Parallel processing of mechanosensory inputs from the locust ovipositor by intersegmental and local interneurones

The neural pathways underlying the processing of signals from locust (Schistocerca gregaria) ovipositor hairs by different classes of interneurones are investigated.Spikes in the sensory neurones from these hairs evoke chemically-mediated, unitary EPSPs with a short and constant latency in six identified non-giant projection interneurones with cell bodies in the terminal abdominal ganglion. Five of these interneurones receive direct inputs from the valves ipsilateral to their neuropilar branches, whereas the other receives direct inputs from valves on both sides. The sensory neurone from a single hair makes divergent connections with several interneurones and those from different hairs make convergent connections with a given interneurone. The amplitude of the EPSPs evoked depends on the position of a hair along the proximal-distal axis of the valve, with sensory neurones from more distal hairs generating larger amplitude EPSPs.Deflection of hairs also excites three of the four giant projection interneurones through polysynaptic pathways and some local interneurones in the terminal abdominal ganglion through monosynaptic connections. Branches of non-giant projection interneurones, local interneurones, but not those of the giant interneurones, overlap the axon terminals of the ovipositor hair afferents in the terminal abdominal ganglion.     

Parallel effects of joint receptors on motor neurones and intersegmental interneurones in the locust

Summary At the distal end of a mesothoracic tibia of the locust,Schistocerca gregaria, is a chordotonal organ which monitors the position and movement of the tarsus relative to the tibia. It contains approximately 35 receptors that variously encode different spatial and temporal parameters (position, velocity and direction of movement). Some excite intersegmental interneurones that respond phasically or tonically, with directional sensitivity to active or imposed movements of the tarsus. Some of these interneurones are also excited by intrinsic movements of the tarsal segments. Others, besides being excited by tarsal proprioceptive inputs, are also excited by exteroreceptors on the tarsus.When stimulated mechanically or electrically, chordotonal afferents evoke excitatory postsynaptic potentials with a central latency of between 0.9 and 1.4 ms simultaneously in the intersegmental interneurones and in tarsal motor neurones. The central arborizations of the afferents, the intersegmental interneurones and the tarsal motor neurones overlap in certain neuropilar regions of the mesothoracic ganglion. Other afferents cause an inhibition of the motor neurones, with a longer and non-consistent latency suggesting the involvement of other intercalated interneurones.These results indicate that proprioceptive inputs from the tarsal joint receptors are transmitted in parallel and monosynaptically to tarsal motor neurones and to the intersegmental interneurones.     

A strand receptor with a central cell body synapses upon spiking local interneurones in the locust

Summary Movements of the femoro-tibial joint of a locust hind leg are monitored by three classes of proprioceptors; a chordotonal organ (Usherwood et al. 1968), multipolar joint receptors (Coillot and Boistel 1968) and a strand receptor innervated by a single afferent with a central cell body (Bräunig 1985). All three classes are excited by imposed or voluntary extension of the tibia. The strand receptor (fe-tiSR) spikes tonically and at a frequency dependent upon the position of the joint whilst the multipolar joint receptors give overlapping information but for a more restricted range. The afferent from the strand receptor makes an excitatory connection with a spiking local interneurone in the midline group of the metathoracic ganglion. The central latency and consistency with which the EPSP follows each sensory spike suggests that the connection is direct. This interneurone also receives convergent inputs from neurones in the chordotonal organ, but not from multipolar joint receptors. Neither the strand receptor nor the multipolar joint receptors apparently synapse upon leg motor neurones that we have tested, in contrast to receptors in the chordotonal organ.     

Responses of spiking local interneurones in the locust to proprioceptive signals from the femoral chordotonal organ

Summary The responses of spiking local interneurones of a ventral midline population in the metathoracic ganglion of the locust,Schistocerca gregaria, to controlled movements of a proprioceptor, the femoral chordotonal organ (FCO) in a hindleg, were revealed by intracellular recording. Afferents from the FCO which signal specific features of the movement or angle of the femoro-tibial joint, can make direct excitatory synapses with particular interneurones in this population (Burrows 1987a).Some interneurones in this population are excited only by flexion, some only by extension, but others by both flexion and extension movements of the femoro-tibial joint. Interneurones excited by one direction of movement may be either unaffected, or inhibited by the opposite movement. The balance between excitation and inhibition is determined by the range over which the movement occurs, and can increase the accuracy of a representation of a movement.The response of some interneurones has tonic components, so that the angle of the joint over a certain range is represented in the frequency of their spikes. Different interneurones respond within different ranges of femoro-tibial angles so that information about the position of the joint is fractionated amongst several members of the population. These interneurones respond to repetitive movements, similar to those used by the locust during walking, with bursts of spikes whose number and frequency are determined by the repetition rate and amplitude of the movement. A brief movement of the FCO may induce effects which persist for many seconds and outlast the changed pattern of afferent spikes. The sign of such an effect depends upon the preceding history of stimulation.Other interneurones respond only to movement so that their response is more phasic. The velocities to which they respond fall within the range of those generated by twitches of the flexor and extensor tibiae muscles and the movements of the tibia during locomotion. Some interneurones respond only to a specific range of velocities because they are inhibited by all other movements. Some interneurones respond to repetitive movements with reliable bursts of spikes, whilst in others the frequency of spikes may be raised but may contain no cyclical information. All, however, produce the largest number of spikes during the first cycle of a repetitive movement.Inputs from the FCO may sum either with excitation generated by direct inputs from exteroceptors or with inhibition produced by other local interneurones as a result of afferent signals.These spiking local interneurones are essential elements in the integration of local reflexes initiated by signals from the FCO. For example, one ensures that the levator tarsi motor neurone is reflexly inhibited when the FCO signals an extension movement. Exteroceptive inputs from the ventral tarsus suppress the spikes in this interneurone and would prevent expression of the reflex when the tarsus is in contact with ground.Abbreviation FCO femoral chordotonal organ     

Descending interneurones from the brain and suboesophageal ganglia and their role in the control of locust behaviour

Lesion and stimulation experiments suggest that the suboesophageal ganglion (SOG) plays a special role in the control of insect behaviour: in bilateral coordination and by maintaining ongoing motor activity. Anatomical observations indicate that there are descending interneurones (DINs) originating in the SOG in addition to those from the brain. An SOG preparation for sampling both types of DIN intracellularly in walking locusts is described. Forty-three units showing activity changes during leg movements and walking were recorded. Using dye injection six were shown to be through-running axons; one was an SOG ascending interneurone; and eight were SOG DINs, 7 contralateral, one ipsilateral. All fired before or during movements and received various sensory inputs. Many gave complex responses to different modalities, several showing directional preferences. Some SOG neurones showed spontaneous changes in activity; activity outlasting movements; or responses to passive as well as active movements. These preliminary results suggest neuronal substrates for the special functions of the SOG in behaviour. They also indicate that DINs, rather than being simple relays, are part of a dynamic network which includes the motor centres. Regulation of complex and subtle aspects of behaviour may be achieved by dynamic and sequential patterns of activity in groups of DINs, some of which may be multifunctional.     

Processing of mechanosensory information from gustatory receptors on a hind leg of the locust

Gustatory receptors (basiconic sensilla) on the legs of the desert locust, Schistocerca gregaria, are innervated by chemosensory afferents and by a mechanosensory afferent. We show, for the first time, that these mechanosensory afferents form an elaborate detector system with the following properties: 1) they have low threshold displacement angles that decrease with increasing stimulus frequency in the range 0.05–1 Hz, 2) they respond phasically to deflections of the receptor shaft and adapt rapidly to repetitive stimulation, 3) they encode the velocity of the stimulus in their spike frequency and have velocity thresholds lower than 1°/s, and 4) they are directionally sensitive, so that stimuli moving proximally towards the coxa elicit the greatest response.The mechanosensory afferents, but not the chemosensory afferents, make apparently monosynaptic connections with spiking local interneurones in a population with somata at the ventral midline of the metathoracic ganglion. They evoke excitatory synaptic potentials that can sum to produce spikes in the spiking local interneurones. Stimulation of the single mechanosensory afferent of a gustatory receptor can also give rise to long lasting depolarizations, or to bursts of excitatory postsynaptic potentials in the interneurones that can persist for several seconds after the afferent spikes. These interneurones are part of the local circuitry involved in the production of local movements of a leg. The mechanosensory afferents from gustatory receptors must, therefore, be considered as part of the complex array of exteroceptors that provide mechanosensory information to these local circuits for use in adjusting, or controlling locomotion.     

Morphology and design of the extensible intersegmental membrane of the female migratory locust

The morphology of the extensible intersegmental membrane (i.s.m.) of the female Locusta migratoria migratorioides is dictated by three main requirements: (a) high extensibility (1500%), (b) low stiffness (5 × 10³ Pa), (c) low Poisson ratio in the plane of the cuticle (0.01 or less). These requirements can be met only by orientating the chitin orthogonally to the direction of extension and having the protein phase uncross-linked and of very low modulus. The Poisson ratio requirement also implies that for the material to be extended at constant volume extreme thinning must occur during extension, giving rise to high shear strains in the direction of extension. The ultrastructure and morphological elements are modified from ‘normal’ cuticle such as to provide for high extensibility (due to unfolding) of the epicuticle and cpidermal cells and a complex system of intracuticular fibres which are probably necessary to retain the topology of the components during high shear straining. No new morphological elements are adduced in this study but the extreme adaptability of those established for other cuticles is illustrated.     

Direct excitation of nonspiking local interneurones by exteroceptors underlies tactile reflexes in the locust

Summary Tactile stimulation of a leg of the locustSchistocerca gregaria can lead to specific reflex movements of that leg. At the same time nonspiking interneurones that are presynaptic to the participating motor neurones are excited or inhibited, suggesting that they are directly involved in these reflexes. The afferent pathways mediating these effects have been examined by recording from individual afferents and nonspiking interneurones.Afferent spikes fromtrichoid orcampaniform sensilla on specific regions of a leg evoke chemically-mediated EPSPs with a constant central latency of about 1.5 ms in certain nonspiking interneurones. The branches of an interneurone and the afferents from which it receives inputs overlap in the neuropil of the ganglion.No afferents have been found to evoke IPSPs directly in the nonspiking interneurones. Instead the inhibition is caused by a population of spiking local interneurones that are themselves excited directly by the afferents, and whose spikes evoke IPSPs in certain nonspiking interneurones.The tactile reflexes can involve movements about one or more joints of the leg, and these coordinated responses are explained by the participation of specific nonspiking interneurones that distribute the sensory inputs to the appropriate sets of motor neurones. For example, when hairs on the dorsal surface of a tarsus are touched, the tarsus is levated. This reflex involves nonspiking local interneurones which are excited directly by these hair afferents and which make direct excitatory connections with the single levator tarsi motor neurone.     

Ultrastructure and development of the extensible abdominal intersegmental membranes of the female migratory locust

The cuticle of the retracted extensible membrane of the female locust is 300 mum thick and below its highly folded epicuticle there is a zone (5 mum thick) of helicoidally oriented laminae of microfibrils (lamellae), an elastomer layer (180 mum thick) of microfibrils with no preferred orientation and a subcuticular zone (1 mum thick). The epidermal cell layer has an extensive system of junctional specializations and pore canals traverse the cuticle to the helicoidally oriented lamellae. In the newly ecdysed adult the elastomer layer is absent and the helicoidally oriented lamellae are incomplete. Essentially the membrane consists of an elastomer layer contained between two stable layers cross-linked by pore canals, one to the other. When in the plasticized state the membrane combines low stiffness with high extensibility and during extension the elastomer layer flows. Recovery is effected by muscles and when the two stable layers have returned to their unstretched states the fluid elastomer is again evenly distributed. There is an increase in the water content and in the volume of the cuticle when it is fully extended. The ultrastructure of the extensible membrane is compared with those of the inextensible membranes from male and female locusts.     

Peaking of response in locust visual interneurones to objects of small angular subtense

One compound eye of an immobilised locust viewed a large screen on to which were projected discs of light for periods of 2 sec every 40 sec. The spike response was counted concurrently in the DCMD and the next largest axon in the contralateral nerve cord connective. The average score for 10 trials, after correction for background was plotted for a series of discs subtending a range of angles from 0.05 to 84°.It was found that the response of the DCMD peaked sharply and consistently at a subtense of 0.3°, and fell away to a low plateau or to zero over the range 2° to 84°. The response could exceed background down to subtenses as low as 0.05° (3′ of arc). The response of the next largest axon also showed an early peak, but it was inhibitory and resembled a mirror image of that of the DCMD, although it did not always coincide, ranging from 0.2 to 0.3°. The response, by contrast with the DCMD, rose to a high level at large subtenses, forming a flat peak.No explanation in optical terms could be found for this peaking at small subtenses, and a scheme is proposed by which a peak response could develop by the interaction of excitatory and inhibitory processes in the optic lobe.The peak value of 0.3° corresponds with the resolution limit for moving periodic patterns repeatedly demonstrated by Burtt and Catton (e.g. 1962, 1969). Such peaking behaviour would serve to lift the response curve of the whole visual system at high spatial frequencies, and thus extend the resolution limit.With stepwise reduction in intensity of a small luminous target there was a steep fall in the DCMD response, but a similar reduction for a large target had only a small effect. This could be explained by assuming that excitatory processes were prevalent for small targets, subtending about 0.3°, whereas for larger targets the excitatory and inhibitory processes came into balance over a wide range of intensities, thereby stabilising the response, at a low level.     

Correlated changes in mechanical properties of the intersegmental membrane and bonding between proteins in the female adult locust

When the female locust ecdyses into the adult instar the intersegmental membranes which will extend when the female digs the oviposition hole are stiff and inextensible. Extensibility of the membranes develops later, and is under the control of the corpus allatum: allatectomy prevents the development of extensibility. The difference in extensibility and stiffness of the membranes before and after sexual maturation can be accounted for as a change in the bonding in the protein matrix of the cuticle. Electrophoretic analysis of the cuticular proteins dissociated by SDS shows that there are five major proteins in the cuticle, and that these do not change when the membrane becomes extensible. The differing responses of the two states of the cuticle to sequential extraction in water, 0·5 M KCl, 7 M urea, and 1·0 M NaOH at 60°C suggest that the change is one of reduction in hydrogen bonding. Simple calculations on mechanical data suggest that in the unstretchable state there is one interchain bond every 370 or so amino acid residues. This may allow sufficiently specific site definition for an enzyme to be involved in the breakdown of hydrogen bonding. An E.M. of a possible enzyme package in the cuticle is presented.     

Tonic effects of stationary luminous slits on the discharge rates of some locust visual interneurones

The presence of an illuminated slit in the visual field of a locust compound eye produced changes in the tonic discharge rate of the DCMD and three other visual interneurones, recorded in a connective. The DCMD discharge peaked initially in the range of low slit subtenses, but over a period of minutes of exposure its character changed so that there was a rise at high subtenses also. When the luminance of a slit of fixed subtense was increased in steps, there was an initial rise then a sharp fall in discharge, indicating an abrupt onset of inhibition. Lateral spread of inhibition could account for the peak in response to slits, at a subtense falling well within the acceptance angle of a single ommatidium. The results show the ability of some visual interneurones to maintain a changed level of discharge in the presence of a stationary object in the visual field of the eye.     

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.