Wind-sensitive interneurones in the terminal ganglion of praying mantids

Neural responses from the wind-sensitive interneuron population in four cockroach species

The wind-sensitive insect cercal sensory system is involved in important behaviors including predator detection and initiating terrestrial escape responses as well as flight maintenance. However, not all insects possessing a cercal system exhibit these behaviors. In cockroaches, wind evokes strong terrestrial escape responses in Periplaneta americana and Blattella germanica, but only weak escape responses in Blaberus craniifer and no escape responses in Gromphadorhina portentosa. Both P. americana and B. craniifer possesses pink flight muscles correlated with flight ability while B. germanica possesses white flight muscles that cannot support flight and G. portentosa lacks wings. These different behavioral combinations could correlate with differences in sensory processing of wind information by the cercal system. In this study, we focused on the wind-sensitive interneurons (WSIs) since they provide input to the premotor/motor neurons that influence terrestrial escape and flight behavior. Using extracellular recordings, we characterized the responses from the WSI population by generating stimulus-response (S-R) curves and examining spike firing rates. Using cluster analysis, we also examined the activity of individual units (four per species, though not necessarily homologous) comprising the population response in each species. Our main results were: (1) all four species possessed ascending WSIs in the abdominal connectives; (2) wind elicited the weakest WSI responses (lowest spike counts and spike rates) in G. portentosa; (3) wind elicited WSI responses in B. craniifer that were greater than P. americana or B. germanica; (4) the activity of four individual units comprising the WSI population response in each species was similar across species.

Neural basis of stimulus-angle-dependent motor control of wind-elicited walking behavior in the cricket Gryllus bimaculatus

Crickets exhibit oriented walking behavior in response to air-current stimuli. Because crickets move in the opposite direction from the stimulus source, this behavior is considered to represent ‘escape behavior’ from an approaching predator. However, details of the stimulus-angle-dependent control of locomotion during the immediate phase, and the neural basis underlying the directional motor control of this behavior remain unclear. In this study, we used a spherical-treadmill system to measure locomotory parameters including trajectory, turn angle and velocity during the immediate phase of responses to air-puff stimuli applied from various angles. Both walking direction and turn angle were correlated with stimulus angle, but their relationships followed different rules. A shorter stimulus also induced directionally-controlled walking, but reduced the yaw rotation in stimulus-angle-dependent turning. These results suggest that neural control of the turn angle requires different sensory information than that required for oriented walking. Hemi-severance of the ventral nerve cords containing descending axons from the cephalic to the prothoracic ganglion abolished stimulus-angle-dependent control, indicating that this control required descending signals from the brain. Furthermore, we selectively ablated identified ascending giant interneurons (GIs) in vivo to examine their functional roles in wind-elicited walking. Ablation of GI8-1 diminished control of the turn angle and decreased walking distance in the initial response. Meanwhile, GI9-1b ablation had no discernible effect on stimulus-angle-dependent control or walking distance, but delayed the reaction time. These results suggest that the ascending signals conveyed by GI8-1 are required for turn-angle control and maintenance of walking behavior, and that GI9-1b is responsible for rapid initiation of walking. It is possible that individual types of GIs separately supply the sensory signals required to control wind-elicited walking.

Active touch in orthopteroid insects: behaviours, multisensory substrates and evolution

Orthopteroid insects (cockroaches, crickets, locusts and related species) allow examination of active sensory processing in a comparative framework. Some orthopteroids possess long, mobile antennae endowed with many chemo- and mechanoreceptors. When the antennae are touched, an animal's response depends upon the identity of the stimulus. For example, contact with a predator may lead to escape, but contact with a conspecific may usually not. Active touch of an approaching object influences the likelihood that a discrimination of identity will be made. Using cockroaches, we have identified specific descending mechanosensory interneurons that trigger antennal-mediated escape. Crucial sensory input to these cells comes from chordotonal organs within the antennal base. However, information from other receptors on the base or the long antennal flagellum allows active touch to modulate escape probability based on stimulus identity. This is conveyed, at least to some extent, by textural information. Guidance of the antennae in active exploration depends on visual information. Some of the visual interneurons and the motor neurons necessary for visuomotor control have been identified. Comparisons across Orthoptera suggest an evolutionary model where subtle changes in the architecture of interneurons, and of sensorimotor control loops, may explain differing levels of vision-touch interaction in the active guidance of behaviour.

The morphology and fine structure of the giant interneurons of the wood cricket Nemobius sylvestris

The structural and ultrastructural characteristics of giant interneurons in the terminal abdominal ganglion of the cricket Nemobius sylvestris were investigated by means of cobalt and fluorescent dye backfilling and transmission electron microscopy. The projections of the 8 eight pairs of the biggest ascending interneurons (giant interneurons) are described in detail. The somata of all interneurons analyzed are located contralateral to their axons, which project to the posterior region of the terminal ganglion and arborise in the cercal glomerulus. Neuron 7-1a is an exception, because its arborisation is restricted to the anterior region of the ganglion. The fine structure of giant interneurons shows typical features of highly active cells. We observed striking indentations in the perineural layer, enabling the somata of the giant interneurons to be very close to the haemolymph. The cercal glomerulus exhibits a high diversity of synaptic contacts (i.e. axo-dendritic, axo-axonic, dendro-axonic, and dendro-dendritic), as well as areas of tight junctions. Electrical synapses seem to be present, as well as mixed synapses. The anatomical organization of the giant interneurons is finally discussed in terms of functional implications and on a comparative basis.

The terminal abdominal ganglion of the wood cricket Nemobius sylvestris

The abdominal cerci of the wood cricket, Nemobius sylvestris, are covered by a variety of hair-like sensilla that differ in length, thickness, and articulation. Fillings from the cercal nerves with cobalt chloride and fluorescent dyes revealed the projection of sensory axons into the terminal abdominal ganglion of the ventral nerve chain. Two projection areas on each side of the terminal abdominal ganglion midline could be identified: a posterior cercal glomerulus and an anterior bristle neuropil. Axons from some cercal sensilla ascend through the connectives to reach the metathoracic ganglionic mass. As their axons pass through each segmental abdominal ganglion, they project medial arborization. Cross-sections of the terminal abdominal ganglion and retrograde fills with cobalt chloride and fluorescent dyes from connectives revealed several small cells and seven pairs of giant ascending interneurons organized symmetrically. Giant somata are located contralateral to their axons (diameters between 20 and 45 mum). The cercal projections overlap extensively with the dendritic fields of the giant interneurons. In the terminal abdominal ganglion, we identified nine longitudinal tracts, two major tracts, and seven smaller ones. The functional implications of the neuranatomical organization of the system are discussed on a comparative basis.

Directional sensitivity of wind-sensitive giant interneurons in the cave cricket Troglophilus neglectus

Unlike the situation in most cockroach and cricket species studied so far, the wind-sensitive cerci of the cave cricket Troglophilus neglectus Krauss (Rhaphidophoridae, Orthoptera) are not oriented parallel to the body axis but perpendicular to it. The effects of this difference on the morphology, and directional sensitivity of cercal giant interneurons (GIs), were investigated. In order to test the hypothesis that the 90 degrees change in cercal orientation causes a corresponding shift in directional sensitivity of GIs, their responses in both the horizontal and vertical planes were tested. One ventral and four dorsal GIs (corresponding to GIs 9-1a and 9-2a, 9-3a, 10-2a, 10-3a of gryllid crickets) were identified. The ventral GI 9-1a of Troglophilus differed somewhat from its cricket homologue in its dendritic arborisation and its directional sensitivity in the horizontal plane. The morphology and horizontal directionality of the dorsal GIs closely resembled that of their counterparts in gryllids. In the vertical plane, the directionality of all GIs tested was similar. They were all excited mainly by wind puffs from the axon-ipsilateral quadrant. The results suggest that directional sensitivity to air currents in the horizontal plane is maintained despite the altered orientation of the cerci. This is presumably due to compensatory modifications in the directional pReferences of the filiform hairs.

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.

Correlation of filiform hair position with sensory afferent morphology and synaptic connections in the second instar cockroach

An attempt is made to relate the distribution of filiform hairs on the cercus of the second instar cockroach, Periplaneta americana, to the morphology and patterns of synaptic connectivity of their afferents. We studied the most distal 25 of the 39 filiform hairs which are commonly present. Filiform afferent arborizations were stained by cobalt filling from the cell body in the cercus. Three fundamental arbor types were found, two similar to those of the first instar medial (M) and lateral (L) afferents, and a third, novel type. L-type arbors could be divided into four subtypes. The most obvious correlate of arbor type is the circumferential position of the hair on the cercus. The proximodistal position of the sensillum within each cercal segment is also a determinant of its arbor. By comparison of hair positions and afferent morphologies, we were able to ascribe homologies between the second instar hairs and members of adult longitudinal hair columns. The patterns of monosynaptic connections between afferents and giant interneurons (GIs) 1, 2, 3, 5, and 6 were determined by recording synaptic potentials in GIs evoked by direct mechanical displacement of individual filiform hairs. Latency from stimulus onset to the rise phase of the first excitatory postsynaptic potential (EPSP) was used as the criterion of monosynapticity. The EPSP amplitudes of the two original L and M afferents are halved in the second instar, in the absence of a significant decrease in GI input resistance. The other afferents can be divided into two basic classes: those which input to GI5 (M-type), and those which input to GI3 and GI6 (L-type). The former is correlated with a central or medial position, while the latter is associated with a group of afferents situated laterally on the cercus. In segments 3 and 4, input to GIs 1 and 2 also correlates with a medial cercal position, however, in the more proximal segments 5 and 6, afferents at all positions input to these interneurons. The occurrence of afferents of identical morphology and similar connectivity in equivalent positions in different segments suggests that each sensory neuron is determined by its two-dimensional position within a segment. The presence of afferents with the same morphology which display proximodistal differences in synaptic connectivity, and of other afferents which have M-type connectivity despite L-type morphology, means that anatomy is generally a poor predictor of synaptic connectivity.

Reidentification of larval interneurons in the pupal stage of the tobacco hornworm, Manduca sexta

The abdominal prolegs are the primary locomotory appendages of Manduca sexta larvae. After the prolegs are lost at pupation, some of the proleg motoneurons die while the survivors are respecified to carry out different functions in the adult moth. As a first step toward investigating the process of functional respecification at the synaptic level, we searched for larval interneurons that affected the activity of proleg motoneurons, and followed these interneurons into the pupal stage. Interneurons were judged to be individually identifiable based on their effects on proleg motoneuron activity and their anatomical features. Seven larval interneurons were identified and placed in five physiological classes based on their effects on proleg motoneurons: ipsilateral excitors, contralateral excitors, ipsilateral inhibitors, contralateral inhibitors, and bilateral inhibitor-excitors. Four of the larval interneurons produced apparently monosynaptic postsynaptic potentials in proleg motoneuron. Of the five larval interneurons that were reidentified in the early pupal stage, two showed minor but consistent structural modifications from the larval stage. Interneurons that produced unitary postsynaptic potentials in larval motoneurons continued to do so in pupal motoneurons. These studies demonstrate that individually identified interneurons can be followed through the larval-pupal transformation, during the initial stages of motoneuron respecification.

White noise analysis of graded response in a wind-sensitive, nonspiking interneuron of the cockroach

1. A novel approach using a Gaussian white noise as stimulus is described which allowed quantitative analysis of neuronal responses in the cercal system of the cockroach, Periplaneta americana. Cerci were stimulated by air displacement which was modulated by a sinusoidal and a white noise signal. During the stimulation, intracellular recordings were made from a uniquely identifiable, nonspiking, local interneuron which locates within the terminal abdominal ganglion. The white noise stimulation was cross-correlated with the evoked response to compute first- and second-order kernels that could define the cell's response dynamics. 2. The interneuron, cell 101, has an exceptionally large transverse neurite that connects two asymmetrical dendritic arborizations located on both sides of the ganglion. 3. The first-order Wiener kernels in cell 101 were biphasic (differentiating). The waveforms of the kernels produced by the ipsilateral and contralateral stimulations were roughly mirror images of each other: the kernels produced by wind stimuli on the side ipsilateral to the cell body of the interneuron are initially depolarized and then hyperpolarized, whereas those on the other side are initially hyperpolarized. The polarity reversal occurred along the midline of the animal's body, and no well-defined kernel was produced by a stimulus directed head on or from the tail. 4. Mean square error (MSE) between the actual response and the model prediction suggests that the linear component in cell 101 comprises half of the cell's total response (MSEs for the linear models were about 50% at preferred directions), whereas the second-order, non-linear component is insignificant. The linear component of the wind-evoked response was bandpass with the preferred frequency of 70-90 Hz. 5. Accounting for a noise, we reasonably assumed that at high frequencies the graded response in cell 101 is linearly related to a modulation of the air displacement and sensitive to the rate of change of the signal (i.e., wind velocity) and the direction of its source. It is suggested that the dynamics of the first-order kernel simply reflect the dynamics of sensory receptors that respond linearly to wind stimulation.

Audition in the praying mantis, Mantis religiosa L.: identification of an interneuron mediating ultrasonic hearing

1. The praying mantis possesses a single ear located in the ventral midline of the metathorax. We have studied the mantis' auditory nervous system using both extracellular and intracellular techniques and have identified anatomically and physiologically a mirror-image pair of interneurons (MR-501-T3) in the metathoracic ganglion that mediates ultrasonic hearing. 2. MR-501-T3 is tuned broadly to ultrasound with best sensitivity (55-60 dB SPL) between 25 and 45 kHz. Its tuning matches closely that of the whole tympanal nerve. 3. The physiological responses of MR-501-T3 are characterized by: (1) a phasic-tonic firing pattern with a distinctive initial burst at 500-800 spikes/s; (2) minimum latencies of 8-12 ms; (3) no spontaneous activity; (4) sigmoid intensity response curves with a small (10 dB) dynamic range; (5) accurate coding of stimulus duration and of repetition rates up to 60 pps. 4. The ascending axon of MR-501-T3 conducts action potentials at 4 m/s, a rate comparable with some giant fiber systems. 5. MR-501-T3 shows no directional capability. Sound from right and left produce identical responses in both cells of the pair. Neither cutting one tympanal nerve nor removing one hemi-ear leads to different responses in the two cells indicating that they must receive a common input, either from the auditory afferents or from interneurons. We present evidence that the two cells are not directly connected. 6. MR-501-T3 is a large, symmetrical cell with its processes primarily in the intermediate neuropil (lateral ring tract). Its integration segment crosses the midline in the supramedian commissure, and the cell body lies dorsally near the entrance of the leg nerve. The axon travels in the dorsal lateral tract and is one of the largest (17 microns) in the connective. 7. Given the strong anatomical similarities between MR-501-T3 and the G and B cells of the locust, these cells may be homologous. 8. We present arguments based on our physiological results and existing behavioral data that MR-501-T3 is part of an ultrasonic warning/escape system in the mantis. As in moths, lacewings, and crickets, this system may provide a defense against nocturnally foraging bats.

Segmental origins of the cricket giant interneuron system

The segmental origins of the cricket giant interneuron system have been studied by staining these neurons with cobalt during the last half of embryonic development. The results demonstrate that the interneurons are derived from three distinct clusters of embryonic neurons that form a serially repeating pattern in each abdominal ganglion. Some of the neurons previously described in adults (Mendenhall and Murphey, '74; Murphey, '85) have been identified in embryos and are described here with respect to this pattern. These neurons include both giant interneurons and several non-giant mechanosensory interneurons that mediate several different sensory modalities. The anatomical organization of this system is compared to similar mechanosensory systems in other insects and crustacea.