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S R Soffe

Publications and source records attributed to S R Soffe.

34 records · Page 2Linked to original sources
PubMed1993

Neuronal control of swimming locomotion: analysis of the pteropod mollusc Clione and embryos of the amphibian Xenopus.

It is rare to be able to explain the behaviour of a whole animal at the level of the properties and connections of characterized CNS neurones. In a marine mollusc, Clione, and a lower vertebrate embryo, Xenopus, it is possible to make intracellular recordings during fictive swimming behaviour. This has allowed us to analyse the operation of two central pattern generators (CPGs) at the cellular level. Although the timeframes over which the two CPGs operate are different, there are significant similarities in their patterns of neural output. A detailed analysis of the neural networks involved reveals that the swimming CPGs of Clione and Xenopus have several common operating principles, which suggests that common mechanisms have evolved to perform similar tasks, despite differences in neuronal 'hardware'.

Animals↗
PubMed1993

Two distinct rhythmic motor patterns are driven by common premotor and motor neurons in a simple vertebrate spinal cord.

Xenopus embryos show two distinct rhythmic motor patterns: swimming and struggling. Both can be generated by spinal cord circuitry and evoked by stimulation of a single skin sensory pathway (Soffe, 1991b). This presents a valuable opportunity to explore mechanisms for vertebrate motor pattern switching. Swimming and struggling have been compared using intracellular recording from spinal neurons in immobilized embryos. Underlying synaptic drive was similar; motoneurons and premotor interneurons were excited in phase with ipsilateral motor root discharge and inhibited in phase with contralateral motor root discharge. Excitation was stronger during struggling and associated with short bursts of impulses, contrasting with single spikes per cycle during swimming. Excitation was reduced in both patterns by local application of 1 mM kynurenic acid, indicating excitatory amino acid mediation. Inhibition was antagonized by 1 microM strychnine, indicating glycine mediation. Many motoneurons (76%) and premotor interneurons (68%) fired during both swimming and struggling, including examples of all three spinal premotor interneuron classes. Most of the remaining motoneurons (20%) and premotor interneurons (24%) fired only during struggling, providing roughly 30% more active neurons than during swimming. To investigate whether new neuronal classes become active during struggling, recordings were made from sensory neurons and sensory interneurons. Rohon-Beard sensory neurons did not fire during either swimming or struggling. Dorsolateral commissural sensory interneurons received rhythmic, strychnine-sensitive inhibition during both swimming and struggling and also did not fire. Neither of these neuronal classes is therefore recruited to the circuitry for struggling. Although behaviorally distinct, Xenopus embryo swimming and struggling motor patterns appear to employ similar synaptic drive. I propose that this reflects the common nature of much of the premotor circuitry that drives them. Extra neurons are recruited to this circuitry during struggling, but only from within classes that also participate in swimming.

Animals↗
PubMed1991

Triggering and gating of motor responses by sensory stimulation: behavioural selection in Xenopus embryos.

Neural mechanisms underlying selection of motor responses are largely unknown in vertebrates. This study shows that in immobilized Xenopus embryos, brief mechanical or electrical stimulation of the trunk skin can trigger sustained fictive swimming, whereas sustained pressure or repetitive electrical stimulation can evoke fictive struggling. These two rhythmic motor patterns are distinct: alternating single motor root spikes propagate from head to tail during swimming; alternating motor root bursts propagate from tail to head during struggling. As both motor patterns can be evoked in embryos with the CNS transected caudal to the cranial roots, the sensory pathway responsible must have direct access to the spinal cord. Rohon-Beard sensory neurons provide the only such pathway known. They respond appropriately to brief stimuli applied to the trunk skin, and also to repetitive electrical stimuli and sustained pressure. The results suggest that Rohon-Beard sensory neurons can both trigger sustained swimming and 'gate in' struggling motor patterns, and thus effect behavioural selection according to their pattern of activity.

Animals↗
PubMed1991

Neuroanatomical and functional analysis of neural tube formation in notochordless Xenopus embryos; laterality of the ventral spinal cord is lost.

Notochordless Xenopus embryos were produced by u.v. irradiation of the uncleaved fertilized egg. The spinal cords were examined using intermediate filament staining for glial cells, retrograde HRP staining for neuronal morphology and an anti-glycinergic antibody to reveal commissural cells and axons. The floorplate cells of the normal cord appear to be absent and their position along the ventral midline of the cord is occupied by motor neurones, Kolmer-Agduhr cells, radial glial cells and a ventrally placed marginal zone containing the longitudinal axons. Motor neurone number is reduced to 15% of control values, and the sensory extramedullary cell number is increased twentyfold. Commissural axons are still able to cross the ventral cord but do so at abnormal angles and some commissural axons continue to grow circumferentially up the contralateral side of the cord rather than turning to grow longitudinally. Extracellular electrophysiological recordings from motor axons reveal that the normal alternation of locomotor activity on the left and right side of the embryo is lost in notochordless animals. These results suggest that the notochord and/or the normal floor plate structure are important for the development of the laterality of spinal cord connections and may influence motor neurone proliferation or differentiation.

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↗
PubMed1991

Centrally generated rhythmic and non-rhythmic behavioural responses in Rana temporaria embryos.

Embryos of the frog Rana temporaria up to and around the time of hatching show a range of rhythmic and non-rhythmic movements. These may occur spontaneously or in response to lightly touching the skin of the trunk or head. The first response to touching one side is usually on the opposite side. Non-rhythmic movements range from weak twitches centred on the mid trunk to strong flexions along much of one side of the body and part of the tail, which result in the animal becoming tightly coiled. Rhythmic movements range from slow, high-amplitude 'lashing' movements to faster, lower-amplitude 'swimming' movements. During rhythmic movements, a wave of bending passes along the animal from head to tail. The longitudinal phase delay in bending is constant for a range of cycle periods (88-193 ms) but is not uniform along the whole body. Bending is maximal along the body and rostral part of the tail, decreases towards the tip of the tail and is lowest at the head. Lateral displacement during rhythmic movements is lowest 0.2 body lengths from the snout, increases rostral and caudal to this level and is highest at the tip of the tail. In animals immobilised with curare, a range of patterns of motor discharge can be recorded in response to stimulation. Non-rhythmic responses range from single spikes to prolonged bursts, usually on the opposite side to the stimulus. Stronger bursts can alternate briefly between the two sides and are never synchronous on both. Episodes of sustained rhythmic activity can be evoked by touch, electrical stimulation of the skin or, rarely, dimming the lights. Cycle periods within each episode can vary considerably but often shorten as activity proceeds. Discharge on the two sides alternates (phase is approximately 0.5). Motor root burst duration correlates with cycle period, bursts being longer at longer cycle periods. Burst onset is delayed caudally, this delay being longer at longer cycle periods. Stimulating one side of the head evokes a large burst of discharge on the opposite side, often followed by sustained rhythmic discharge. These responses in immobilised animals are judged to constitute centrally generated correlates of the main behavioural responses of R. temporaria embryos.

Animals↗
PubMed1987

Ionic and pharmacological properties of reciprocal inhibition in Xenopus embryo motoneurones.

1. Properties of rhythmic, compound mid-cycle inhibitory post-synaptic potentials (i.p.s.p.s), which constitute one of the three main synaptic drives to motoneurones during fictive swimming in Xenopus embryos, have been examined using ionic and pharmacological manipulation. 2. Mid-cycle i.p.s.p.s are Cl- dependent. They are reversed by intracellular Cl- injection and attenuated by lowered extracellular Cl- concentration. 3. In response to bath application of 100 microM-glycine or 100 microM-gamma-aminobutyric acid (GABA), motoneurones show a decrease in cell input resistance of 24 +/- 2.9 M omega (mean +/- S.E. of mean) or 16 +/- 3.7% and 26 +/- 6.0 M omega or 14 +/- 2.0% respectively. This is associated with a weak hyperpolarization or depolarization of 0 +/- 1.5 mV and -3 +/- 1.4 mV respectively. Both responses can be made strongly depolarizing by intracellular Cl- injection. 4. The response to glycine is blocked by 1 microM-strychnine but is largely unaffected by bicuculline below 50 microM. The response to GABA is largely blocked by 10 microM-bicuculline but is unaffected by 1 microM-strychnine. Both strychnine and bicuculline are therefore specific antagonists in the amphibian embryo preparation. Glycine and GABA are both partially antagonized by 10 microM-picrotoxin. 5. Mid-cycle i.p.s.p.s recorded in motoneurones during fictive swimming are reduced in amplitude by 0.5-1 microM-strychnine but are largely unaffected by 40 microM-bicuculline. In embryos immobilized by ventral root transection, 100 microM-tubocurarine, a likely GABA antagonist in the embryo, has no effect on mid-cycle inhibition. Glycine is suggested to be the probable transmitter released by commissural interneurones and mediating mid-cycle inhibition during fictive swimming, acting to increase conductance of Cl-.

Action Potentials↗
PubMed1985

Synaptic potentials in motoneurons during fictive swimming in spinal Xenopus embryos.

Embryos spinalized at the 3rd to 6th postotic myotome and immobilized in 10(-4) M tubocurarine can respond to a brief skin stimulus with motor root activity suitable for swimming. Embryos spinalized at the more caudal levels give shorter episodes of fictive swimming. We have previously described the synaptic inputs to motoneurons during fictive swimming in intact embryos (23). In the present paper we look to see if similar synaptic inputs are present in spinal embryos and are therefore spinal in origin. All motoneuron firing during fictive swimming is associated with a tonic depolarization that falls away slowly once firing stops, is increased by hyperpolarizing current, and is reduced by depolarizing current. A slow depolarizing potential evoked by lower levels of skin stimulation has similar properties and rate of fall. In 1-2 mM PDA, an excitatory amino acid antagonist, only a small remnant of the depolarization remains, and motoneuron firing stops. The NMDA antagonist 50 microM APV reduces the depolarization less but also blocks firing. Motoneurons fire one spike per swimming cycle, in phase with nearby motor root discharge. Spikes are preceded by a depolarizing prepotential. This increases with hyperpolarizing current, which can block the spike to reveal an underlying depolarizing potential. In phase with motor root discharge on the opposite side of the body, motoneurons receive a midcycle inhibitory postsynaptic potential, which increases with depolarizing current, decreases with hyperpolarizing current, and is blocked by 10(-6) M strychnine. Strychnine, 5 X 10(-7) M, leads first to broadening of motor root bursts then to loss of the alternating swimming pattern of activity, which is replaced by synchronous bursts on both sides of the body. We conclude that the synaptic inputs to motoneurons during fictive swimming in spinal embryos are very similar in properties and pharmacology to those in intact embryos. These inputs, including the tonic depolarization always associated with motoneuron firing during swimming, must be at least partly spinal in origin.

Amino Acids↗
PubMed1984

Activity of commissural interneurons in spinal cord of Xenopus embryos.

Horseradish peroxidase- (HRP) filled microelectrodes have been used to examine the anatomy and physiology of "commissural interneurons," a morphologically defined class of spinal cord interneuron in Xenopus laevis embryos. Commissural interneurons have unipolar cell bodies in the dorsal half of the spinal cord. Their dendrites lie in the mid to ventral parts of the lateral tracts and their axons cross the cord ventrally, T branch, and ascend and descend on the opposite side of the cord. Recordings were made from animals immobilized in tubocurarine and responding to natural stimulation with three patterns of fictive motor activity. During episodes of fictive swimming, commissural interneurons are phasically excited to fire 1 spike/cycle in phase with motor discharge on the same side and receive a midcycle inhibitory postsynaptic potential (IPSP) in phase with motor discharge on the opposite side. Rhythmic activity is superimposed on a background depolarization. During periods of synchrony, phasic excitatory input doubles in frequency so that cells fire with half the swimming cycle period. The background depolarization is generally stronger than during swimming. During periods of fictive struggling, evoked by electrical stimulation of the skin, commissural interneurons fire a burst of spikes per cycle, cells being relatively hyperpolarized when motoneurons on the opposite side are active. In response to ipsilateral skin stimulation, some cells receive an IPSP at a latency of 12-20 ms. This precedes the onset of fictive locomotion. We discuss how anatomy and activity of commissural interneurons is suitable for a reciprocal inhibitory role.

Animals↗
PubMed1982

Activity of myotomal motoneurons during fictive swimming in frog embryos.

1. Dye-filled microelectrodes have been used to identify and to examine the electrical activity of spinal cord motoneurons during fictive swimming in amphibian embryos. 2. Impaled neurons all had ventral cell bodies, dorsal, lateral, or ventral dendrites, and most showed either damaged or complete peripheral axons projecting out to the myotomes. It was rarely possible to identify cells by recording 1:1 motor root spikes evoked by intracellular current pulses. 3. During fictive swimming, motoneurons are tonically depolarized, fire 1 spike per swimming cycle, and are inhibited in phase with motor root activity on the opposite side. Motoneurons can also fire synchronously on the two sides, at double the normal swimming frequency. They occasionally also show a pattern of lower frequency alternating activity in which there is a prolonged burst of discharge on each cycle.

Animals↗
PubMed1982

Tonic and phasic synaptic input to spinal cord motoneurons during fictive locomotion in frog embryos.

1. In curarized, late developmental stage Xenopus embryos, episodes of rhythmic motor root discharge, termed fictive swimming (17), may be evoked by touch or by dimming the lights, as in unparalyzed animals. Motoneurons are tonically depolarized throughout each episode, are phasically excited to fire 1 spike per cycle, and receive a midcycle inhibitory postsynaptic potential (IPSP) in phase with motor root activity on the opposite side. 2. Rostral hemisection of the spinal cord abolishes motor root discharge on the operated side caudal to the cut but leaves activity on the intact side unaffected. In motoneurons, the tonic depolarization is abolished on the hemisected side but is still present on the intact side. This is evidence that the tonic depolarization is a descending drive. 3. Midcycle IPSPs normally seen in motoneurons during fictive swimming are abolished by rostral hemisection of the opposite side of the cord but are still recorded on the cut side. The simplest conclusion is that the inhibitory interneurons responsible lie on the opposite side of the spinal cord to the motoneurons they inhibit, and so represent a reciprocal inhibitory pathway. 4. The phasic excitatory postsynaptic potentials (EPSPs), which drive motoneuron spikes during swimming, are still present on the intact side of a rostrally hemisected cord but are abolished on the operated side. We conclude that the excitatory interneurons responsible lie on the same side of the cord as the motoneurons they excite.

Animals↗
PubMed1979

Environmental osmolarity and neurosecretory neurones in Lymnaea stagnalis (L.).

Cell body volume and Alcian blue-Alcian Yellow staining properties of neurosecretory neurones in the brain of Lymnaea stagnalis were compared for snails kept in de-ionised water and standard tapwater. In the same experiment, the ionic content of the blood, blood volume and body weight and environmental ionic composition were measured. Five days of immersion in de-ionised water resulted in significant decreases in body weight, blood volume and blood, Na+ and Cl- concentrations but no change in blood Ca2+, K+,and HCO3- concentrations, compared with controls. No consistently significant differences across the 5 day period were found in cell body volumes for Dark Green Cells, Yellow Cells or Light Green Cells (used as a control) when these volumes were compared for large numbers of cells from snails kept in de-ionised water and standard tapwater. However, the number of Yellow Cells which could be counted in snails kept in de-ionised water was lower than the number from standard tapwater by day 2 of the experiment and lower for Yellow-green Cells by day 5. We interpret this lower number to be the result of depletion of Alcian blue-Alcian Yellow stained neurosecretory material in these cells which made them impossible to distinguish. This was confirmed by examination of visceral Yellow Cells which could be identified on the basis of known location close to the visceral-right parietal connective.

Animals↗
PubMedDate not supplied

Modelling inter-segmental coordination of neuronal oscillators: synaptic mechanisms for uni-directional coupling during swimming in Xenopus tadpoles.

Locomotion requires longitudinal co-ordination. We have examined uni-directional synaptic coupling processes between two classes of neuronal network oscillators: autonomously active "intrinsic" oscillators, and "potential" oscillators that lack sufficient excitatory drive for autonomous activity. We model such oscillator networks in the bilaterally-symmetrical, Xenopus tadpole spinal cord circuits that co-ordinate swimming. "Glutamate" coupling EPSPs can entrain a second oscillator of lower frequency provided their strength is sufficient. Fast (AMPA) EPSPs advance spiking on each cycle, while slow (NMDA) EPSPs increase frequency over many cycles. EPSPs can also enable rhythmicity in "potential" oscillators and entrain them. IPSPs operate primarily on a cycle-by-cycle basis. They can advance or delay spiking to entrain a second "intrinsic" oscillator with higher, equal or lower frequency. Bilaterally symmetrical coupling connections operate twice per cycle: once in each half-cycle, on each side of the receiving oscillator. Excitatory and inhibitory coupling allow entrainment in complimentary areas of parameter space.

Action Potentials↗