In vivo responses of paired giant mechanoreceptor neurons in Aplysia abdominal ganglion

Two neurons with cell bodies symmetrically located in the abdominal ganglion and giant axons in the left (L1) and right (R1) pleurovisceral connectives of Aplysia californica were examined in vivo and in vitro. Direct stimulation of R1 and L1 in the intact animal does not elicit any observable behavior, suggesting that they are neither motoneurons nor command neurons. These cells respond in vivo to sudden onset mechanical stimulation of widespread regions of the body. R1 and L1 spikes are initiated in at least three different loci: (1) the peripheral axon in the foot, (2) the neuropil of the pleural and/or pedal ganglion, and (3) the neuropil of the abdominal ganglion. Furthermore, R1 and L1 probably have two different mechanisms for spike initiation: (1) sensory (foot), and (2) synaptic (abominal and/or head ganglia). The different loci for spike initiation account for the bidirectional conduction of R1 and L1 spikes. As sensory (mechanoreceptor) neurons, R1 and L1 have peripheral axons in the ipsilateral posterior pedal nerve, show low threshold responses to stimulation of the ipsilateral posterior foot, they are rapidly adapting their responses do not decrease with repetion, and they are not blocked by high Mg⁺⁺/low Ca⁺⁺ solutions. As synaptically-driven neurons, R1 and L1 have widespread bilateral responsiveness, their responses decrease with repetition and their inputs are blocked with high Mg⁺⁺/low Ca⁺⁺ solutions. These neurons integrate sensory and synaptic inputs and conduct bidirectionally, however, their output connections must be specified before their behavioral function can be understood.     

Anatomy and physiology of neurons composing the commissural ring nerve of the cricket, Acheta domesticus

The commissural ring nerve (RN) of the cricket Acheta domesticus links together the two cercal motor nerves of the terminal abdominal ganglion. It contains the axons of almost 100 neurons including two bilateral clusters of eight to 13 ventrolateral neurons and approximately 75 neurons with midline somata within the terminal abdominal ganglion. The ventrolateral neurons have an ipsilateral dendritic arborization within the dorsal neuropil of the ganglion and their axons use the RN as a commissure in order to enter the contralateral nerves of the tenth ganglionic neuromere. In contrast, most midline neurons have bifurcating axons projecting bilaterally into the neuropil of the ganglion as well as into the RN where they often branch extensively before entering the contralateral tenth nerves. Most RN neurons have small, non-spiking somata with spike initiation zones distant from the soma. Many midline neurons also produce double-peaked spikes in their somata, indicative of multiple spike initiation zones. Spontaneous neuronal activity recorded extracellularly from the RN reveals several units, some with variable firing patterns, but none responding to sensory stimuli. The RN is primarily composed of small (50 nm diameter) axon profiles with a few large (0.5-1 microm diameter) profiles. Occasionally, profiles of nerve terminals containing primarily small clear vesicles and a few large dense vesicles are observed. These vesicles can sometimes be clustered about an active zone. We conclude that the primary function of the RN is to serve as a peripheral nerve commissure and that its role as a neurohemal organ is negligible. J. Exp. Zool. 286:350-366, 2000.Copyright 2000 Wiley-Liss, Inc.     

Pedal serotonergic neurons modulate the synaptic input of withdrawal interneurons ofHelix

A group of serotonergic cells, located in the pedal ganglia ofHelix lucorum, modulates synaptic responses of neurons involved in withdrawal behavior. Extracellular or intracellular stimulation of these serotonergic cells leads to facilitation of spike responses to noxious stimuli in the putative command neurons for withdrawal behavior. Noxious tactile stimuli elicit an increase in background spiking frequency in the modulatory neurons and a corresponding increase in stimulus-evoked spike responses in withdrawal interneurons. The serotonergic neurons have processes in the neuropil of the parieto-visceral ganglia complex, consistent with their putative role in modulating the activity of giant parietal interneurons, which send processes to the same neuropil and to the pedal ganglia. The serotonergic cells respond to noxious tactile and chemical stimuli. Although the group as a whole respond to noxious stimuli applied to any part of the body, most cells respond more to ipsilateral than contralateral stimulation, and exhibit differences in receptive areas. Intracellular investigation revealed electrical coupling between serotonergic neurons which could underlie the recruitment of members of the group not responding to a given noxious stimulus.     

Penis-retractor muscle of Aplysia: excitatory motor neurons

Cobalt axonal iontophoresis and intracellular recordings were used to identify a cluster of several motor neurons innervating the penis-retractor muscle of Aplysia. Intracellularly recorded motor neuron action potentials elicited direct, one-for-one, constant latency excitatory junctional potentials (ejps) in individual muscle fibers. The axons of motor neurons could be recorded extracellularly in the penis-retractor nerve and stimulation of the nerve backfired the motor neurons. Perfusion of the ganglion, the muscle, or both with solutions of either increased Mg++/decreased Ca++ or increased Ca++ sea water indicated that the presumed motor neuron impaled was not a sensory cell and that interneurons were not intercalated in the pathway. Innervation of muscle fibers was found to be functionally polyneuronal and diffuse. The ejps were found to undergo marked facilitation with repetitive motor-neuron stimulation. The motor neurons were isolated in a distinct cluster in the right pedal ganglion. Their electrical activity was characterized by spontaneous irregular action potentials and a moderate input of postsynaptic potentials.     

Serotonin immunoreactivity of neurons in the gastropod Aplysia californica

Serotonergic neurons and axons were mapped in the central ganglia of Aplysia californica using antiserotonin antibody on intact ganglia and on serial sections. Immunoreactive axons and processes were present in all ganglia and nerves, and distinct somata were detected in all ganglia except the buccal and pleural ganglia. The cells stained included known serotonergic neurons: the giant cerebral neurons and the RB cells of the abdominal ganglion. The area of the abdominal ganglion where interneurons are located which produce facilitation during the gill withdrawal reflex was carefully examined for antiserotonin immunoreactive neurons. None were found, but two bilaterally symmetric pairs of immunoreactive axons were identified which descend from the contralateral cerebral or pedal ganglion to abdominal ganglion. Because of the continuous proximity of this pair of axons, they could be recognized and traced into the abdominal ganglion neuropil in each preparation. If serotonin is a facilitating transmitter in the abdominal ganglion, these and other antiserotonin immunoreactive axons in the pleuroabdominal connectives may be implicated in this facilitation.     

Properties of central motor neurons exciting locomotory cilia inTritonia diomedea

Summary A pair of large, identifiable neurons (Pd 21), one in each pedal ganglion, can excite previously inactive locomotory cilia on the sole of the foot ofTritonia diomedea (Audesirk, 1978; Fig. 3). These neurons exert their effect via axons which innervate the foot and are probably central motor neurons for pedal cilia. IntactTritonia are stimulated to crawl by the application of 1.5 M NaCl to the tail, and conversely usually stop crawling when the chemosensitive oral veil is touched with food (sea whip,Virgularia sp.). The Pd 21 neurons are excited by 1.5 M NaCl applied externally to the tail, and are inhibited by sea whip touch to the oral veil (Figs. 4 and 5). When aTritonia performs its escape swim, the cilia move strongly, and the Pd 21 neurons fire bursts of spikes in phase with dorsal flexions (Figs. 6 and 7). After a swim, aTritonia rapidly crawls along the substrate; during this time the spiking rate of the Pd 21s is greatly accelerated. Interneurons thought to drive swim bursts produce monosynaptic EPSPs in the Pd 21s (Fig. 8). The Pd 21s are coordinated in their spike activity by synaptic activity which is synchronous in the two neurons regardless of the site of external stimulation, and by electrical coupling between the two cells via axons in a pedal commissure (Figs. 9 and 10). The coupling coefficient for passively conducted potentials is quite high, about 0.15, despite an axon 8 to 12 mm long separating the two cells.Abbreviations BPSP biphasic postsynaptic potential - SW sea water     

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)     

The topological organization of primary afferents in the terminal ganglion of crayfish,Procambarus clarkii

Summary The central projections of primary afferents in the terminal ganglion of the crayfish can be seen when an axonal filling with nickel chloride with subsequent silver intensification was used for identification. We describe here the topological relationships of the projections to the landmark structures of the neuropil.The terminal ganglion has five pairs of sensory nerves associated with the mechanosensory hairs and internal proprioceptors. The projection fields of the primary sensory neurons in the nerves Rl and R2 are almost entirely restricted to the ipsilateral half of the ganglion, whereas those of the nerves R3, R4 and R5 cross the midline to form three sensory commissures, A6SCI, A7SCI and A7SCII. The projection fields are segregated from each other, although all are restricted to the ventral neuropil which lies under the ventral intermediate tract (VIT). The intersegmental projections that ascend via the connective ipsilateral to their origins could be observed. This pattern of projection correlates well with the receptive fields exhibited by several mechanosensory interneurons on the body surface of the final segment.     

Binocular visual integration in the crustacean nervous system

Although the behavioral repertoire of crustaceans is largely guided by visual information their visual nervous system has been little explored. In search for central mechanisms of visual integration, this study was aimed at identifying and characterizing brain neurons in the crab involved in binocular visual processing. The study was performed in the intact animal, by recording intracellularly the response to visual stimuli of neurons from one of the two optic lobes. Identified neurons recorded from the medulla (second optic neuropil), which include sustaining neurons, dimming neurons, depolarizing and hyperpolarizing tonic neurons and on-off neurons, all presented exclusively monocular (ipsilateral) responses. In contrast, all wide field movement detector neurons recorded from the lobula (third optic neuropil) responded to moving stimuli presented to the ipsilateral and to the contralateral eye. In these cells, the responses evoked by ipsilateral or contralateral stimulation were almost identical, as revealed by analysing the number and amplitude of the elicited postsynaptic potentials and spikes, and the ability to habituate upon repeated visual stimulation. The results demonstrate that in crustaceans important binocular processing takes place at the level of the lobula.     

Transplantation of neurons reveals processing areas and rules for synaptic connectivity in the cricket nervous system

In order to assess the nature of spatial cues in determining the characteristic projection sites of sensory neurons in the CNS, we have transplanted sensory neurons of the cricket Acheta domesticus to ectopic locations. Thoracic campaniform sensilla (CS) function as proprioceptors and project to an intermediate layer of neuropil in thoracic ganglia while cercal CS transduce tactile information and project into a ventral layer in the terminal abdominal ganglion (TAG). When transplanted to ectopic locations, these afferents retain their modality-specific projection in the host ganglion and terminate in the layer of neuropil homologous to that of their ganglion of origin. Thus, thoracic CS neurons project to intermediate neuropil when transplanted to the abdomen and cercal CS neurons project to a ventral layer of neuropil when transplanted to the thorax. We conclude that CS can be separated into two classes based on their characteristic axonal projections within each segmental ganglion. We also found that the sensory neurons innervating tactile hairs project to ventral neuropil in any ganglion they encounter after transplantation. Ectopic sensory neurons can form functional synaptic connections with identified interneurons located within the host ganglia. The new contacts formed by these ectopic sensory neurons can be with normal targets, which arborize within the same layer of neuropil in each segmental ganglion, or with novel targets, which lack dendrites in the normal ganglion and are thus normally unavailable for synaptogenesis. These observations suggest that a limited set of molecular markers are utilized for cell–cell recognition in each segmentally homologous ganglion. Regenerating sensory neurons can recognize novel postsynaptic neurons if they have dendrites in the appropriate layer of neuropil. We suggest that spatial constraints produced by the segmentation and the modality-specific layering of the nervous system have a pivotal role in determining synaptic specificity. © 1993 John Wiley & Sons, Inc.     

Unit responses of the reticular and ventral anterior thalamic nuclei of cats to orbitofrontal cortical stimulation

Responses of 98 neurons of the reticular (R) and 72 neurons of the ventral anterior (VA) thalamic nuclei to stimulation of various zones of the orbitofrontal cortex were investigated in acute experiments on cats immobilized with D-tubocurarine. Not all zones of this cortex were found to be connected equally closely with R and VA. Most of the R (82.7%) and VA (66.7%) neurons responded to stimulation of the proreal gyrus, and fewest (37.3 and 48.9%, respectively) to stimulation of the posterior orbital gyrus. Among the responding neurons, 85.2–86.3% of R cells and 78.2–81.2% of VA cells were excited by cortical stimulation and the rest were inhibited. Excitation was expressed as the appearance of a single spike or of discharges of varied duration in response to each stimulus. The latent period of the spike responses varied from 0.5 to 55.0 msec and the minimal latent period of the discharges was 0.8 msec and its maximal value over 500 msec. The spike frequency in the discharge was 120–250/sec. Unit responses consisting of spikes with a latent period of under 1.3 msec and, it is assumed, some of the responses with a latent period of under 4.0 msec were antidromic. The axons of some R and VA neurons were shown to form branches terminating in different zones of the orbitofrontal cortex.     

Metamorphosis of wing motor system in the silk moth,Bombyx mori L. (Lepidoptera: Bombycidae): Anatomy of the sensory and motor neurons that innervate larval mesothoracic dorsal musculature,stretch receptors,and epidermis

Anatomy of dorsal mesothoracic structures, such as muscles, sensory organs, and innervation, was studied in the silkworm, Bombyx mori L. (Lepidoptera : Bombycidae), and compared with the adult wing motor system. Musculature and nerve innervation were investigated by dissection and electron micrograph; and central projection of sensory fibers and morphology of somata and dendrites of motor neurons by cobalt back-filling, followed by silver intensification. There are 23 muscle bundles (DLM) and 2 stretch receptors (SR). The DLMs, SRs, and epidermis are innervated by a branch of the dorsal nerve trunk emerging from the mesothoracic ganglion (MSG). The branch bifurcates into a dorsal sensory branch of about 300 sensory fibers and a dorsal motor branch of 14 fibers. The sensory fibers project mainly to a longitudinal portion near the mid line in the ventral neuropil of MSG and the metathoracic ganglion. Several fibers extend into the prothoracic ganglion (PG) and a few into the subesophageal and 1st abdominal ganglia. At least 13 (probably 14) motor neurons send axons to DLMs: 9 (probably 10) in PG, and 4 in MSG. Their dendrites are located mostly on the dorsoipsilateral side of the neuropil, but several branches cross the mid line and give rise to many fine branches on the contralateral side. Comparison between the larval (present study) and adult motor system shows a significant similarity in the musculature, peripheral nerve pattern, and motor neurons with some peculiarities.     

Involvement of pedal peptide in locomotion in Aplysia: modulation of foot muscle contractions

Pedal peptide (Pep) is a 15-amino-acid neuropeptide that is localized within the Aplysia central nervous system (CNS) predominantly to a broad band of neurons in each pedal ganglion. Pep-neurons were identified by intracellular staining and immunocytology or by radioimmunoassay (RIA) of extracts from identified neurons. RIA reveals that 97% of all Pep-like immunoreactivity (IR-Pep) in pedal nerves is found in the three nerves that innervate the foot. Nearly every Pep-neuron sends an axon out at least one of these three nerves. Application of Pep to foot muscle causes an increase in the amplitude and relaxation rate of contractions driven by nerve stimulation or intracellular stimulation of pedal motor neurons. The increase in relaxation rate was the predominant effect. Intracellular recording in "split-foot" preparations reveals that Pep-neurons increase their overall firing rates and fire in bursts with each step during locomotion. Recovery of IR-Pep from foot perfusate following pedal nerve stimulation increases in a frequency-dependent fashion. Thus it appears that one function of Pep-neurons is to modulate foot muscle contractility during locomotion in Aplysia.     

Sensory Projections from Ectopic Appendages in an insect: Inherent Specificity and Influence of Location

Peripheral and central pathfinding by sensory axons from appendages was investigated in the fly Sarcophaga bullata . (a) Supernumerary appendages (haltere, wing, antenna and leg) were produced by imaginal disc transplantation at various ectopic sites, (b) Leg neuropil was deafferented by leg disc extirpation and in its place another leg disc was implanted. (c) The basal stalk of a leg disc connecting it with the thoracic ganglion was transected. Using cobalt chloride and HRP backfilling methods the pathways taken by the afferents from these experimentally altered appendages was examined. The results indicate that the larval nerves and the imaginal disc stalks act as guides for growing axons to locate their correct entry sites within the ventral ganglion. In the absence of these guides the axons follow any peripheral nerve, such as abdominal nerve, and enter the ganglion at inappropriate sites. However, within the ganglion they take particular routes, almost identical to those taken by axons from in situ appendages suggesting the existance of some kind of a labelled pathway. Deafferentation does not make the leg neuropil more attractive to ingrowing ectopic sensory axons.     

A reflex behavior mediated by monosynaptic connections between hair afferents and motoneurons in the larval tobacco hornworm,Manduca sexta

In the tobacco hornworm caterpillar, tactile stimulation of sensory hairs located on the tip of a proleg (the planta) evokes ipsilateral or bilateral retraction of the prolegs in that segment. We have used electrophysiological and anatomical methods to investigate the excitatory neural pathways linking the planta hair afferents and the proleg retractor motoneurons (MNs). An important technical innovation was the development of an isolated proleg and desheathed ganglion preparation that permits rapid and reversible ionic manipulations and drug applications. Action potentials (spikes) in individual planta hair afferents produce time-locked excitatory postsynaptic potentials (EPSPs) in ipsilateral proleg MNs which appear to be chemically-mediated and monosynaptic: the EPSPs have a short and constant latency, they follow afferent spikes without failure, they are reversibly abolished in elevated Mg++ saline, and they persist in saline with elevated Mg++ and Ca++ levels. Planta hair afferents also excite ipsilateral MNs by polysynaptic pathways, and their excitation of contralateral proleg MNs is exclusively polysynaptic. Cobalt-staining of the proleg MNs and planta hair afferents shows that the afferents terminate in ventral neuropil, and the proleg MNs have an unusual ventral projection into this region. The ventral projection is on the ipsilateral side, which is consistent with the electrophysiological finding that time-locked EPSPs are found only from ipsilateral hairs. Two factors that contribute to the strong monosynaptic excitation of proleg MNs by ipsilateral planta hairs are the convergence of many hair afferents onto each MN, and the facilitation shown at each afferent-MN synapse. At least 6 afferents converge on each MN, and at short interspike intervals the afferent-evoked EPSPs are enhanced by as much as 400% by homosynaptic facilitation. The EPSP is abolished reversibly by the cholinergic antagonists curare and atropine, suggesting that the neurotransmitter at the synapse is acetylcholine (ACh). This is of particular interest because the ACh receptors of tobacco-feeding Manduca larvae are reported to be less nicotine-sensitive than those of other insects.     

Projection pattern of sensory neurons in the central nervous system of a homeotic mutation of the moth Manduca sexta

Octopod (Octo) is a mutation of the moth Manduca sexta, which transforms the first abdominal segment (A1) in the anterior direction. Mutant animals are characterized by the appearance of homeotic thoracic-like legs on A1. We exploited this mutation to determine what rules might be used in specifying the fates of sensory neurons located on the body surface of larval Manduca. Mechanical stimulation of homeotic leg sensilla did not cause reflexive movements of the homeotic legs, but elicited responses similar to those observed following stimulation of ventral A1 body wall hairs. Intracellular recordings demonstrated that several of the motoneurons in the A1 ganglion received inputs from the homeotic sensory hairs. The responses of these motoneurons to stimulation of homeotic sensilla resembled their responses to stimulation of ventral body wall sensilla. Cobalt fills revealed that the mutation transformed the segmental projection pattern of only the sensory neurons located on the ventral surface of A1, resulting in a greater number with intersegmental projection patterns typical of sensory neurons found on the thoracic body wall. Many of the sensory neurons on the homeotic legs had intersegmental projection patterns typical of abdominal sensory neurons: an anteriorly directed projection terminating in the third thoracic ganglion (T3). Once this projection reached T3, however, it mimicked the projections of the thoracic leg sensory neurons. These results demonstrate that the same rules are not used in the establishment of the intersegmental and leg-specific projection patterns. Segmental identity influences the intersegmental projection pattern of the sensory neurons of Manduca, whereas the leg-specific projections are consistent with a role for positional information in determining their pattern. © 1995 John Wiley & Sons, Inc.     

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.     

FMRF-amide-like immunoreactive efferent fibers and FMRF-amide suppression of pacemaker neurons in eyes of Bulla

The eyes of certain marine gastropods including Aplysia and Bulla, contain circadian pacemakers, which produce a circadian rhythm of autogenous compound action potential (CAP) activity. The CAPs are generated by the synchronous spike discharge of a distinctive population of retinal pacemaker neurons whose axons convey the CAP activity to the CNS. When CAP activity is recorded from a preparation with eyes attached to the CNS, the CAP activity is modulated by efferent activity. In this study we have identified FMRF-amide-like immunoreactive efferent axons in the optic nerves of Bulla. These axons arborize in the basal retinal neuropil adjacent to the pacemaker neurons and are in a position to make synaptic contacts with their dendrites. Similar immunoreactive fibers are not observed in Aplysia eyes. Exogenous FMRF-amide at micromolar concentrations suppresses ongoing CAP activity in isolated eyes but does not suppress the ERG or phase shift the circadian rhythm of CAP activity. Intracellular recordings from the retinal pacemaker neurons reveal that FMRF-amide hyperpolarizes the membrane potential, suppresses spike discharge, and decreases the input resistance, suggesting that a K conductance is increased. Electrical stimulation of the region of the cerebral ganglion that contains FMRF-amide immunoreactive neurons suppresses ongoing CAP activity. All these results are consistent with the idea that the FMRF-amide immunoreactive central neurons and their axons provide a pathway for efferent modulation of the CAP rhythm generated by the retinal pacemaker neurons.     

The sensory basis for the trap-jaw mechanism in the ant Odontomachus bauri

Ants of the ponerine genus Odontomachus have evolved a mechanism that allows them to instantaneously close their long mandibles to catch prey or defend themselves. This trap-jaw action is triggered by contact of trigger hairs with a potential prey item. Two of these long mechanosensory hair sensilla reside proximally on each mandible and are supplied by giant sensory cells.Extracellular recordings demonstrate that the sensory cells respond to tactile stimulation. Their phasic responses encode amplitude and velocity of hair-deflection away from the midline, but not hair position. The discharge of action potentials follows stimulus frequencies of more than 300 Hz. During sinusoidal stimulation, the cells adapt very little, sustain discharge rates of more than 200 Hz for more than 20 s, and reach peak spike rates of about 450 Hz.The afferent axons of these sensory cells give rise to huge axon terminals within the suboesophageal ganglion. One of the afferents has a prominent contralateral branch, the other is confined to ipsilateral neuropil. Anatomical data indicate that the 4 afferents may be coupled and may serve as the substrate for a very fast reflex.Abbreviations HRP horseradish peroxidase - LGS lateral giant sensillum - MGS median giant sensillum - SEM scanning electron microscopy - SOG suboesophageal ganglion     

Localization of glutamate-like immunoreactive neurons in the central and peripheral nervous system of the adult and developing pond snail, Lymnaea stagnalis

We investigated the distribution and projection patterns of central and peripheral glutamate-like immunoreactive (GLU-LIR) neurons in the adult and developing nervous system of Lymnaea. Altogether, 50-60 GLU-LIR neurons are present in the adult central nervous system. GLU-LIR labeling is shown in the interganglionic bundle system and at the varicosities in neuropil of the central ganglia. In the periphery, the foot, lip, and tentacle contain numerous GLU-LIR bipolar sensory neurons. In the juvenile Lymnaea, GLU-LIR elements at the periphery display a pattern of distribution similar to that seen in adults, whereas labeled neurons increase in number in the different ganglia of the central nervous system from juvenile stage P1 up to adulthood. During embryogenesis, GLU-LIR innervation can be detected first at the 50% stage of embryonic development (the E50% stage) in the neuropil of the cerebral and pedal ganglia, followed by the emergence of labeled pedal nerve roots at the E75% stage. Before hatching, at the E90% stage, a few GLU-LIR sensory cells can be found in the caudal foot region. Our findings indicate a wide range of occurrence and a broad role for glutamate in the gastropod nervous system; hence they provide a basis for future studies on glutamatergic events in networks underlying different behaviors.