The role of delayed excitation in the co-ordination of some metathoracic flight motoneurons of a locust

Summary Intracellular recordings have been made from the somata of two metathoracic flight motoneurons, one innervating an elevator muscle of the hindwing, the tergosternal muscle 113 and the other a depressor, the first basalar muscle 127. The locust,Ghortoicetes terminifera was mounted ventral side uppermost with the thorax restrained and opened for access to the thoracic ganglia. Patterns of electrical activity recorded from the thoracic muscles were similar to those shown by a locust during flight when tethered in a more normal posture. In flight the left and right 113 motoneurons each receive a single impulse together at every stroke of the wing, with the 127 muscles active in approximate antiphase. A spike in a 113 motoneuron causes a delayed wave of excitation simultaneously upon itself and its contralateral partner (Fig. 2). The epsp's which form these waves summate and may cause a spike which follows the original one with a delay equal to the wingbeat period. The delayed excitation of the contralateral motoneuron is of larger amplitude than the ipsilateral one so that spikes in either motoneuron must activate separate but symmetrical pathways. A single spike may cause multiple waves in either motoneuron, each separated by intervals equal to the wingbeat period (Fig. 3). In the pathway must be neurons capable of reverberation.A spike in a 113 motoneuron causes a delayed excitation of the ipsilateral 127 motoneuron so that its membrane potential is lowered antiphasically to that of 113 (Fig. 17). A spike in a 127 motoneuron has no effect on the 113 motoneurons. In flight these pathways causing delayed excitation may co-ordinate the motoneurons.The left and right 113 motoneurons receive common synaptic inputs from at least two sources (Fig. 8). These occur as bursts of epsp's at intervals approximately equal to or multiples of the wingbeat period and in the absence of flight. Epsp's of sufficient amplitude cause a spike in the motoneuron which is in the correct phase in the flight pattern relative to any other active motoneurons (Fig. 9). During sustained flight epsp's contribute to the wave of depolarization that the motoneuron undergoes at each wingbeat (Fig. 11). In the absence of the epsp's the motoneuron does not oscillate on its own. At the end of flight bursts of epsp's may continue at the flight frequency long after all activity in the muscles has ceased.Beit Memorial Research Fellow.     

Development of the gin trap reflex inManduca sexta: a comparison of larval and pupal motor responses

1. Responses of motor neurons in larvae and pupae of Manduca sexta to stimulation of tactile sensory neurons were measured in both semi-intact, and isolated nerve cord preparations. These motor neurons innervate abdominal intersegmental muscles which are involved in the production of a general flexion reflex in the larva, and the closure reflex of the pupal gin traps. 2. Larval motor neurons respond to stimulation of sensory neurons innervating abdominal mechanosensory hairs with prolonged, tonic excitation ipsilaterally, and either weak excitation or inhibition contralaterally (Figs. 4A, 6). 3. Pupae respond to tactile stimulation of mechanosensory hairs within the gin traps with a rapid closure reflex. Motor neurons which innervate muscles ipsilateral to the stimulus exhibit a large depolarization, high frequency firing, and abrupt termination (Figs. 2, 4B). Generally, contralateral motor neurons fire antiphasically to the ipsilateral motor neurons, producing a characteristic triphasic firing pattern (Figs. 7, 8) which is not seen in the larva. 4. Pupal motor neurons can also respond to sensory stimulation with other types of patterns, including rotational responses (Fig. 3A), gin trap opening reflexes (Fig. 3B), and 'flip-flop' responses (Fig. 9). 5. Pupal motor neurons, like larval motor neurons, do not show oscillatory responses to tonic current injection, nor do motor neurons of either stage appear to interact synaptically with one another. Most pupal motor neurons also exhibit i-V properties similar to those of larval motor neurons (Table 1; Fig. 10). Some pupal motor neurons, however, show a marked non-linear response to depolarizing current injection (Fig. 11).     

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     

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     

Developmental changes in the expression level of connexin36 in the rat retina

Many noctuid moth species perceive ultrasound via tympanic ears that are located at the metathorax. Whereas the neural processing of auditory information is well studied at the peripheral and first synaptic level, little is known about the features characterizing higher order sound-sensitive neurons in the moth brain. During intracellular recordings from the lateral protocerebrum in the brain of three noctuid moth species, Heliothis virescens, Helicoverpa armigera and Helicoverpa assulta, we found an assembly of neurons responding to transient sound pulses of broad bandwidth. The majority of the auditory neurons ascended from the ventral cord and ramified densely within the anterior region of the ventro-lateral protocerebrum. The physiological and morphological characteristics of these auditory neurons were similar. We detected one additional sound-sensitive neuron, a brain interneuron with its soma positioned near the calyces of mushroom bodies and with numerous neuronal processes in the ventro-lateral protocerebrum. Mass-staining of ventral-cord neurons supported the assumption that the ventro-lateral region of the moth brain was the main target for the auditory projections ascending from the ventral cord.     

Physiology and pharmacology of acetylcholinergic responses of interneurons in the antennal lobes of the mothManduca sexta

1. Neurons in the antennal lobe (AL) of the moth Manduca sexta respond to the application, via pressure injection into the neuropil, of acetylcholine (ACh). When synaptic transmission is not blocked, both excitatory (Fig. 2) and inhibitory (Fig. 3) responses are seen. 2. Responses to ACh appear to be receptor-mediated, as they are associated with an increase in input conductance (Figs. 2B and 3B) and are dose-dependent (Fig. 2 C). 3. All neurons responsive to ACh are also excited by nicotine. Responses to nicotine are stronger and more prolonged than responses to ACh (Fig. 4C). No responses are observed to the muscarinic agonist, oxotremorine (Fig. 4 B). 4. Curare blocks responses of AL neurons to applied ACh, while atropine and dexetimide are only weakly effective at reducing ACh responses (Figs. 5 and 6). 5. Curare is also more effective than atropine or dexetimide at reducing synaptically-mediated responses of AL neurons (Fig. 7). 6. In one AL neuron, bicuculline methiodide (BMI) blocked the IPSP produced by electrical stimulation of the antennal nerve, but it did not reduce the inhibitory response to application of ACh (Fig. 8).     

Physiological characteristics and reflex activation of DUM (octopaminergic) neurons of locust metathoracic ganglion

Intracellular recordings were made from single or pairs of somata of the dorsal unpaired median (DUM) neurons of the metathoracic ganglion of the locust Schistocerca gregaria and the grasshopper Romalea microptera, during reflex actions, direct electric excitation and orthodromic and antidromic neural stimulation. Some, possibly all, of these neurons are unique, identifiable individuals in regard to their targets, which are specific peripheral muscles. Their physiological properties and the ways they are activated synaptically are, however, similar. Large, overshooting action potentials, comprising three components, occur. The first component in time is small and represents an excitatory synaptic potential for orthodromic stimulation or an axon spike (AS) for antidromic stimulation, electrotonically conducted into the soma. The second component is larger, being an electrotonically conducted integrating segment spike (ISS). The final component is the soma spike (SS). Neither AS nor ISS have a late positive phase, but there is a large, prolonged one for SS. The latter, combined with rapid accommodation, determine a low maximum firing rate for the neurons. Most nerves entering the ganglion make excitatory inputs onto each DUM neuron, which is readily driven to spike by electric excitation of either connective. There is a great deal of spontaneous excitatory synaptic input to each DUM neuron and a high proportion of it is common. Although there is no detectable electrical coupling between the cells, there is about 30% synchronous firing, apparently due to the common inputs; independent excitation and inhibition also occur. All sensory modalities tested have inputs to the neurons, which tend to fire constantly at a low rate (1 per 3–4 sec). In reflex actions, DUM neurons tend to fire before motor output occurs. It is suggested that the cells will be found to have many functions serving a general role comparable to that achieved by the release of adrenaline in vertebrates.     

Modulation of flight by the giant interneurons of the cockroach

In this paper, I have examined the behavioral functions of feedback loops between the cockroach (Periplaneta americana) giant interneurons (GIs) and the flight thoracic rhythm generator.

Multiple feedback loops in the flying cockroach: excitation of the dorsal and inhibition of the ventral giant interneurons

1. In a tethered cockroach (Periplaneta americana) whose wings have been cut back to stumps, it is possible to elicit brief sequences of flight-like activity by puffing wind on the animal's body. 2. During such brief sequences, rhythmic bursts of action potentials coming from the thorax at the wingbeat frequency, descend the abdominal nerve cord to the last abdominal ganglion (A6). This descending rhythm is often accompanied by an ascending rhythm (Fig. 2). 3. Intracellular recording during flight-like activity from identified ascending giant interneurons, and from some unidentified descending axons in the abdominal nerve cord, shows that: (a) ventral giant interneurons (vGIs) remain silent (Fig. 3); (b) dorsal giant interneurons (dGIs) are activated at the onset of the flight-like activity and remain active sporadically throughout the flight sequence (Fig.4); (c) some descending axons in the abdominal nerve cord show rhythmic activity phase-locked to the flight rhythm (Fig. 5). 4. Also during such brief sequences, the cercal nerves, running from the cerci (paired, posterior, wind sensitive appendages) to the last abdominal ganglion, show rhythmic activity at the wingbeat frequency (Fig. 6). This includes activity of some motor axons controlling vibratory cercal movements and of some sensory axons. 5. More prolonged flight sequences were elicited in cockroaches whose wings were not cut and which flew in front of a wind tunnel (Fig. 1B). 6. In these more prolonged flight sequences, the number of ascending spikes per burst was greater than that seen in the wingless preparation (Fig. 8; compare to Fig. 2). Recordings from both ventral and dorsal GIs show that: in spite of the ongoing wind from both the tunnel and the beating wings, which is far above threshold for the vGIs in a resting cockroach, the vGIs are entirely silent during flight. Moreover, the vGIs response to strong wind puffs that normally evoke maximal GI responses is reduced by a mean of 86% during flight (Fig. 9). The dGIs are active in a strong rhythm (Figs. 11 and 12). 7. Three sources appear to contribute to the ascending dGI rhythm (1) the axons carrying the rhythmic descending bursts; (2) the rhythmic sensory activity resulting from the cercal vibration; and (3) the sensory activity resulting from rhythmic wind gusts produced by the wingbeat and detected by the cerci. The contribution of each source has been tested alone while removing the other two (Figs. 13 and 14). Such experiments suggest that all 3 feedback loops are involved in rhythmically exciting the dGIs (Fig. 15).     

Intracellular activity in cricket neurons during generation of song patterns

Summary During production of song patterns by the semi-isolated CNS of Gryllus campestris, intracellullar recordings were made in fibers of the mesothoracic ganglion, including synaptic areas of identified wing opener and closer motor neurons. The normal calling song pattern and some transitional songs toward courtship and toward aggression were generated by the CNS in the absence of any phasic sensory timing (Figs. 1, 4). Intracellular activity of the opener motor neurons was characterized by an absence of events in the interchirp interval, an EPSP underlying each burst, and an IPSP following the burst if the closer motor neurons were to be activated (Fig. 1). Intracellular activity of the closer motor neurons was characterized by an absence of events in the interchirp interval, an IPSP immediately following the onset of the opener motor neuron burst, and an EPSP after the IPSP (Figs. 2, 3). Units were found which fired in a burst during the period when both the opener and closer motor neurons were inhibited (Fig. 5). Complementary sets of units were found which displayed an oscillation of activity at the chirp rhythm but not at the pulse rhythm (Fig. 6). Gaps in the calling song were observed whose characteristics indicated that motor neuron activity was neither required for, nor effective in, resetting the chirp timing oscillator (Fig. 8). A possible model for the song generating mechanism is outlined.     

The neurophysiology of larval firefly luminescence: Direct activation through four bifurcating (DUM) neurons

Summary The paired lanterns of the larval fireflyPhoturis versicolor are bilaterally innervated by four dorsal unpaired median (DUM) neurons the somata of which are found in the terminal abdominal ganglion (A8) and which stain with Neutral Red (Fig. 1A). Both intra- and extracellularly recorded activity in these neurons is always associated with a bilateral glow response, or BGR (Figs. 3 and 4). Luminescence cannot be initiated or maintained in the absence of DUM neuron excitation. Furthermore, there is a linear causative relationship between the frequency of DUM neuron activity and the amplitude of the resultant BGR (Figs. 6 and 7).Due to the intrinsic bilateral morphology, firefly DUM neurons may be antidromically activated through either lantern nerve, resulting in the initiation of luminescence in the contralateral lantern (Figs. 8 and 9). This activation is unaffected by high Mg⁺⁺ saline indicating that the DUM neurons provide a direct pathway for conduction through the ganglion (Fig. 9). The DUM neurons receive synaptic input from axons descending through both anterior connectives, however, stimulation of only one connective results in a BGR since excitation is carried to both sides of the periphery through the bilateral axons.Firefly DUM neurons exhibit physiological qualities typical of neurosecretory cells: spikes are characterized by a slow time course and a long and deep afterhyperpolarization (Fig. 10). This is consistent with the observation that spontaneous firing rates are usually below 3 Hz, but nevertheless elicit a strong BGR (Figs. 3 and 5). The physiological evidence presented in this study correlates well with the morphological, pharmacological and biochemical evidence compiled from previous studies, which indicates that the four DUM neurons represent the sole photomotor output from the central nervous system to the larval lanterns. Evidence is discussed which indicates that these effects are mediated throught the release of octopamine, long presumed to be the lantern neurotransmitter. These results, therefore, describe a novel and unexpected role for DUM neurons in regulating an unusual invertebrate effector tissue and further expands the growing list of functions for octopamine in neural control mechanisms.Abbreviations A1-A7 first through seventh abdominal ganglia - A8 terminal abdominal ganglion - DUM dorsal unpaired median - BGR bilateral glow response     

Neuronal control of swimming in the medicinal leech

Summary The cell bodies and function of twelve neurons whose impulse pattern is clearly related to that of the swimming rhythm were identified in the segmental ganglion of the leech. These include excitatory and inhibitory motor neurons of the dorsal and ventral longitudinal muscles and the excitatory flattener motor neuron of the dorsoventral muscles. During swimming the membrane potential of these cells oscillates between a depolarized and a hyperpolarized phase. The activity of this ensemble of cells is sufficient to account for the contractile rhythm of the swimming animal. The following connections were found between these motor neurons. Electrotonic junctions link: (1) bilaterally homologous cells; (2) excitors of the dorsal longitudinal muscles; (3) excitors of the ventral longitudinal muscles; (4) inhibitors of both dorsal and ventral longitudinal muscles. The dorsal inhibitors project via an inhibitory pathway to the dorsal excitors, and the ventral inhibitor projects via an inhibitory pathway to the ventral excitors. The membrane potential oscillation of the excitors is at least partly attributable to the phasic inhibitory synaptic input which they receive from the inhibitors. The excitatory shortener motor neuron of the entire longitudinal musculature is maintained in an inactive state during swimming. This control is achieved by rectifying electrotonic junctions linking this neuron to the dorsal and ventral excitors. These junctions allow passage of only depolarizing current from the shortener to the dorsal and ventral excitors and of only hyperpolarizing current in the reverse direction. Furthermore, both dorsal and ventral inhibitors project via inhibitory pathways to the shortener neuron.We are greatly indebted to Ann Stuart for advice and help in this study, and for communicating to us some unpublished findings. We thank Elizabeth Mullenbach for excellent technical assistance.This research was supported by grant GB 31933 X from the National Science Foundation, and by Public Health Service Research grant GM 17866 and Training Grant GM 01389 from the Institute for General Medical Sciences.     

Acoustic irradiation produced by flying moths (Lepidoptera,Noctuidae)

Characteristics of acoustic waves accompanying the flight of noctuid moths (Noctuidae) were measured. The low-frequency part of the spectrum is formed of a series of up to 17 harmonics of the wingbeat frequency (30–50 Hz) with a general tendency toward the decrease in the spectral density and the increase in the sound frequency. The root-mean-square level of the sound pressure from flapping wings was found to be 70–78 dB SPL. Besides low-frequency components, the flight of moths was accompanied by short ultrasonic pulses, which appeared with every wingbeat. Most of the spectral energy was concentrated within a range of 7–150 kHz with the main peaks at 60–110 kHz. The short-term pulses were divided into two or more subpulses with different spectra. The high-frequency pulses were produced at two phases of the wingbeat cycle: during the pronation of the wings at the highest point and at the beginning of their upward movement from the lowest point. In most of the specimens tested, the peak amplitude of sounds varied from 55 to 65 dB SPL at a distance of 6 cm from the insect body. However, in nine noctuid species, no high-frequency acoustic components were recorded. In these experiments, the acoustic flow from the flying moth within a frequency range of 2 to 20 kHz did not exceed the self-noise level of the microphone amplifier (RMS 18 dB SPL). Probable mechanisms of the high frequency acoustic emission during flight, the effect of these sounds on the auditory sensitivity of moths, and the possibility of their self-revealing to insectivorous bats are discussed. In addition, spectral characteristics of the moth echolocation clicks were more precisely determined within the higher frequency range (>100 kHz).     

Comparison of motor patterns in the intact and deafferented flight system of the locust

Summary The activity of flight interneurons was recorded intracellularly in intact, tethered flying locusts (Locusta migratoria) and after removal of sensory input from the wing receptors. Depolarization patterns and spike discharges were characterized and compared for the two situations.In general, depressor interneurons (n=6) showed only minor changes in their activity as a result of deafferentation (Fig. 1). Exceptions were interneurons 308 and 506 (Fig. 2). By contrast, all but one of the elevator interneurons (n=9) produced distinctly different depolarization patterns in intact locusts and following deafferentation. Three different groups of elevator interneurons were found (excluding the one exceptional neuron, Fig. 6). (i) One group of interneurons (n=4) produced different, superthreshold depolarizations in intact and deafferented animals (Fig. 3). Characteristic, biphasic depolarizations were recorded from these fibres at lower wingbeat frequencies in the intact situation but only single, delayed potentials were recorded after deafferentation. (ii) The second group of interneurons (n=3) exhibited distinct rhythmic activity only in intact animals. After deafferentation their depolarizations were small and often below the threshold for spike initiation (Fig. 4). (iii) One interneuron produced rhythmic flight motor oscillations only after deafferentation. In intact locusts the membrane potential of this neuron showed very small oscillations and remained subthreshold (Fig. 5).Four main conclusions emerge from these data. (i) The activity of elevator interneurons is under greater sensory control than that of the depressors. This confirms the results of our previous electromyographic and motoneuronal analyses, (ii) A considerable portion of elevator activity is generated as a result of phasic sensory feedback. An essential input is from the hindwing tegulae (Table 1; Pearson and Wolf 1988). (iii) The activity of depressor interneurons appears to be determined by central mechanisms to a major extent. (iv) Different sets of central neurons appear to be involved in flight pattern generation in intact and deafferented locusts —although the two sets share many common elements.Abbreviations EMG electromyogram - PSP postsynaptic potential (EPSP excitatory andIPSP inhibitory)     

The chronotron: a neuron that learns to fire temporally precise spike patterns

In many cases, neurons process information carried by the precise timings of spikes. Here we show how neurons can learn to generate specific temporally precise output spikes in response to input patterns of spikes having precise timings, thus processing and memorizing information that is entirely temporally coded, both as input and as output. We introduce two new supervised learning rules for spiking neurons with temporal coding of information (chronotrons), one that provides high memory capacity (E-learning), and one that has a higher biological plausibility (I-learning). With I-learning, the neuron learns to fire the target spike trains through synaptic changes that are proportional to the synaptic currents at the timings of real and target output spikes. We study these learning rules in computer simulations where we train integrate-and-fire neurons. Both learning rules allow neurons to fire at the desired timings, with sub-millisecond precision. We show how chronotrons can learn to classify their inputs, by firing identical, temporally precise spike trains for different inputs belonging to the same class. When the input is noisy, the classification also leads to noise reduction. We compute lower bounds for the memory capacity of chronotrons and explore the influence of various parameters on chronotrons' performance. The chronotrons can model neurons that encode information in the time of the first spike relative to the onset of salient stimuli or neurons in oscillatory networks that encode information in the phases of spikes relative to the background oscillation. Our results show that firing one spike per cycle optimizes memory capacity in neurons encoding information in the phase of firing relative to a background rhythm.     

Local inhibitor of the crayfish telson-flexor motor giant neurons: morphology and physiology

The motor circuits that control telson flexion in the crayfish (Procambarus clarkii) include a curiously arranged sub-circuit: a premotor 'command' neuron excites a motor neuron via a trisynaptic pathway, but also inhibits (and prevents firing of) the motor neuron via a shorter latency pathway (Kramer et al. 1981 a). The premotor and motor neurons in this circuit have been previously identified (Kramer et al. 1981 a; Dumont and Wine 1985a, b; see Fig. 1). We have now identified a local interneuron that inhibits the motor neurons. The cell we studied is called the 'C' cell because of its distinctive structure (Figs. 2, 3). A single pair of bilaterally homologous C-cells was found in the last (6th) abdominal ganglion. The C-cells are invariably dye coupled to one another following injections of lucifer yellow into either one of them, and are frequently dye coupled to smaller axons in the 2nd, 3rd, and 6th nerves. In addition, some of the extensive branches of the C-cell extend out into the 6th nerve, where they are in close proximity to the axons of the motor neurons they inhibit (Fig. 3). Two kinds of evidence established that the C-cell directly inhibits the motor neurons. First, when simultaneous recordings were made from the C-cell and the motor neurons, spikes in the C-cell, no matter how evoked, were invariably followed, within 1.5 ms, by depolarizing IPSPs in the motor neuron (Fig. 6). Second, when the C-cell was hyperpolarized so that it could not fire, that same IPSP in the motor neuron was abolished (Fig. 6). The inhibitory pathway to the motor neurons must be fired at short latency in order to prevent firing caused by the trisynaptic excitatory input (Fig. 1). The C-cells were fired at short latency (less than 3 ms) by impulses in either of the escape command cells (Fig. 4), and at even shorter latency by impulses in the Segmental Giant of the 6th ganglion (SG6) (Fig. 5). It has been established elsewhere that the SGs are a major output pathway of the escape command cells; our results suggest that they may be the pathway for command-evoked firing of the C-cell. The C-cells are also excited by two descending, non-giant, flexion premotor neurons, called I2 and I3 (Fig. 5). The EPSPs from a single I2 or I3 impulse were subthreshold, but temporal and spatial summation of EPSPs from the non-giant pathway sometimes fired the C-cells.(ABSTRACT TRUNCATED AT 400 WORDS)     

Early functional impairment of sensory-motor connectivity in a mouse model of spinal muscular atrophy

To define alterations of neuronal connectivity that occur during motor neuron degeneration, we characterized the function and structure of spinal circuitry in spinal muscular atrophy (SMA) model mice. SMA motor neurons show reduced proprioceptive reflexes that correlate with decreased number and function of synapses on motor neuron somata and proximal dendrites. These abnormalities occur at an early stage of disease in motor neurons innervating proximal hindlimb muscles and medial motor neurons innervating axial muscles, but only at end-stage disease in motor neurons innervating distal hindlimb muscles. Motor neuron loss follows afferent synapse loss with the same temporal and topographical pattern. Trichostatin A, which improves motor behavior and survival of SMA mice, partially restores spinal reflexes, illustrating the reversibility of these synaptic defects. Deafferentation of motor neurons is an early event in SMA and may be a primary cause of motor dysfunction that is amenable to therapeutic intervention.     

Organization of the stomatogastric ganglion of the spiny lobster

Summary The Stomatogastric ganglion ofPanulirus interruptus contains about 30 neurons, and controls the movements of the lobster's stomach. When experimentally isolated, the ganglion continues to generate complex rhythmic patterns of activity in its motor neurons which are similar to those seen in intact animals.In this paper, we describe the synaptic organization of a group of six neurons which drive the stomach's lateral teeth (Figs. 2, 6). This group includes four motor neurons and two interneurons, all but one of which were recorded and stimulated with intracellular microelectrodes.One pair of synergistic motor neurons, LGN and MGN, are electrotonically coupled and reciprocally inhibitory (Figs. 9, 12). A second pair of synergistic motor neurons, the LPGNs, are antagonists of LGN and MGN. The LPGNs are electrotonically coupled (Fig. 14), and are both inhibited by LGN and MGN (Figs. 8, 11). The LPGNs inhibit MGN (Fig. 15) but not LGN. One of the two interneurons in the ganglion, Int 1, reciprocally inhibits both LGN and MGN (Figs. 10, 13). The other interneuron, Int 2, excites Int 1 and inhibits the LPGNs (Fig. 16). The synaptic connections observed in the ganglion are reflected in the spontaneous activity recorded from the isolated ganglion and from intact animals.From the known synaptic organization and observations on the physiological properties of each of the neurons, we have formulated some hypotheses about the pattern-generating mechanism. We found no evidence that any of the neurons are endogenous bursters.We thank D. Kennedy, Eve Marder, and D. Russell for criticizing early drafts of these papers, Nina Pollack and Betty Jorgensen for expert technical assistance, Diane Newsome, SanDee Newcomb, and Pattie Macpherson for typing the many drafts. The authors' research is supported by grant number NS-09322 from N.I.H. and by the Alfred P. Sloan Foundation. B. M. is an NINDS-NIH postdoctoral fellow.     

In vivo visualization of individual neurons in arthropod ganglia facilitates intracellular neuropil recording

Summary A simple method for the in vivo visualization of dye filled cells by laser illumination is used to characterize neurons in situ in the segmentai ganglia of the locust and the crayfish (Fig. 1). Neuron visualization provides the structural information necessary for identification of cells during an ongoing physiological experiment (Figs. 2, 3). Sequential penetrations of soma and neuropil as well as simultaneous double neuropil penetrations of spiking and nonspiking cells are facilitated by the visual control afforded by neuron visualization (Figs. 4, 5, 6). Furthermore, neuron visualization allows the sampling of cellular properties at multiple, predetermined sites in the dendritic and axonal arbors of identified neurons (Fig. 7) and aids in establishing synaptic connectivity through double neuropil recordings (Fig. 8).     

Patterned synaptic drive to locust flight motoneurons after hemisection of thoracic ganglia

Intracellular recordings were carried out on locust flight motoneurons after hemisection of individual thoracic ganglia. With the exception of minimal surgical manipulations, the animals were intact and able to perform tethered flight. Analysis of the synaptic drive recorded in the motoneurons during flight motor activity revealed the extent to which ganglion hemisection influenced the premotor rhythm generating network.