Auditory interneurons in locusts produce directional head and abdomen movements

Locusts (Locusta migratoria) were stimulated with pulses of pure tones of frequencies between 5 kHz and 25 kHz. Interneurons responding to these stimuli (auditory interneurons) were recorded intracellularly and identified by dye injection. Their output functions were investigated by injection of depolarizing current during simultaneous registration of components of flight steering behavior of the animals, i.e. movements of the head and the abdomen and flight activity. Three different types of effects were found, corresponding to 3 functional classes of interneurons:

Involvement of the suboesophageal and thoracic ganglia in the control of antennal movements in crickets

In crickets (Gryllus campestris, Gryllus bimaculatus) the contribution of the suboesophageal ganglia (SOG) and thoracic ganglia to the generation of antennal movements during visual tracking, walking and flight was investigated by the transection of connectives. Transection of one circumoesophageal connective abolished the movements and postures of the antenna ipsilateral to the lesion, while the contralateral antenna behaved normally. Simple antennal reflexes remained. Transection of one neck connective reduced fast components of antennal movements during tracking and walking. During flight the ipsilateral antenna could not be maintained in a prolonged forward position. Antennal movements during tracking and walking appeared normal after transection of one connective between pro- and mesothoracic ganglia. However, the antennal flight posture required uninterrupted connections between brain and mesothoracic ganglion. The ablation of more posterior ganglia had no effect on the antennal behaviours investigated. Recordings from an antennal motor nerve revealed a unilateral net excitation relayed via the SOG to the brain. Two ascending interneurones with activity closely correlated with antennal movements are candidates for such a relay function. The data show that the brain is not sufficient to generate antennal movements and postures as integral parts of several behaviours. The SOG and the thoracic ganglia are required in addition. Accepted: 12 March 1997     

Intersegmental interneurons serving larval and pupal mechanosensory reflexes in the moth Manduca sexta

1. Intersegmental interneurons (INs) that participate in the larval bending reflex and the pupal gin trap closure reflex were identified in the isolated ventral nerve cord of Manduca sexta. INs 305, 504, and 703 show qualitatively different responses in the pupa than in the larva to electrical stimulation of sensory neurons that are retained during the larval-pupal transition to serve both reflexes. Action potentials produced by current injected into the 3 interneurons excite motor neurons that are directly involved in the larval and pupal reflexes. The excitation of the motor neurons is not associated with EPSPs at a fixed latency following action potentials in the interneurons, and thus there do not seem to be direct synaptic connections between the interneurons and the motor neurons. 2. IN 305 (Fig. 2) has a lateral soma, processes in most of the dorsal neuropil ipsilateral to the soma, and a crossing neurite that gives rise to a single contralateral descending axon. IN 305 is excited by stimulation of the sensory nerve ipsilateral to its soma in the larva and the pupa. Stimulation of the sensory nerve contralateral to its soma produces an inhibitory response in the larva, but a mixed excitatory/inhibitory response to the identical stimulus in the pupa. 3. IN 504 (Fig. 3) has a lateral soma, processes throughout most of the neuropil ipsilateral to the soma, and a crossing neurite that bifurcates to give rise to a process extending to the caudal limit of the neuropil and an ascending axon. IN 504 is excited by stimulation of the sensory nerve ipsilateral to its soma in both larvae and pupae, while the response to stimulation of the sensory nerve contralateral to its soma is inhibitory in the larva but mixed (excitatory/inhibitory) in the pupa. 4. IN 703 has a large antero-lateral soma, a neurite that extends across to the contralateral side giving rise to processes located primarily dorsally in both ipsilateral and contralateral neuropils, and two axons that ascend and descend in the connectives contralateral to the soma (Fig. 4). IN 703 responds to stimulation of the sensory nerves on either side of the ganglion, but the form of the response changes during the larval-pupal transition. In the larva, the response consists of very phasic (0-2 spikes) excitation, but in the pupa there is a prolonged excitation that greatly outlasts the stimulus (Fig. 6).(ABSTRACT TRUNCATED AT 400 WORDS)     

Suboesophageal neurons involved in head movements and feeding in locusts.

The projections of nerves 6 and 7 of the locust suboesophageal ganglion (SOG) were stained by axonal filling with cobalt chloride. Nerve 6 contains two motoneurons which innervate neck muscles 50 and 51. Sensory neurons innervating hairs on the dorso-occipital region of the head also enter the ganglion through nerve 6 and terminate in a small bilateral plexus. The projections of the head hairs in nerve 6 do not overlap the arborizations of the motoneurons or the neurons of nerve 7, but lie in the same area as descending sensory neurons from wind-sensitive hairs of the front of the head. One branch of nerve 7 (7B) contains two fibres which innervate the salivary gland. These 'salivary' neurons (labelled SN1 and SN2) have their cell bodies in the ganglion. The second branch, 7A, contains sensory neurons from the submentum of the labium, which form four sensory plexuses, two dorsal and two ventral. The sensory plexuses from the submentum have specific regions of overlap with the salivary neurons and with the neck muscle motoneurons. We interpret these as indicating a flow of information from labial receptors signalling head and mouthpart movement to neurons involved in salivation and head movement. We further postulate that the anatomical separation of the various sensory plexuses is indicative of functional localization within the ganglion.     

Neural control of three-dimensional eye and head movements

Although the eyes and head can potentially rotate about any three-dimensional axis during orienting gaze shifts, behavioral recordings have shown that certain lawful strategies--such as Listing's law and Donders' law--determine which axis is used for a particular sensory input. Here, we review recent advances in understanding the neuromuscular mechanisms for these laws, the neural mechanisms that control three-dimensional head posture, and the neural mechanisms that coordinate three-dimensional eye orientation with head motion. Finally, we consider how the brain copes with the perceptual consequences of these motor acts.     

Learning the optimal control of coordinated eye and head movements

Various optimality principles have been proposed to explain the characteristics of coordinated eye and head movements during visual orienting behavior. At the same time, researchers have suggested several neural models to underly the generation of saccades, but these do not include online learning as a mechanism of optimization. Here, we suggest an open-loop neural controller with a local adaptation mechanism that minimizes a proposed cost function. Simulations show that the characteristics of coordinated eye and head movements generated by this model match the experimental data in many aspects, including the relationship between amplitude, duration and peak velocity in head-restrained and the relative contribution of eye and head to the total gaze shift in head-free conditions. Our model is a first step towards bringing together an optimality principle and an incremental local learning mechanism into a unified control scheme for coordinated eye and head movements.     

Intersegmental variation of afferent pathways to giant interneurons of the earthworm,Lumbricus terrestris L.

Summary We have investigated the connectivity of four classes of mechanosensory afferents to giant interneurons in the earthwormLumbricus. Three of these classes of afferents change their specification for connection to medial giant (MGF) and lateral giant (LGF) fibers along the length of the animal. Near the caudal end, stimulation of touch, pressure and small tactile fibers generates excitatory post-synaptic potentials, epsp's, in the two LGF's but not in the MGF. Near the rostral end these afferents produce much smaller epsp's in the LGFs but produce large epsp's in the MGF. In the middle region of the animal an overlap region exists where both giant fibers receive approximately equal inputs from these afferents. The amplitude of these inputs are reduced compared to the maxima seen at either end. The fourth class of sensory afferents investigated, the stretch neurons, have no synaptic effect on the giant fibers anywhere in the nerve cord.These results explain at least part of the basis, in neuronal connectivity, for the differences in response to tactile stimulation of the head and tail segments previously characterized in terms of behavior and giant fiber impulse activity. In this system developmental mechanisms generating synaptic connectivity patterns have coded certain classes of homologous afferent neurons and interneurons to make different connections in different segments.Abbreviations MGF medial giant fiber - LGF lateral giant fiber - SN1 first segmental root - SN2 second segmental root - SN3 third segmental root - RIN giant interneuron     

Synchronous firing by specific pairs of cercal giant interneurons in crickets encodes wind direction

One prominent stimulus to evoke an escape response in crickets is the detection of air movement, such as would result from an attacking predator. Wind is detected by the cercal sensory system that consists of hundreds of sensory cells at the base of filiform hairs. These sensory cells relay information to about a dozen cercal giant and non-giant interneurons. The response of cercal sensory cells depends both, on the intensity and the direction of the wind. Spike trains of cercal giant interneurons then convey the information about wind direction and intensity to the central nervous system. Extracellular recording of multiple cercal giant interneurons shows that certain interneuron pairs fire synchronously if a wind comes from a particular direction. We demonstrate here that directional tuning curves of synchronously firing pairs of interneurons are sharper than those of single interneurons. Moreover, the sum total of all synchronously firing pairs eventually covers all wind directions. The sharpness of the tuning curves in synchronously firing pairs results from excitatory and inhibitory input from the cercal sensory neurons. Our results suggest, that synchronous firing of specific pairs of cercal giant interneurons encodes the wind direction. This was further supported by behavioral analyses.     

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.     

Effect of auditory deafferentation on the synaptic connectivity of a pair of identified interneurons in adult field crickets

In adult crickets, Teleogryllus oceanicus, unilateral auditory deafferentation causes the medial dendrites of an afferent-deprived, identified auditory interneuron (Int-1) in the prothoracic ganglion to sprout and form new functional connections in the contralateral auditory neuropil. The establishment of these new functional connections by the deafferented Int-1, however, does not appear to affect the physiological responses of Int-1's homolog on the intact side of the prothoracic ganglion which also innervates this auditory neuropil. Thus it appears that the sprouting dendrites of the deafferented Int-1 are not functionally competing with those of the intact Int-1 for synaptic connections in the remaining auditory neuropil following unilateral deafferentation in adult crickets. Moreover, we demonstrate that auditory function is restored to the afferent-deprived Int-1 within 4-6 days following deafferentation, when few branches of Int-1's medial dendrites can be seen to have sprouted. The strength of the physiological responses and extent of dendritic sprouting in the deafferented Int-1 progressively increase with time following deafferentation. By 28 days following deafferentation, most of the normal physiological responses of Int-1 to auditory stimuli have been restored in the deafferented Int-1, and the medial dendrites of the deafferented Int-1 have clearly sprouted and grown across into the contralateral auditory afferent field. The strength of the physiological responses of the deafferented Int-1 to auditory stimuli and extent of dendritic sprouting in the deafferented Int-1 are greater in crickets deafferented as juveniles than as adults. Thus, neuronal plasticity persists in Int-1 following sensory deprivation from the earliest juvenile stages through adulthood.     

Mechanisms of experience dependent control of aggression in crickets

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Postural adjustments to head movements in the cat

Head movements induced by motor cortex stimulation in the cat are accompanied by variations in the vertical force exerted by each limb. These postural responses were found to show stereotyped patterns: with head dorsiflexions an increase was observed in the force exerted by the anterior limbs and a decrease at the posterior limb level. From comparison between the latencies of the force variations, the beginning of head acceleration, and EMG activity in the limb extensor muscles, it was concluded that triggering of these postural responses is not reflex, but depends on the same command as the movement itself. This early response might be a means of avoiding the downward movement of the trunk which would otherwise result from the reaction force corresponding to the upward head movement.     

Hormonal control of reproduction and reproductive behavior in crickets

In Orthoptera, the endocrine control of reproduction has been investigated mainly in Acrididae. Gryllidae are also good models for the study of hormonal control of reproduction and reproductive behavior. In this review, special attention will be focused on the house cricket, Acheta domesticus. In the house cricket, vitellogenesis is controlled according to the classical model described for most insect orders. However, whereas allatectomy completely abolished oocyte growth in A. domesticus, it did not totally suppress ovarian development and egg-laying in Teleogryllus commodus, Gryllus bimaculatus, and G. campestris. In the Gryllidae studied thus far, juvenile hormone (JH) is not needed for mating behavior. In adult A. domesticus, the expression of oviposition movement, although independent of the presence of the ovaries, is strictly controlled by JH III. Recent findings suggest hormonal actions on the central nervous system of the house cricket, and provide a stimulating basis for further research on the respective involvement of hormones and nervous system in the control of reproductive behavior in adult crickets. Arch. Insect Biochem. Physiol. 35:393–404, 1997. © 1997 Wiley-Liss, Inc.