Comparison of motor patterns in the intact and deafferented flight system of the locust

Summary The activity of flight interneurons was recorded intracellularly in intact, tethered flying locusts (Locusta migratoria) and after removal of sensory input from the wing receptors. Depolarization patterns and spike discharges were characterized and compared for the two situations.In general, depressor interneurons (n=6) showed only minor changes in their activity as a result of deafferentation (Fig. 1). Exceptions were interneurons 308 and 506 (Fig. 2). By contrast, all but one of the elevator interneurons (n=9) produced distinctly different depolarization patterns in intact locusts and following deafferentation. Three different groups of elevator interneurons were found (excluding the one exceptional neuron, Fig. 6). (i) One group of interneurons (n=4) produced different, superthreshold depolarizations in intact and deafferented animals (Fig. 3). Characteristic, biphasic depolarizations were recorded from these fibres at lower wingbeat frequencies in the intact situation but only single, delayed potentials were recorded after deafferentation. (ii) The second group of interneurons (n=3) exhibited distinct rhythmic activity only in intact animals. After deafferentation their depolarizations were small and often below the threshold for spike initiation (Fig. 4). (iii) One interneuron produced rhythmic flight motor oscillations only after deafferentation. In intact locusts the membrane potential of this neuron showed very small oscillations and remained subthreshold (Fig. 5).Four main conclusions emerge from these data. (i) The activity of elevator interneurons is under greater sensory control than that of the depressors. This confirms the results of our previous electromyographic and motoneuronal analyses, (ii) A considerable portion of elevator activity is generated as a result of phasic sensory feedback. An essential input is from the hindwing tegulae (Table 1; Pearson and Wolf 1988). (iii) The activity of depressor interneurons appears to be determined by central mechanisms to a major extent. (iv) Different sets of central neurons appear to be involved in flight pattern generation in intact and deafferented locusts —although the two sets share many common elements.Abbreviations EMG electromyogram - PSP postsynaptic potential (EPSP excitatory andIPSP inhibitory)     

The sizes of motoneurons supplying hindlimb muscles in the mouse

Motoneurons supplying identified muscle groups in the mouse spinal cord were labelled by retrograde transport of horseradish peroxidase. The size of motoneurons was estimated by measuring perimeter and cross-sectional area at the level of the nucleolus for the following seven major muscle groups: quadriceps femoris, adductors and gracilis, gluteal musculature, hamstring muscles, posterior crural musculature, anterolateral crural musculature and intrinsic musculature of the foot. The qualitative observation of two size ranges of motoneuron was supported by the measurements. Frequency distribution histograms of motoneuronal cross sectional area were bimodal for all motoneuronal groups except for the foot musculature. The population parameters and proportions for the six bimodal histograms were estimated by the method of maximum likelihood. It was found that the mean area of the small neuron component, which were presumed to be gamma motoneurons, was similar for the six bimodal systems. In contrast to this the mean area of the large neuron component, presumed to be alpha motoneurons, was found to be different for the six bimodal systems; motoneurons supplying more proximal muscles showed a larger mean area than those supplying distal muscles. The mean area of both components was unaffected by survival time and this was interpreted as indicating that changes in survival time did not label greater numbers of small or large motoneurons. The proportion of motoneurons in the small neuron component was found to vary from 9 to 27%.     

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.     

Output connections of a wind sensitive interneurone with motor neurones innervating flight steering muscles in the locust

Summary The output connections of a bilaterally symmetrical pair of wind-sensitive interneurones (called A4I1) were determined in a non-flying locust (Schistocerca gregaria). Direct inputs from sensory neurones of specific prosternai and head hairs initiate spikes in these interneurones in the prothoracic ganglion.The interneurone with its axon in the right connective makes direct, excitatory connections with the two mesothoracic motor neurones innervating the pleuroaxillary (pleuroalar, M85) muscle of the right forewing, but not with the comparable motor neurones of the left forewing. The connections can evoke motor spikes.The interneurones also exert a powerful, but indirect effect on the homologous metathoracic pleuroaxillary motor neurones (muscle 114), and a weaker, indirect effect on subalar motor neurones of the hindwings. No connections or effects were found with other flight motor neurones, or motor neurones innervating hindleg muscles, including common inhibitor 1 which also innervates the pleuroaxillary muscle.One thoracic interneurone with its cell body in the right half of the mesothoracic ganglion and with its axon projecting ipsilaterally to the metathoracic ganglion receives a direct input from the right A4I1 interneurone.These restricted output connections suggest a role for the A4I1 interneurones in flight steering.Abbreviations DCMD descending contralateral movement detector - EPSP excitatory postsynaptic potential - TCG tritocerebral commissure giant (interneurone)     

Interaction of lipophorin with the plasma membrane of locust flight muscles

Binding of high-density lipophorin (HDLp) to a plasma membrane preparation of locust flight muscle tissue was studied using a radiolabelled ligand binding assay and ligand blotting techniques. Analysis at 33 degrees C of the concentration-dependent total binding of tritium-labelled HDLp ([3H]HDLp) to the membrane preparation revealed the presence of a single specific binding site with an equilibrium dissociation constant of Kd = 9 (+/- 2) X 10(-7) M and a maximal binding capacity of 84 (+/- 10) ng X (micrograms protein)-1. Unlabelled HDLp as well as unlabelled low-density lipophorin (LDLp) competed with [3H]HDLp for binding to the identified binding site. In addition, ligand blotting demonstrated that both HDLp and LDLp bind specifically to a 30-kDa protein in the plasma membrane preparation, suggesting the involvement of this protein in the binding of lipophorins to the isolated membranes. A possible relationship between the identified binding of lipophorins and the observed co-purification of lipophorin lipase activity with the plasma membranes is discussed.     

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.

The distribution of motoneurons supplying hind limb muscles in the clawed toad, Xenopus laevis

The distribution of motoneurons in the lumbar spinal cord (spinal segments 8-10) of the clawed toad, Xenopus laevis, was studied with the horseradish peroxidase technique. In a total of 13 different hind limb muscles this tracer was applied in a slow-release gel. Motoneurons innervating a particular hind limb muscle were clustered in longitudinally arranged motor pools. Motor pools of different muscles did show considerable overlap both in the rostrocaudal and transverse plane. But, the various motor pools clearly show a somatotopic organization of motoneurons even in such a condensed lumbar spinal cord as in Xenopus laevis. Motoneurons innervating more distally positioned muscles are generally found in more caudal segments, while proximal muscles (with the exception of the m. adductor magnus) are supplied by motoneurons more or less throughout the lumbar enlargement. Flexor muscles usually are innervated by motoneurons situated ventrolaterally in the ventral horn, extensor muscles by dorsomedially found motoneurons. This pattern is particularly apparent for proximal (thigh) muscles, less so for more distal (shank and foot) muscles. The present data are in keeping with those obtained with the retrograde cell degeneration technique in ranid frogs and are consistent with observations in other tetrapods, although a more clear separation of motor pools is evident in "higher" vertebrates such as birds and mammals.     

The role of delayed excitation in the co-ordination of some metathoracic flight motoneurons of a locust

Summary Intracellular recordings have been made from the somata of two metathoracic flight motoneurons, one innervating an elevator muscle of the hindwing, the tergosternal muscle 113 and the other a depressor, the first basalar muscle 127. The locust,Ghortoicetes terminifera was mounted ventral side uppermost with the thorax restrained and opened for access to the thoracic ganglia. Patterns of electrical activity recorded from the thoracic muscles were similar to those shown by a locust during flight when tethered in a more normal posture. In flight the left and right 113 motoneurons each receive a single impulse together at every stroke of the wing, with the 127 muscles active in approximate antiphase. A spike in a 113 motoneuron causes a delayed wave of excitation simultaneously upon itself and its contralateral partner (Fig. 2). The epsp's which form these waves summate and may cause a spike which follows the original one with a delay equal to the wingbeat period. The delayed excitation of the contralateral motoneuron is of larger amplitude than the ipsilateral one so that spikes in either motoneuron must activate separate but symmetrical pathways. A single spike may cause multiple waves in either motoneuron, each separated by intervals equal to the wingbeat period (Fig. 3). In the pathway must be neurons capable of reverberation.A spike in a 113 motoneuron causes a delayed excitation of the ipsilateral 127 motoneuron so that its membrane potential is lowered antiphasically to that of 113 (Fig. 17). A spike in a 127 motoneuron has no effect on the 113 motoneurons. In flight these pathways causing delayed excitation may co-ordinate the motoneurons.The left and right 113 motoneurons receive common synaptic inputs from at least two sources (Fig. 8). These occur as bursts of epsp's at intervals approximately equal to or multiples of the wingbeat period and in the absence of flight. Epsp's of sufficient amplitude cause a spike in the motoneuron which is in the correct phase in the flight pattern relative to any other active motoneurons (Fig. 9). During sustained flight epsp's contribute to the wave of depolarization that the motoneuron undergoes at each wingbeat (Fig. 11). In the absence of the epsp's the motoneuron does not oscillate on its own. At the end of flight bursts of epsp's may continue at the flight frequency long after all activity in the muscles has ceased.Beit Memorial Research Fellow.     

Effect of denervation on enzyme levels in flight muscles of the adult migratory locust

The metathoracic dorso-longitudinal muscles of adult males of Locusta migratoria were denervated. Twenty days after denervation wet weight, protein content and the specific activities of enzymes, representative of aerobic glycolytic and β-oxidative pathways, and citric acid and glycerophosphate cycles are reduced in a statistically significant way. In contrast, the specific activity of lactate dehydrogenase is increased. It is demonstrated that locusts, in which the metathoracic dorso-longitudinal muscles are denervated on one side only, constitute an experimental system with proper experimental and control muscles.     

Co-ordinating interneurones of the locust which convey two patterns of motor commands: their connexions with flight motoneurones.

1. Some flight motoneurones receive two superimposed rhythms of depolarizing synaptic potentials when the locust is not flying; a slow rhythm which is invariably linked to the expiratory phase of ventilation, and a fast rhythm with a period of about 50 ms which is similar to the wingbeat period in flight. 2. By recording simultaneously from groups of motoneurones, the synaptic potentials which underly these rhythms have been revealed in 30 flight motoneurones in the three thoracic ganglia. The inputs occur in elevator motoneurones and some depressors but are of lower amplitude in the latter. The inputs have not been found in leg motoneurones. 3. The rhythmic depolarizations are usually subthreshold but sum with sensory inputs to evoke spikes in flight motoneurones at intervals equal to or multiples of the wingbeat period in flight. 4. Both rhythms originate in the metathoracic ganglion and are mediated by the same interneurones. They can be adequately explained by supposing that there are two symmetrical interneurones which each make widespread connexions with left and right flight motoneurones in the three ganglia. 5. The slow rhythm is coded in the overall burst of interneurone spikes during expiration and the fast rhythm in the interval between the spikes of a burst.     

Interactions of suboesophageal ganglion and frontal ganglion motor patterns in the locust

Although locust feeding has been well studied, our understanding of the neural basis of feeding-related motor patterns is still far from complete. This paper focuses on interactions between the pattern of rhythmic movements of the mouth appendages, governed by the suboesophageal ganglion (SOG), and the foregut movements, controlled by the frontal ganglion (FG), in the desert locust. In vitro simultaneous extracellular nerve recordings were made from totally isolated ganglia as well as from fully interconnected SOG-FG and brain-SOG-FG preparations. SOG-confined bath application of the nitric oxide donor, SNP, or the phosphodiesterase antagonist, IBMX, each followed by the muscarinic agonist pilocarpine, consistently induced robust fictive motor patterns in the SOG. This was observed in both isolated and interconnected preparations. In the brain-SOG-FG configuration the SOG-confined modulator application had an indirect excitatory effect on spontaneous FG rhythmic activity. Correlation between fictive motor patterns of the two ganglia was demonstrated by simultaneous changes in burst frequency. These interactions were found to be brain-mediated. Our results indicate the presence of intricate neuromodulation-mediated circuit interactions, even in the absence of sensory inputs. These interactions may be instrumental in generating the complex rhythmic motor patterns of the mandibles and gut muscles during locust feeding or ecdysis-related air swallowing.