Connections of the forewing tegulae in the locust flight system and their modification following partial deafferentation

Central pattern generating networks in insect locomotion

Central pattern generators (CPGs) are neural circuits that based on their connectivity can generate rhythmic and patterned output in the absence of rhythmic external inputs. This property makes CPGs crucial elements in the generation of many kinds of rhythmic motor behaviors in insects, such as flying, walking, swimming, or crawling. Arguably representing the most diverse group of animals, insects utilize at least one of these types of locomotion during one stage of their ontogenesis. Insects have been extensively used to study the neural basis of rhythmic motor behaviors, and particularly the structure and operation of CPGs involved in locomotion. Here, we review insect locomotion with regard to flying, walking, and crawling, and we discuss the contribution of central pattern generation to these three forms of locomotion. In each case, we compare and contrast the topology and structure of the CPGs, and we point out how these factors are involved in the generation of the respective motor pattern. We focus on the importance of sensory information for establishing a functional motor output and we indicate behavior-specific adaptations. Furthermore, we report on the mechanisms underlying coordination between different body parts. Last but not least, by reviewing the state-of-the-art knowledge concerning the role of CPGs in insect locomotion, we endeavor to create a common ground, upon which future research in the field of motor control in insects can build.

From injury to full repair: nerve regeneration and functional recovery in the common octopus, Octopus vulgaris

Spontaneous nerve regeneration in cephalopod molluscs occurs in a relative short time after injury, achieving functional recovery of lost capacity. In particular, transection of the pallial nerve in the common octopus (Octopus vulgaris) determines the loss and subsequent restoration of two functions fundamental for survival, i.e. breathing and skin patterning, the latter involved in communication between animals and concealment. The phenomena occurring after lesion have been investigated in a series of previous studies, but a complete analysis of the changes taking place at the level of the axons and the effects on the animals' appearance during the whole regenerative process is still missing. Our goal was to determine the course of events following injury, from impairment to full recovery. Through imaging of the traced damaged nerves, we were able to characterize the pathways followed by fibres during regeneration and end-target re-innervation, while electrophysiology and behavioural observations highlighted the regaining of functional connections between the central brain and periphery, using the contralateral nerve in the same animal as an internal control. The final architecture of a fully regenerated pallial nerve does not exactly mirror the original structure; however, functionality returns to match the phenotype of an intact octopus with no observable impact on the behaviour of the animal. Our findings provide new important scenarios for the study of regeneration in cephalopods and highlight the octopus pallial nerve as a valuable 'model' among invertebrates.

Recovery of locomotion after injury in Drosophila melanogaster depends on proprioception

Locomotion is necessary for survival in most animal species. However, injuries to the appendages mediating locomotion are common. We assess the recovery of walking in Drosophila melanogaster following leg amputation. Whereas flies pre-amputation explore open arenas in a symmetric fashion on average, foreleg amputation induces a strong turning bias away from the side of the amputation. However, we find that unbiased walking behavior returns over time in wild-type flies, while recovery is significantly impaired in proprioceptive mutants. To identify the biomechanical basis of this locomotor impairment and recovery, we then examine individual leg motion (gait) at a fine scale. A minimal mathematical model that links neurodynamics to body mechanics during walking shows that redistributing leg forces between the right and left side enables the observed recovery. Altogether, our study suggests that proprioceptive input from the intact limbs plays a crucial role in the behavioral plasticity associated with locomotor recovery after injury.

Quantification of dendritic and axonal growth after injury to the auditory system of the adult cricket Gryllus bimaculatus

Dendrite and axon growth and branching during development are regulated by a complex set of intracellular and external signals. However, the cues that maintain or influence adult neuronal morphology are less well understood. Injury and deafferentation tend to have negative effects on adult nervous systems. An interesting example of injury-induced compensatory growth is seen in the cricket, Gryllus bimaculatus. After unilateral loss of an ear in the adult cricket, auditory neurons within the central nervous system (CNS) sprout to compensate for the injury. Specifically, after being deafferented, ascending neurons (AN-1 and AN-2) send dendrites across the midline of the prothoracic ganglion where they receive input from auditory afferents that project through the contralateral auditory nerve (N5). Deafferentation also triggers contralateral N5 axonal growth. In this study, we quantified AN dendritic and N5 axonal growth at 30 h, as well as at 3, 5, 7, 14, and 20 days after deafferentation in adult crickets. Significant differences in the rates of dendritic growth between males and females were noted. In females, dendritic growth rates were non-linear; a rapid burst of dendritic extension in the first few days was followed by a plateau reached at 3 days after deafferentation. In males, however, dendritic growth rates were linear, with dendrites growing steadily over time and reaching lengths, on average, twice as long as in females. On the other hand, rates of N5 axonal growth showed no significant sexual dimorphism and were linear. Within each animal, the growth rates of dendrites and axons were not correlated, indicating that independent factors likely influence dendritic and axonal growth in response to injury in this system. Our findings provide a basis for future study of the cellular features that allow differing dendrite and axon growth patterns as well as sexually dimorphic dendritic growth in response to deafferentation.

Central projections of the wing afferents in the hawkmoth, Agrius convolvuli

Flight behaviors in various insect species are closely correlated with their mechanical and neuronal properties. Compared to locusts and flies which have been intensively studied, moths have "intermediate" properties in terms of the neurogenic muscle activations, power generation by indirect muscles, and two-winged-insect-like flapping behavior. Despite these unique characteristics, little is known about the neuronal mechanisms of flight control in moths. We investigated projections of the wing mechanosensory afferents in the central nervous system (CNS) of the hawkmoth, Agrius convolvuli, because the mechanosensory proprioceptive feedback has an essential role for flight control and would be presumably optimized for insect species. We conducted anterograde staining of nine afferent nerves from the fore- and hindwings. All of these afferents projected into the prothoracic, mesothoracic and metathoracic ganglia (TG1, 2 and 3) and had ascending fibers to the head ganglia. Prominent projection areas in the TG1-3 and suboesophageal ganglion (SOG) were common between the forewing, hindwing and contralateral forewing afferents, suggesting that information from different wings are converged at multiple levels presumably for coordinating wing flapping. On the other hand, differences of projections between the fore- and hindwing afferents were observed especially in projection areas of the tegulae in the TG1 and contralateral projections of the anterior forewing nerve in the TGs and SOG, which would reflect functional differences between corresponding mechanoreceptors on each wing. Afferents comprising groups of the campaniform sensilla at the wing bases had prominent ascending pathways to the SOG, resembling the head-neck motor system for gaze control in flies. Double staining of the wing afferents and flight or neck motoneurons also indicated potential connectivity between them. Our results suggest multiple roles of the wing proprioceptive feedback for flight and provide the anatomical basis for further understanding of neuronal mechanisms of the flight system in moths.

Changes in the auditory neuropil after deafferentation in adult grasshoppers (Schistocerca gregaria)

Nervous systems are capable of structural adjustments. Such plastic changes also occur in the auditory system of the locust Schistocerca gregaria in which a deafferentation leads to compensatory mechanisms, such as collateral sprouting of interneurons. In this study we further investigated lesion related changes in the major auditory neuropil, the median ventral association center (mVAC) of the metathoracic ganglion. The auditory sensory organ of adult locusts was unilaterally extirpated and the mVAC was histologically and immunocytochemically analyzed until 20 days postoperative. Measurements of the neuropil area in transverse sections showed a decrease in size. The putative transmitter of the afferents, acetylcholine, was investigated by acetylcholinesterase histochemistry. Comparisons of staining intensities in the intact and deafferentated mVAC indicated that the amount of acetylcholinesterase in the deafferentated mVAC decreased shortly after the operation. Both, the decreases in size of the mVAC as well as that in acetylcholinesterase histochemistry were only less than 10% compared to the controls. The immunoreactivity against the neurotransmitters gamma-amino butyric acid and serotonin was not influenced by the deafferentation.

Functional recovery of aimed scratching movements after a graded proprioceptive manipulation

To demonstrate the role of proprioceptive feedback in aimed limb movements, we induced graded changes in the signals provided by the principal receptor in a leg of a locust. The femoro-tibial chordotonal organ (FCO) of the hindleg monitors extension and flexion movements of the tibia and provides the main source of proprioceptive feedback about tibial kinematics. The FCO apodeme (tendon) was surgically shortened by different amounts to provide a systematic bias to this feedback, and aimed scratching movements were analyzed over the week after surgery. Shortening the apodeme led to increased firing of sensory neurons of the FCO at flexed joint angles and is thus functionally similar to flexing the tibia. Immediately after surgery, limb movements shifted dorsally and posteriorly, driven by overextension of the femoro-tibial joint and changes at other joints of the limb. The extent of tibial overextension reflected the extent of apodeme shortening. Overextension would tend to renormalize the FCO feedback signal and can be explained by known interjoint reflex pathways. Our data demonstrate that proprioceptive feedback provides a graded signal that is used to control these aimed limb movements. Over the course of 7 d after surgery, there was a functional recovery in aiming as the overall patterns of movement returned toward control values driven by reciprocal compensatory changes at two joints. The sensory to motor pathways are monosynaptic and oligosynaptic in this system, thus providing us with a powerful opportunity to investigate further the sensorimotor transformations and plasticity of aimed limb movements.

Frequency control of motor patterning by negative sensory feedback

The sensory system plays a key role in the generation of behavior by providing the nervous system with information about the environment and feedback about body movements such that motor output can continuously be adapted to changing circumstances. Although the effects of sensory organs on nervous system function have been demonstrated in many systems, the impact of sensory activity has rarely been studied in conditions in which motor output and sensory activity can interact as they do in behaving animals. In such situations, emergent properties may surface and govern the characteristics of the motor system. We studied the dynamics of sensorimotor interaction with a combination of electrophysiological experiments and computational modeling in the locust flight pattern generator, including its sensory components. The locust flight motor output is produced by a central pattern generator that interacts with phasic sensory feedback from the tegula, a proprioceptor that signals downstroke movement of the wing. We modeled the flight control system, and we tested the model predictions by replacing tegula feedback in the animal with artificial feedback through computer-controlled electric stimulation of the appropriate sensory nerves. With reference to the cycle frequency in the locust flight rhythm, our results show that motor patterns can be regulated via the variation of sensory feedback loops. In closed-loop conditions, tegula feedback strength determines cycle frequency in the model and the biological preparation such that stronger feedback results in lower frequencies. This regulatory mechanism appears to be a general emergent property of negative feedback systems.

Descending control of turning behavior in the cockroach, Blaberus discoidalis

Legged locomotion has evolved as the most effective form of movement through unpredictable and tortuous environments. Upon encountering an obstacle, an animal must evaluate the object with its sense organs then use the information it acquires to direct appropriate transitional behaviors, such as turning. Previous studies using genetic and surgical lesions implicate the central body complex (CBC) in control of such transitional behaviors of various insects. In this study, lesions of the CBC and surrounding brain regions were used to examine the effects of damage on turning in free-moving and tethered cockroaches. Lesions were performed either as sagittal incisions or by inserting small pieces of foil into regions of the brain. Locomotor behaviors of intact and lesioned animals were compared using high speed video and kinematic analysis. The lesions locations were determined through histological methods. Sagittal lesions to the CBC often result in continuous or incorrect turns. Foil lesions in the CBC also increase the probability that individuals will show turning deficits. The location and degree of the lesion had a strong effect on the animal's ability to turn. These data strongly suggest that the CBC mediates the effects of head sense organs that produce changes in the direction of walking.

Insulin-like peptides are not involved in maturation or functional recovery of neural circuits in the locust flight system

We sought to manipulate maturation and functional recovery of locust flight circuitry by treating locusts with pharmacological doses of bovine anti-insulin and insulin. Anti-insulin treatment of maturing locusts caused reduced growth of the thoracic nervous system, lower body weight, and softer cuticles compared with control locusts. We were unable to block either maturation or recovery of flight circuitry with anti-insulin. We propose that insulin-related peptides are involved in growth and cuticular changes during adult maturation, but have no role in promoting neuronal sprouting during this period or as a result of injury.Key words: insulin, maturation, functional recovery, proprioceptors, flight.

Identified nerve cells and insect behavior

Studies of insect identified neurons over the past 25 years have provided some of the very best data on sensorimotor integration; tracing information flow from sensory to motor networks. General principles have emerged that have increased the sophistication with which we now understand both sensory processing and motor control. Two overarching themes have emerged from studies of identified sensory interneurons. First, within a species, there are profound differences in neuronal organization associated with both the sex and the social experience of the individual. Second, single neurons exhibit some surprisingly rich examples of computational sophistication in terms of (a) temporal dynamics (coding superimposed upon circadian and shorter-term rhythms), and also (b) what Kenneth Roeder called "neural parsimony": that optimal information can be encoded, and complex acts of sensorimotor coordination can be mediated, by small ensembles of cells. Insect motor systems have proven to be relatively complex, and so studies of their organization typically have not yielded completely defined circuits as are known from some other invertebrates. However, several important findings have emerged. Analysis of neuronal oscillators for rhythmic behavior have delineated a profound influence of sensory feedback on interneuronal circuits: they are not only modulated by feedback, but may be substantially reconfigured. Additionally, insect motor circuits provide potent examples of neuronal restructuring during an organism's lifetime, as well as insights on how circuits have been modified across evolutionary time. Several areas where future advances seem likely to occur include: molecular genetic analyses, neuroecological syntheses, and neuroinformatics--the use of digital resources to organize databases with information on identified nerve cells and behavior.

Tegula function during free locust flight in relation to motor pattern, flight speed and aerodynamic output

Tegulae are complex proprioceptors at the wing base of locusts. Deafferentation of the tegulae causes a lack of specific phasic information related to the wing downstroke and the timing of the upstroke. Employing telemetry during free flight of the locust Schistocerca gregaria, we investigated the consequences of tegula ablation on free flight parameters including motor patterns (wingbeat frequency and the relationship between the activation of flight muscle antagonists), free flight speed and aerodynamic output. We investigated the role of the tegula pairs of both wings on the motor pattern generated in free-flying locusts. We show that the tegula organs are not essential for generating the motor patterns necessary for free flight. However, they are required for increasing the motor output to give additional effective lifting power during adaptive behaviour. We also investigated long-term changes in the free flight parameters after tegula ablation. The recovery of the adult flight system revealed in the present study suggests that there is adaptation to the loss of proprioceptive information; this argues for a full functional and behavioural recovery of the flight system of the locust under closed-loop conditions.

Plasticity of synaptic connections in sensory-motor pathways of the adult locust flight system

We investigated possible roles of retrograde signals and competitive interactions in the lesion-induced reorganization of synaptic contacts in the locust CNS. Neuronal plasticity is elicited in the adult flight system by removal of afferents from the tegula, a mechanoreceptor organ at the base of the wing. We severed one hindwing organ and studied the resulting rearrangement of synaptic contacts between flight interneurons and afferent neurons from the remaining three tegulae (2 forewing, 1 hindwing). This was done by electric stimulation of afferents and intracellular recording from interneurons (and occasionally motoneurons). Two to three weeks after unilateral tegula lesion, connections between tegula afferents and flight interneurons were altered in the following way. 1) Axons from the forewing tegula on the operated side had established new synaptic contacts with metathoracic elevator interneurons. In addition, the amplitude of compound excitatory postsynaptic potentials elicited by electric stimulation was increased, indicating that a larger number of afferents connected to any given interneuron. 2) On the side contralateral to the lesion, connectivity between axons from the forewing tegula and elevator interneurons was decreased. 3) The efficacy of the (remaining) hindwing afferents appeared to be increased with regard to both synaptic transmission to interneurons and impact on flight motor pattern. 4) Flight motoneurons, which are normally restricted to the ipsilateral hemiganglion, sprouted across the ganglion midline after unilateral tegula removal and apparently established new synaptic contacts with tegula afferents on that side. The changes on the operated side are interpreted as occupation of synaptic space vacated on the interneurons by the severed hindwing afferents. On the contralateral side, the changes in synaptic contact must be elicited by retrograde signals from bilaterally arborizing flight interneurons, because tegula projections remain strictly ipsilateral. The pattern of changes suggests competitive interactions between forewing and hindwing afferents. The present investigation thus presents evidence that the CNS of the mature locust is capable of extensive synaptic rearrangement in response to injury and indicates for the first time the action of retrograde signals from interneurons.

Structure of the forewing stretch receptor axon in immature and mature adult locusts

During the first 2 weeks following imaginal ecdysis, the wingbeat frequency of Locusta migratoria doubles, and the activity of the forewing stretch receptor (fSR), in response to wing elevation, increases. We examined the three-dimensional structure of the centrally projecting axon of the fSR during adult maturation to determine if there are changes in the branching geometry. We found that changes occur in the mesothoracic projection (IISR Meso). Here, there was a significant increase in the volume of the projection from 2.3 x 10(4) +/- 0.2 x 10(4) microns 3 in immature locusts to 6.0 x 10(4) +/- 1.2 x 10(4) microns 3 in mature locusts. There were also significant increases in the total length, the number of branch points, the number of axonal swellings, and the diameters of first- and second-order branches of the projection. No significant changes were observed in the prothoracic projection (IISR Pro), and the only significant change observed in IISR Meta was negative allometric growth relative to IISR Meso. These results demonstrate that during adult maturation, growth of the fSR axon is heteromorphic between different ganglionic projections and that there is a potential increase in the connectivity of IISR Meso to other flight neurons in the mesothoracic ganglion. We suggest that this may be a mechanism for maintaining the efficacy of afferent input to flight interneurons that are also growing during maturation.

THE LOCUST TEGULA: SIGNIFICANCE FOR FLIGHT RHYTHM GENERATION, WING MOVEMENT CONTROL AND AERODYNAMIC FORCE PRODUCTION

The tegula, a complex sense organ associated with the wing base of the locust, plays an important role in the generation of the flight motor pattern. Here its function in the control of wing movement and aerodynamic force production is described.

The vertical component of forewing movement was monitored while recording intracellularly from flight motoneurones during stationary flight. First, in accordance with previous electrophysiological results, stimulation of hindwing tegula afferents was found to reset the wingstroke to the elevation phase in a well-coordinated manner. Second, recordings made before and after removal of fore- and hindwing tegulae were compared. This comparison demonstrated that the delayed onset of elevator motoneurone activity caused by tegula removal is accompanied by a corresponding delay in the upstroke movement of the wings.

The consequences of this delayed upstroke for aerodynamic force production were investigated by monitoring wing movements and lift generation simultaneously. A marked decrease in net lift generation was observed following tegula removal. Recordings of wing pronation indicate that this decrease in lift is primarily due to the delayed upstroke movement - that is, to a delay of the wings near the aerodynamically unfavourable downstroke position.

It is concluded that the tegula of the locust hindwing signals to the nervous system the impending completion of the wing downstroke and allows initiation of the upstroke movement immediately after the wings have reached the lower reversal point of the wingstroke. The functional significance of tegula feedback and central rhythm generation for locust flight control are discussed.

A light insensitive method for contrast enhancement of insect neurons filled with a cobalt-lysine complex

Modified protocols for cobalt-filling and silver intensification of neurons in the larval and adult stages of the moth, Manduca sexta, have led to improved neuronal visualization and minimal background staining. In particular, long distance projecting multisegmental interneurons, originating in the pterothoracic or terminal abdominal ganglion, were best visualized when a cobalt:lysine complex was used to fill hemi-connectives for several days at 4 C. Ganglia closest to the placement of tracer, which became flooded with cobalt:lysine during the filling period, were removed from the insect. This step eliminated the artifactual filling of neurons that may have taken up the tracer from such pooled regions. This led to a more accurate assessment of whether a multisegmental interneuron projected through the full length of nerve cord to the original site of tracer placement. The protocol for light insensitive silver intensification of cobalt-filled neurons was modified to include an important pH adjustment. NaOH was used to alter the pH of the protective colloid, sodium tungstate, to 10.4 or greater in solution. Especially in larvae, our techniques produced intensely stained cobalt-filled neurons within ganglia that remained transparent and relatively free of nonspecific silver deposition.

Neuronal basis of behavior

In addition to describing behavior in terms of neuronal properties and interconnections, some studies are using these well defined neuronal circuits to see how the circuits interact, how they develop, and how they are modified by experience, hormones and neuromodulators. The ready availability of computers and computational techniques has helped in some efforts, as have improvements in physiological and morphological techniques. The major insights, however, still come from experiments that ask clear and direct questions. This review highlights some of the promising approaches and suggests some general features of how neuronal circuits produce behavior.

Reorganization of sensory regulation of locust flight after partial deafferentation

Previous investigations have shown that the flight motor pattern of the mature locust (Locusta migratoria L.) relies heavily on the input of the hindwing tegulae. Removal of the hindwing tegulae results in an immediate change in the motor pattern: the wingbeat frequency (WBF) decreases and the interval between the activity of depressor and elevator muscles (D-E interval) increases. In contrast, removal of the forewing tegulae has little effect on the motor pattern. Here we report adaptive modifications in the flight system that occur after the removal of the hindwing tegulae. Over a period of about 2 weeks following hindwing tegula removal, the flight motor pattern progressively returned towards normal, and in about 80% of the animals recovery of the flight motor pattern was complete. We describe the changes in the activity pattern of flight muscles and in the patterns of depolarizations in flight motoneurons and flight interneurons associated with this recovery. In contrast to the situation in the intact animal, the activity of the forewing tegulae is necessary in recovered animals for the generation of the motor pattern. Removal of the forewing tegulae in recovered animals resulted in similar changes in the flight motor pattern as were observed in intact animals after the removal of the hindwing tegulae. Furthermore, electrical stimulation of forewing tegula afferents in recovered animals produced similar resetting effects on the motor pattern as electrical stimulation of the hindwing tegulae afferents in intact animals. From these observations we conclude that recovery is due to the functional replacement of the removed hindwing tegulae by input from the forewing tegulae.