Biomedical subjects

S Grillner

Publications and source records attributed to S Grillner.

At least 109 records · Page 6Linked to original sources

PubMed1993

Visual input affects the response to roll in reticulospinal neurons of the lamprey.

A body orientation with the dorsal side up is usually maintained by lampreys during locomotion. Of crucial importance for this is the vestibular-driven control system. A visual input can affect the body orientation: illumination of one eye during swimming evokes roll tilt towards the source of light. The aim of the present study was to investigate the interaction of visual and vestibular inputs in reticulospinal (RS) neurons of the brainstem. The RS system is the main descending system transmitting information from the brainstem to the spinal cord. The response of neurons in the middle rhombencephalic reticular nucleus to a unilateral non-patterned optic input was investigated, as well as the influence of this input on the response of RS neurons to vestibular stimulation (roll tilt). Experiments were carried out on a brainstem preparation with intact labyrinths and, in some cases, intact eyes. Illumination of one eye or electrical stimulation of the optic nerve (10 Hz) resulted in an activation of RS neurons preferentially on the ipsilateral side of the brainstem. The same result was obtained after ablation of the optic tectum, demonstrating that there are asymmetrical visual projections to the lower brainstem which do not involve the tectum. Stimulation of the optic nerve strongly affected the vestibular response in RS neurons. As a rule RS neurons are silent at the normal (dorsal-side-up) orientation of the brainstem and become active with contralateral roll tilt. During continuous optic nerve stimulation, however, the RS neurons on the side of stimulation fire during normal orientation of the brainstem, and the response to contralateral roll tilt increases considerably in many neurons.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1993

Effects of serotonin on fictive locomotion coordinated by a neural network deprived of NMDA receptor-mediated cellular properties.

The neural network coordinating fictive locomotion in the isolated lamprey spinal cord can be activated by either N-methyl-D-aspartate (NMDA) or kainate/AMPA receptors. Serotonin (5-HT) is known to affect the pattern of fictive locomotion induced by NMDA receptor activation by increasing spike rate within a burst, increasing burst duration, and increasing the intersegmental delay. Two cellular mechanisms contribute to these 5-HT induced effects: The after-hyperpolarization following individual action potentials is reduced and the depolarizing plateau induced by NMDA is prolonged. Both of these serve as important burst-terminating factors in the locomotor network. In order to isolate these two mechanisms, the 5-HT effect on plateau potentials was eliminated by applying 5-HT during fictive locomotion induced by activation of kainate/AMPA receptors. In the absence of NMDA-mediated plateau potentials, the 5-HT-induced increase in burst duration and cycle period was greater than that previously reported during NMDA-induced fictive locomotion. In addition, the prolonged burst period of a single side was subdivided into brief multiple bursts with a shorter cycle period than the control reciprocal burst activity. Intracellular recordings of spinal neurons combined with chloride injection to reverse inhibitory post-synaptic potentials revealed that, in a proportion of these cells, the bursting within a single side may be generated solely or predominantly by phasic excitation.

2-Amino-5-phosphonovalerate↗

PubMed1993

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.

Animals↗

PubMed1993

The spinal GABA system modulates burst frequency and intersegmental coordination in the lamprey: differential effects of GABAA and GABAB receptors.

1. The effect of spinal GABAergic neurons on the segmental neuronal network generating locomotion has been analyzed in the lamprey spinal cord in vitro. It is shown that gamma-aminobutyric acid (GABA)A- and GABAB-mediated effects influence the burst frequency and the intersegmental coordination and that the GABA system is active during normal locomotor activity. 2. Fictive locomotor activity was induced by superfusing the spinal cord with a Ringer solution containing N-methyl-D-aspartate (NMDA, 150 microM). The efferent locomotor activity was recorded by suction electrodes from the ventral roots or intracellularly from interneurons or motoneurons. If a GABA uptake blocker was added to the perfusate, the burst rate decreased. This effect was counteracted by GABAB receptor blockade by phaclofen or 2-(OH)-saclofen. If instead a GABAB receptor agonist (baclofen) was added during fictive locomotion, a depression of the burst rate occurred. It was concluded that a GABAB receptor activation due to an endogenous release of GABA caused a depression of the burst activity with a maintained well-coordinated locomotor activity. 3. If a GABAA receptor antagonist (bicuculline) is applied during fictive locomotion elicited by NMDA, a certain increase of the burst rate occurred. Conversely, if a selective GABAA agonist (muscimol) was administered, the burst rate decreased. Similarly, if the GABAA receptor activity was potentiated by activation of a benzodiazepine site by diazepam, the burst rate was reduced. If, however the GABAergic effect was first enhanced by an uptake blocker (nipecotic acid), an administration of a GABAA antagonist (bicuculline) increased the burst rate, but in addition, the burst pattern became less regular with recurrent shorter periods without clear reciprocal burst activity. The GABAA receptor activity appears important for the rate control and for permitting a regular burst pattern. 4. The intersegmental coordination in the lamprey is characterized by a rostrocaudal constant phase lag of approximately 1% of the cycle duration between the activation of consecutive segments during forward swimming. This rostrocaudal phase lag can be reversed during backward swimming, which can be induced also experimentally in the isolated spinal cord by providing a higher excitability to the caudal segments. In a split-bath configuration, a GABA uptake blocker or a GABAB agonist was administered to the rostral part of the spinal cord, which caused a reversal of the phase lag as during backward swimming. If GABAA receptors were blocked under similar conditions, the intersegmental coordination became irregular. It is concluded that an increased GABA activity in a spinal cord region can modify the intersegmental coordination.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1993

Computer simulations of NMDA and non-NMDA receptor-mediated synaptic drive: sensory and supraspinal modulation of neurons and small networks.

1. The segmental locomotor network in lamprey can generate the rhythmic burst pattern underlying locomotion when it is driven via synaptic glutamate receptors. Lower rates of activity can be evoked by activation of N-methyl-D-aspartate (NMDA) receptors, whereas a rapid activity can only be induced by non-NMDA receptors [kainate/alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)]. The reticulospinal and sensory inputs are known to act via both NMDA and non-NMDA receptors, but it is unclear how these inputs can provide an appropriate control of the locomotor rate. We have examined the effects of different types of excitatory synaptic input to neurons of the locomotor network with the use of a computer-simulated electrical neuron model, with Na+, K+, Ca(2+)-dependent K+ channels, and with inherent oscillatory properties linked to the NMDA conductance. Synapses were modeled as a modulated ionic conductance in the membrane of the postsynaptic cell comprising a voltage-dependent NMDA component (Na+, K+, Ca2+ conductances) of long duration, and/or a non-NMDA component (Na+, K+ conductance) of short duration. 2. By using two neurons to drive a postsynaptic cell with non-NMDA-type synapses, a continuous range of firing frequencies could be evoked in the postsynaptic cell, by altering the firing rate of the presynaptic cells. If a single presynaptic neuron was used, there was a tendency toward spike synchronization between the pre- and postsynaptic cells. 3. When a postsynaptic neuron was driven via NMDA synapses, an oscillatory burst activity could be evoked. The rate of the oscillations was, however, little affected by the presynaptic firing rate. When a drive neuron with mixed (NMDA and non-NMDA) synapses was used, the rate of the oscillations could be changed within a limited frequency range by altering the presynaptic firing rate. By adding another larger drive neuron, having a larger rheobase current and mixed synapses with smaller relative NMDA components, the frequency range of the postsynaptic oscillations could be markedly increased. The frequency range depended on the parameters selected for each of the two types of mixed synapses. 4. A small rhythm-generating neuronal network, comprising six cells connected as the principal interneurons of the lamprey spinal locomotor network, was used to test the role of a tonic NMDA and non-NMDA receptor activation to drive the network and produce bursting.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1993

GABAB receptor activation causes a depression of low- and high-voltage-activated Ca2+ currents, postinhibitory rebound, and postspike afterhyperpolarization in lamprey neurons.

1. Activation of gamma-aminobutyric acid-B (GABAB) receptors during N-methyl-D-aspartate (NMDA)-induced fictive locomotor activity in the lamprey spinal cord reduces the burst frequency and changes the intersegmental coordination. Presynaptic inhibition of both the excitatory and inhibitory synaptic transmission from spinal premotor interneurons occurs through GABAB receptor activation. To further analyze the cellular mechanisms underlying the GABABergic modulation of the locomotor network, the present study investigates somatodendritic effects of GABAB receptor activation on interneurons and motoneurons in the lamprey spinal cord in vitro using single-electrode current- and voltage-clamp techniques. 2. High- (HVA) and low- (LVA) voltage-activated calcium currents were studied with single-electrode voltage clamp when Na+ and K+ currents were blocked--using tetrodotoxin, tetraethylammonium (TEA), and CsCl electrodes--after substituting Ca2+ with Ba2+. Cobalt-sensitive inward barium currents, activated at -50 mV, became larger when the holding potential was set to a more hyperpolarized level, thus suggesting the existence of an LVA calcium current. The presence of cobalt-sensitive inward barium currents activated at -30 and -10 mV suggests the existence of an HVA calcium current. GABAB receptor activation (baclofen) reduced the peak amplitude of both the LVA and HVA Ca2+ component. 3. The late phase of the afterhyperpolarization (AHP), which follows the action potential, was reduced in amplitude by cobalt, thus lending further support to the notion that the Ca2+ influx, and the subsequent activation of Ca(2+)-dependent K+ channels (KCa2+), constitutes the major part of the AHP generation. Application of the GABAB agonist baclofen also reduced the peak amplitude of the AHP in interneurons and motoneurons, and this reduction was counteracted by the GABAB antagonist 2(OH)saclofen. Baclofen reduced the duration of action potentials broadened by TEA, thus suggesting that the Ca2+ inflow was reduced. Intracellular injection of the GTP analogue GTP gamma S also reduced the duration of the action potential and the peak amplitude of the AHP in TEA, thus supporting the notion that a GTP-binding protein (G-protein)-mediated GABAB receptor activation reduced the calcium inflow, leading to less activation of KCa channels and, consequently, to a smaller peak amplitude of the AHP. 4. Baclofen suppressed the subthreshold depolarization induced by a depolarizing current pulse injection without affecting either the spike threshold or the resting membrane conductance.(ABSTRACT TRUNCATED AT 400 WORDS)

Afferent Pathways↗

PubMed1993

Voltage-clamp frequency domain analysis of NMDA-activated neurons.

1. Voltage and current-clamp steps were added to a sum of sine waves to measure the tetrodotoxin-insensitive membrane properties of neurons in the intact lamprey spinal cord. A systems analysis in the frequency domain was carried out on two types of cells that have very different morphologies in order to investigate the structural dependence of their electrophysiological properties. The method explicitly takes into account the geometrical shapes of (i) nearly spherical dorsal cells with one or two processes and (ii) motoneurons and interneurons that have branched dendritic structures. Impedance functions were analysed to obtain the cable properties of these in situ neurons. These measurements show that branched neurons are not isopotential and, therefore, a conventional voltage-clamp analysis is not valid. 2. The electrophysiological data from branched neurons were curve-fitted with a lumped soma-equivalent cylinder model consisting of eight equal compartments coupled to an isopotential cell body to obtain membrane parameters for both passive and active properties. The analysis provides a quantitative description of both the passive electrical properties imposed by the geometrical structure of neurons and the voltage-dependent ionic conductances determined by ion channel kinetics. The model fitting of dorsal cells was dominated by a one-compartment resistance and capacitance in parallel (RC) corresponding to the spherical, non-branched shape of these cells. Branched neurons required a model that contained both an RC compartment and a cable that reflected the structure of the cells. At rest, the electrotonic length of the cable was about two. Uniformly distributed voltage-dependent ionic conductance sites were adequate to describe the data at different membrane potentials. 3. The frequency domain admittance method in conjunction with a step voltage clamp was used to control and measure the oscillatory behavior induced by N-methyl-D-aspartate (NMDA) on lamprey spinal cord neurons. Voltage-clamp currents and impedance functions were measured at different membrane potentials. The impedance functions had a voltage-dependent resonance and phase shift characteristic of a negative conductance. These measurements provide a quantitative analysis of the conductances induced by NMDA in central neurons of the lamprey spinal cord and directly establish the basis of the non-linear oscillatory behavior previously observed in the presence of NMDA. NMDA was shown specifically to activate a negative and a positive conductance, both of which were markedly affected by the membrane potential. It is shown that the net current in the presence of NMDA must be considered as the algebraic sum of currents in opposite directions.(ABSTRACT TRUNCATED AT 400 WORDS)

Amino Acids↗

PubMed1992

Subcellular distribution of serotonin in the lamprey spinal cord.

The subcellular distribution of serotonin (5-hydroxytryptamine; 5-HT) in the lamprey (Ichtyomyzon unicuspis, Lampetra fluviatilis) spinal cord was investigated by using ultracentrifugation on continuous density gradients combined with an electron microscopic analysis of the gradients and of immunostained tissue. Endogenous 5-HT was analyzed by high-performance liquid chromatography with electrochemical detection. After differential centrifugation, the highest levels of 5-HT were found in the particulate fractions. After ultracentrifugation of lysed synaptosomal fractions on continuous sucrose gradients and the subsequent sedimentation of the individual fractions, 5-HT showed a biphasic distribution in the gradient. The two peaks corresponded to 0.30-0.40 M and 0.85-1.05 M sucrose. Electron microscopy of intact tissue showed that some of the boutons were strongly immunoreactive to 5-HT with dense precipitates over large granular vesicles. The area around these large vesicles, however, also showed reaction product. Large granular vesicles could be clearly distinguished in the immunostained axonal varicosities. In tissue not processed for 5-HT immunoreactivity it was seen that the varicosities contained not only large dense-cored vesicles, but also small agranular vesicles. An electron microscopical analysis of the subcellular fractions revealed that the fraction corresponding to the "light" 5-HT peak contained numerous vesicular structures, which in most cases were electron lucent. In the "heavy" fractions, nerve ending particles containing vesicles of various sizes were observed. The results suggest that 5-HT in the lamprey spinal cord may be distributed in more than one subcellular compartment which, apart from the cytosol, possibly corresponds to small and large synaptic vesicles.

Animals↗

PubMed1992

Two types of motoneurons supplying dorsal fin muscles in lamprey and their activity during fictive locomotion.

The location and dendritic morphology of motoneurons supplying the dorsal fin muscles were studied in the lamprey spinal cord (Ichthyomyzon unicuspis). Motoneurons were retrogradely labelled after injection of HRP into the fin muscles or after its application on the cut ends of the ventral roots. HRP-labelled cells were subsequently reconstructed, in the horizontal and/or transverse planes. Fin motoneurons were also injected intracellularly with Lucifer Yellow and their detailed three-dimensional structure was analysed by confocal laser-scanning microscopy. Unlike myotomal motoneurons, which are closely spaced in the lateral cell column, fin motoneurons were distributed along the spinal cord separately or in pairs. They could be distinguished from motoneurons supplying trunk muscles by having a limited number of dendrites in the lateral part of the spinal cord. In addition, some fin motoneurons extend their dendrites into the dorsal column. The motor cells innervating fin muscles were divided into two types based on their dendritic morphology. Type I have a widespread dendritic tree in the rostrocaudal direction and, with few exceptions, completely restricted to the ipsilateral side. A proportion (25%) of these cells have dendrites extending into the dorsal column. Type II fin motoneurons extended their dendrites both ipsi- and contra-laterally. The contra-lateral dendrites pass below and above the central canal. The dendrites send off branches into the dorsal columns on both the ipsi- and the contra-lateral sides. Electron microscopic analysis showed that both type I and type II fin motoneurons receive numerous synaptic contacts from dorsal column axons. During fictive locomotion both types of motoneurons are active in antiphase in relation to myotomal motoneurons and to the main locomotor burst.

Animals↗

PubMed1992

Spino-bulbar neurons convey information to the brainstem about different phases of the locomotor cycle in the lamprey.

Lamprey retriculospinal neurons show phasic oscillations of their membrane potential during fictive locomotion. This modulation originates from the spinal cord locomotor networks. The aim of the present study was to elucidate the pattern of discharge of the spino-bulbar axons responsible for this modulation. Experiments were performed on in vitro brainstem/spinal cord preparations. Two baths were formed in the recording chamber. The caudal one was perfused with 150 microM N-methyl-D-aspartate (NMDA) solution to induce fictive locomotion. The rostral bath containing the brain and the first 3-5 segments of the spinal cord was exposed to a 0 Ca2+ + 2.6 mM Mn2+ solution to block synaptic transmission and therefore to abolish any rhythmic descending activity. Spinobulbar axons were recorded intracellularly at the level of the brain/spinal cord junction. They exhibited phasic discharges correlated with the ongoing motor activity in the caudal pool. Some discharged in phase with either the ipsilateral or the contralateral ventral root bursts, others with either of the transition phases between these two bursts. These spinal cells with ascending axons, running in the ventrolateral spinal cord, may be important for modulating the activity of supraspinal neurons to match the ongoing locomotor activity.

Action Potentials↗

PubMed1992

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

To investigate the effects 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.

Animals↗

PubMed1992

Synaptic potentials and transfer functions of lamprey spinal neurons.

1. Electrotonic and chemical synaptic potentials were measured as a function of frequency of presynaptic action potentials. Over the frequency range from 0.02 to 10 Hz, the electrotonic synaptic potential was constant, while the chemical synaptic potential decreased in magnitude. Above 10 Hz, both synaptic events decreased in magnitude consistent with filtering by the dendritic structures. 2. Electrotonic synaptic transfer functions from 0.5 to 100 Hz were measured for the I1 reticulospinal Müller axon to spinal neuron electrotonic synaptic junction of the lamprey spinal cord using paired recordings from the pre-synaptic terminals and the post-synaptic neurons. In addition to this two-point synaptic transfer function, individual single point impedance functions of both the post-synaptic soma and the pre-synaptic axon terminal were measured. 3. The measured functions were interpreted with a computational model based on a three dimensional reconstruction of a Lucifer yellow filled motoneuron. Simulations of the model for a synaptic location of the I1 synapse were consistent with the measured synaptic transfer functions. 4. Synaptic potentials were simulated for inputs on dendrites near the I1 axon as well as distal dendritic regions. The high frequency filtering increased as the synaptic location was moved from the soma to the periphery, but the potential response on distal dendrites was larger than would have been predicted from the end of the equivalent cylinder of a Rall model that was used to fit soma impedance functions. 5. Electrotonic post-synaptic potentials were enhanced by the activation of a TTX-sensitive negative conductance.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1992

Computer simulation of the segmental neural network generating locomotion in lamprey by using populations of network interneurons.

Realistic computer simulations of the experimentally established local spinal cord neural network generating swimming in the lamprey have been performed. Populations of network interneurons were used in which cellular properties, like cell size and membrane conductance including voltage dependent ion channels were randomly distributed around experimentally obtained mean values, as were synaptic conductances (kainate/AMPA, NMDA, glycine) and delays. This population model displayed more robust burst activity over a wider frequency range than the more simple subsample model used previously, and the pattern of interneuronal activity was appropriate. The strength of the reciprocal inhibition played a very important role in the regulation of burst frequency, and just by changing the inhibitory bias the entire physiological range could be covered. At the lower frequency range of bursting the segmental excitatory interneurons provide stability as does the activation of voltage dependent NMDA receptors. Spike frequency adaptation by means of summation of afterhyperpolarization (AHP) serves as a major burst terminating factor, and at lower rates the membrane properties conferred by the NMDA receptor activation. The lateral interneurons were not of critical importance for the burst termination. They may, however, be of particular importance for inducing a rapid burst termination during for instance steering and righting reactions. Several cellular factors combine to provide a secure and stable motor pattern in the entire frequency range.

Animals↗

PubMed1992

Vestibular control of swimming in lamprey. II. Characteristics of spatial sensitivity of reticulospinal neurons.

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)

Animals↗

PubMed1992

Vestibular control of swimming in lamprey. III. Activity of vestibular afferents: convergence of vestibular inputs on reticulospinal neurons.

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 degrees by rotation (360 degrees) 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 degrees tilt to the ipsilateral side, the next group in size responded at 180 degrees, and only a few afferents were activated by contralateral roll. For pitch, there are groups responding with maximal sensitivity at 90 degrees nose-up, 90 degrees nose-down and at 180 degrees. 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 nose-down. RS neurons from the anterior and middle rhombencephalic nuclei (ARRN and MRRN) were recorded before and after unilateral transection of the VIII nerve.(ABSTRACT TRUNCATED AT 250 WORDS)

Afferent Pathways↗

PubMed1992

Apamin blocks the slow AHP in lamprey and delays termination of locomotor bursts.

The effects of apamin on the slow afterhyperpolarization (sAHP) in spinal neurones and on the frequency of rhythmic bursting during fictive locomotion were investigated in the lamprey spinal cord in vitro. Apamin, which is a selective blocker of a small conductance KCa channel responsible for the sAHP in many types of neurones, was also found to reduce the sAHP in lamprey neurones. The summation of the sAHP is considered to be an important burst terminating factor in the spinal locomotor network and thereby to regulate the frequency of fictive locomotion. In support of this view, apamin was found to reduce the frequency of rhythmic bursting during fictive locomotion induced by kainate and NMDA. Serotonin, which has previously been shown to reduce the sAHP and slow the rate of rhythmic bursting, may therefore act, at least in part, on apamin-sensitive KCa channels.

Action Potentials↗

PubMed1992

Neural mechanisms of intersegmental coordination in lamprey: local excitability changes modify the phase coupling along the spinal cord.

1. To elucidate the neural mechanisms responsible for coordinating undulatory locomotor movements, the intersegmental phase lag was analyzed from ventral roots along the spinal cord during fictive swimming. It was induced by bath application of N-methyl-D-aspartate (NMDA) in in vitro preparations of lamprey spinal cord, while the excitability of different segments were modified. The phase lag between consecutive segments during normal forward swimming is 1% of the cycle duration in a broad range of values. Rostral segments are activated before more caudal ones. 2. Under control conditions, whole preparations (12-24 segment long; n = 22) were perfused with NMDA solutions of the same concentration (100-150 microM). The intersegmental phase lag values varied in a continuous range with a single peak around a median value of forward +0.74% per segment (range: forward +2.23% to backward -0.97%). 3. To examine whether excitability differences along the spinal cord could modify the intersegmental phase lag, different levels of excitatory amino acids (NMDA) were applied to spinal cord preparations positioned in a partitioned chamber. Different portions of the cord could be perfused separately by NMDA solutions of different concentrations (50-150 microM). If rostral segments were perfused with the higher NMDA solution, the lag was inevitably in the forward direction. Conversely, if the caudal portion was perfused with the higher NMDA solution, caudally located ventral roots became activated before the rostral ventral roots in a caudorostral succession, thus reversing the direction of the fictive swimming wave to propagate as during backward swimming. If the middle portion was perfused by the highest NMDA solution, this portion instead became leading, and the activity propagated from this point in both the rostral and the caudal directions. The portion located in the pool with highest NMDA concentration always gave rise to a "leading" segment. 4. When a portion of the preparation was perfused with an NMDA solution of a high concentration (75-150 microM), the cycle duration was close to that recorded when the whole preparation was perfused with the same high NMDA solution. The ensemble cycle duration is, therefore, largely determined by the leading segment. 5. The phase lag changes were not restricted to the region around the barrier separating pools with different NMDA solutions.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1992

Local serotonergic modulation of calcium-dependent potassium channels controls intersegmental coordination in the lamprey spinal cord.

1. The intersegmental coordination during undulatory locomotion in lamprey is characterized by a constant phase lag between consecutive segments, that is, the ratio between the intersegmental time lag and the cycle duration remains constant. It is shown that the spinal 5-HT (serotonin) system can, in a graded fashion, control the phase lag value from a rostrocaudal to a caudorostral lag corresponding to a reversed direction of swimming. These effects can be explained by a 5-HT-induced depression of Ca(2+)-dependent K+ channels (KCa channels) in network neurons. 2. The actions of the spinal 5-HT system were analyzed in the lamprey spinal cord preparation in vitro. Fictive swimming was induced by bath application of N-methyl-D-aspartate (NMDA). The intersegmental phase lag between ventral root burst activities was measured along the ipsilateral side of the spinal cord. The chamber with the preparation was partitioned into two pools so that the rostral and caudal halves of the preparation could be perfused independently with solutions containing the same level of NMDA (100-150 microM) with or without additional 5-HT or a 5-HT uptake blocker (citalopram). 3. Addition of 5-HT to one of these partitioned pools changed the intersegmental phase lag in this pool, whereas the cycle duration remained unchanged. It was determined by the activity in the "non-5-HT" pool. Addition of 5-HT to the caudal pool resulted in an increased rostrocaudal phase lag. When 5-HT was added to the rostral pool, on the other hand, the phase lag shifted direction to a backward coordination.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗