The morphology of a population of thoracic intersegmental interneurones in the locust

Quadrupedal gaits in hexapod animals - inter-leg coordination in free-walking adult stick insects

The analysis of inter-leg coordination in insect walking is generally a study of six-legged locomotion. For decades, the stick insect Carausius morosus has been instrumental for unravelling the rules and mechanisms that control leg coordination in hexapeds. We analysed inter-leg coordination in C. morosus that freely walked on straight paths on plane surfaces with different slopes. Consecutive 1.7 s sections were assigned inter-leg coordination patterns (which we call gaits) based on footfall patterns. Regular gaits, i.e. wave, tetrapod or tripod gaits, occurred in different proportions depending on surface slopes. Tetrapod gaits were observed most frequently, wave gaits only occurred on 90 deg inclining slopes and tripod gaits occurred most often on 15 deg declining slopes, i.e. in 40% of the sections. Depending on the slope, 36-66% of the sections were assigned irregular gaits. Irregular gaits were mostly due to multiple stepping by the front legs, which is perhaps probing behaviour, not phase coupled to the middle legs' cycles. In irregular gaits, middle leg and hindleg coordination was regular, related to quadrupedal walk and wave gaits. Apparently, front legs uncouple from and couple to the walking system without compromising middle leg and hindleg coordination. In front leg amputees, the remaining legs were strictly coordinated. In hindleg and middle leg amputees, the front legs continued multiple stepping. The coordination of middle leg amputees was maladapted, with front legs and hindlegs performing multiple steps or ipsilateral legs being in simultaneous swing. Thus, afferent information from middle legs might be necessary for a regular hindleg stepping pattern.

Intersegmental transfer of sensory signals in the stick insect leg muscle control system

Intersegmental coordination during locomotion in legged animals arises from mechanical couplings and the exchange of neuronal information between legs. Here, the information flow from a single leg sense organ of the stick insect Cuniculina impigra onto motoneurons and interneurons of other legs was investigated. The femoral chordotonal organ (fCO) of the right middle leg, which measures posture and movement of the femur-tibia joint, was stimulated, and the responses of the tibial motoneuron pools of the other legs were recorded. In resting animals, fCO signals did not affect motoneuronal activity in neighboring legs. When the locomotor system was activated and antagonistic motoneurons were bursting in alternation, fCO stimuli facilitated transitions from flexor to extensor activity and vice versa in the contralateral leg. Following pharmacological treatment with picrotoxin, a blocker of GABA-ergic inhibition, the tibial motoneurons of all legs showed specific responses to signals from the middle leg fCO. For the contralateral middle leg we show that fCO signals encoding velocity and position of the tibia were processed by those identified local premotor nonspiking interneurons known to contribute to posture and movement control during standing and voluntary leg movements. Interneurons received both excitatory and inhibitory inputs, so that the response of some interneurons supported the motoneuronal output, while others opposed it. Our results demonstrate that sensory information from the fCO specifically affects the motoneuronal activity of other legs and that the layer of premotor nonspiking interneurons is a site of interaction between local proprioceptive sensory signals and proprioceptive signals from other legs.

Intersegmental coordination of walking movements in stick insects

Locomotion requires the coordination of movements across body segments, which in walking animals is expressed as gaits. We studied the underlying neural mechanisms of this coordination in a semi-intact walking preparation of the stick insect Carausius morosus. During walking of a single front leg on a treadmill, leg motoneuron (MN) activity tonically increased and became rhythmically modulated in the ipsilateral deafferented and deefferented mesothoracic (middle leg) ganglion. The pattern of modulation was correlated with the front leg cycle and specific for a given MN pool, although it was not consistent with functional leg movements for all MN pools. In an isolated preparation of a pair of ganglia, where one ganglion was made rhythmically active by application of pilocarpine, we found no evidence for coupling between segmental central pattern generators (CPGs) that could account for the modulation of MN activity observed in the semi-intact walking preparation. However, a third preparation provided evidence that signals from the front leg's femoral chordotonal organ (fCO) influenced activity of ipsilateral MNs in the adjacent mesothoracic ganglion. These intersegmental signals could be partially responsible for the observed MN activity modulation during front leg walking. While afferent signals from a single walking front leg modulate the activity of MNs in the adjacent segment, additional afferent signals, local or from contralateral or posterior legs, might be necessary to produce the functional motor pattern observed in freely walking animals.

Distribution of GABA-like immunoreactive neurons in insects suggests lineage homology

Gamma-aminobutyric acid (GABA) is an important inhibitory neurotransmitter in vertebrates and invertebrates (Sattelle [1990] Adv. Insect Physiol. 22:1-113). The GABA phenotype is lineally determined in postembryonic neurons in the tobacco hawkmoth, Manduca sexta (Witten and Truman, [1991] J. Neurosci. 11:1980-1989) and is restricted to six identifiable postembryonic lineages in the moth's thoracic hemiganglia. We used a comparative approach to determine whether this distinct clustering of GABAergic neurons is conserved in Insecta. In the nine orders of insects surveyed (Thysanura, Odonata, Orthoptera, Isoptera, Hemiptera, Coleoptera, Diptera, Lepidoptera, and Hymenoptera), GABA-like immunoreactive neurons within a thoracic hemiganglion were clustered into six distinct groups that occupied positions similar to the six postembryonic lineages in Manduca. On the basis of cell body position and axon trajectories, we suggest that these are indeed homologous lineage groups and that the lineal origins of the GABAergic cells have been very conservative through insect evolution. The distinctive clustering of GABA-positive cells is shared with crustaceans (Mulloney and Hall [1990] J. Comp. Neurol. 291:383-394; Homberg et al. [1993] Cell Tissue Res. 271:279-288) but is not found in the centipede Lithobius forficulatus. There is a two- to threefold increase in numbers of thoracic neurons between the flightless Thysanura and the most advanced orders of insects. Using the GABA clusters as indicators of specific lineages, we find that only selected lineages have significantly contributed to this increase in neuronal numbers.

Morphology and physiology of abdominal projection interneurones in the locust with mechanosensory inputs from ovipositor hair receptors

The anatomy and physiological properties of eight non-giant projection interneurones which originate from the locust terminal abdominal ganglion and receive wind and tactile inputs from ovipositor hair receptors are described. Their cell bodies (diameter 25-40 mu m) are clustered in the anterolateral region of the eighth abdominal neuromere, and their axons ascend through either the contralateral or the ipsilateral connective to more anterior abdominal ganglia. In contrast to the giant interneurones, they have small-diameter axons and are not sensitive to cercal hair wind inputs. According to their arborisation pattern within the terminal abdominal ganglion, the non-giant projection interneurones can be divided into those with main central arborisations in the ventral neuropil (anterolateral interneurones 1-6, ALIN1-ALIN6) and those with arborisations in the dorsal neuropil (ALIN7 and ALIN8). Interneurones of the first type possess four to six secondary neurites, which form a dense dendritic field in the ventral neuropil, either contralaterally or ipsilaterally to their soma. Two interneurones have contralaterally ascending axons and main dendritic fields contralateral to their soma. Two interneurones have contralaterally ascending axons and ipsilateral main dendritic fields. One interneurone has an ipsilaterally ascending axon and an ipsilateral main dendritic field. The primary neurites of interneurones with contralateral axons transverse the ganglion through dorsal commissure I. Five interneurones have unilateral ventral dendritic fields. One interneurone possesses bilateral ventral branches. Some interneurones project only in the eighth abdominal neuromere, whereas others send branches posteriorly into the neuropil of the ninth abdominal neuromere. Interneurones of the second type send three to four secondary neurites to the dorsal neuropil of the eighth and ninth abdominal neuromeres. One interneurone has an ascending axon in the ipsilateral connective and the other in the contralateral connective. The axons of the projection interneurones pass through a lateral or dorsal tract to the seventh abdominal ganglion. Their axonal projections are sparse, remain ipsilateral to the axons, and are confined to the dorsomedial neuropil. ALIN1-ALIN7 are depolarised and spike in response to wind and direct mechanical deflection of trichoid sensilla on both left and right ovipositor valves. They respond with more spikes to stimulation of hairs on the ventral valve ipsilateral to their main dendritic field. ALIN8, in contrast, shows a delayed inhibitory/excitatory response.

Intersegmental ascending interneurons controlling uropod movements of the crayfish Procambarus clarkii

The premotor effects of intersegmental ascending interneurons upon uropod motor neurones in the crayfish Procambarus clarkii (Girard) are examined with intracellular recording and staining techniques. We show that many ascending interneurones can affect the activity of the antagonistic opener and closer motor neurones in the terminal ganglion. Based upon soma position, ascending interneurones are divided into three groups of rostral, medial, and caudal interneurones. Twenty-four ascending interneurones are characterized physiologically according to their inputs from the tailfan and their output effects on the uropod motor neurones of both sides. Each interneurone is identifiable as a unique individual by means of overall shape, soma position, number of main branches, the commissure in which primary neurites cross the midline, axon position in the 5th-6th abdominal connective and physiological responses. They are classified into six classes; coactivating, coinhibiting, reciprocally closing, reciprocally opening, variably effective, and not effective interneurones, according to their premotor effects on the uropod motor neurones. These ascending interneurones seem to act as multifunctional units conveying sensory information from the tailfan to the anterior abdominal ganglia and, at the same time, influencing the uropod motor pattern in the terminal abdominal ganglion.

GABA-like immunoreactivity in a population of locust intersegmental interneurones and their inputs

Intracellular labelling of locust intersegmental interneurones with lucifer yellow or horseradish peroxidase was carried out in combination with light and electron microscope immunocytochemistry by using an antibody raised against gamma amino butyric acid (GABA). Fifteen percent (four out of 27) of intracellularly stained interneurones showed GABA-like immunoreactivity. This is in agreement with previous physiological observations that 20% of the interneurones in this population make inhibitory output connections in the metathoracic ganglion. GABA-like immunoreactivity was also found in processes presynaptic to the interneurones in the mesothoracic ganglion. The presence of such immunoreactive inputs onto the intersegmental interneurones correlates well with physiological evidence that their receptive fields are in part shaped by direct input from GABA-ergic spiking local interneurones.

Morphology of a population of mechanosensory ascending interneurones in the metathoracic ganglion of the locust

A population of ascending interneurones with cell bodies in the metathoracic ganglion of the locust is described. Interneurones are characterised by their morphology (revealed by intracellular cobalt injection) and by their physiological responses to afferent stimulation. All interneurones have their somata in the ventral cortex of the ganglion, in an area just posterior to the medial tracheae. On the basis of gross morphology the interneurones can be divided into three groups: (1) those that have a main area of fine neurites ipsilateral to the soma and an ipsilateral ascending axon; (2) interneurones that also have their main neurites ipsilateral to the soma but have a contralateral ascending axon, and (3) interneurones that have their main stimulation of sensory receptors on the hind leg ipsilateral to the main neurites. Interneurones receiving excitatory inputs from tactile hairs on the hind leg have branches in the most ventral neuropil, whereas interneurones receiving input from leg proprioceptors have branches in the more intermediate and dorsolateral neuropil. The branching pattern of an interneurone and the size and position of the receptive field on the leg are correlated. Interneurones with restricted branching patterns have restricted receptive fields. The position of the ventral branching reflects the position of the receptive field on the leg. An interneurone with a receptive field restricted to the femur has ventral branches in an anterior position in the ganglion; an interneurone with a receptive field restricted to the tarsus has ventral branches in a more posterior position.

Motor neurons of grasshopper metathoracic ganglion occur in stereotypic anatomical groups

Anatomical groups containing identified motor neurons of the main muscles of the legs and the wings are described in a segmental ganglion of the adult grasshopper. The groups occur reproducibly in ganglia of different individuals and are a simplifying and organizing feature of ganglionic morphology. The motor neurons within each group have cell bodies near each other in the cortex of the ganglion and primary neurites that enter the ganglionic core as a discrete bundle. The primary neurite bundles are distinctive in shape and position and have the same composition in every individual, despite variations in the positions of the cell bodies of the contributing motor neurons. The primary neurite bundle of a group is separate from those of other groups and separate from bundles of motor axons that exit or sensory axons that enter the ganglion. Each group of cell bodies in the cortex appears from light microscope examination to be held separately within a glial surround. Areas of glial cell cytoplasm may extend considerably beyond the boundaries of the neuronal cell bodies, to give shape and structural integrity to the cortex. Similarities between the morphology of the adult groups reported here and the descriptions by others of embryonic and larval nervous systems suggest to us that the motor neurons of each group are the progeny of a single neuroblast.

The tritocerebral commissure 'dwarf' (TCD): a major GABA-immunoreactive descending interneuron in the locust

The minor branch of the tritocerebral commissure of the locust, Locusta migratoria, contains only two axons which are from interneurons in the brain descending to the ventral cord ganglia. The smaller of these two neurons, the tritocerebral commissure dwarf (TCD), is immunoreactive to GABA, suggesting that it may be an inhibitory interneuron. We have exploited the accessibility of its axon in the commissure, first, to fill it with cobalt to define its morphology, and second, to record its input characteristics. It has a cell body and arborization of fine branches in the deutocerebrum of the brain, its axon passes contralateral through the tritocerebral commissure and it forms bilateral arborizations in the suboesophageal and three thoracic ganglia. It receives mechanosensory input from many regions of the ipsilateral body and head, and it is sensitive to illumination levels, generally showing greater spontaneous activity in the dark. It is one of the largest GABA-immunoreactive descending interneurons in the locust, suggesting it plays a prominent role in behaviour. Since it is easily accessible for physiological recording, its roles in circuits for particular components of behaviour should be amenable to investigation.

The development of GABA-like immunoreactivity in the thoracic ganglia of the locust Schistocerca gregaria

The development of GABA-like immunoreactivity was investigated in embryonic and juvenile locusts using an antibody raised against GABA-protein conjugates. GABA-like immunoreactivity was first detectable in the neuropile of embryonic ganglia at 55% development, and in neuronal somata at 62% development. The total number of immunoreactive somata increased between 62% and 85% embryonic development, and followed an anterio-posterior pattern of expression. At 85% development, the number of immunoreactive somata reached adult levels and no change in number was then seen. In embryonic stages and first and second juvenile instars two dorsal and four ventral groups of somata were labeled in all three thoracic ganglia, whilst in later juvenile instars one of the dorsal groups was visible as a separate entity only in the metathoracic ganglion. These early patterns were modified by alterations in the positions of some of the groups during late embryogenesis and during juvenile development to produce the adult pattern. The results show that the development of GABA expression is similar to that of other neurotransmitters. The characteristics of the development of immunoreactivity indicate that some of these immunoreactive clusters may be derived from clonally related neurones. Finally, we demonstrate the presence of immunoreactive somata and processes in embryos, which correspond to those of identified local and intersegmental interneurones studied in the adult.

A population of ascending intersegmental interneurones in the locust with mechanosensory inputs from a hind leg

A population of some 35 intersegmental interneurones with somata in the metathoracic ganglion has been characterized by intracellular recording and staining. These interneurones integrate signals from extero- and proprioceptors on a hind leg. The somata are clustered in an anterior and lateral region of the dorsal cortex, and the axons project to more anterior ganglia in either the ipsilateral or contralateral connectives. Some of these interneurones are excited by afferents from a proprioceptor at the femorotibial joint, the femoral chordotonal organ. An afferent spike evokes a chemically mediated EPSP in an interneurone with a latency and consistency that suggest that the connection is direct. An individual interneurone codes particular features of the movement about the femorotibial joint, responding to flexion, extension, or both directions of movement with either phasic or tonic responses. These interneurones have an extensive field of fine branches ipsilateral to the hind leg from which they receive input. These branches are in lateral and intermediate regions of neuropil to which the afferents of the chordotonal organ also project. Axonal branches, from either an ipsilateral and contralateral axon, are sparse and varicose and occur in dorsal neuropil. Other interneurones are excited by afferents from exteroceptive hairs (trichoid sensilla). An individual interneurone is excited by a particular array of hairs on specific regions of a hind leg. The connections between the afferents and the interneurones appear direct. These interneurones have a dense and compact array of fine branches ipsilateral to the hind leg from which they receive input. These branches are in the most ventral region of neuropil, to which the hair afferents also project. Branches from the ipsilateral axons are sparse and varicose and occur in more dorsal neuropil. The interneurones can thus provide the more anterior ganglia with precise information about the movement of a joint in a hind leg and of the location of an exteroceptive stimulus. This information would be of importance in ensuring the correct co-ordination of the legs during walking.

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

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.