Biomedical subjects

K T Sillar

Publications and source records attributed to K T Sillar.

35 records · Page 2Linked to original sources

PubMed1994

Synaptic specificity: development of locomotor rhythmicity.

The basic neural networks that generate rhythmic locomotor movements in vertebrates are present in the spinal cord at very early stages in ontogeny, and these pre-existing circuits are then modified during development to perform more complex adult locomotor functions. Current research on the mechanisms responsible for the maturation of early locomotor circuits suggests that the descent of modulatory brainstem control systems may be particularly important for restructuring immature spinal networks so that they impart greater flexibility and control of their motor output.

Animals↗

PubMed1994

Oscillatory membrane properties of spinal cord neurons that are active during fictive swimming in Rana temporaria embryos.

We have sought evidence for intrinsic oscillatory membrane properties in spinal cord neurons which participate in fictive locomotion in Rana temporaria embryos. In the presence of tetrodotoxin to synaptically isolate intracellularly recorded neurons, the bath application of N-methyl-d-aspartate (NMDA) depolarizes neurons by 20 to 30 mV. It does not, however, elicit continuous membrane potential oscillations, as have been found in homologous spinal neurons of the lamprey. However, addition of the neuromodulatory amine 5-HT to the bathing medium rapidly induces repetitive large scale (up to 40mV) oscillations in membrane potential. These oscillations depend upon the presence of magnesium ions and are abolished by the NMDA antagonist, APV. They do not occur when 5-HT is applied in the absence of NMDA. Expression of oscillatory membrane behaviour thus appears to rely upon an interaction between 5-HT and NMDA receptors. Neuroanatomical evidence shows that endogenous 5-HT-containing descending projections are present in the ventrolateral margins of the spinal cord at this stage in development and could therefore trigger these oscillations during normal motor behaviour. Preliminary pharmacological studies support the conclusion that activation of 5-HT1a receptors is involved in the induction process.

Animals↗

PubMed1994

Electrical coupling and intrinsic neuronal oscillations in Rana temporaria spinal cord.

In the presence of TTX and NMDA, spinal cord neurons in Rana temporaria embryos generate membrane potential oscillations, but only when 5-HT is added to the perfusate. These oscillations are voltage-dependent due to magnesium block of the NMDA receptor ionophore and can vary in amplitude between 0 and 40 mV with imposed membrane polarization. In contrast, the intrinsic frequency of the oscillations is unaffected by changes in membrane potential. This could result from electrical coupling amongst homonymous motoneurons. Here we present initial evidence for such connections and discuss their implications for the segmental control of rhythmic motor behaviour.

Animals↗

PubMed1994

Presynaptic inhibition of primary afferent transmitter release by 5-hydroxytryptamine at a mechanosensory synapse in the vertebrate spinal cord.

The effects of the neuromodulatory monoamine 5-HT (serotonin) on a cutaneous mechanosensory (Rohon-Beard, R-B neuron) pathway in the spinal cord of postembryonic Xenopus laevis tadpoles have been examined. In paralyzed animals, exogenous 5-HT at 1-10 microM reversibly inhibits (within 1-2 min) the activation of fictive swimming in response to electrical stimulation of R-B free nerve endings in the skin. At threshold stimulus intensities for swimming under control conditions, intracellularly recorded EPSPs in contralateral motoneurons are completely abolished by 5-HT without any obvious change in neuronal conductance or membrane potential. However, increasing the stimulus voltage can activate swimming with enhanced motor burst discharge on each cycle (Sillar et al., 1992). This suggested that 5-HT inhibits the swim-initiating pathway rather than the motor rhythm-generating circuitry itself. Extracellular recordings from the central projections of R-B neurons indicated that the amine does not impair the generation of mechanoafferent impulses or their propagation into the spinal cord. However, 5-HT application blocks impulse activity in dorsolaterally positioned sensory interneurons (DLis) that are contacted by R-B neurons, suggesting that 5-HT acts at R-B to DLi synapses in the dorsal cord. By recording with microelectrodes from DLis, we find that skin stimulus-evoked EPSPs at this first-order synapse in the swim-initiating pathway are reversibly suppressed by 5-HT. No obvious change in DLi membrane potential or conductance could be detected during the inhibition, suggesting a presynaptic site of action for 5-HT. To investigate this suggestion further, the effects of 5-HT on the spontaneous release of R-B sensory transmitter (excitatory amino acid, EAA) were examined, again by recording postsynaptically from DLis. In quiescent preparations, DLis receive spontaneous glycinergic, GABAergic, and EAA receptor-mediated PSPs. The inhibitory potentials are abolished by strychnine and curare, respectively. The excitatory potentials that remain are not blocked by application of the calcium channel blocker cadmium chloride at 1 mM, but are suppressed by the EAA receptor antagonist kynurenic acid. They therefore resemble the TTX-resistant EPSPs described previously in Xenopus DLis (Sillar and Roberts, 1991), which are presumed to arise from the spontaneous liberation of EAA transmitter from R-B terminals. Bath application of 5-HT dramatically reduces the rate of occurrence of these spontaneous EPSPs consistent with a presynaptic locus for the inhibitory effects of 5-HT.(ABSTRACT TRUNCATED AT 400 WORDS)

Afferent Pathways↗

PubMed1993

Control of frequency during swimming in Xenopus embryos: a study on interneuronal recruitment in a spinal rhythm generator.

1. In Xenopus embryos, the frequency of natural and fictive swimming usually drops slowly as swimming continues but can increase following stimulation of the skin or dimming of the illumination. We have investigated whether such increases are associated with an increase in the number of neurones active at higher frequencies. 2. Recordings from ventral presumed motoneurones show that these were reliably active at all swimming frequencies. 3. Recordings from more dorsal presumed interneurones showed that in the majority of these firing probability decreased as a function of swimming frequency. Dye-filled microelectrodes were used to show that some of these neurones had the anatomy of known classes of excitatory and inhibitory premotor interneurones. 4. If skin stimulation is given at appropriate phases of the swimming cycle, it can lead to a transient increase in frequency. Recordings from silent premotor interneurones during such stimulation show that they can be recruited to fire during the post-stimulus frequency increases. 5. It was possible that spike failure in the interneurones could have been due to damage by the recording microelectrodes. We therefore measured the amplitudes and probability of occurrence of rhythmic 'on-cycle' IPSPs which occur in sensory interneurones and 'on-cycle' IPSPs which sometimes occur in motoneurones during fictive swimming. Both decreased in amplitude and could fail as frequency dropped, providing further evidence that the number of inhibitory interneurones firing on each cycle of swimming is a function of frequency. 6. We conclude that premotor rhythm-generating interneurones are not active on all cycles of swimming and that their probability of firing action potentials increases with swimming frequency. This suggests that swimming frequency is determined in part by the number of premotor interneurones which are active.

Animals↗

PubMed1992

Modulation of swimming rhythmicity by 5-hydroxytryptamine during post-embryonic development in Xenopus laevis.

During the first 24 h of post-embryonic development in Xenopus laevis, a rapid change in the neural activity underlying swimming occurs in which the duration of ventral root discharge on each cycle increases from a single compound impulse to discrete bursts of activity. Moreover, this change in motor output progresses rostrocaudally, suggesting that it could result from the influence of a descending neural pathway upon the spinal rhythm-generating circuitry during early post-embryonic development. To begin to examine whether serotonergic neurons of brainstem raphe nuclei might have a role in this swimming development, we have studied the effects of 5-hydroxytryptamine (5HT) on fictive swimming in embryonic and larval animals. As previously demonstrated for other vertebrate locomotor rhythms, we find that bath-applied 5HT enhances the duration of motor activity on each cycle of larval fictive swimming. In addition, our results show that the sensitivity of the swimming rhythm to exogenous 5HT follows a strict rostrocaudal gradient. In young embryos (stages 32-36) 5HT does not affect the duration of ventral root impulses per cycle; by the time of hatching (stage 37/38), rostral but not caudal discharge is enhanced, and by stage 42 (24 h post-hatching) 5HT can increase motor burst durations along most of the length of the animal. These reversible changes induced by bath-applied 5HT closely resemble the normal rostrocaudal development of burst discharge during swimming in animals some 12 h older.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1992

The post-embryonic development of cell properties and synaptic drive underlying locomotor rhythm generation in Xenopus larvae.

In the first 24 h of post-embryonic development, the motor rhythm underlying swimming in Xenopus laevis tadpoles changes from brief (ca. 7 ms) ventral root discharge in each cycle to bursts of activity lasting around 20 ms (Sillar et al. 1991). Because individual motoneurons in the spinal cord of newly hatched embryos normally fire only a single impulse per cycle, two possible changes underly the transition to motor bursts seen in larval ventral roots; desynchronization of neurons in a given ventral root which continue to fire once per cycle, or the developmental acquisition of a multiple spike capability in individual motoneurons. Here we have recorded intracellularly from ventrally positioned spinal neurons, presumed to be myotomal motoneurons, in stage 37/38 embryos and 24 h later in development in stage 42 larvae. We find that (i) larval neurons are able to fire more than one impulse per cycle of fictive swimming activity; (ii) unlike in the embryo, they generally will fire multiple impulses in response to injected depolarizing current; (iii) the synaptic drive to motoneurons during swimming increases dramatically in complexity, although it still consists of alternating phases of synaptic excitation and chloride-dependent inhibition, superimposed upon tonic synaptic depolarization. The results therefore suggest a developmental change in the membrane properties of rhythmically active neurons as a major factor in the post-embryonic development of swimming in Xenopus larvae. This change appears to occur in premotor rhythm generating interneurons as well as in the motoneurons themselves and may satisfy a demand for behavioural flexibility that allows larvae to survive in a complex and changing environment.

Animals↗

PubMed1992

Identified proprioceptive afferents and motor rhythm entrainment in the crayfish walking system.

1. In crayfish, Pacifastacus leniusculus, remotion of a walking leg stretches the thoraco-coxal (TC) muscle receptor organ (TCMRO), located at the leg's articulation with the thorax. In vitro, alternate stretch and release of the fourth leg's TCMRO entrained the centrally generated rhythmic motor output to that leg, with the remotor phase of the rhythm entraining to TCMRO stretch, the promoter phase to release. This coordination of motor bursts to afferent input corresponds to that of active, rhythmic movements in vivo. 2. Entrainment was rapid in onset (stable coordination resulting within the first or second stimulus cycle) and was relatively phase-constant (whatever the stimulus frequency, during 1:1 entrainment, remotor bursts began near the onset of stretch and promotor bursts began near the onset of release). Outside the range of 1:1 entrainment, 2:1, 1:2, and 1:3 coordination ratios (rhythm:stimulus) were encountered. Resetting by phasic stimulation of the TCMRO was complete and probabilistic: effective stimuli triggered rapid transitions between the two burst phases. 3. The TCMRO is innervated by two afferents, the nonspiking S and T fibers, which generate graded depolarizing receptor potentials in response to stretch. During proprioceptive entrainment, the more phasic T fiber depolarized and hyperpolarized more rapidly or in advance of the more tonic S fiber. These receptor potentials were modified differently in the two afferents by interaction with central synaptic inputs that were phase-locked to the entrained motor rhythm. 4. Injecting slow sinusoidal current into either afferent alone could entrain motor rhythms: promoter phase bursts were entrained to depolarization of the S fiber or hyperpolarization of the T fiber, whereas the converse response was obtained for remotor phase bursts. 5. During proprioceptive entrainment, tonic hyperpolarization of the S fiber weakened entrained promotor bursts and allowed remotor burst durations to increase. Hyperpolarizing the T fiber weakened entrained remotor bursts and allowed promotor bursts to occur during stretch. These results suggest that the staggered receptor potentials of the two afferents alternately excite opposite burst phases of the rhythm during proprioceptive entrainment. 6. Injecting brief current pulses into either afferent perturbed the timing of entrained bursts in opposite ways, suggesting that, during proprioceptive entrainment, the membrane potential trajectories of the two afferents have reciprocal triggering effects on burst transitions. 7. We infer that entrainment results from 1) complete resetting of burst transitions in a two-phase central oscillator, 2) opposing feedback pathways mediated by a phasic and a tonic afferent, 3) temporally staggered afferent receptor potentials, and 4) the ability of afferent receptor potentials to trigger burst transitions.

Animals↗

PubMed1992

The role of premotor interneurons in phase-dependent modulation of a cutaneous reflex during swimming in Xenopus laevis embryos.

Phase-dependent reflex modulation during fictive "swimming" in Xenopus laevis embryos has been examined with intracellular recordings from rhythmically active spinal neurons. (1) At rest, cutaneous trunk or tail skin stimulation evokes EPSPs in motoneurons and premotor excitatory and inhibitory interneurons of the opposite motor system. During swimming, these EPSPs can only be evoked during the depolarized phase of activity and can then produce extra action potentials that lead to phase-dependent reflexes in ventral roots. On the stimulated side, IPSPs are evoked in rhythmic neurons that can block centrally generated action potentials if the stimulus coincides with the inhibited phase of the swimming cycle. This inhibition suppresses ventral root discharge in a phase-dependent manner. (2) The presence of premotor interneurons in the crossed reflex pathway suggests two parallel routes for cutaneous excitation to reach the motoneurons, one direct and the other indirect through excitatory premotor interneurons. During swimming, the crossed excitation through both routes is gated by the rhythm-generating circuit to allow summation in motoneurons only during the depolarized phase of the swim cycle. (3) Following phase-dependent reflexes, the frequency of swimming is raised for several cycles, a phenomenon that requires sensory activation of premotor rhythm-generating interneurons. The results provide evidence on the role of identified premotor spinal interneurons in phase-dependent reflex modulation.

Animals↗

PubMed1991

The development of swimming rhythmicity in post-embryonic Xenopus laevis.

The post-embryonic development of 'fictive' swimming in immobilized Xenopus laevis tadpoles has been examined during the first day of larval life. In Xenopus embryos (stage 37-38; Nieuwkoop & Faber 1956), the rhythmic ventral root activity underlying swimming occurs as single brief (ca. 7 ms) compound impulses on each cycle. However, by stage 42 (about 24 h after hatching), ventral root discharge consists of bursts lasting around 20 ms per cycle. In addition to increased burst duration in each cycle of larval swimming, the range of cycle periods within an episode increases, although mean period values (ca. 70-80 ms) remain similar to those of the younger animal. Consequently, motoneurons at developmental stage 42 are active during swimming for a greater percentage (ca. 25%) of cycle time than at stage 37-38 (ca. 10%). Developmental stage 40 (ca. 12 h post-hatching) is an intermediate stage in rhythm development. Ventral root discharge varies from bursts of 10-20 ms at the start of an episode to embryonic (ca. 7 ms) spikes at the end of an episode. Furthermore, discharge varies from bursts of activity in rostral segments of stage 40 larvae to 7 ms spikes more caudally, as in embryos. The data thus suggest that Xenopus swimming rhythmicity develops relatively rapidly, along a rostrocaudal gradient, and may involve acquisition of multiple spiking in spinal neurons.

Animals↗

PubMed1991

Segregation of NMDA and non-NMDA receptors at separate synaptic contacts: evidence from spontaneous EPSPs in Xenopus embryo spinal neurons.

Many excitatory amino acid (EAA)-mediated synaptic potentials are dual-component as a result of the simultaneous activation of N-methyl-D-aspartate (NMDA) and non-NMDA receptor subtypes, the two major classes of EAA receptor in vertebrates. This raises the question of whether the two receptor types are located separately or together at individual synaptic contacts. Support for the segregation of NMDA and non-NMDA receptors in discrete anatomical patches arises from the observation that the fast and slow components of dual-component potentials mediated via NMDA and non-NMDA receptor types can fail independently. We have obtained further support for this by investigating the spontaneous release of EAA neurotransmitter at sensory synapses in the spinal cord of Xenopus laevis embryos. We report the occurrence of spontaneous TTX-resistant EPSPs in sensory interneurons that are mediated by EAA receptors. These spontaneous potentials share the same pharmacological sensitivities as EPSPs evoked by skin sensory afferents, being blocked by kynurenic acid and reduced by (+-)-2-amino-5-phosphonovaleric acid (APV). The spontaneous EPSPs differ from evoked EPSPs in their time courses: while evoked EPSPs are almost exclusively of the dual-component variety, the spontaneous EPSPs are predominantly either fast or slow. These data suggest that spontaneous EPSPs reflect release of EAA neurotransmitter at synaptic contacts overlying homogeneous populations of either NMDA or non-NMDA receptors. Their relatively large size, up to 50% or more of the amplitude of unitary EPSPs evoked by stimulation, also suggests that synapses between skin afferents and sensory interneurons may comprise relatively few points of synaptic contact.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1991

Spinal pattern generation and sensory gating mechanisms.

Sensory gating mechanisms are deployed during vertebrate locomotion to ensure that adaptive and appropriate motor responses to afferent input occur during all phases of the movement cycle. Recent animal studies on the integration of cutaneous information have investigated the roles of interneurones in sensory gating. Premotor interneurones, rhythmically active during locomotion, as well as 'sensory' interneurones appear to be intimately involved in sensory gating, receiving synaptic inputs from the spinal rhythm generator to gate the flow of sensory information in the spinal cord.

Animals↗

PubMed1991

Patterns of synaptic drive to ventrally located spinal neurones in Rana temporaria embryos during rhythmic and non-rhythmic motor responses.

1. Intracellular recordings have been made from ventrally located neurones in the spinal cord of Rana temporaria embryos at around the time of hatching. Both short-latency 'reflex' and more prolonged rhythmic motor responses can be elicited by stimulation of the skin in immobilized embryos. Initial responses to single-sided skin stimuli usually involve excitation of neurones on the opposite side and strychnine-sensitive inhibition of neurones on the same side. Less reliable responses to dimming the lights also involve initial excitation on one side associated with inhibition on the opposite side. 2. Intracellular recordings from single neurones during rhythmic activity show that on each cycle the same neurone can fire one or many spikes during the course of a single evoked or spontaneous episode. Bursts occur at longer cycle periods, generally at the start of episodes; single spikes occur at shorter cycle periods, generally later in episodes. 3. During sustained rhythmic responses, neuronal membrane potential is generally depolarised and returns gradually to its resting level at the end of the episode. During the episode, relatively depolarising phases of synaptic excitation alternate with relatively hyperpolarising phases of chloride-dependent synaptic inhibition. Cell input resistance is reduced by around 50% throughout each episode. Within each cycle, input resistance is reduced further during the hyperpolarising phase than during the depolarising phase. 4. Rhythmic excitation and inhibition of ventrally located neurones appears to be similar throughout the whole range of cycle periods, supporting the suggestion that a single rhythm-generating system with a wide 'permissive' range drives rhythmic movements in R. temporaria embryos.

Animals↗

PubMed1988

A neuronal mechanism for sensory gating during locomotion in a vertebrate.

The response of the foot to touch during walking depends on whether it is in the air or on the ground. In most animals, reflex responses to external stimuli are similarly adapted to their timing in the locomotor cycle, but there is only fragmentary information about the neural mechanisms involved. In arthropods, reflex modulation can occur in the sensory receptors themselves and in neurons that discharge during locomotion. By recording with dye-filled microelectrodes from neurons in the spinal cord of frog embryos, we describe reflex modulation at the level of sensory interneurons. Sensory inputs from skin receptors excite a specific class of spinal sensory interneuron whose activity leads to reflex bending of the body away from the stimulus. During swimming, these inputs are gated by rhythmic postsynaptic inhibition, so that sensory drive reaches motor neurons only at phases in the locomotor cycle when the resulting contraction would likewise turn the embryo away from the stimulated side. Such gating of sensory pathways could be a general feature of all locomotor systems where responses to sensory stimuli need to be adapted to the phase of locomotion.

Action Potentials↗

PubMed1988

Unmyelinated cutaneous afferent neurons activate two types of excitatory amino acid receptor in the spinal cord of Xenopus laevis embryos.

The trunk and tail skin of Xenopus laevis embryos near the time of hatching is innervated by the mechanoreceptive free nerve endings of Rohon-Beard neurons, a homogeneous class of cutaneous primary afferent fibers. Rohon-Beard neurons have cell bodies and axons in the dorsal spinal cord, where they monosynaptically excite a population of dorsolaterally situated interneurons (Clarke and Roberts, 1984). EPSPs can be recorded in these dorsolateral interneurons following electrical stimulation of the unmyelinated neurites of Rohon-Beard neurons in the skin. The EPSPs are dual component, consisting of separate fast and slow potentials that are usually evoked synchronously and that closely resemble those described previously in Xenopus and lamprey motoneurons (Dale and Roberts, 1985; Dale and Grillner, 1986). The excitation of dorsolateral interneurons by Rohon-Beard neurons is reduced by the bath application of excitatory amino acid antagonists. Kynurenic acid suppresses both the fast and slow components of the EPSPs, while both (+/-)-2-amino-5-phosphonovaleric acid (APV) and 1 mM magnesium reduce the slow component but have little or no effect on the peak amplitude of the EPSPs. These data suggest that Rohon-Beard neurons release an excitatory amino acid neurotransmitter, which acts simultaneously at both N-methyl-D-aspartate (NMDA) and non-NMDA receptor types. This is the first direct demonstration of dual-component excitatory amino acid-mediated synaptic transmission from cutaneous primary afferent neurons in the vertebrate spinal cord. The bath application of the agonists NMDA, kainate, or quisqualate in salines containing 1 microM TTX depolarized the interneurons and reduced their input resistance, which suggests that the interneurons possess all 3 types of excitatory amino acid receptor. Kynurenic acid strongly inhibits responses to NMDA and kainate, but is relatively less effective against the larger responses of quisqualate in this system.

2-Amino-5-phosphonovalerate↗

PubMed1986

Central input to primary afferent neurons in crayfish, Pacifastacus leniusculus, is correlated with rhythmic motor output of thoracic ganglia.

A preparation is described in which the thoracic ganglia of the crayfish are isolated together with the thoracocoxal muscle receptor organ (TCMRO) of the fourth leg. This preparation allows intracellular analysis of both centrally generated and reflex activity in leg motor neurons (MNs). The isolated thoracic ganglia can spontaneously generate a rhythmic motor pattern resembling that used during forward walking (Fig. 4). This involves the reciprocal activity of promotor and remotor MNs, with levator MNs firing in phase with promotor bursts. Stretch of the TCMRO in quiescent preparations evokes a resistance reflex in promotor MNs (Fig. 6). In more active preparations the response is variable and often becomes an assistance reflex, with excitation of remotor MNs on stretch (Fig. 7). When rhythmic motor patterns occur, the neuropilar processes of the S and T fibers receive central inputs that are strongly correlated with the oscillatory drive to the MNs and probably have the same origin (Figs. 8 and 9). Central inputs to the S and T fibers occur in opposite phases within a cycle of rhythmic motor output. The S fiber is depolarized in phase with promotor MNs and the T fiber in phase with remotor activity. The input to the T fiber is shown to be a chemical synaptic drive that has a reversal potential approximately 14 mV more depolarized than the fiber's resting membrane potential. This input substantially modulates the amplitude and waveform of passively propagated receptor potentials generated by TCMRO stretch (Fig. 11). It is argued that the central inputs to the TCMRO afferents will modulate proprioceptive feedback resulting from voluntary movements.

Afferent Pathways↗

PubMed1986

Phase-dependent reversal of reflexes mediated by the thoracocoxal muscle receptor organ in the crayfish, Pacifastacus leniusculus.

Both negative feedback, resistance reflexes and positive feedback, assistance reflexes are mediated by the thoracocoxal muscle receptor organ (TCMRO) in the crayfish, depending on the central excitability of the preparation. In this paper we present evidence that the velocity-sensitive afferent T fiber of the TCMRO may elicit either resistance or assistance reflexes in different preparations. In preparations displaying assistance reflexes, the S and T fibers of the TCMRO exert reciprocal effects on leg motor neurons (MNs). The S fiber excites promotor MNs (negative feedback) and inhibits remotor MNs, the T fiber excites remotor MNs (positive feedback) and inhibits promotor MNs. During reciprocal motor output of promotor and remotor MNs, reflexes mediated by the TCMRO are modulated in a phase-dependent manner. The TCMRO excites promotor MNs during their active phases (negative feedback) but inhibits them during their reciprocal phases. Remotor MNs are excited by the TCMRO during their active phases (positive feedback). It is proposed that depolarizing central inputs that occur in the S and T fibers at opposite phases of the motor output cycle (21) facilitate the output effects of each afferent in alternation, effectively mediating a phase-dependent shift between the effects of one afferent and the other. The implications of central modulation of reflex pathways and the possible functions of positive and negative feedback reflexes during locomotion are discussed.

Animals↗