excitatory, and they were inhibited by the ipsilateral Müller cell I1. CC3 interneurons could have contralateral dendrites and bifurcating axons, and they had lower average axonal conduction velocities than CC1 and CC2 interneurons. 7. Inhibitory CC interneurons may be important for motor coordination in the lamprey. Movements of the lamprey body during reflexes and swimming consist of contraction and relaxation of myotomal muscles on opposite sides of the body. By being coactive with ipsilateral myotomal motoneurons, inhibitory CC interneurons could contribute to the inhibition of contralateral motoneurons during these movements.     

The trigeminal sensory relay to reticulospinal neurones in lampreys

This study was carried out to identify lamprey neurones relaying trigeminal sensory inputs to reticulospinal cells. Double labeling with fluorescent tracers was used in vitro. Fluorescein-conjugated dextran amines were applied to the proximal stump of the cut trigeminal nerve on both sides, and Texas Red-conjugated dextran amines were injected unilaterally in the middle (MRRN) or the posterior (PRRN) rhombencephalic reticular nuclei. Texas Red retrogradely labeled cells were found ipsi- and contralateral to each injection. Any of these cells with the soma or at least a major dendrite among the fluorescein-labeled trigeminal afferent axons was considered a candidate relay cell. Of these two possibilities, only cells with their soma among the fluorescein-labeled trigeminal afferents were found. The candidate relay cells projecting to the MRRN were mostly clustered at the caudal vestibular nerve level within the trigeminal descending tract, whereas the majority of those projecting to the PRRN were located more caudally. The diameter of candidate relay cells ranged from 9.2 to 24.6 mum and 9.2 to 46.1 mum, after MRRN and PRRN injections, respectively. A possible relay function for these cells was tested with electrophysiological experiments. The intracellular responses to trigeminal nerve stimulation were recorded in reticulospinal cells under control conditions and after ejections of a combination of glutamate ionotropic receptor antagonists over the candidate relay cells in small areas along the sulcus limitans. The synaptic responses elicited in MRRN reticulospinal cells were maximally depressed when ejections were made at the level of the vestibular nerve, in accord with the anatomical data. The synaptic responses in PRRN reticulospinal cells showed maximal depression when ejections were made slightly more caudally. Altogether, these results suggest that cells located within the trigeminal descending tract and projecting to reticular nuclei are likely to be the sensory trigeminal relays to reticulospinal neurones in lampreys.     

Vestibulo-reticular projections in adult lamprey: their role in locomotion

This study describes the anatomical projections from vestibular secondary neurons to reticulospinal neurons in the adult lamprey and the modulation of vestibular inputs during fictive locomotion. Anatomical tracers were applied in the posterior (PRRN) and middle rhombencephalic reticular nuclei as well as to the proximal stumps of cut vestibular nerve branches to identify the neurons projecting to the reticular nuclei that were in close proximity with vestibular primary afferents. Labeled neurons were found in the intermediate (ION) and posterior (PON) octavomotor nuclei, and were more numerous on the side of the injection (around 56-87 and 101-107 for the ION and the PON, respectively). Morphologies varied but cells were mostly round or oval. Axonal projections from the PON formed a dense bundle, whereas those from the ION were less densely packed. Based on their morphology and the distribution of their projections, most vestibulo-reticular neurons were presumed to be vestibulospinal cells. Reticulospinal cells from the PRRN were recorded intracellularly in the in vitro brainstem-spinal cord preparation and large excitatory post-synaptic potentials (EPSPs) were evoked following stimulation of the ipsilateral anterior and the contralateral posterior branches of the vestibular nerves, whereas inhibitory post-synaptic potentials (IPSPs) or smaller EPSPs were elicited by stimulation of the ipsilateral posterior or of the contralateral anterior branches. During fictive locomotion, both the excitatory and the inhibitory responses displayed phasic changes in amplitude such that the amplitude of the EPSPs was minimal when the spinal cord activity switched from the ipsilateral to the contralateral side of the recorded reticulospinal cell. The IPSPs were then of maximal amplitude. We propose that this modulation could serve to reduce the influence of vestibular inputs in response to head movements during locomotion.     

Spinal locomotor inputs to individually identified reticulospinal neurons in the lamprey

Locomotor feedback signals from the spinal cord to descending brain stem neurons were examined in the lamprey using the uniquely identifiable reticulospinal neurons, the Müller and Mauthner cells. The same identified reticulospinal neurons were recorded in several preparations, under reduced conditions, to address whether an identified reticulospinal neuron shows similar locomotor-related oscillation timing from animal to animal and whether these timing signals can differ significantly from other identified reticulospinal neurons. Intracellular recordings of membrane potential in identified neurons were made in an isolated brain stem-spinal cord preparation with a high-divalent cation solution on the brain stem to suppress indirect neural pathways and with D-glutamate perfusion to the spinal cord to induce fictive swimming. Under these conditions, the identified reticulospinal neurons show significant clustering of the timings of the peaks and troughs of their locomotor-related oscillations. Whereas most identified neurons oscillated in phase with locomotor bursting in ipsilateral ventral roots of the rostral spinal cord, the B1 Müller cell, which has an ipsilateral descending axon, and the Mauthner cell, which has a contralateral descending axon, both had oscillation peaks that were out of phase with the ipsilateral ventral roots. The differences in oscillation timing appear to be due to differences in synaptic input sources as shown by cross-correlations of fast synaptic activity in pairs of Müller cells. Since the main source of the locomotor input under these experimental conditions is ascending neurons in the spinal cord, these experiments suggest that individual reticulospinal neurons can receive locomotor signals from different subsets of these ascending neurons. This result may indicate that the locomotor feedback signals from the spinal locomotor networks are matched in some way to the motor output functions of the individual reticulospinal neurons, which include command signals for turning and for compensatory movements.     

Responses of reticulospinal neurons in the lamprey to lateral turns

When swimming, the lamprey maintains a definite orientation of its body in the vertical planes, in relation to the gravity vector, as the result of postural vestibular reflexes. Do the vestibular-driven mechanisms also play a role in the control of the direction of swimming in the horizontal (yaw) plane, in which the gravity cannot be used as a reference direction? In the present study, we addressed this question by recording responses to lateral turns in reticulospinal (RS) neurons mediating vestibulospinal reflexes. In intact lampreys, the activity of axons of RS neurons was recorded in the spinal cord by implanted electrodes. Vestibular stimulation was performed by periodical turns of the animal in the yaw plane (60 degrees peak to peak). It was found that the majority of responding RS neurons were activated by the contralateral turn. By removing one labyrinth, we found that yaw responses in RS neurons were driven mainly by input from the contralateral labyrinth. We suggest that these neurons, when activated by the contralateral turn, will elicit the ipsilateral turn and thus will compensate for perturbations of the rectilinear swimming caused by external factors. It is also known that unilateral eye illumination elicits a contralateral turn in the yaw plane (negative phototaxis). We found that a portion of RS neurons were activated by the contralateral eye illumination. By eliciting an ipsilateral turn, these neurons could mediate the negative phototaxis.     

Vestibular control of swimming in lamprey

1. Experiments were carried out on an in vitro preparation of the lamprey brainstem isolated together with intact labyrinths. Responses of reticulospinal neurons from different brainstem reticular nuclei (mesencephalic, MRN; anterior rhombencephalic, ARRN; middle rhombencephalic, MRRN; and posterior rhombencephalic, PRRN) to rotation of the preparation (0 degrees-360 degrees) either in the sagittal plane (pitch tilt, or nose up-down movement) or in the transverse plane (roll tilt, or left-right inclination) were recorded. 2. Responses to roll tilt were qualitatively similar in all nuclei: contralateral side down tilt (in relation to the location of the neuron in the brain) caused an activation of reticulospinal neurons. The angular thresholds for activation differed, however, between nuclei as well as the angle at which the maximal activity occurred. The maximal response for MRN was at 45 degrees, for MRRN and PRRN at 90 degrees, for ARRN at 180 degrees. Thus, the zones of spatial sensitivity differed in different nuclei, and they covered the whole range of possible inclinations in the transverse plane. 3. Responses to pitch tilt were not uniform in the different nuclei. MRN neurons responded preferentially in the range of 45 degrees-90 degrees nose-up inclinations, but a proportion of the cells responded in the range of 45 degrees-90 degrees nose-down inclinations. The ARRN neurons had their maximal response when the brain was turned to a dorsal side-down position (180 degrees). In the MRRN, three subgroups of neurons could be distinguished, the first responding at around 90 degrees nose-down, the second responding at around 90 degrees nose-up and the third responding in both zones. However, the activation in the nose-up zone was less robust: responses in this zone were present only in approximately one half of the experiments. Finally, the PRRN neurons were found to be very heterogeneous, with their zones of sensitivity being distributed throughout the whole space (0 degrees-360 degrees). Thus, also in the sagittal plane, the zones of spatial sensitivity in the different nuclei covered the whole range of possible inclinations. 4. Long-term recording of MRRN neurons having the zone of sensitivity around 90 degrees nose-up showed that this response was rather unstable. Its amplitude varied considerably and could disappear with time to reappear later. These results, together with the fact that in a part of the experiments the MRRN neurons responded only in the 90 degrees nose-down zone (see above), leads us to suggest that the system of spatial orientation can dynamically re-organize.(ABSTRACT TRUNCATED AT 400 WORDS)     

Movements and muscle activity initiated by brain locomotor areas in semi-intact preparations from larval lamprey

In in vitro brain/spinal cord preparations from larval lamprey, locomotor-like ventral root burst activity can be initiated by pharmacological (i.e., "chemical") microstimulation in several brain areas: rostrolateral rhombencephalon (RLR); dorsolateral mesencephalon (DLM); ventromedial diencephalon (VMD); and reticular nuclei. However, the quality and symmetry of rhythmic movements that would result from this in vitro burst activity have not been investigated in detail. In the present study, pharmacological microstimulation was applied to the above brain locomotor areas in semi-intact preparations from larval lamprey. First, bilateral pharmacological microstimulation in the VMD, DLM, or RLR initiated symmetrical swimming movements and coordinated muscle burst activity that were virtually identical to those during free swimming in whole animals. Unilateral microstimulation in these brain areas usually elicited asymmetrical undulatory movements. Second, with synaptic transmission blocked in the brain, bilateral pharmacological microstimulation in parts of the anterior (ARRN), middle (MRRN), or posterior (PRRN) rhombencephalic reticular nucleus also initiated symmetrical swimming movements and muscle burst activity. Stimulation in effective sites in the ARRN or PRRN initiated higher-frequency locomotor movements than stimulation in effective sites in the MRRN. Unilateral stimulation in reticular nuclei elicited asymmetrical rhythmic undulations or uncoordinated movements. The present study is the first to demonstrate in the lamprey that stimulation in higher-order locomotor areas (RLR, VMD, DLM) or reticular nuclei initiates and sustains symmetrical, well-coordinated locomotor movements and muscle activity. Finally, bilateral stimulation was a more physiologically realistic test of the function of these brain areas than unilateral stimulation.     

Reticulospinal neurons receive direct spinobulbar inputs during locomotor activity in lamprey

Reticulospinal neurons of the lamprey brain stem receive rhythmic input from the spinal cord during locomotor activity. The goal of the present study was to determine whether such spinal input has a direct component to reticulospinal neurons or depends on brain stem interneurons. To answer this question, an in vitro lamprey brain stem-spinal cord preparation was used with a diffusion barrier placed caudal to the obex, separating the experimental chamber into two baths. Locomotor activity was induced in the spinal cord by perfusion of d-glutamate or N-methyl-dl-aspartate into the spinal cord bath. The brain stem bath was first perfused with normal Ringer solution followed by a high-Ca(2+), -Mg(2+) solution, which reduced polysynaptic transmission. The amplitudes of membrane potential oscillations of reticulospinal neurons in the posterior and middle rhombencephalic reticular nuclei (PRRN and MRRN, respectively) recorded with sharp intracellular microelectrodes did not significantly change from normal to high-divalent solution. This finding suggests a large part of the spinal input creating the oscillations is direct to the reticulospinal neurons. Application of strychnine to the high-Ca(2+), -Mg(2+) solution decreased membrane potential oscillation amplitude, and injection of Cl(-) reversed presumed inhibitory postsynaptic potentials, indicating a role for direct spinal inhibitory inputs. Although reduced, the persistence of oscillations in strychnine suggests that spinal excitatory inputs also contribute to the oscillations. Thus it was concluded that both excitatory and inhibitory spinal neurons provide direct rhythmic inputs to reticulospinal cells of the PRRN and MRRN during locomotor activity. These inputs provide reticulospinal cells with information regarding the activity of the spinal locomotor networks.     

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.     

Spinobulbar neurons in lamprey: cellular properties and synaptic interactions

An in vitro preparation of the nervous system of the lamprey, a lower vertebrate, was used to characterize the properties of spinal neurons with axons projecting to the brain stem [i.e., spinobulbar (SB) neurons)]. To identify SB neurons, extracellular electrodes on each side of the spinal cord near the obex recorded the axonal spikes of neurons impaled with sharp intracellular microelectrodes in the rostral spinal cord. The ascending spinal neurons (n = 144) included those with ipsilateral (iSB) (63/144), contralateral (cSB) (77/144), or bilateral (bSB) (4/144) axonal projections to the brain stem. Intracellular injection of biocytin revealed that the SB neurons had small- to medium-size somata and most had dendrites confined to the ipsilateral side of the cord, although about half of the cSB neurons also had contralateral dendrites. Most SB neurons had multiple axonal branches including descending axons. Electrophysiologically, the SB neurons were similar to other lamprey spinal neurons, firing spikes throughout long depolarizing pulses with some spike-frequency adaptation. Paired intracellular recordings between SB and reticulospinal (RS) neurons revealed that SB neurons made either excitatory or inhibitory synapses on RS neurons and the SB neurons received excitatory input from RS neurons. Mutual excitation and feedback inhibition between pairs of RS and SB neurons were observed. The SB neurons also received excitatory inputs from primary mechanosensory neurons (dorsal cells), and these same SB neurons were rhythmically active during fictive swimming, indicating that SB neurons convey both sensory and locomotor network information to the brain stem.     

Visual self-motion perception during head turns

Extra-retinal information is critical in the interpretation of visual input during self-motion. Turning our eyes and head to track objects displaces the retinal image but does not affect our ability to navigate because we use extra-retinal information to compensate for these displacements. We showed observers animated displays depicting their forward motion through a scene. They perceived the simulated self-motion accurately while smoothly shifting the gaze by turning the head, but not when the same gaze shift was simulated in the display; this indicates that the visual system also uses extra-retinal information during head turns. Additional experiments compared self-motion judgments during active and passive head turns, passive rotations of the body and rotations of the body with head fixed in space. We found that accurate perception during active head turns is mediated by contributions from three extra-retinal cues: vestibular canal stimulation, neck proprioception and an efference copy of the motor command to turn the head.     

Responses of cat mesencephalic reticulospinal neurons to stimulation of superior colliculus,pericruciate cortex,and neck muscle afferents

Summary Neurons were recorded extracellularly in the mesencephalic reticular formation outside the interstitial nucleus of Cajal in cerebellectomized cats anesthetized with chloralose. Reticulospinal neurons were identified by antidromic stimulation of the upper cervical segments. Stimulation in the deep layers of the ipsilateral superior colliculus evoked firing in 36% of reticulospinal neurons. For many neurons thresholds for activation were high in the intermediate tectal layers and declined as the electrodes entered the underlying tegmentum. However, low threshold points were found above the deep fiber layer within the superior colliculus for some cells. Stimulation of the contralateral superior colliculus excited 10% of neurons and thresholds for activation were high above the deep fiber layer for all neurons. Stimulation of the ipsilateral and contralateral pericruciate cortex excited 39 and 21% of neurons, respectively. The lowest threshold area was found in the frontal eye fields. Sixteen percent of neurons received excitation from neck muscle afferents (C₂ biventer-cervicis) bilaterally. Comparison of responses between mesencephalic reticulospinal neurons and interstitiospinal neurons (Fukushima et al. 1981) showed that responses of the two groups of neurons were similar when the pericruciate cortex and neck muscle afferents were stimulated. However, a difference was observed in tectal responses, since low threshold points were rarely observed above the deep fiber layer for interstitiospinal neurons.Supported in part by a Grant-in-Aid for Scientific Research (No. 477063) from The Ministry of Education, Science, and Culture of Japan     

Responses of reticulospinal neurons in intact lamprey to vestibular and visual inputs

A lamprey maintains the dorsal-side-up orientation due to the activity of postural control system driven by vestibular input. Visual input can affect the body orientation: illumination of one eye evokes ipsilateral roll tilt. An important element of the postural network is the reticulospinal (RS) neurons transmitting commands from the brain stem to the spinal cord. Here we describe responses to vestibular and visual stimuli in RS neurons of the intact lamprey. We recorded activity from the axons of larger RS neurons with six extracellular electrodes chronically implanted on the surface of the spinal cord. From these multielectrode recordings of mass activity, discharges in individual axons were extracted by means of a spike-sorting program, and the axon position in the spinal cord and its conduction velocity were determined. Vestibular stimulation was performed by rotating the animal around its longitudinal axis in steps of 45 degrees through 360 degrees. Nonpatterned visual stimulation was performed by unilateral eye illumination. All RS neurons were classified into two groups depending on their pattern of response to vestibular and visual stimuli; the groups also differed in the axon position in the spinal cord and its conduction velocity. Each group consisted of two symmetrical, left and right, subgroups. In group 1 neurons, rotation of the animal evoked both dynamic and static responses; these responses were much larger when rotation was directed toward the contralateral labyrinth, and the dynamic responses to stepwise rotation occurred at any initial orientation of the animal, but they were more pronounced within the angular zone of 0-135 degrees. The zone of static responses approximately coincided with the zone of pronounced dynamic responses. The group 1 neurons received excitatory input from the ipsilateral eye and inhibitory input from the contralateral eye. When vestibular stimulation was combined with illumination of the ipsilateral eye, both dynamic and static vestibular responses were augmented. Contralateral eye illumination caused a decrease of both types of responses. Group 2 neurons responded dynamically to rotation in both directions throughout 360 degrees. They received excitatory inputs from both eyes. Axons of the group 2 neurons had higher conduction velocity and were located more medially in the spinal cord as compared with the group 1 neurons. We suggest that the reticulospinal neurons of group 1 constitute an essential part of the postural network in the lamprey. They transmit orientation-dependent command signals to the spinal cord causing postural corrections. The role of these neurons is discussed in relation to the model of the roll control system formulated in our previous studies.     

Sound localization in anurans. I. Evidence of binaural interaction in dorsal medullary nucleus of bullfrogs (Rana catesbeiana).

1. The response patterns of single cells to monaural and binaural acoustic stimuli were studied in the dorsal medullary nucleus of the bullfrog (Rana catesbeiana). This nucleus represents the first ascending center in the anuran's central auditory nervous system. 2. Of the 142 cells isolated, 75 units responded only to monaural stimulation. Approximately 80% of these monaural cells could be excited by the ipsilateral ear, while the remaining 20% received their excitatory input from the contralateral ear. The other 67 units responded to binaural stimuli. Of these binaural cells, 14 could be excited by either contralateral or ipsilateral stimuli, and the threshold and best excitatory frequency were similar for each ear (EE). The other 53 binaural cells (EI) could be excited by stimulation of one ear and inhibited by stimulation of the other ear; for almost all of these cells the contralateral ear was excitatory and the ipsilateral ear was inhibitory. The best inhibitory frequency for one ear was approximately the same as the best excitatory frequency for the other ear, and the threshold for inhibition was near the threshold for excitation. 3. The tuning curves for all of the cells in the dorsal medullary nucleus were unimodal with "Q" values ranging from 0.4 to 4. The excitatory thresholds were widely scattered between 22 and 115 dB SPL. 4. The distribution of best excitatory frequencies for the monaural cells comprised three groups: 200-300, 500-800, and 900-1,600 Hz. The best excitatory frequencies of the binaural cells were scattered over this entire range, with a broad peak around 200-800 Hz. 5. Approximately 80% of the cells in the dorsal nucleus responded tonically throughout the duration of an excitatory tone burst. The remaining 20% of the cells responded phasically during the transient stages of a tone burst over a wide intensity range. 6. Response latencies were compared for the two types of monaural cells to tones at their best exciatatory frequencies at 10 dB above threshold. The latencies for the contralaterally excitable cells were just a few milliseconds longer than the latencies for the ipsilaterally excitable cells. For binaural cells the latency for contralateral stimulation was only 1-2 ms longer than for ipsilateral stimulation. It was concluded that the contralateral input to the dorsal medullary nucleus is not of efferent descending origin from higher auditory centers. 7. All of the binaural EI cells were sensitive to small interaural intensity differences and many were also sensitive to minute interaural time differences.These cells likely play a role in localization of sounds of significance to anurans.     

Defining the excitatory neurons that drive the locomotor rhythm in a simple vertebrate: insights into the origin of reticulospinal control

Important questions remain about the origin of the excitation that drives locomotion in vertebrates and the roles played by reticulospinal neurons. In young Xenopus tadpoles, paired whole-cell recordings reveal reticulospinal neurons that directly excite swimming circuit neurons in the brainstem and spinal cord. They form part of a column of neurons (dINs) with ipsilateral descending projections which fire reliably and rhythmically in time with swimming. We ask if, at this early stage of development, these reticulospinal neurons are themselves the primary source of rhythmic drive to spinal cord neurons on each cycle of swimming. Loose-patch recordings in the hindbrain and spinal cord from neurons active during fictive swimming distinguished dINs from other neurons by spike shape. These recordings showed that reticulospinal dINs in the caudal hindbrain (rhombomeres 7–8) fire significantly earlier on each swimming cycle than other, ipsilateral, swimming circuit neurons. Whole-cell recordings showed that fast EPSCs typically precede, and probably drive, spikes in most swimming circuit neurons. However, the earliest-firing reticulospinal dINs spike too soon to be driven by underlying fast EPSCs. We propose that rebound following reciprocal inhibition can contribute to early reticulospinal dIN firing during swimming and show rebound firing in dINs following evoked, reciprocal inhibitory PSPs. Our results define reticulospinal neurons that are the source of the primary, descending, rhythmic excitation that drives spinal cord neurons to fire during swimming. These neurons are an integral part of the rhythm generating circuitry. We discuss the origin of these reticulospinal neurons as specialised members of a longitudinally distributed population of excitatory interneurons extending from the brainstem into the spinal cord.     

Modulatory effect of substance P to the brain stem locomotor command in lampreys

Substance P initiates locomotion when injected in the brain stem of mammals. This study examined the possible role of this peptide on the supraspinal locomotor command system in lampreys. Substance P was bath applied or locally injected into an in vitro isolated brain stem, and the effects of the drug were examined on reticulospinal cells and on the occurrence of swimming in a semi-intact preparation. Bath applications of substance P induced sustained depolarizations occurring rhythmically in intracellularly recorded reticulospinal cells. Spiking activity was superimposed on the depolarizations and swimming was induced. The sustained depolarizations were abolished by tetrodotoxin, and substance P did not affect the membrane resistance of reticulospinal cells nor their firing properties, suggesting that it did not directly effect reticulospinal cells. To establish where the effects were exerted, successive lesions of the brain stem were made as well as local applications of the drug in the brain stem. Removing the mesencephalon abolished the sustained depolarizations, whereas large ejections of the drug in the mesencephalon excited reticulospinal cells and elicited bouts of swimming. More local injections into the mesencephalic locomotor region (MLR) also elicited swimming. After an injection of substance P, the current threshold needed to induce locomotion by MLR stimulation was decreased, and the size of the postsynaptic responses of reticulospinal cells to MLR stimulation was increased. Substance P also reduced the frequency of miniature spontaneous postsynaptic currents in reticulospinal cells. Taken together, these results suggest that substance P plays a neuromodulatory role on the brain stem locomotor networks of lampreys.     

Vestibular control of swimming in lamprey

Summary Experiments were carried out on the in vitro preparation of the lamprey brainstem isolated together with the labyrinths. The brain orientation in space could be changed in steps of 45° by rotation (360°) around the longitudinal axis (roll) or the transverse axis (pitch). Vestibular afferents in the VIII nerve, or reticulospinal (RS) neurons, were recorded extracellularly during roll and pitch. Two main types of afferents could be distinguished. Presumed otolith afferents responded both to a change of position and to a maintained new position. These afferents were classified in several groups according to the position of their zone of sensitivity. For roll, the largest group had their maximal sensitivity around 90° tilt to the ipsilateral side, the next group in size responded at 180°, and only a few afferents were activated by contralateral roll. For pitch, there are groups responding with maximal sensitivity at 90° nose-up, 90° nose-down and at 180°. A minority of afferents were active when the brainstem was in a normal position, i.e. horizontal, with the dorsal side up. Another type of afferent responded only by a high-frequency burst to a change of brain orientation. They were classified as canal afferents in analogy with other species. All tested canal afferents responded to rotation towards ipsi-side down. Pitch tilt revealed two groups that responded to rotation towards either nose-up or nosedown. RS neurons from the anterior and middle rhombencephalic nuclei (ARRN and MRRN) were recorded before and after unilateral transection of the VIII nerve. For ARRN neurons, the inputs from the ipsi- and contralateral labyrinths were found to be almost equivalent, while for MRRN neurons these inputs contributed differently to the cells' response to tilt.