Reconfiguration of the Respiratory Network at the Onset
of Locust Flight
Jan-Marino Ramirez
Department of Organismal Biology and Anatomy, The University of Chicago, Chicago, Illinois 60637
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ABSTRACT |
Ramirez, Jan-Marino. Reconfiguration of the respiratory network at the onset of locust flight. J. Neurophysiol. 80: 3137-3147, 1998. The respiratory interneurons 377, 378, 379 and 576 were identified within the suboesophageal ganglion (SOG) of the locust. Intracellular stimulation of these neurons excited the auxillary muscle 59 (M59), a muscle that is involved in the control of thoracic pumping in the locust. Like M59, these interneurons did not discharge during each respiratory cycle. However, the SOG interneurons were part of the respiratory rhythm generator because brief intracellular stimulation of these interneurons reset the respiratory rhythm and tonic stimulation increased the frequency of respiratory activity. At the onset of flight, the respiratory input into M59 and the SOG interneurons was suppressed, and these neurons discharged in phase with wing depression while abdominal pumping movements remained rhythmically active in phase with the slower respiratory rhythm (Fig. 9). The suppression of the respiratory input during flight seems to be mediated by the SOG interneuron 388. This interneuron was tonically activated during flight, and intracellular current injection suppressed the respiratory rhythmic input into M59. We conclude that the respiratory rhythm generator is reconfigured at flight onset. As part of the rhythm-generating network, the interneurons in the SOG are uncoupled from the rest of the respiratory network and discharge in phase with the flight rhythm. Because these SOG interneurons have a strong influence on thoracic pumping, we propose that this neural reconfiguration leads to a behavioral reconfiguration. In the quiescent state, thoracic pumping is coupled to the abdominal pumping movements and has auxillary functions. During flight, thoracic pumping is coupled to the flight rhythm and provides the major ventilatory movements during this energy-demanding locomotor behavior.
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INTRODUCTION |
The respiratory control system of invertebrates and vertebrates must be capable of interacting with other neuronal networks in a state-dependent manner. It has to adapt to an alteration in the animal's behavioral state to 1) maintain constant oxygen supply and 2) coordinate the muscles that control the airflow with muscles that are activated during the new behavioral act. Oxygen demand is particularly high during locomotion. Thus two strategies were described in various respiratory systems: 1) the recruitment of additional auxillary muscles, which increases the airflow (van Lunteren and Dick 1997), and 2) the synchronous activation of muscles involved in respiration and locomotion, which is an energy-efficient mechanism to utilize locomotor activity for controlling airflow (Bramble and Jenkins 1993
; Funk et al. 1992a
,b
; Paterson et al. 1987
). Adaptive mechanisms are also critical during behaviors that involve the activation of respiratory muscles in a multifunctional manner. Respiratory muscles are activated during vocalization, vomiting, coughing, sneezing, and swallowing, and it is essential to control the airflow by resetting the respiratory system (Dick et al. 1993
; Jean et al. 1997
; Larson et al. 1994
; Martin et al. 1994
; Miller and Yates 1993
; Shannon et al. 1997
; Widdicombe 1995
). This raises the general issue of how the coordination of the respiratory system is controlled by the nervous system.
In mammals the same neurons in the ventral respiratory group (VRG) of the medulla are activated during breathing and nonrespiratory behaviors (Chiao et al. 1994
; Dawid-Milner et al. 1993
; Fukuda and Koga 1995
; Gestreau et al. 1996
; Yajima and Larson 1993
). The activation pattern of respiratory neurons changes considerably during these nonrespiratory behaviors, suggesting that the respiratory network is reconfigured (Grelot et al. 1993
; Koga and Fukuda 1994
). Therefore it was proposed that principles of network reconfiguration as demonstrated in great detail for the stomatogastric system of crustacean should also apply for the respiratory system (Dickinson 1995
; Dickinson et al. 1990
; Meyrand et al. 1991
, 1994
; Weimann et al. 1991
). However, in a mammalian network it is difficult to proof that the same neurons that are activated during breathing and nonrespiratory behaviors are also part of the rhythm-generating network. Although there was great progress in localizing the site for respiratory rhythm generation in mammals (Ramirez et al. 1998
; Rekling and Feldman 1998
; Smith et al. 1991
) and lower vertebrates (Togerson et al. 1997), it remains unkown which neurons are part of the oscillator and which are not. This level of investigation is more easily achieved in invertebrates.
Here the respiratory system of the locust was used to address at the single cell level the issue of how the respiratory rhythm-generating network behaves during a state-dependent alteration. Although the locust respiratory system is not homologous with the vertebrate respiratory network, it has to fulfill similar functional roles. It has to maintain constant oxygen supply and coordinate respiratory and nonrespiratory muscles during a behavioral change. As in mammals auxillary respiratory muscles are recruited in a state-dependent manner.
In the quiescent locust auxillary muscles in the prothorax and neck can enhance the airflow, which is normally provided by abdominal pumping movements (Miller 1960a
). However, the relative importance of abdominal and auxillary pumping movements changes during flight. Abdominal pumping movements alone become insufficient and even nonessential for oxygen supply, whereas prothoracic and neck muscles become the major respiratory muscles during flight (Miller 1960a
). A muscle that is critical for prothoracic pumping is the muscle 59 (Miller 1960a; Snodgrass 1929
). So far it is unknown how auxillary muscles such as motoneuron 59 are controlled by the CNS. However, it was suggested that the head ganglia play a major role in controlling these muscles. Miller (1960a)
demonstrated that carbon dioxide receptors on the head increase abdominal pumping, induce neck and prothoracic ventilation, and cause contraction of spiracle I (Miller and Mills 1976
). This effect might be mediated by neurons in the suboesophageal ganglian (SOG). We previously reported one neuron in this ganglion, which was part of the respiratory oscillator (Ramirez and Pearson 1989a
), and similar neurons were also identified in crickets (Otto and Campan 1978
; Otto and Hennig 1993
; Otto and Weber 1982
). Furthermore, lesion experiments have shown that the SOG strongly affects the ventilation frequency (Huber 1960
). In this study, additional respiratory neurons are characterized within the SOG, and it will be demonstrated that these neurons control the activity of auxillary motoneuron 59. To address the issue of how these SOG neurons behave during a behavioral change their activity was examined at the onset of flight. As will be demonstrated the SOG neurons, which are elements of the respiratory oscillator, are also activated in the flight rhythm, indicating that portions of the respiratory oscillator must be reconfigured in a state-dependent manner. This neural reconfiguration seems to adapt the prothoracic pumping movements to a different functional role during flight.

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| FIG. 1.
Auxillary muscle 59 is activated in phase with expiratory activity as demonstrated by simultaneous electromyography (EMG) recordings from muscle 59 (top traces) and abdominal muscle 179 (active in phase with expiration, bottom traces, A). However, even in the same animal, activity in M59 can cease for several cycles (B). Thus M59 is not always activated during each respiratory cycle. C: camera lucida drawing from the motoneuron innervating muscle 59. The neuron was stained intracellularly with Lucifer yellow. D: intracellular recording from motoneuron 59 (top trace) reveals a rapid onset of activity (top trace) that coincides with the activation of the abdominal, expiratory muscle 179 (bottom trace).
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METHODS |
Adult male and female Locusta migratoria were kept in crowded colonies with a light/dark cycle of 12 h. Experiments were performed at room temperature (22-25°C).
Preparation and recording techniques
Animals were mounted dorsal side up on a cork board after the removal of the wings and legs. Details of the preparation were published previously (Ramirez and Pearson 1989a
,b
), so only a brief account is given. The thorax was opened with a dorsal incision, and the gut and small muscles as well as the tentorium over the ganglia were removed. The mesothoracic and suboesophageal ganglia were supported by a stainless steel platform and kept moist by a saline described by Robertson and Pearson (1982)
. Flight was induced by a frontal wind stimulus of 3-4 m/s.
Rhythmic activity in the flight system was monitored by electromyograph (EMG) recordings of the subalar muscle 129, a hindwing depressor. Respiratory activity was monitored by EMG recordings from either the inspiratory muscle 177 or the expiratory muscle 179. Recordings from these abdominal muscles were obtained by inserting low resistance glass electrodes (500 k
) filled with potassium acetate (1 mol l
1) into these muscles. In some instances, respiratory activity was monitored by a hook electrode placed on either a median nerve or one of nerves 8, 9, or 10 of the metathoracic ganglion. The activity patterns in these nerves were described by Lewis et al. (1973)
.
Intracellular recordings were obtained with sharp electrodes (40-200 m
) from the neuropile. Electrodes were filled with a 5% solution of Lucifer yellow in distilled water. Neurons were filled by passing negative current (4-7 nA) for
30 min, and ganglia were processed as previously described (Robertson and Pearson 1982
). Neurons were identified and numbered according to the three-digit nomenclature introduced by Robertson and Pearson (1982
, 1983)
.
Neuronal activity was recorded on a DC tape recorder and analyzed off-line. The resetting effect of a neuron was examined by injecting depolarizing current pulses (200-300 ms, 2-5 nA) while simultaneously observing the effect on the respiratory rhythm recorded extracellularly from a respiratory muscle. The effect on the respiratory rhythm was evaluated as described by Ramirez and Pearson (1989a
,b
). For the suboesophageal neurons a respiratory cycle was taken as the time between the onset of two consecutive bursts of activity in the expiratory motor nerve or muscle. Phase 0 refers to the onset of activity in this expiratory nerve. Resetting curves were obtained by subtracting the duration of the cycle in which the current pulse was applied (t2) from the duration of the undisturbed cycle immediately preceding this cycle (t1). This difference was divided by t1. The phase where the stimulus was applied was calculated by dividing the interval between the onset of the cycle in which the stimulus was applied and the onset of the stimulus pulse by the duration of the preceding undisturbed cycle t1 (see Ramirez and Pearson 1989b
). Each neuron was recorded in >10 animals, which was sufficient to assess in a qualitative manner, whether there was an effect on the respiratory rhythm or not. However, because of the variability in the respiratory rhythm among animals and in the number of action potentials evoked by current injection, I did not attempt to obtain an overall quantification of the data, particularly because reset curves were often incomplete. As also explained by Ramirez and Pearson (1989a
,b
), phase response curves could be shifted by as much as 0.2. A possible explanation for this variability is the variable excitation levels of a neuron or the recording from different sites in the neuron. Therefore diagrams in this study can only represent qualitative information on the effect of current injection on the respiratory rhythm. This variability in the resetting effect is shown for three resetting curves obtained in different preparations for the neuron 378.
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RESULTS |
Activity of the auxillary muscle 59
Muscle 59 (Snodgrass 1929
) is an important muscle for thoracic pumping and causes a retraction of the prothorax (Miller 1960a
). In the quiescent locust muscle 59 had an activity pattern that is consistent with its auxillary respiratory function. As demonstrated in Fig. 1A muscle 59 (top trace) discharged in phase with abdominal muscle 179, which is known to be activated during expiration (bottom trace). However, in many cases the activity of muscle 59 was weak and did occur not during every expiratory phase (Fig. 1B). However, in all instances the activity of muscle 59 coincided with abdominal expiration. The motoneuron innervating muscle 59 is localized within the mesothoracic ganglion (Fig. 1C) (Honegger et al. 1985
). Its cell body was positioned in the anterior one-half of the ganglion, contralateral to its efferent axon that projected into nerve 1. The dendrites of the motoneuron were restricted to the contralateral one-half of the ganglion, i.e., the side of the ganglion where the axon projects into the peripheral nerve (Fig. 1A). As expected from the extracellular recordings, motoneuron 59 discharged in phase with abdominal expiration (Fig. 1D). Its rapid onset of activity coincided with the onset of activity in the expiratory muscle.
Interneurons within the suboesophageal ganglion
The suboesophageal ganglion contained some neurons that discharged in phase with expiration. Interneuron 378 (Fig. 2A) had a cell body on the ventral midline of the anterior labial neuromere and a contralateral axon descending at least into the mesothoracic ganglion. The bilateral arborizations extended through the maxillary, mandibulary, and labial neuromere. All neurons of this type (n = 14) had a very thin process ascending into the anterior connective (Fig. 2A). Another secondary process projected into the ipsilateral connective, which however did not continue into the prothoracic ganglion.

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| FIG. 2.
Suboesophageal ganglion interneuron 378 is activated in a variable manner. A: camera lucida drawing of the interneuron 378. B: intracellularly recorded interneuron 378 (top trace) is activated in phase with expiration as indicated by the extracellular recording from abdominal muscle 179 (Exp; bottom trace). C: interneuron 378 (top trace) received 2 components of phasic synaptic input. A weak plateau-like depolarization in phase with abdominal expiration (M179, Exp, bottom trace), this component was also seen in the absence of activity in muscle 59 (middle trace); an additional depolarization that correlated with the activation of muscle 59. D: in some instances, during weak ventilation, interneuron 378 (top trace) was only rhythmically active during muscle 59 discharge (bottom trace). The recordings in this B-D were obtained from different preparations.
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Interneuron 378 discharged in phase with expiration (Fig. 2B). During intense ventilation, the peak activity of this interneuron coincided with the onset of activity in the abdominal expiratory muscle. During the expiratory cycle the rhythmic depolarization decremented (Fig. 2B). The activity of interneuron 378 was variable and state dependent. During weak ventilation variability in the interneuron 378 correlated with variability in the motoneuron 59. As shown in Fig. 2C, the neuron received two components of synaptic input, a plateau-like depolarization (2nd cycle in Fig. 2C), which was also seen in the absence of activity in muscle 59. When muscle 59 was active, 378 received an additional component of depolarizing input (Fig. 2C, top trace) that coincided in time with the activation of muscle 59. Note that this additional input coincided with increased activity in the abdominal muscle (Fig. 2C, bottom trace). As described for interneuron 377 (Ramirez and Pearson 1989a
), interneuron 378 was sometimes not activated in a pronounced rhythmic manner during weak ventilation (Fig. 2D). Only when muscle 59 was activated vigorously did interneuron 378 discharge in phase with this auxillary muscle (Fig. 2D, bottom trace).
Interneuron 379 (Fig. 3A) was also activated in a variable manner. The soma of this interneuron lies laterally in the maxillary neuromere of the SOG. The axon descends contralaterally at least to the metathoracic ganglion. The bilateral arborization extends through all SOG neuromeres. This neurons was mostly inactive during weak respiration and only activated during increased activity in muscle 59 and increased activity in the expiratory muscle (Fig. 3B).

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| FIG. 3.
Suboesophageal ganglion interneurons 379 and 576 were also activated in phase with expiration. A: camera lucida drawing of interneuron 379. B: interneuron 379 (top trace) was rhythmically activated only during increased activity in muscle 59 (middle trace). Note that no obvious rhythmic input was observed during weak abdominal activity (bottom trace, Exp, expiratory muscle M179). C: camera lucida drawing of the ascending interneuron 576. D: intracellularly recorded interneuron 576 (top trace) is rapidly depolarized at the onset of each expiration, as indicated by the simultaneous EMG recording from muscle 179 (Exp, bottom trace).
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Interneuron 576 was characterized by a cell body localized at the ventral midline of the labial neuromere. Its axon ascends in the connectives contralateral to the cell body at least into the tritocerebrum. The processes of 576 are symmetrically distributed within the labial and maxillary neuromeres (Fig. 3C). In 1 of 19 cases, a physiologically and anatomically similar neuron was stained with an ascending and descending axon. Interneuron 576 discharged in phase with expiration (Fig. 3D). The neuron was activated in a decrementing pattern, i.e., its major activity occurred at the onset of expiration. Maximal inhibition of the neuron occurred just preceding expiration.
SOG neurons affect the activity of muscle 59
All interneurons in the SOG had not only an activity pattern that corresponded with the activity of the auxillary muscle 59 but they also affected the activity of this muscle. Examples for each identified SOG neuron are shown in Fig. 4. The bridge balance of the amplifier was not adjusted properly; thus a 0.1-Hz filter was used to show the evoked activity in these neurons. During this stimulation respiratory-related modulation of rhythmic activity persisted, but the neurons discharged also during the inspiratory phase. In all examined cases with a simultaneous recording of the muscle 59 and abdominal muscle (576, n = 2; 377, n = 4; 378, n = 3; 379, n = 1), 1-3 nA of depolarizing current injection led to the activation of muscle 59 as well as to an increased activity in abdominal expiratory muscles (Fig. 4, A-D). However, the number of experiments was too low to allow a quantitative evaluation of this phenomenon. However, as qualitatively shown in Fig. 4B, the activity of the inspiratory muscle was decreased during the activation of the SOG neuron. Note also that the frequency of respiratory rhythmic activity increased, indicating that these SOG neurons affected the respiratory rhythm generator. The excitatory effect on motoneuron 59 was most likely indirect. Simultaneous recordings from 378 and motoneuron 59 revealed that action potentials in SOG neurons were not followed in a 1:1 manner by excitatory postsynaptic potentials in motoneuron 59. However, this simultaneous recording was successful in only one experiment and was therefore not further evaluated.

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| FIG. 4.
Suboesophageal ganglion interneurons excite muscle 59. Intracellular current injection into interneurons 576 (A, top trace), 377 (B, top trace), 378 (C, top trace), and 379 (D, top trace) evoked increased activity in muscle 59 (middle traces) and abdominal muscles (A, C, and D: Exp, M179; B: Insp, opener muscle of spiracle 3). B, C, and D: note that the frequency of respiratory rhythmic activity increased in response to the stimulation of suboesophageal ganglion interneurons. Vertical calibration: A, 50 mV; B, 25 mV; C, 100 mV; D, 50 mV.
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SOG interneurons as elements of the respiratory rhythm generator
In accordance with previous studies (e.g., Friesen et al. 1978
; Ramirez and Pearson 1989a
; Weeks 1981
) a neuron can be considered an element of the respiratory rhythm generator if 1) it is rhythmically polarized in phase with the respiratory rhythm and 2) current injection into this neuron can shift the phase of the respiratory rhythm (i.e., reset the rhythm).
All neurons described in this study were able to reset the respiratory rhythm. Brief current injection into the interneuron 378 reset the respiratory rhythm and caused an early onset of expiration (Fig. 5A). The third trace indicates expiratory activity in an unaffected sequence. This experiments was performed in three different experiments, which clearly demonstrated that there was an overlap in the resetting properties. The reset curve obtained from three individual 378 neurons are indicated in Fig. 5B,
,
, and
. A reset curve was also obtained for interneuron 379 (Fig. 5C). Current injection into this interneuron always induced a shortening of the respiratory cycle. The shortening between phase 0.4 and 1 was consistent with the shortening effect of 378 at approximately the same phase range. Current injections into interneuron 576 resulted in a prolongation of the respiratory cycle, when applied in the phase range 0-0.6 (Fig. 5D). A shortening of the respiratory cycle was induced when current pulses were applied between phase 0.6 and 0.8 (Fig. 5D).

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| FIG. 5.
Stimulation of the suboesophageal ganglion interneurons 378, 576, and 379 reset the respiratory rhythm. A: intracellular current injection into the identified interneuron 378 (top trace) causes a shortening of the respiratory cycle as indicated by a simultaneous EMG recording of muscle 179 (Exp, middle trace). The bottom trace represents unaffected respiratory cylces to better demonstrate the evoked shortening effect. B: reset curve obtained from 3 individual animals. The procedure to obtain these curves was described previously by Ramirez and Pearson (1989a ,b ). C: reset curve obtained from interneuron 379. D: reset curve obtained from 2 individual neurons 576.
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Alteration in the activity pattern of SOG neurons and motoneuron 59 at the onset of flight
At the onset of flight, motoneuron 59 changed its activity pattern and started to discharge in phase with wing depression (Fig. 6A). Each wingbeat cycle led to a rhythmic depolarization in the motoneuron, which was accompanied by the generation of two to four action potentials. Thus thoracic pumping switched from being in phase with abdominal pumping in the quiescent locust to an activity occurring in phase with the wing depressor rhythm.

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| FIG. 6.
Alteration in the activity pattern of suboesophageal ganglion interneurons and motoneuron 59 at the onset of flight. Motoneuron 59 (top trace) and the interneurons 377 (B, top trace), 378 (C, top trace), 379 (D, top trace), and 576 (E, top trace) were activated in phase with wing depression as indicated by the simultaneous EMG recording from subalar muscle 129 (all panels, bottom traces). Vertical calibration bar: 10 mV (A-E); Horizontal calibration bar: (A) 200 ms, (B) 120 ms, (C) 100 ms, (D) 100 ms, and (E) 120 ms.
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The SOG interneurons 377 (Fig. 6B), 378 (Fig. 6C), 379 (Fig. 6D), and 576 (Fig. 6E) exhibited a similar change in their activity and locked on to the flight rhythm. In all neurons the phase-locked depolarization started 0-7 ms before and ceased 25-40 ms after subalar EMG activity (muscle 129, a depressor; Fig. 6). Simultaneous intracellular recordings from suboesophageal ganglion interneurons (377, 378, and 379; Fig. 7) and thoracic depressor motoneurons reveal that the SOG interneurons reached the peak of rhythmic depolarizations after the peak of depolarizations in thoracic motoneurons. The first action potential in the interneurons often coincided with the last action potentials in the depressor motoneurons (Fig. 7, A and B). Thus, based on the activation time, it is unlikely that these interneurons could contribute to the activation of depressor activity in the flight rhythm generator. Simultaneous intracellular recordings with thoracic elevator motoneurons reveal that the last action potentials in the SOG interneurons often coincided with the first action potentials in elevator motoneurons (shown for IN 379, Fig. 7C). Thus these neurons are activated during depression but discharge into the interphase between wing depression and elevation, an activation pattern that is distinct from the known flight interneurons within the thoracic flight rhythm generator (compare, e.g., Robertson and Pearson 1982
). Each wingbeat led to a rhythmic depolarization with one to six interneuronal action potentials. However, the activation pattern of these neurons differed at the onset of wind-induced flight rhythmic activity. The interneurons 377 and 576 were depolarized just after the onset of wind (presumably by the wind sensory stimulus). The activity started before the first elevator activity and also before interneuron 379. Interneuron 379 was strongly depolarized 40-60 ms after wind onset. This latency was remarkably long and variable. Thus the flight initiating wind information must have reached the thoracic ganglia before this interneuron was excited. The activation of interneuron 379 coincided with the activation of elevator neurons (Fig. 7C). The ascending mesothoracic interneuron 404, which is known to be involved in the initiation of flight (Pearson et al. 1985
; Ramirez 1988
), was excited 10-12 ms before the activation of interneuron 379 as revealed by simultaneous intracellular recordings (not shown). Interneuron 378 was hyperpolarized before the onset of flight (Fig. 7A).

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| FIG. 7.
Simultaneous intracellular recordings from suboesophageal ganglion (SOG) interneurons 377 (A, top trace), 378 (B, top trace), and 379 (C, top trace) and thoracic depressor motoneurons (A and B, DEP, bottom traces) and a thoracic elevator motoneuron (C, EL, bottom trace). Note that the SOG interneurons discharge in the interphase between wing depression and elevation. Vertical calibration bar: (A-C, top and bottom traces) 10 mV; horizontal calibration bar (A) 80 ms, (B) 100 ms, and (C) 60 ms. Recordings in A-C were obtained from different preparations.
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After a flight sequence SOG neurons resumed respiratory rhythmic activity. However, the onset of the first expiratory burst, which followed flight termination, was variable. As shown in Fig. 8, the first expiratory burst followed either shortly after flight termination (Fig. 8A), directly after flight termination (Fig. 8B), or relatively late after flight termination (Fig. 8C). Thus there seems to be no strict coupling between both motor patterns at the offset of flight. This differs from the onset of flight, which was always associated with a reset of the respiratory rhythm in the abdomen (Ramirez and Pearson 1989b
).

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| FIG. 8.
Activity of SOG neurons 576 (A, top trace) and 379 (B and C, top traces) recorded simultaneously with expiratory nerve activity (A) and EMG activity from muscle 129 at the termination of flight. Vertical calibration bar: (A-C) 15 mV; horizontal calibration bar (A) 180 ms and (B and C) 360 ms. The recordings were obtained from different preparations.
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Suppression of the respiratory rhythmic input into the SOG neurons during flight
In the SOG neurons, the respiratory rhythmic depolarizations that occurred in the quiesent animal were in all cases suppressed at the onset of flight. As examplified for interneuron 576 (Fig. 9A), there was an inhibition of the respiratory depolarization after the wind stimulus (arrow). The interneuron was then rhythmically active in phase with wing depression. The initial suppression of the respiratory activity was most likely induced already by the wind stimulus. In some cases as shown in Fig. 9B the respiratory rhythmic input was reduced even if no flight was initiated. Toward the end of the flight sequence the respiratory and flight rhythmic depolarizations overlapped as indicated by the gray shading (Fig. 9A). Each filled block indicates a wing depression.

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| FIG. 9.
Respiratory rhythmic input into the interneuron 576 is suppressed during flight, which was induced by a wind puff (arrow). A: intracellular recording from interneuron 576 obtained subsequently in the quiescent locust (A, top trace, shading indicates respiratory rhythmic depolarization), at the onset of flight (A, 2nd trace, shading indicates the beginning of the respiratory rhythmic depolarization that preceded the onset of flight), during flight (A, 3rd trace), and at the end of flight (A, 4th trace). Fourth trace: each wing depression as reflected in a fast depolarization in interneuron 576 is indicated by a black block to better illustrate the overlap between respiratory (shaded area) and flight rhythmic input (blocks). B: initial suppression of the respiratory depolarizing input in interneuron 576 (top trace) was most likely elicited by the wind stimulus (bottom trace, recording from the magnetic valve that controlled the air stream). Note that a suppression of the respiratory input was also evident if a wind stimulus did not induce flight rhythmic activity (middle trace, EMG from muscle 129).
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Tonic interneuron 388 suppresses the respiratory rhythmic input to motoneuron 59
A possible candidate for a neuron that might be involved in the suppression of the respiratory rhythmic input in these neurons is interneuron 388 (Ramirez 1988
) (Fig. 10). As described previously, this interneuron received indirect excitatory input from the flight initiating interneurons 404 (Pearson et al. 1985
). Interneuron 388 is tonically active during flight (see, e.g., Fig. 10B) and causes a slight decrease in the respiratory rate when stimulated (Ramirez and Pearson 1989b
). Electrical stimulation of 388, as shown in Fig. 10, caused a dramatic suppression of the respiratory rhythmic input in motoneuron 59 (Fig. 10D). In fact, only a very slight increase in its activity was sufficient to greatly reduce the respiratory rhythmic activity in this auxillary motoneuron. Given that this interneuron is tonically activated at a much higher rate during flight (Fig. 10B) it is likely that this tonic interneuron will contribute to the suppression of the respiratory rhythmic input to this motoneuron. Unfortunately, we did not succeed in simultaneously recording from the respiratory rhythmic SOG interneurons and this tonically active SOG interneuron to test whether 388 is also responsible for suppressing the respiratory rhythmic input to these neural elements of the respiratory rhythm generator.

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| FIG. 10.
A likely candidate mediating the suppression of respiratory rhythmic input, the suboesophageal ganglion interneuron 388. Simultaneous intracellular recordings revealed that action potentials in interneuron 388 were not followed in a 1:1 manner by excitatory postsynaptic potentials in motoneuron 59, suggesting that both neurons are connected indirectly. This indirect excitatory connection is symbolized in A by a broken line. B: during flight, which was evoked by a wind stimulus (bottom trace), interneuron 388 is tonically activated (top trace), whereas the simultaneously recorded motoneuron 59 (2nd trace) is rhythmically active in phase with wing depression as indicated by the simultaneous EMG recording from subalar muscle 129 (3rd trace). C: simultaneous intracellular recording obtained from interneuron 388 (top trace) and motoneuron 59 (bottom trace) in a quiescent locust. D: intracellular stimulation of 388 (top trace) induced a complete suppression of the respiratory depolarizing input in motoneuron 59 (bottom trace). The bridge in interneuron 388 was not balanced properly. Thus no action potentials are visible during positive current injection (+DC and arrow) into interneuron 388. E: slight current-induced increase (+DC and arrow) in the spontaneous activity of interneuron 388 (top trace) decreased significantly the respiratory rhythmic input into motoneuron 59 (bottom trace).
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DISCUSSION |
In the quiescent locust, ventilation is achieved primarily by the rhythmic activation of the abdomen, which leads to an airflow within the tracheal trunks (Hustert 1975
; Lewis et al. 1973
; Miller 1960a
,b
). At the onset of flight the respiratory rhythm driving these abdominal pumping movements is reset, i.e., expiratory interneurons and motoneurons located within the abdominal and thoracic ganglia are initially inhibited (Fig. 11B, bottom left panel) and inspiratory interneurons are excited (Ramirez and Pearson 1989b
). Neurons in the abdominal and thoracic ganglia maintain respiratory rhythmic activity at a higher frequency (Fig. 11A) throughout the entire flight sequence (Fig. 11B, bottom left panel) (Ramirez and Pearson 1989b
). The interneurons controlling the abdominal pumping movements are part of the respiratory rhythm generator (Ramirez and Pearson 1989a
). The reset of the respiratory rhythm and the increase in the respiratory rate is, at least partly, due to a feed-forward mechanism. Interneurons involved in controlling the alterations of the respiratory system also contribute to the initiation of flight (Ramirez 1988
; Ramirez and Pearson 1989b
). A similar strategy, i.e., an increase in the respiratory rate in anticipation of need, was also described in the respiratory system of mammals (DiMarco et al. 1983; Eldridge et al. 1981
, 1985
; Feldman 1986
).

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| FIG. 11.
Reconfiguration of the respiratory system at the onset of flight. A: bar histogram illustrating the alteration of respiratory activity in thoracic and abdominal muscles at the onset of flight. Left 2 bars: frequency of respiratory activity as measured from abdominal muscles in 5 quiescent (middle bar) and in the same 5 tegula and stretch-receptor deafferented flying animals (left bar). Data were obtained by measuring in each animal 5-8 respiratory cycles before and 1-2 cycles during flight. Right bar: frequency of rhythmic activity as measured from a depressor muscle (129) during flight. Data were obtained from 5 animals during flight by measuring in each individual 5-10 wing beat cycles. B: muscle 59 (B, top panel, top trace, extracellular recording) discharges in the quiescent animal in phase with abdominal expiratory muscles (B, top panel, bottom trace, extracellular recording) and during flight (B, bottom right panel, top trace, intracellular recording from 59) in phase with a depressor muscle (B, bottom right panel, bottom trace, EMG from muscle 129: DEP). In contrast, abdominal respiratory activity does not become flight-rhythmically active, as demonstrated by an intracellular recording from an expiratory motoneuron (B, left bottom panel, top trace EXP) obtained simultaneously with an EMG recording from a depressor muscle (B, left bottom panel, bottom trace, DEP, muscle 83). Expiratory activity is prematurely terminated at the onset of flight as was described in detail by Ramirez and Pearson 1989 . Vertical calibration bar: (B) bottom left panel, top trace: 18 mV; (B) bottom right panel, top trace: 10 mV; horizontal calibration bar: (B) top panel: 650 ms; bottom left panel: 1 s, bottom right panel: 310 ms.
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In this study I demonstrated that the respiratory rhythm generator is not only reset at the onset of flight but is also reconfigured. A group of identified interneurons within the suboesophageal ganglion was shown to participate in the generation of the respiratory rhythm in the quiescent locust. When electrically stimulated, these neurons were able to reset the respiratory rhythm and increase the frequency of respiratory rhythmic activity. At the onset of flight the respiratory rhythmic input into these interneurons was suppressed and they discharged in phase with the flight rhythm. Although these interneurons were able to excite thoracic flight interneurons (Ramirez, unpublished observation) it was not possible to show that they became elements of the flight rhythm generator itself. In no case was it possible to reset the flight rhythm by stimulating a SOG neuron. Thus these SOG neurons seem not to be involved in the generation of the flight rhythm. However, because stimulation of these neurons affects the activity of motoneuron 59, they seem to have an important function in the control of thoracic pumping movements. These thoracic pumping movements are rhythmically activated in phase with expiratory abdominal activity in the quiescent animal (Fig. 11B, top panel) and in phase with wing depressors during flight (Fig. 11B, bottom right panel).
It was previously described that the muscle 59 is involved in the control of thoracic pumping, which has an auxillary function in the quiescent animal (Miller 1960a
; Miller and Mills 1976
). It was demonstrated in this study that its activation is consistent with its auxillary role. During weak ventilation, this muscle was often inactive but was recruited during vigorous ventilation. A recruitment of additional respiratory muscles during increased oxygen demand is also a common principle in the control of mammalian breathing (e.g., Ainsworth et al. 1989
).
In locusts, the recruitment of additional muscles is accompanied by the recruitment of additional elements of the respiratory rhythm generator, a principle that was previously demonstrated (Ramirez and Pearson 1989a
) and that was also described in this study (e.g., for the interneuron 378). This leads to the important conclusion that the number of active elements in a rhythm-generating network is state dependent, a principle that was described in various rhythm-generating networks, including Clione swimming system (Arshavsky et al. 1989
) and lobster stomatogastric system (Hooper et al. 1990
). The principle that the number of active elements of a rhythm generator may be state dependent has interesting implications for mammalian respiration. For this system it was demonstrated that a relatively small portion of the ventrolateral medulla (the pre-Bötzinger complex) is essential and sufficient for the generation of the respiratory rhythm (Ramirez and Richter 1996
; Ramirez et al. 1998
; Rekling and Feldman 1998
; Richter et al. 1997
; Smith et al. 1991
). However, it is unknown whether the extension of the respiratory rhythm generator changes in a state-dependent manner in an intact behaving mammal. This is a likely possibility because we know that respiratory neurons are widely distributed in the nervous system. Such neurons were described, e.g., in the so-called VRG around the nucleus ambiguus, the dorsal respiratory group within the nucleus of the solitary tract, and the Kölliker-Fuse region in the pons (Bianchi et al. 1995
; Dick et al. 1994
).
Another important aspect of this study is that the change in the rhythmicity of auxillary muscles, such as muscle 59, involves a reconfiguration of the respiratory rhythm-generating network itself. Here it was demonstrated that the SOG neurons that contributed in the quiescent animal to the generation of the respiratory rhythm became rhythmically active in phase with the flight rhythm. This is a different strategy than that found in the control of bifunctional muscles that are activated during walking and flight in the locust (Ramirez and Pearson 1988
). In the control of these muscles there was neither a reconfiguration of portions of the flight nor a walking-generating network. Instead, both rhythm-generating networks were functionally independent and were converging only at the motoneuronal level to activate the bifunctional muscles during the two different behaviors. Thus, with respect to the control of bi- or multifunctional muscles, it is important to examine at the level of the rhythm-generating network whether portions of the rhythm generator are reconfigured or whether independent rhythm-generating networks converge at the motoneuronal level. As mentioned in the INTRODUCTION, this is a critical issue in the control of mammalian respiratory muscles that are commonly activated in a variety of nonrespiratory behaviors. In the future it should be possible to dissect the different portions of the mammalian respiratory network to solve one of the most exciting problems in the field, i.e., the interaction of various mammalian neural networks known to be localized within the brain stem, including the neural networks for breathing, gasping, swallowing, mastication, or vomiting.
 |
ACKNOWLEDGEMENTS |
This work was supported in part by an award to the University of Chicago's Division of Biological Sciences under the Research Resources Program for Medical Schools of the Howard Hughes Medical Institute.
 |
FOOTNOTES |
Received 10 October 1997; accepted in final form 26 August 1998.
 |
REFERENCES |
-
AINSWORTH, D. M.,
SMITH, C. A.,
EICKER, S. W.,
HENDERSON, K. S.,
DEMPSEY, J. A.
The effects of locomotion on respiratory muscle activity in the awake dog.
Resp. Physiol.
78: 145-162, 1989.[Medline]
-
ARSHAVSKY, Y. I.,
ORLOVSKI, G. N.,
PANCHIN, Y. V.,
PAVLOVA, G. A.
Control of locomotion in marine mollusc Clione limacina. VII. Reexamination of type 12 interneurons.
Exp. Brain Res.
78: 398-406, 1989.[Medline]
-
BIANCHI, A. L.,
DENAVIT-SAUBIE, M.,
CHAMPAGNAT, J.
Central control of breathing in mammals: neuronal circuitry, membrane properties and neurotransmitters.
Physiol. Rev.
75: 1-45, 1995.[Free Full Text]
-
BRAMBLE, D. M.,
JENKINS, F. A.
Mammalian locomotor-respiratory integration: implications for diaphragmatic and pulmonary design.
Science
262: 235-240, 1993.[Medline]
-
CHIAO, G. Z.,
LARSON, C. R.,
YAJIMA, Y.,
KO, P.,
KAHRILAS, P. J.
Neuronal activity in nucleus ambiguous during deglutition and vocalization in conscious monkeys.
Exp. Brain Res.
100: 29-38, 1994.[Medline]
-
DAWID-MILNER, M. S.,
LARA, J. P.,
MILAN, A.,
GONZALES-BARON, S.
Activity of inspiratory neurons of the ambiguus complex during cough in the spontaneously breathing decerebrate cat.
Exp. Physiol.
78: 835-838, 1993.[Abstract]
-
DICK, T. E.,
BELLINGHAM, M. C.,
RICHTER, D. W.
Pontine respiratory neurons in anaestized cats.
Brain Res.
636: 259-269, 1994.[Medline]
-
DICK, T. E.,
OKU, Y.,
ROMANIUK, J. R.,
CHERNIAK, N. S.
Interaction between central pattern generators for breathing and swallowing in the cat.
J. Physiol. (Lond.)
465: 715-730, 1993.[Abstract]
-
DICKINSON, P. S.
Interactions among neural networks for behaviour.
Curr. Opin. Neurobiol.
5: 792-798, 1995.[Medline]
-
DICKINSON, P. S.,
MECSAS, C.,
MARDER, E.
Neuropeptide fusion of two motor pattern generator circuits.
Nature
334: 155-158, 1990.
-
DIMARCO, A. F.,
ROMANIUK, J. R.,
VON
EULER, C., AND YAMAMOTO, Y. Immediate changes in ventilation and respiratory pattern associated with onset and cessation of locomotion in the cat.
J. Physiol. (Lond.)
343: 1-16, 1983.[Abstract]
-
ELDRIDGE, F. L.,
MILLHORN, D. E.,
KILEY, J. P.,
WALDROP, T. G.
Stimulation by central command of locomotion, respiration and circulation during exercise.
Resp. Physiol.
59: 313-337, 1985.[Medline]
-
ELDRIDGE, F. L.,
MILLHORN, D. E.,
WALDROP, T. G.
Exercise hyperpnea and locomotion: parallel activation from the hypothalamus.
Science
211: 844-846, 1981.[Medline]
-
FELDMAN, J. L.
Neurophysiology of breathing in mammals.
In: Handbook of Physiology. The Nervous System. Intrinsic Regulatory System in the Brain. Washington, DC: Am. Physiol. Soc., 1986, sect. 1, vol. IV, p. 463.
-
FRIESEN, W. O.,
POON, M.,
STENT, G. S.
Neuronal control of swimming in the medicinal leech. IV. Identification of a network of oscillatory interneurones.
J. Exp. Biol.
75: 25-44, 1978.[Abstract]
-
FUKUDA, H.,
KOGA, T.
Stimulation of glossopharyngeal and laryngeal nerve afferents induces expulsion only when it is applied during retching in paralyzed decerebrate dogs.
Neurosci. Lett.
193: 117-120, 1995.[Medline]
-
FUNK, G. D.,
MILSOM, W. K.,
STEEVES, J. D.
Coordination of wingbeat and respiration in the Canada geese. I. Passive wing flapping.
J. Appl. Physiol.
73: 1014-1024, 1992a.[Abstract/Free Full Text]
-
FUNK, G. D.,
STEEVES, J. D.,
MILSOM, W. K.
Coordination of wingbeat and respiration in birds. II. "Fictive" flight.
J. Appl. Physiol.
73: 1025-1033, 1992b.[Abstract/Free Full Text]
-
GESTREAU, C.,
MILANO, S.,
BIANCHI, A. L.,
GRELOT, L.
Activity of dorsal respiratory group inspiratory neurons during laryngeal-induced fictive coughing and swallowing in decerebrate cats.
Exp. Brain Res.
108: 247-456, 1996.[Medline]
-
GRELOT, L.,
MILANO, S.,
PORTILLO, F.,
MILLER, A. D.
Respiratory interneurons of the lower cervical (C4-C5) cord: membrane potential changes during fictive coughing, vomiting, and swallowing in the decerebrate cat.
Pflügers Arch.
425: 313-320, 1993.[Medline]
-
HONEGGER, H. W.,
ALTMAN, J. S.,
KIEN, J.,
MÜLLER-TAUTZ, R.,
POLLERBERG, E. A
comparative study of neck muscle motoneurons in a cricket and a locust.
J. Comp. Neurol.
230: 517-535, 1985.
-
HOOPER, S. L.,
MOULINS, M.,
NONNOTTE, L.
Sensory input induces long lasting changes in the output of the lobster pyloric network.
J. Neurophysiol.
64: 155-157, 1990.
-
HUBER, F.
Experimentelle Untersuchungen zur nervösen Atmungsregulation der Orthopteren (Saltatoria: Gryllidae).
Zeitsch. Vergl. Physiol.
44: 60-132, 1960.
-
HUSTERT, R.
Neuromuscular co-ordination and proprioceptive control of rhythmical abdominal ventilation in intact Locusta migratoria migratorioides.
J. Comp. Physiol.
97: 159-179, 1975.
-
JEAN, A.,
CAR, A.,
KESSLER, J.-P.
Brainstem organization of swallowing and its interaction with respiration.
In: Neural Control of Respiratory Muscles, edited by
A. D. Miller,
A. L. Bianchi,
and B. D. Bishop
Boca Raton, FL: CRC, 1997, p. 223-237.
-
KOGA, T.,
FUKUDA, H.
Bulbospinal augmenting inspiratory neurons may participate in contractions of the diaphragm during vomiting in decerebrate dogs.
Neurosci. Lett.
180: 257-260, 1994.[Medline]
-
LARSON, C. R.,
YAJIMA, Y.,
KO, P.
Modification in activity of medullary respiratory-related neurons for vocalization and swallowing.
J. Neurophysiol.
71: 2294-2304, 1994.[Abstract/Free Full Text]
-
LEWIS, G. W.,
MILLER, P. L.,
MILLS, P. S.
Neuro-muscular mechanisms of abdominal pumping in the locust.
J. Exp. Biol.
59: 149-168, 1973.
-
MARTIN, B. J.,
LOGEMANN, J. A.,
SHAKER, R.,
DODDS, W. J.
Coordination between respiration and swallowing: respiratory phase relationships and temporal integration.
J. Appl. Physiol.
76: 714-723, 1994.[Abstract/Free Full Text]
-
MEYRAND, P.,
SIMMERS, J.,
MOULINS, M.
Construction of a pattern-generating circuit with neurons of different networks.
Nature
351: 60-63, 1991.[Medline]
-
MEYRAND, P.,
SIMMERS, J.,
MOULINS, M.
Dynamic construction of a neural network from multiple pattern generators in the lobster stomatogastric nervous system.
J. Neurosci.
14: 630-644, 1994.[Abstract]
-
MILLER, A. D.,
YATES, B. J.
Evaluation of role of the upper cervial inspiratory neurones in respiration, emesis and cough.
Brain Res.
606: 143-147, 1993.[Medline]
-
MILLER, P. L.
Respiration in the desert locust. I. The control of ventilation.
J. Exp. Biol.
37: 224-236, 1960a.
-
MILLER, P. L.
Respiration in the desert locust. II. The control of the spiracles.
J. Exp. Biol.
37: 237-263, 1960b.
-
MILLER, P. L. AND MILLS, P. S. Some aspects of the development of breathing in the locust. In: Perspectives in Experimental Biology, edited by P. S. Davis. 1976, vol. I, p. 199-208.
-
OKU, Y.,
TANAKA, I.,
EZURE, K.
Activity of bulbar respiratory neurons during fictive coughing and swallowing in the decerebrate cat.
J. Physiol. (Lond.)
480: 309-324, 1994.[Abstract]
-
OTTO, D.,
CAMPAN, R.
Descending interneurons from the cricket suboesophageal ganglion.
Naturwissenschaften
65: 491, 1978.
-
OTTO, D.,
HENNIG, R. M.
Interneurons descending from the cricket suboesophageal ganglion control stridulation and ventilation.
Naturwissenschaften
80: 36-38, 1993.
-
OTTO, D.,
WEBER, T.
Interneurons descending from the cricket cephalic ganglia that discharge in the pattern of two motor rhythms.
J. Comp. Physiol.
148: 209-219, 1982.
-
PATERSON, D. J.,
WOOD, G. A.,
MARSHALL, R. N.,
MORTON, A. R.,
HARRISON, A.B.C.
Entrainment of respiratory frequency to exercise rhythm during hypoxia.
J. Appl. Physiol.
62: 1767-1772, 1987.[Abstract/Free Full Text]
-
PEARSON, K. G.,
REYE, D. N.,
PARSONS, D. W.,
BICKER, G.
Flight initiating interneurons in the locust.
J. Neurophysiol.
53: 910-934, 1985.[Abstract/Free Full Text]
-
RAMIREZ, J. M.
Interneurons in the suboesophageal ganglion of the locust associated with flight initiation.
J. Comp. Physiol.
162: 669-685, 1988.
-
RAMIREZ, J. M.,
PEARSON, K. G.
Generation of motor patterns for walking and flight in motoneurons supplying bifunctional muscles in the locust.
J. Neurobiol.
19: 257-282, 1988.[Medline]
-
RAMIREZ, J. M.,
PEARSON, K. G.
Distribution of intersegmental interneurones that can reset the respiratory rhythm of the locust.
J. Exp. Biol.
141: 151-176, 1989a.
-
RAMIREZ, J. M.,
PEARSON, K. G.
Alteration of the respiratory system at the onset of locust flight. I. Abdominal pumping.
J. Exp. Biol.
42: 401-424, 1989b.
-
RAMIREZ, J. M.,
RICHTER, D. W.
The neuronal mechanisms of respiratory rhythm generation.
Curr. Opin. Neurobiol.
6: 817-825, 1996.[Medline]
-
RAMIREZ, J. M.,
SCHWARZACHER, S. W.,
PIERREFICHE, O.,
OLIVERA, B. M.,
RICHTER, D. W.
Selective lesioning of the cat pre-Bötzinger complex in vivo eliminates breathing but not gasping.
J. Physiol. (Lond.)
507: 895-907, 1998.[Abstract/Free Full Text]
-
REKLING, J. C.,
FELDMAN, J. L.
Prebötzinger complex and pacemaker neurons: Hypothesized site and kernel for respiratory rhythm generation.
Annu. Rev. Physiol.
60: 385-405, 1998.[Medline]
-
RICHTER, D. W.,
BALLANYI, K.,
RAMIREZ, J. M.
Respiratory rhythm generation.
In: Neural Control of Respiratory Muscles, edited by
A. D. Miller,
A. L. Bianchi,
and B. D. Bishop
Boca Raton, FL: CRC, 1997, p. 119-130.
-
ROBERTSON, R. M.,
PEARSON, K. G. A
preparation for the intracellular analysis of neuronal activity during flight in the locust.
J. Comp. Physiol.
146: 311-320, 1982.
-
ROBERTSON, R. M.,
PEARSON, K. G.
Interneurons in flight system of the locust: distribution, connections and resetting properties.
J. Comp. Neurol.
215: 33-50, 1983.[Medline]
-
SHANNON, R.,
BOLSER, D. C.,
LINDSEY, B. G.
Neural control of coughing and sneezing.
In: Neural Control of Respiratory Muscles, edited by
A. D. Miller,
A. L. Bianchi,
and B. D. Bishop
Boca Raton, FL: CRC, 1997, p. 213-222.
-
SMITH, J. C.,
ELLENBERGER, H.,
BALLANYI, K.,
RICHTER, D. W.,
FELDMAN, J. L.
Pre-Bötzinger complex: a brainstem region that may generate respiratory rhythm in mammals.
Science
254: 726-729, 1991.[Medline]
-
SNODGRASS, R. E.
The thoracic mechanism of a grasshopper and its antecedents.
Smithsonian Misc. Collect.
82: 1-111, 1929.
-
TORGERSON, C. S.,
GDOVIN, M. J.,
KOGO, N.,
REMMERS, J. E.
Depth profiles of pH and PO2 in the in vitro brainstem preparation of the tadpole Rana catesbeiana.
Resp. Physiol.
108: 205-213, 1997.[Medline]
-
VAN LUNTEREN, E.,
DICK, T. E.
The muscles of the upper airway and accessory respiatory muscles.
In: Neural Control of Respiratory Muscles, edited by
A. D. Miller,
A. L. Bianchi,
and B. D. Bishop
Boca Raton, FL: CRC, 1997, p. 47-58.
-
WEEKS, J. C.
Neuronal basis of leech swimming: separation of swim initiation, pattern generation and intersegmental coordination by selective lesions.
J. Neurophysiol.
45: 698-723, 1981.[Free Full Text]
-
WEIMANN, J. M.,
MEYRAND, P.,
MARDER, E.
Neurons that form multiple pattern generators: Identification and multiple activity patterns of gastric/pyloric neurons in the crab stomatogastric system.
J. Neurophysiol.
65: 111-122, 1991.[Abstract/Free Full Text]
-
WIDDICOMBE, J. G.
Neurophysiology of the cough reflex.
Eur. Resp. J.
8: 1193-1202, 1995.[Abstract/Free Full Text]
-
YAJIMA, Y.,
LARSON, C. R.
Multifunctional properties of ambiguous neurons identified electrophysiologically during vocalization in the awake monkey.
J. Neurophysiol.
70: 529-540, 1993.[Abstract/Free Full Text]