Unravelling intra- and intersegmental neuronal connectivity between central pattern generating networks in a multi-legged locomotor system

AbstractAnimal walking results from a complex interplay of central pattern generating networks (CPGs), local sensory signals expressing position, velocity and forces generated in the legs, and coordinating signals between neighboring ones. In the stick insect, in particular, intra- and intersegmental coordination is conveyed by these sensory signals. The rhythmic activity of the CPGs, hence of the legs, can be modified by the aforementioned sensory signals. However, the precise nature of the interaction between the CPGs and these sensory signals has remained largely unknown. Experimental methods aiming at finding out details of these interactions, often apply the muscarinic acetylcholine receptor agonist, pilocarpine in order to induce rhythmic activity in the CPGs, hence in the motoneurons of the segmental ganglia. Using this general approach, we removed the influence of sensory signals and investigated the putative connections between CPGs associated with the coxa-trochanter (CTr)-joint in the different segments (legs) in more detail. The experimental data underwent phase-difference analysis and Dynamic Causal Modelling (DCM). These methods can uncover the underlying coupling structure and strength between pairs of segmental ganglia (CPGs). We set up different coupling schemes (models) for DCM and compared them using Bayesian Model Selection (BMS). Models with contralateral connections in each segment and ipsilateral connections on both sides, as well as the coupling from the meta- to the ipsilateral prothoracic ganglion were preferred by BMS to all other types of models tested. Moreover, the intrasegmental coupling strength in the mesothoracic ganglion was the strongest and most stable in all three ganglia.

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The Role of Sensory Signals From the Insect Coxa-Trochanteral Joint in Controlling Motor Activity of the Femur-Tibia Joint

Journal of Neurophysiology ◽

10.1152/jn.2001.85.2.594 ◽

2001 ◽

Vol 85 (2) ◽

pp. 594-604 ◽

Cited By ~ 75

Author(s):

Turgay Akay ◽

Ulrich Bässler ◽

Petra Gerharz ◽

Ansgar Büschges

Keyword(s):

Rhythmic Activity ◽

Stick Insect ◽

Active Movement ◽

Inhibitory Influence ◽

Locomotor System ◽

Sensory Signals ◽

Receptor Organ ◽

Motoneuron Activity ◽

Locomotor Patterns

Interjoint coordination in multi-jointed limbs is essential for the generation of functional locomotor patterns. Here we have focused on the role that sensory signals from the coxa-trochanteral (CT) joint play in patterning motoneuronal activity of the femur-tibia (FT) joint in the stick insect middle leg. This question is of interest because when the locomotor system is active, movement signals from the FT joint are known to contribute to patterning of activity of the central rhythm-generating networks governing the CT joint. We investigated the influence of femoral levation and depression on the activity of tibial motoneurons. When the locomotor system was active, levation of the femur often induced a decrease or inactivation of tibial extensor activity while flexor motoneurons were activated. Depression of the femur had no systematic influence on tibial motoneurons. Ablation experiments revealed that this interjoint influence was not mediated by signals from movement and/or position sensitive receptors at the CT joint, i.e., trochanteral hairplate, rhombal hairplate, or internal levator receptor organ. Instead the influence was initiated by sensory signals from a field of campaniform sensillae, situated on the proximal femur (fCS). Selective stimulation of these fCS produced barrages of inhibitory postsynaptic potentials (IPSPs) in tibial extensor motoneurons and activated tibial flexor motoneurons. During pharmacologically activated rhythmic activity of the otherwise isolated mesothoracic ganglion (pilocarpine, 5 × 10− 4 M), deafferented except for the CT joint, levation of the femur as well had an inhibitory influence on tibial extensor motoneurons. However, the influence of femoral levation on the rhythm generated was rather labile and only sometimes a reset of the rhythm was induced. In none of the preparations could entrainment of rhythmicity by femoral movement be achieved, suggesting that sensory signals from the CT joint only weakly affect central rhythm-generating networks of the FT joint. Finally, we analyzed the role of sensory signals from the fCS during walking by recording motoneuronal activity in the single middle leg preparation with fCS intact and after their removal. These experiments showed that fCS activity plays an important role in generating tibial motoneuron activity during the stance phase of walking.

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Localization of muscarinic acetylcholine receptor dependent rhythm generating modules in the Drosophila larval locomotor network

10.1101/2021.03.08.432150 ◽

2021 ◽

Author(s):

Julius Jonaitis ◽

James MacLeod ◽

Stefan R. Pulver

Keyword(s):

Acetylcholine Receptor ◽

Muscarinic Acetylcholine Receptor ◽

Rhythmic Activity ◽

Rhythm Generation ◽

Motor Patterns ◽

Locomotor System ◽

Muscarinic Acetylcholine ◽

Bath Application ◽

The Central Nervous System ◽

Control Locomotion

AbstractMechanisms of rhythm generation have been extensively studied in motor systems that control locomotion over terrain in limbed animals; however, much less is known about rhythm generation in soft-bodied terrestrial animals. Here we explored how muscarinic acetylcholine receptor (mAChR) dependent rhythm generating networks are distributed in the central nervous system (CNS) of soft-bodied Drosophila larvae. We measured fictive motor patterns in isolated CNS preparations using a combination of Ca2+ imaging and electrophysiology while manipulating mAChR signalling pharmacologically. Bath application of the mAChR agonist oxotremorine potentiated rhythm generation in distal regions of the isolated CNS, whereas application of the mAChR antagonist scopolamine suppressed rhythm generation in these regions. Oxotremorine raised baseline Ca2+ levels and potentiated rhythmic activity in isolated posterior abdominal CNS segments as well as isolated anterior brain and thoracic regions, but did not induce rhythmic activity in isolated anterior abdominal segments. Bath application of scopolamine to reduced preparations lowered baseline Ca2+ levels and abolished rhythmic activity. These results suggest the presence of a bimodal gradient of rhythmogenicity in the larval CNS, with mAChR dependent rhythm generating networks in distal regions separated by medial segments with severely reduced rhythmogenic abilities. This work furthers our understanding of motor control in soft-bodied locomotion and provides a foundation for study of rhythm generating networks in an emerging genetically tractable locomotor system.

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Intra- and intersegmental influences among central pattern generating networks in the walking system of the stick insect

Journal of Neurophysiology ◽

10.1152/jn.00321.2017 ◽

2017 ◽

Vol 118 (4) ◽

pp. 2296-2310 ◽

Cited By ~ 22

Author(s):

Charalampos Mantziaris ◽

Till Bockemühl ◽

Philip Holmes ◽

Anke Borgmann ◽

Silvia Daun ◽

...

Keyword(s):

Neural Networks ◽

Sensory Feedback ◽

Muscarinic Acetylcholine Receptor ◽

Central Pattern Generators ◽

Sensory Information ◽

Direct Interaction ◽

Stick Insect ◽

Weakly Coupled ◽

The Neural Networks ◽

Pattern Generators

To efficiently move around, animals need to coordinate their limbs. Proper, context-dependent coupling among the neural networks underlying leg movement is necessary for generating intersegmental coordination. In the slow-walking stick insect, local sensory information is very important for shaping coordination. However, central coupling mechanisms among segmental central pattern generators (CPGs) may also contribute to this. Here, we analyzed the interactions between contralateral networks that drive the depressor trochanteris muscle of the legs in both isolated and interconnected deafferented thoracic ganglia of the stick insect on application of pilocarpine, a muscarinic acetylcholine receptor agonist. Our results show that depressor CPG activity is only weakly coupled between all segments. Intrasegmental phase relationships differ between the three isolated ganglia, and they are modified and stabilized when ganglia are interconnected. However, the coordination patterns that emerge do not resemble those observed during walking. Our findings are in line with recent studies and highlight the influence of sensory input on coordination in slowly walking insects. Finally, as a direct interaction between depressor CPG networks and contralateral motoneurons could not be observed, we hypothesize that coupling is based on interactions at the level of CPG interneurons. NEW & NOTEWORTHY Maintaining functional interleg coordination is vitally important as animals locomote through changing environments. The relative importance of central mechanisms vs. sensory feedback in this process is not well understood. We analyzed coordination among the neural networks generating leg movements in stick insect preparations lacking phasic sensory feedback. Under these conditions, the networks governing different legs were only weakly coupled. In stick insect, central connections alone are thus insufficient to produce the leg coordination observed behaviorally.

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Signals From Load Sensors Underlie Interjoint Coordination During Stepping Movements of the Stick Insect Leg

Journal of Neurophysiology ◽

10.1152/jn.01271.2003 ◽

2004 ◽

Vol 92 (1) ◽

pp. 42-51 ◽

Cited By ~ 62

Author(s):

Turgay Akay ◽

Sebastian Haehn ◽

Josef Schmitz ◽

Ansgar Büschges

Keyword(s):

Stick Insect ◽

Swing Phase ◽

Campaniform Sensilla ◽

Locomotor System ◽

Interjoint Coordination ◽

Stepping Pattern ◽

Tightly Coupled ◽

Motoneuron Activity ◽

Walking Stick ◽

Stimulation Of

During stance and swing phase of a walking stick insect, the retractor coxae (RetCx) and protractor coxae (ProCx) motoneurons and muscles supplying the thorax-coxa (TC)-joint generate backward and forward movements of the leg. Their activity is tightly coupled to the movement of the more distal leg segments, i.e., femur, tibia, and tarsus. We used the single middle leg preparation to study how this coupling is generated. With only the distal leg segments of the middle leg being free to move, motoneuronal activity of the de-afferented and -efferented TC-joint is similarly coupled to leg stepping. RetCx motoneurons are active during stance and ProCx motoneurons during swing. We studied whether sensory signals are involved in this coordination of TC-joint motoneuronal activity. Ablation of the load measuring campaniform sensilla (CS) revealed that they substantially contribute to the coupling of TC-joint motoneuronal activity to leg stepping. Individually ablating trochanteral and femoral CS revealed the trochanteral CS to be necessary for establishing the coupling between leg stepping and coxal motoneuron activity. When the locomotor system was active and generated alternating bursts of activity in ProCx and RetCx motoneurons, stimulation of the CS by rearward bending of the femur in otherwise de-afferented mesothoracic ganglion terminated ongoing ProCx motoneuronal activity and initiated RetCx motoneuronal activity. We show that cuticular strain signals from the trochanteral CS play a major role in shaping TC-joint motoneuronal activity during walking and contribute to their coordination with the stepping pattern of the distal leg joints. We present a model for the sensory control of timing of motoneuronal activity in walking movements of the single middle leg.

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Role of Proprioceptive Signals From an Insect Femur-Tibia Joint in Patterning Motoneuronal Activity of an Adjacent Leg Joint

Journal of Neurophysiology ◽

10.1152/jn.1999.81.4.1856 ◽

1999 ◽

Vol 81 (4) ◽

pp. 1856-1865 ◽

Cited By ~ 65

Author(s):

Dietmar Hess ◽

Ansgar Büschges

Keyword(s):

Movement Control ◽

Stick Insect ◽

Control Mode ◽

Chordotonal Organ ◽

Joint Control ◽

Muscarinic Agonist ◽

Behavioral State ◽

Sensory Signals ◽

Motoneuron Activity

Role of proprioceptive signals from an insect femur-tibia joint in patterning motoneuronal activity of an adjacent leg joint. Interjoint reflex function of the insect leg contributes to postural control at rest or to movement control during locomotor movements. In the stick insect ( Carausius morosus), we investigated the role that sensory signals from the femoral chordotonal organ (fCO), the transducer of the femur-tibia (FT) joint, play in patterning motoneuronal activity in the adjacent coxa-trochanteral (CT) joint when the joint control networks are in the movement control mode of the active behavioral state. In the active behavioral state, sensory signals from the fCO induced transitions of activity between antagonistic motoneuron pools, i.e., the levator trochanteris and the depressor trochanteris motoneurons. As such, elongation of the fCO, signaling flexion of the FT joint, terminated depressor motoneuron activity and initiated activity in levator motoneurons. Relaxation of the fCO, signaling extension of the FT joint, induced the opposite transition by initiating depressor motoneuron activity and terminating levator motoneuron activity. This interjoint influence of sensory signals from the fCO was independent of the generation of the intrajoint reflex reversal in the FT joint, i.e., the “active reaction,” which is released by elongation signals from the fCO. The generation of these transitions in activity of trochanteral motoneurons barely depended on position or velocity signals from the fCO. This contrasts with the situation in the resting behavioral state when interjoint reflex action markedly depends on actual fCO stimulus parameters, i.e., position and velocity signals. In the active behavioral state, movement signals from the fCO obviously trigger or release centrally generated transitions in motoneuron activity, e.g., by affecting central rhythm generating networks driving trochanteral motoneuron pools. This conclusion was tested by stimulating the fCO in “fictive rhythmic” preparations, activated by the muscarinic agonist pilocarpine in the otherwise isolated and deafferented mesothoracic ganglion. In this situation, sensory signals from the fCO did in fact reset and entrain rhythmic activity in trochanteral motoneurons. The results indicate for the first time that when the stick insect locomotor system is active, sensory signals from the proprioceptor of one leg joint, i.e., the fCO, pattern motor activity in an adjacent leg joint, i.e., the CT joint, by affecting the central rhythm generating network driving the motoneurons of the adjacent joint.

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Ultrastructure of the prothoracic ganglion and connectives of the stick insect in relation to function

Journal of Insect Physiology ◽

10.1016/0022-1910(71)90154-5 ◽

1971 ◽

Vol 17 (8) ◽

pp. 1451-1469 ◽

Cited By ~ 12

Author(s):

H. Huddart

Keyword(s):

Stick Insect ◽

Prothoracic Ganglion

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Cholinergic Currents in Leg Motoneurons of Carausius morosus

Journal of Neurophysiology ◽

10.1152/jn.00963.2009 ◽

2010 ◽

Vol 103 (5) ◽

pp. 2770-2782 ◽

Cited By ~ 14

Author(s):

Eugênio E. Oliveira ◽

Andreas Pippow ◽

Vincent L. Salgado ◽

Ansgar Büschges ◽

Joachim Schmidt ◽

...

Keyword(s):

Nicotinic Acetylcholine Receptors ◽

Acetylcholine Receptors ◽

Muscarinic Acetylcholine Receptor ◽

Stick Insect ◽

Fluorescent Label ◽

Receptor Ligands ◽

Nicotinic Acetylcholine ◽

Carausius Morosus ◽

Unequivocal Identification

We used patch-clamp recordings and fast optical Ca2+ imaging to characterize an acetylcholine-induced current ( IACh) in leg motoneurons of the stick insect Carausius morosus. Our long-term goal is to better understand the synaptic and integrative properties of the leg sensory-motor system, which has served extremely successfully as a model to study basic principles of walking and locomotion on the network level. The experiments were performed under biophysically controlled conditions on freshly dissociated leg motoneurons to avoid secondary effects from the network. To allow for unequivocal identification, the leg motoneurons were backfilled with a fluorescent label through the main leg nerve prior to cell dissociation. In 87% of the motoneurons, IACh consisted of a fast-desensitizing ( IACh1) and a slow-desensitizing component ( IACh2), both of which were concentration dependent, with EC50 values of 3.7 × 10−5 and 2.0 × 10−5 M, respectively. Ca2+ imaging revealed that a considerable portion of IACh (∼18%) is carried by Ca2+, suggesting that IACh, besides mediating fast synaptic transmission, could also induce Ca2+-dependent processes. Using specific nicotinic and muscarinic acetylcholine receptor ligands, we showed that IACh was exclusively mediated by nicotinic acetylcholine receptors. Distinct concentration–response relations of IACh1 and IACh2 for these ligands indicated that they are mediated by different types of nicotinic acetylcholine receptors.

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Rhythmic patterns in the thoracic nerve cord of the stick insect induced by pilocarpine

Journal of Experimental Biology ◽

10.1242/jeb.198.2.435 ◽

1995 ◽

Vol 198 (2) ◽

pp. 435-456 ◽

Cited By ~ 7

Author(s):

A Büschges ◽

J Schmitz ◽

U Bässler

Keyword(s):

Phase Transitions ◽

Nerve Cord ◽

Rhythmic Activity ◽

Stick Insect ◽

Burst Duration ◽

Cycle Period ◽

Muscarinic Agonist ◽

Thoracic Nerve ◽

Similar Range

Bath application of the muscarinic agonist pilocarpine onto the deafferented stick insect thoracic nerve cord induced long-lasting rhythmic activity in leg motoneurones. Rhythmicity was induced at concentrations as low as 1x10(-4) mol l-1 pilocarpine. The most stable rhythms were reliably elicited at concentrations from 2x10(-3) mol l-1 to 5x10(-3) mol l-1. Rhythmicity could be completely abolished by application of atropine. The rhythm in antagonistic motoneurone pools of the three proximal leg joints, the subcoxal, the coxo-trochanteral (CT) and the femoro-tibial (FT), was strictly alternating. In the subcoxal motoneurones, the rhythm was characterised by the retractor burst duration being correlated with cycle period, whereas the protractor burst duration was almost independent of it. The cycle periods of the rhythms in the subcoxal and CT motoneurone pools were in a similar range for a given preparation. In contrast, the rhythm exhibited by motoneurones supplying the FT joint often had about half the duration. The pilocarpine-induced rhythm was generated independently in each hemiganglion. There was no strict intersegmental coupling, although the protractor motoneurone pools of the three thoracic ganglia tended to be active in phase. There was no stereotyped cycle-to-cycle coupling in the activities of the motoneurone pools of the subcoxal joint, the CT joint and the FT joint in an isolated mesothoracic ganglion. However, three distinct 'spontaneous, recurrent patterns' (SRPs) of motoneuronal activity were reliably generated. Within each pattern, there was strong coupling of the activity of the motoneurone pools. The SRPs resembled the motor output during step-phase transitions in walking: for example, the most often generated SRP (SRP1) was exclusively exhibited coincident with a burst of the fast depressor trochanteris motoneurone. During this burst, there was a switch from subcoxal protractor to retractor activity after a constant latency. The activity of the FT joint extensor motoneurones was strongly decreased during SRP1. SRP1 thus qualitatively resembled the motoneuronal activity during the transition from swing to stance of the middle legs in forward walking. Hence, we refer to SRPs as 'fictive step-phase transitions'. In intact, restrained animals, application of pilocarpine also induced alternating activity in antagonistic motoneurone pools supplying the proximal leg joints. However, there were marked differences from the deafferented preparation. For example, SRP1 was not generated in the latter situation. However, if the ipsilateral main leg nerve was cut, SRP1s reliably occurred. Our results on the rhythmicity in leg motoneurone pools of deafferented preparations demonstrate central coupling in the activity of the leg motoneurones that might be incorporated into the generation of locomotion in vivo.

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