Parallel motor pathways from thoracic interneurons of the ventral giant interneurons system of the cockroach,Periplaneta americana

The data described here complete the principal components of the cockroach wind-mediated escape circuit form cercal afferents to leg motor neurons. It was previously known that the cercal afferents excite ventral giant interneurons which then conduct information on wind stimuli to thoracic ganglia. The ventral giant interneurons connect to a large population of interneurons in the thoracic ganglia which, in turn, are capable of exciting motor neurons that control leg movements. Thoracic interneurons that receive constant short latency inputs from ventral giant interneurons have been referred to as type A thoracic interneurons (TI_As). In this paper, we demonstrate that the motor response of TI_As occurs in adjacent ganglia as well as in the ganglion of origin for the TI_A. We then describe the pathway from TI_As to motor neurons in both ganglia. Our observations reveal complex interactions between thoracic interneurons and leg motor neurons. Two parallel pathways exist. TI_As excite leg motor neurons directly and via local interneurons. Latency and amplitude of post-synaptic potentials (PSPs) in motor neurons and local interneurons either in the ganglion of origin or in adjacent ganglia are all similar. However, the sign of the responses recorded in local interneurons (LI) and motor neurons varies according to the TI_A subpopulation based on the location of their cell bodies. One group, the dorsal posterior group, (DPGs) has dorsal cell bodies, whereas the other group, the ventral median cells, (VMC) has ventral cell bodies. All DPG interneurons either excited postsynaptic cells or failed to show any connection at all. In contrast, all VMC interneurons either inhibited postsynaptic cells or failed to show any connection. It appears that the TI_As utilize directional wind information from the ventral giant interneurons to make a decision on the optimal direction of escape. The output connections, which project not only to cells within the ganglion of origin but also to adjacent ganglia and perhaps beyond, could allow this decision to be made throughout the thoracic ganglia as a single unit. However, nothing in these connections indicates a mechanism for making appropriate coordinated leg movements. Because each pair of legs plays a unique role in the turn, this coordination should be controlled by circuits didicated to each leg. We suggest that this is accomplished by local interneurons between TI_As and leg motor neurons.     

Parallel motor pathways from thoracic interneurons of the ventral giant interneuron system of the cockroach, Periplaneta americana

The data described here complete the principal components of the cockroach wind-mediated escape circuit from cercal afferents to leg motor neurons. It was previously known that the cercal afferents excite ventral giant interneurons which then conduct information on wind stimuli to thoracic ganglia. The ventral giant interneurons connect to a large population of interneurons in the thoracic ganglia which, in turn, are capable of exciting motor neurons that control leg movements. Thoracic interneurons that receive constant short latency inputs from ventral giant interneurons have been referred to as type A thoracic interneurons (TIAs). In this paper, we demonstrate that the motor response of TIAs occurs in adjacent ganglia as well as in the ganglion of origin for the TIA. We then describe the pathway from TIAs to motor neurons in both ganglia. Our observations reveal complex interactions between thoracic interneurons and leg motor neurons. Two parallel pathways exist. TIAs excite leg motor neurons directly and via local interneurons. Latency and amplitude of post-synaptic potentials (PSPs) in motor neurons and local interneurons either in the ganglion of origin or in adjacent ganglia are all similar. However, the sign of the responses recorded in local interneurons (LI) and motor neurons varies according to the TIA subpopulation based on the location of their cell bodies. One group, the dorsal posterior group, (DPGs) has dorsal cell bodies, whereas the other group, the ventral median cells, (VMC) has ventral cell bodies. All DPG interneurons either excited postsynaptic cells or failed to show any connection at all. In contrast, all VMC interneurons either inhibited postsynaptic cells or failed to show any connection. It appears that the TIAs utilize directional wind information from the ventral giant interneurons to make a decision on the optimal direction of escape. The output connections, which project not only to cells within the ganglion of origin but also to adjacent ganglia and perhaps beyond, could allow this decision to be made throughout the thoracic ganglia as a single unit. However, nothing in these connections indicates a mechanism for making appropriate coordinated leg movements. Because each pair of legs plays a unique role in the turn, this coordination should be controlled by circuits dedicated to each leg. We suggest that this is accomplished by local interneurons between TIAs and leg motor neurons.     

Response of thoracic interneurons to tactile stimulation in the cockroach,Periplaneta americana

Receåfindings indicate that cockroaches escape in response to tactile stimulation as well as they do in response to the classic wind puff stimulus. The thoracic interneurons that receive inputs from ventral giant interneurons also respond to tactile stimulation and therefore, represent a potential site of convergence between wind and tactile stimulation as well as other sensory modalities. In this article, we characterize the tactile response of these interneurons, which are referred to as type-A thoracic interneurons (TI_As). In response to tactile stimulation of the body cuticle, TI_As typically respond with a short latency biphasic depolarization which often passes threshold for action potentials. The biphasic response is not typical of responses to wind stimulation nor of tactile stimulation of the antennae. It is also not seen in tactile responses of thoracic interneurons that are not part of the TI_A group. The responses of individual TI_As to stimulation of various body locations were mapped. The left-right directional properties of TI_As are consistent with their responses to wind puffs from various different directions. Cells that respond equally well to wind from the left and right side also respond equally well to tactile stimuli on the left and right side of the animal's body. In contrast, cells that are biased to wind on one side are also biased to tactile stimulation on the same side. In general, tactile responses directed at body cuticle are phasic rather than tonic, occurring both when the tactile stimulator is depressed and released. The response reflects stimulus strength and follows repeated stimulation quite well. However, the first phase of the biphasic response is more robust during high-frequency stimulation than the second phase. TI_As also respond to antennal stimulation. However, here the response characteristics are complicated by the fact that movement of either antenna evokes descending activity in both left and right thoracic connectives. The data suggest that the TI_As make up a multimodal site of sensory convergence that is capable of generating an oriental escape turn in response to any one of several sensory cues. 1994 John Wiley & Sons, Inc.     

Characterization of tactile-sensitive interneurons in the abdominal ganglia of the cockroach,Periplaneta americana

Tactile stimulation of an insect's abdomen evokes various behaviors including grooming and vigorous escape responses. We tested a sample of 37 tactile-sensitive abdominal interneurons for various morphological and physiological characteristics, including their ability to excite thoracic interneurons that are known to integrate wind information conducted by giant interneurons in the classical escape response. The results suggest that abdominal tactile-sensitive interneurons are heterogeneous both in anatomical and physiological properties. In general, these cells are very small interganglionic interneurons that respond to tactile stimulation at more than one abdominal segment. However, the larger population contained virtually all types of cells. Some projected anteriorly, others posteriorly, and still others projected in both directions. For most cells, the soma was on the side opposite to their axons, but in 24% of the cells it was on the same side. Patterns of dendritic arbors also varied among cells. However, tactile sensitivity was in general consistent with the morphological bias noted in dendritic branch patterns. We were able to document the existence of tactile abdominal interneurons that connect directly to thoracic interneurons involved in escape (TI_As). However, instances of demonstrated connectivity were rare. One cell that did show connectivity (AI652) was characterized in detail, and its properties were appropriate for conducting tactile signals in a directional escape system. The dendritic arbors were biased to the side that was ipsilateral to the cell's soma and axon. As a result, this cell's abdominal inputs and thoracic outputs are on the same side. This pattern is appropriate for generating the sensory fields recorded previously in TI_As. Its axon was located in the ventral median tract, which should bring it close to the integrating region of the TI_As. © 1998 John Wiley & Sons, Inc. J Neurobiol 34: 227–241, 1998     

Effects of paraoxon on spike initiation and conduction block in the giant interneurons of the American cockroach, Periplaneta americana

1. The effects of paraoxon were studied on spike initiation and conduction in the giant interneurons (GIs) of the American cockroach, using electrophysiological techniques. 2. Paraoxon treatment induced high-frequency bursts in GI axons. During these bursts, overshooting spikes recorded in the sixth abdominal ganglion were replaced, in phase, by small, decremental potentials. 3. These small potentials were not EPSPs since current injection could modulate their frequency. 4. An analysis of anteriorly conducted spikes indicates that the site of spike initiation is located near the dendritic region of the GI and is unchanged by paraoxon treatment.     

Electrophysiology and anatomy of medulla interneurons in the optic lobe of the cockroach,Periplaneta americana

1. Medulla interneurons of the optic lobe of P. americana were studied to determine their spectral properties. These neurons exhibited tonic firing which changed with monochromatic broadfield illumination of the ipsilateral eye. The response patterns of these neurons were analyzed by inferring their relation to the ultraviolet (UV) and green (G) photoreceptor groups of the eye. Their anatomy was described after injection of Lucifer yellow. 2. Broadband neurons received either excitatory or inhibitory input from both UV and G receptors. These neurons were not strictly sensitive to luminosity levels and had large cell bodies in the central rind of the medulla and wide dendritic arbors in the medulla neuropil. 3. Narrow band neurons received input from predominantly one receptor type. Their spectral sensitivity curves were more finely tuned than those of the primary receptors presumably due to neural interactions within the optic lobe. 4. Color opponent neurons were inhibited by UV and excited by G inputs in their sustained response. Under certain conditions, some of these neurons also showed G inhibition. These neurons suggested the presence of a subsystem involved in color vision. 5. Broadband, narrow band and color opponent properties were seen in some single neurons when tested over a 5-6 log unit range of intensity. The responses of some of these neurons changed when stimulus duration was increased. These findings indicated that functional classification for these neurons was dependent on stimulus intensity and duration. 6. Polarizational sensitivity was tested in preliminary experiments. Two neurons responded to the movement and direction of polarized light.     

Nonlinear cable properties of the giant axon of the cockroach Periplaneta americana

The steady state nonlinear properties of the giant axon membrane of the cockroach Periplaneta americana were studied by means of intracellular electrodes. The resistivity of this membrane markedly decreases in response to small subthreshold depolarizations. The specific slope resistance is reduced by twofold at 5 mV depolarization and by a factor of 14 at 20 mV depolarization. As a result, the spatial decay, V(X), of depolarizing potentials is enhanced when compared with the passive (exponential) decay. This enhancement is maximal at a distance of 1-1.5 mm from a point of subthreshold (0-20 mV) depolarizing perturbation. At that distance, the difference between the actual potential and the potential expected in the passive axon is approximately 30%. The effects of membrane rectification on V(X) were analyzed quantitatively with a novel derivation based on Cole's theorem, which enables one to calculate V(X) directly from the input current-voltage (I0-V) relation of a long axon. It is shown that when the experimental I0-V curve is replotted as (I0Rin)-1 against V (where Rin is the input resistance at the resting potential), the integral between any two potentials (V1 greater than V2) on this curve is the distance, in units of the resting space constant, over which V1 attenuates to V2. Excellent agreement was found between the experimental V(X) and the predicted value based solely on the input I0-V relation. The results demonstrate that the rectifying properties of the giant axon membrane must be taken into account when the electrotonic spread of even small subthreshold potentials is studied, and that, in the steady state, this behavior can be extracted from measurements at a single point. The effect of rectification on synaptic efficacy is also discussed.     

Organization of giant interneuron projections in thoracic ganglia of the cockroach Periplaneta americana

We have investigated the structural organization of the wind-sensitive giant interneurons in the thoracic ganglia of adult American cockroaches. These seven bilaterally paired interneurons have long been thought to play a role in directing the wind-elicited escape response of the animal. Each of the giant interneurons was labeled individually by intracellular injection of cobaltic hexamine chloride. An individual giant interneuron could be reliably identified from animal to animal based on its branching pattern in thoracic ganglia. The axons of the giant interneurons are situated on each side of the nerve cord in two recognizable subgroups. Comparisons of the axonal arbors of the dorsal and ventral subgroups showed that they project into distinct but partly overlapping regions of thoracic ganglia. Three of the giant interneurons were found to have axonal arbors that cross the longitudinal midline of thoracic and abdominal ganglia. Bilateral pairs of these giant interneurons were labeled concomitantly, and many of the individual neurites from these cells appeared to be closely apposed. All these morphological characteristics are discussed in relation to the connectivity and functional significance of the giant interneurons.     

The central morphology of the giant interneurons and their spatial relationship with the thoracic motorneurons in the cockroach, Periplaneta americana (Insecta)

Three groups of giant fibers are found in the cockroach ventral nerve cord. A latero-dorsal group (dorsal GIs), a latero-ventral group (ventral GIs) and a medio-ventral group. The morphology of all three groups of fibers within the thoracic ganglia is described. The morphology of the dorsal and ventral GI pathways in the abdominal and suboesophageal ganglia is also described. The projection patterns of the neurons in each ganglion are remarkably similar which suggests a common function. When motorneurons 5rl (depressor) and 6Br4 (levator) are stained simultaneously with the dorsal and ventral GI groups, some branches from both motor and giant neurons converge. The branching of the remaining medio-ventral group of fibers and their proximity to areas receiving motorneuronal input suggests that these are the small diameter axons described by Dagan and Parnas (1970).     

Classical olfactory conditioning in the cockroach Periplaneta americana

We established a classical conditioning procedure for the cockroach, Periplaneta americana, by which odors were associated with reward or punishment. Cockroaches underwent differential conditioning trials in which peppermint odor was associated with sucrose solution and vanilla odor was associated with saline solution. Odor preference of cockroaches was tested by allowing them to choose between peppermint and vanilla sources. Cockroaches that had undergone one set of differential conditioning trials exhibited a significantly greater preference for peppermint odor than did untrained cockroaches. Memory formed by three sets of differential conditioning trials, with an inter-trial interval of 5 min, was retained at least 4 days after conditioning. This conditioning procedure was effective even for cockroaches that had been harnessed in plastic tubes. This study shows, for the first time in hemimetaborous insects, that both freely moving and harnessed insects are capable of forming olfactory memory by classical conditioning procedure. This procedure may be useful for future electrophysiological and pharmacological studies aimed at elucidation of neural mechanisms underlying olfactory learning and memory.     

Octopamine-like immunoreactivity in the dorsal unpaired median (DUM) neurons innervating the accessory gland of the male cockroach Periplaneta americana

The musculature of the mushroom-shaped accessory gland receives innervation from trunks 5C1 of the phallic nerves, which arise from the posterior part of the terminal abdominal ganglion of the male cockroach Periplaneta americana. Anterograde cobalt filling through trunks 5C1 with the subsequent precipitating procedure has shown the fine innervation of the accessory gland. By retrograde cobalt filling through the same trunks, different types of cells have been mapped in the terminal abdominal ganglion. About 25 dorsal unpaired median (DUM) neurons have been identified among them. About 36 octopamine-like immunoreactive DUM neurons with large somata have been characterized in whole-mount preparations of the terminal abdominal ganglion. The combination of the cobalt-filling technique with immunohistochemical mapping of cells suggests an octopaminergic innervation of the musculature of the accessory gland by DUM neurons.     

Localization of ventral giant interneuron connections to the ventral median branch of thoracic interneurons in the cockroach

A detailed morphological study was performed to localize the probable sites of connections between two identified populations of interneurons (ventral giant interneurons and type-A thoracic interneurons) in the cockroach. Type-A thoracic interneurons (TIAS) appear to play an important role in orienting the cockroach during wind-mediated escape. However, their large number, approximately 100 neurons, precludes analyzing each cell's role electrophysiologically. The TIAS are characterized by a prominent branch located on one or both sides of the ventral median (VM) region of the thoracic ganglion in which their soma resides. The presence of this ventral median branch can be used to predict connectivity with left or right ventral giant interneurons (vGIs) (Ritzmann and Pollack, 1988) and is correlated with the TIA's directional response to wind (Westin, Ritzmann, and Goddard, 1988), suggesting that this is the locus of synaptic connection. Two approaches were employed to address this hypothesis. Morphological overlap of differentially labelled cells (ethidium bromide, Lucifer Yellow) was examined at the light microscopic level to locate areas of possible synaptic contact. Experiments were also performed in which one-half of the vGI input to the TIAs was surgically removed early in postembryonic development. Although no changes in the overall branching pattern were observed, the VM branches on the operated side were significantly shorter than were those on the unoperated side. Thoracic interneurons that do not receive inputs from vGIs were unaffected by this surgery. The data reported here thereby confirm previous observations by localizing the vGI inputs specifically to the VM branch, and provide a morphological cue for predicting connectivity and function.     

Wind-activated thoracic interneurons of the cockroach: II. Patterns of connection from ventral giant interneurons

A number of thoracic interneurons (TIs) have been found to receive inputs from ventral giant interneurons (vGIs). Each of these cells responds to wind with short latency depolarizations. The previous paper described response properties of several TIs to wind stimuli, including those excited by vGIs. The data showed a correlation between the shape of the TI's wind fields and its morphology. The presence of ventral branches located near the midline of the ganglion predicts a strong response to wind on that side. These ventral median (VM) branches are in the proper location to permit overlap with processes from vGIs. Here we describe the patterns of connections between individual vGIs and 13 of the thoracic interneurons located in the meso- and metathoracic ganglia. A correlation was found between the presence of VM branches and excitation by vGIs. TIs were only excited by vGIs on the side(s) on which VM branches exist. However, presence of a VM branch does not imply that all vGIs on that side make connections with the TI. Summation was found between various vGIs that excited each individual thoracic interneuron. In unilateral thoracic interneurons, no sign of inhibition was found from vGIs on the sides opposite that which contained excitatory vGI axons. Neither was there any evidence of inhibition from dorsal giant interneurons. In addition preliminary evidence indicated that left-right homologues do not inhibit one another. Thus, the data suggest that directional wind fields are primarily the result of selective connection from specific vGIs.     

Neuromodulation of flight initiation by octopamine in the cockroach Periplaneta americana

We have tested the effect of a known insect neuromodulator, octopamine, on flight initiation in the cockroach. Using minimally dissected animals, we found that octopamine lowered the threshold for windevoked initiation of flight when applied to either of two major synaptic sites in the flight circuitry: 1) the last abdominal ganglion, where wind-sensitive neurons from the cerci excite dorsal giant interneurons, or 2) the metathoracic ganglion, where the dorsal giant interneurons activate interneurons and motoneurons which are involved in producing the rhythmic flight motor pattern in the flight muscles (Fig. 2).Correlated with this change in flight initiation threshold, we found that octopamine applied to the last abdominal ganglion increased the number of action potentials produced by individual dorsal giant interneurons when recruiting the cereal wind-sensitive neurons with wind puffs (Figs. 3, 4, 5) or with extracellular stimulation of their axons (Fig. 6). Octopamine increases the excitability of the giant interneurons (Figs. 7, 8). Also, when we stimulated individual dorsal giant interneurons intracellularly, the number of action potentials needed to initiate flight was reduced when octopamine was applied to the metathoracic ganglion (Fig. 9).Abbreviations EMG electromyogram - dGIs dorsal giant interneurons - GI giant interneuron - A6 sixth abdominal ganglion - T3 third thoracic ganglion - EPSP excitatory postsynaptic potential     

The effect of single giant interneuron lesions on wind-evoked motor responses in the cockroach,Periplaneta americana

In order to determine the contribution of individual giant interneurons (GIs) to wind-evoked motor outputs, responses were recorded from depressor and levator motor neurons in the cockroach Periplaneta americana to wind puffs of different directions. The depressor response was generally stronger to wind from the ipsilateral rear than to wind from the contralateral rear. The levator response was more variable but was more often stronger to wind from the contralateral rear than to wind from the ipsilateral rear. These results are as expected based on behavioral responses to wind puffs. (Camhi and Tom, 1978). The depressor response was nearly or totally abolished by severing the ipsilateral connective of the nerve cord but was little affected by severing the contralateral connective. The levator reponse was greatly reduced for some angles by severing the ipsilateral connective and for other angles by severing the contralateral connective. Blocking conduction in the ipsilateral GI 5 nearly or completely abolished the depressor response to wind from the ipsilateral rear. This is the first time that a GI has been shown to be necessary, not just sufficient, for a given motor output.