Connections between descending visual interneurons and metathoracic motoneurons in the locust

Summary Connections between the four DMD neurons and metathoracic motoneurons in the locustSchistocerca were examined by recording extracellularly from the interneurons in the pro-mesothoracic connectives and intracellularly from seventeen motoneurons. A DIMD or DCMD spike causes an EPSP in the fast extensor tibiae motoneuron, which can be modified by changing the membrane potential. The EPSP always follows spikes at frequencies up to 200 Hz and with a latency of 0.9 ms, suggesting that the connections are monosynaptic and chemically mediated. EPSPs from the DIMD or DCMD arrive at the same time, their axons having the same conduction velocity, and appear simultaneously in the fast extensor tibiae motoneurons on both sides of the ganglion. There is spatial and temporal summation between the inputs but on no occasion did the motoneurons spike. Three inhibitory neurons are depolarized by DMD inputs and may on occasion spike, but it is not known whether these connections are direct. Similarly the slow excitatory motoneuron to the anterior coxal adductor muscle is hyperpolarized by DMD input. Other leg, flight or ventilatory motoneurons examined received no inputs from the DMD neurons. The connections shown are consistent with the hypothesis that the DMD neurons are in some way involved with initiation of a jump, but to achieve this must act synergistically with other inputs. This work was supported in part by USPHS grant No. NS 09404-03 to C.H.F.R. Dr. Rowell wishes to thank Dr. J. Phillipson for the use of facilities in the Oxford Department of Zoology during sabbatical leave.     

The role of proprioception for motor learning in locust flight

In a muscle-specific flight simulator (simulator driven by muscle action potentials) locusts (Locusta migratoria) show motor learning by which steering performance of the closed-loop muscles is improved. The role of proprioceptive feedback for this motor learning has been studied. Closed-loop muscles were cut in order to disable proprioceptive feedback of their contractions. Since there are no proprioceptors within the muscles, this is a muscle-specific deafferentation. Cut muscles are still activated during flight and their action potentials can be used for controlling the flight simulator. With cut muscles in closed-loop, steering is less reliable as can be seen from the frequent oscillations of the yaw angle. However, periods of stable flight indicate that deafferented muscles are still, in principle, functional for steering. Open-loop yaw stimuli reveal that steering reactions in cut muscles are weaker and have a longer delay than intact muscles. This is responsible for the oscillations observed in closed-loop flight. Intact muscles can take over from cut muscles in order to re-establish stable closed-loop flight. This shows that proprioceptive mechanisms for learning are muscle specific. A hypothetical scheme is presented to explain the role of proprioception for motor learning.     

Patterned synaptic drive to locust flight motoneurons after hemisection of thoracic ganglia

Intracellular recordings were carried out on locust flight motoneurons after hemisection of individual thoracic ganglia. With the exception of minimal surgical manipulations, the animals were intact and able to perform tethered flight. Analysis of the synaptic drive recorded in the motoneurons during flight motor activity revealed the extent to which ganglion hemisection influenced the premotor rhythm generating network.

Range fractionation in the locust metathoracic femoral chordotonal organ

Summary Insect femoral chordotonal organs are internal proprioceptors which monitor the position and movements of the femur-tibia joint of the leg. The locust (Locusta migratoria) metathoracic femoral chordotonal organ is composed of approximately 100 neurones with a variety of response properties. In this study intracellular recordings were used to examine the range fractionation of phasic and tonic responses to tibial movements. Some neurones responded across the full range of leg angles, while others had restricted response ranges, and could therefore act as labeled lines. Neurones with maximal firing at mid-angles are described for the first time in a locust femoral chordotonal organ. Responses are discussed in terms of underlying structural constraints on signal transduction.Abbreviation (mt) FCO (metathoracic) femoral chordotonal organ     

Generation of motor patterns for walking and flight in motoneurons supplying bifunctional muscles in the locust

In the flight system of Locusta migratoria certain muscles move a wing and a leg (bifunctional muscles) and are active during the performance of walking and flight. A preparation that allowed intracellular recordings during these behaviors was developed to analyze the activity of motoneurons supplying these and other muscles. Motoneurons innervating bifunctional muscles were active during walking and flight, whereas motoneurons innervating unifunctional flight muscles were active only during flight. Both motor patterns, walking and flight, were sometimes generated simultaneously in our preparation. In bifunctional motoneurons the two patterns were superimposed, whereas in unifunctional motoneurons only the flight motor pattern was observed. All flight interneurons we examined were either inactive or tonically inhibited during walking. All interneurons that were strongly modulated during walking were either inactive, inhibited, or only weakly modulated during flight. Anatomical investigations showed that unifunctional flight motoneurons have their main processes in the extreme dorsal region of neuropil. With the exception of the second basalar motoneurons, all bifunctional motoneurons have their processes extending more ventrally in the neuropil. Flight interneurons have their processes restricted to the dorsal neuropil. Interneurons that were rhythmically active during walking had their processes distributed more ventrally. We conclude that motoneurons innervating bifunctional muscles are active during both motor patterns, walking and flight, and that these patterns are produced by two distinct interneuronal networks. The pattern-generating network for flight appears to be located in the extreme dorsal regions of the thoracic ganglia, and the network for walking is located more ventrally.     

The dorsal, unpaired, median neurons of the locust metathoracic ganglion

Neurons having large cell bodies in the anterior dorsal median cluster in the metathoracic ganglion of the locust Schistocerca gregaria and the grasshopper Romalea microptera were studied by direct dye injection and reverse filling combined with elyctrical stimulation and recording. Eight, possibly nine, are of the unpaired type, with a T branch leading into left and right axons that leave the ganglion to terminate in muscles. Another six are probably paired, and may be interneurons. Five of the 8 or 9 unpaired neurons have one axonal branch in both N4 and N5, on both sides: the others have but a single branch. One of the nine, DUMETi, has left and right axons exclusively innervating the jumping muscles, and another, DUMDL, has left and right axons exclusively innervating the dorsal longitudinal flight muscles. Neither the locations, sizes or numbers of somata, nor their locations were as constant as is the case for ordinary ventral motoneurons.     

Responses and locations of neurones in the locust metathoracic femoral chordotonal organ

Summary Insect legs possess chordotonal organs which monitor leg angle, and the direction, velocity and acceleration of leg movements. The locust metathoracic femoral chordotonal organ (mtFCO) has previously been studied morphologically and physiologically, but no detailed analysis of the responses of individual neurones, and their location in the organ has so far been produced. By recording from, and staining mtFCO neurones I have been able to compile for the first time such a map. The distribution of neurone somata in the locust mtFCO is more complex than previously thought: receptors sensitive to both stretch and relaxation of the apodeme are distributed throughout the organ. Seventeen response types were encountered. Neurones with a particular response type have somata in comparable locations within the mtFCO. Comparisons are made between the response types found in the stick insect and those in the locust. The possible functions of some of the responses are discussed.Abbreviation (mt)FCO (metathoracic) femoral chordotonal organ - F-T femur-tibia     

FMRFamide-like immunoreactivity in the metathoracic ganglion of the locust (Schistocerca gregaria)

Summary The distribution of FMRFamide-like immunoreactivity in the metathoracic ganglion of the locust, Schistocerca gregaria, has been investigated in serial semithin transverse sections with the use of the peroxidase-antiperoxidase (PAP) technique. The topographical distribution of approximately 120 immunopositive neurons was established. Antiserum against bovine pancreatic polypeptide (BPP) stains the same ganglionic cells as FMRFamide-antiserum, yet this staining is largely blocked after preabsorption to FMRFamide. A comparison of these results with those from other studies suggests that there may be more than one type of endogenous RFamide-like peptide.     

K homeostasis and central pattern generation in the metathoracic ganglion of the locust

Stress-induced arrest of ventilatory motor pattern generation is tightly correlated with an abrupt increase in extracellular potassium concentration ([K⁺]_o) within the metathoracic neuropil of the locust, Locusta migratoria. Na⁺/K⁺-ATPase inhibition with ouabain elicits repetitive surges of [K⁺]_o that coincide with arrest and recovery of motor activity. Here we show that ouabain induces repetitive [K⁺]_o events in a concentration-dependent manner. 10⁻⁵ M, 10⁻⁴ M, and 10⁻³ M ouabain was bath-applied in semi-intact locust preparations. 10⁻⁴ M and 10⁻³ M ouabain reliably induced repetitive [K⁺]_o events whereas 10⁻⁵ M ouabain had no significant effect. In comparison to 10⁻⁴ M ouabain, 10⁻³ M ouabain increased the number and hastened the time to onset of repetitive [K⁺]_o waves, prolonged [K⁺]_o event duration, increased resting [K⁺]_o, and diminished the absolute value of [K⁺]_o waves. Recovery of motor patterning following [K⁺]_o events was less likely in 10⁻³ M ouabain. In addition, we show that K⁺ channel inhibition using TEA suppressed the onset and decreased the amplitude of ouabain-induced repetitive [K⁺]_o waves. Our results demonstrate that ventilatory circuit function in the locust CNS is dependent on the balance between mechanisms of [K⁺] accumulation and [K⁺] clearance. We suggest that with an imbalance in favour of accumulation the system tends towards a bistable state with transitions mediated by positive feedback involving voltage-dependent K⁺ channels.     

The regulation of glycolysis in perfused locust flight muscle

Concentrations of glycolytic intermediates, amino acids and possible regulator substances were measured in extracts from locust thoracic muscles perfused under different conditions. The conversion of [(14)C]glucose into intermediates and CO(2) by muscle preparations was also followed. When muscles perfused with glucose were made anaerobic changes in metabolite concentrations occurred that could be accounted for by an activation of phosphofructokinase and pyruvate kinase. When butyrate and glucose were present in the perfusion medium the rate of glycolytic flux was lower than with glucose alone, and the aldolase reaction appeared to be inhibited. When butyrate alone was supplied to the muscle the concentrations of most glycolytic intermediates were similar to those found when glucose was supplied. Iodoacetate caused changes in concentrations of intermediates that appeared to result from inhibition of glyceraldehyde 3-phosphate dehydrogenase. Fluoroacetate-poisoned muscles showed a high citrate concentration, but no obvious site of inhibition by citrate was apparent in the glycolytic pathway. Mechanisms for control of glycolysis in locust flight muscle are discussed and related to the known properties of isolated enzymes. It is proposed that trehalase, hexokinase, phosphofructokinase, aldolase, and pyruvate kinase may be control enzymes in this tissue.     

Physiological and morphological properties of the metathoracic common inhibitory neuron of the locust

Summary Intracellular recordings were made from the soma of the metathoracic common inhibitory neuron of the locustsSchistocerca andChortoicetes. The soma is passively invaded by a spike of 2–5 mV in amplitude. The response of the common inhibitor to a variety of different inputs was studied. Tests for coupling between the common inhibitory and excitatory motoneurons to the same or antagonistic muscles were made by simultaneous recordings from pairs of neuron somata. No low resistance or synaptically mediated coupling was found. The somata of the two common inhibitory neurons which supply muscles on opposite sides of the body lie together on the ventral surface of the ganglion on the mid-line (Fig. 6). They are not coupled in any way. Cobalt chloride injected into the common inhibitor has shown it to have an extensive and complex dendritic tree confined to the ipsilateral half of the ganglion (Fig. 8). A single branch extends into the mesothoracic ganglion. There are differences in the branching patterns of the dendrites in different animals (Fig. 10).Beit Memorial Research Fellow.