RESPIRATORY RESPONSES TO STIMULATION OF AFFERENT VAGAL FIBRES IN RABBITS

Transcription

1 ACTA NEUROBIOL. EXP. 1980, 40: RESPIRATORY RESPONSES TO STIMULATION OF AFFERENT VAGAL FIBRES IN RABBITS W. A. KARCZEWSKI, E. NASEONSKA and J. R. ROMANIUK Department of Neurophysiology, Medical Research Centre, Polish Academy of Sciences Warsaw, Poland Abstract. In the experiments with 43 paralyzed rabbits ventilated artificially under general anaesthesia with halothane the effect of electrical stimulation of the central end of the vagus nerve on respiratory pattern was investigated. The analysis of respiratory responses to varying parameters of stimulation, and comparison of results with the experiments in which stimulation of pulmonary receptors was used, confirmed the view that the activities transmitted along the thick myelinated fibres of the vagus nerve exerted an inhibitory effect on the generation of inspiratory activity, while the activities transmitted along thin myelinated fibres accelerated the respiratory rate. The integrative processes transforming the activity of thick fibres had a time constant shorter by two orders than the integration of responses to stimulation of thin fibres. INTRODUCTIOK Electrical stimulation of the vagus nerve is a frequently usedand frequently criticized - method of studying the central mechanisms of regulation of the respiratory rhythm and volume. Selective stimulation of various fibres of this mixed nerve is very difficult and the obtained results are often controversial. It is, however, being used despite this drawback because, frankly speaking, there is no good method of selective, "physiological" stimulation of various pulmonary receptors, and in investigations of the central neuronal connections (latency

2 measurements) it is the method of choice. It was not the intention of the authors of the present work to demonstrate the validity of this method in the investigations of vagal influence on rhythm and volume of breathing, but to find out what information can be obtained with its cautious use. The results of earlier works of Hammouda and Wilson (16, 17), Boyd and Maaske (7), Wyss (38), and many recent reports on the electrical stimulation of the vagus nerve form the basis of many present concepts of respiratory pattern regulation, although some of the conclusions reached in these papers have been controversial, e.g. the view that high frequency of discharges transmitted along vagal fibres exerts an inhibitory effect, while low-frequency discharges stimulate the generation of the respiratory rhythm (38). According to Hammouda and Wilson (16, 17) there are two types of fibres, one transmitting excitatory information, the other inhibitory information in relation to respiratory frequency. The results obtained in this work indicate evidently that appropriate application of electrical stimulation of the vagus nerve may lead to results and conclusions of some value and may explain the basis of the activity of central mechanisms processing the vagal information. METHODS The experiments were carried out with 43 adult male rabbits weighing kg. Premedication was obtained by injecting neuroleptanalgesic agents (fentanyl, Richter, mg/kg and dehydrobenzperidol, Richter, 1.25 mgkg) into the marginal vein of the pinna. The animals were placed in supine position on a heated operating table. Tracheostomy was routinely performed and the femoral artery was cannulated for measurements of the arterial blood pressure and for blood sampling for determination of PaC02, PaOz and ph. The animals were paralyzed with gallamine (Tricuran, Germed) 5 mglkg and were connected to a respirator (Medipan) whose arbitrarily chosen parameters approached the parameters of rabbit ventilation before induction of paralysis. After starting artificial ventilation halothane (Halan, Germed) general anaesthesia was induced through a vaporizer (Halopar, Farum) in amounts of 0.7 volo/o in a mixture of air with oxygen (from 1 : 1 to 3 : 1). Both vagus nerves were cut in their cervical parts and the left one (or both) was desheathed. After dissecting the left and right Cg roots of the phrenic nerve they were cut and desheathed, and

3 multifibre activity was recorded from the left root, and in most experiments the right root was divided until single-fibre activity was obtained. Bipolar silver electrodes were used for stimulation and recording. During the experiment the following procedures were routinely performed : 1) heparin (1,500 IU) was given into the femoral artery to prevent blood clotting; 2) gallamine (5 mg/kg) was administered every 2 h; 3) several deep inflations of the lungs were done at intervals of several minutes to prevent a fall in lung compliance; 4) arterial blood pressure was monitored with a Statham P23DB pressure gauge and EK-4 Farum electromanometer. Gasometric determinations were done with a Radiometer BMS-3 unit. The end-tidal C02 concentration was monitored continuously with Capnograph Godart; 5) body temperature was measured in the rectum and normally kept at a level of 37-38OC with a heating pad. One vagus nerve was stimulated electrically using Tonnies stimulator with stimulus-isolation unit, or Cobrabid and Zopan PGP-4 stimulators with Cobrabid SIU 5 isolation unit. The electrical activity of the phrenic nerve was amplified with a Tektronix 3A9 differential preamplifier and integrated at 70 ms time constant (Medipan integrator - see 3). The activity of a single neuron of the phrenic nerve was transformed by means of an instantaneous frequency meter (Medipan). The firing rate of the motoneuron (f) was measured at the peak of inspiration. The activity of the phrenic nerve, integrated activity of the phrenic nerve (INT.PHR.), stimulus marking, single phrenic motoneurone activity or its instantaneous frequency, end tidal C02 concentration and arterial blood pressure were recorded from a multi-beam Tektronix 565 oscilloscope screen with a OK-3 Medipan camera on photosensitive paper, and on a 6-channel recorder (Medipan). The first four variables were recorded also on a 4-channil magnetic tape recorder RM-1040 (Unitra ZRK). Changes in PaC02 value were obtained by changing the parameters of artificial ventilation (hypoventilation or hyperventilation). Body temperature changes were induced by external heating of the animals. Using a special device transforming the signal of integrated activity of the phrenic nerve, it was possible to obtain electric impulses appearing at the beginning and end of inspiration. These impulses were used for plotting sequential histograms of the duration of inspiration (TI), expiration (TE) and the duration o'f the cyclse (T) with a special-

4 purpose Anops 101 (ZBMM PW) digital computer. Stimulation with volleys of impulses during inspiration and expiration was possible owing to application of a PGP-4 Zopan stimulator gated by means of a rectangular impulse, equal in duration to TI or TE. Compound action potentials of the vagus nerve were obtained as follows: a bipolar recording electrode was placed 1.5 to 2.5 cm rostra1 to the stimulating electrode, the obtained signal was amplified (Tektronix 3A9 preamplifier) and averaged using the Anops 101 computer, which was triggered by the same stimulus. The duration of a single stimulating pulse was 0.5 ms, the frequency of pulses was from 1 to 300/s, the intensity of stimulus was from 0.01 to 1.0 V. The effect of electrical stimulation of afferent vagal fibres on the respiratory resistance of the larynx was studied in 7 rabbits breathing spontaneously and with intact left vagus nerve under general anaesthesia obtained with intravenous 40 /a urethane solution with chloralose (1.2 glkg -/- 50 mg/kg respectively). For measurements of respiratory resistance the method described by Stransky et al. (34) was applied. Since in preliminary experiments (30) it had been observed that the maximal change of the respiratory resistance in response to stimulation takes place at the frequency of 40 pulsesls, this frequency was used in the present investigations. RESULTS The basic problem in the studies of vagal influence on the regulation of respiratory rhythm and volume is whether the observed respiratory responses are due to stimulation of pulmonary stretch receptors (1) or irritant receptors (26) (in experiments with electrical stimulation C fibres can be excluded because of their much higher threshold for excitation - 35), or whether they are a result of stimulation of several types of lung receptors simultaneously. The activity of pulmonary stretch receptors is transmitted along thick myelinated fibres and that of irritant receptors along thin1 myelinated fibres; the threshold of these fibres during electrical stimulation is similar (29, 35, 37). The investigations of compound action potentials based on differences of conduction rates along nerve fibres are the main method for checking which type of fibre is being stimulated. In the present work, besides investigations of compound action potentials, several other respiratory responses to different conditions of stimulation were assessed.

5 Effect of electrical stimulation of the vagus nerve during expiration on the duration of this phase In the first phase of the experiment stimulation at a frequency of 40 pulses/s was applied only during expiration. Changes in the parameters of the respiratory rhythm were studied increasing the intensity of stimuli (in mv) after each series of stimulations. The stimulation was begun with 100 mv and, after it reached a certain level of intensity, a prolongation of the expiratory phase was observed. With further increasing stimulation intensity (increments of about 100 mv) this phase became ever longer. After reaching a certain threshold value of stimulation intensity, a change was observed in the respiratory response - the prolongation of the expiratory phase became steadily shorter. A further rise in stimulation intensity led to shortening of the expiratory pause below the control value, and this shortening was the more pronounced the higher was the intensity of stimulation. The study of compound action potentials showed in the latter case the presence of A6 wave (on the descending slope of Aa (Ap) wave). For making sure whether the reaction was not caused by spatial sunimation of a greater number of identical fibres resulting from increased intensity of stimulation, both vagus nerves were stimulated separately with the same intensity which caused prolongation of TE, and then both vagus nerves were stimulated simultaneously. The rections showed summationprolongation of TE was thus a sum of single responses (Fig. 1) It is worth stressing that using "a" stimulation it was found that TE prolongation was proportional to the frequency of stimulation (Fig. 2). In all experiments carried out so far it was found that the effect of stimulation intensity rise (only a rise of the same stimulation type, that is a or a + 6) could be evoked by increasing the frequency of stimulating pulses or by stimulation of both vagus nerves simultaneously. These results point out that both temporal and spatial summation occur in this experiment. Stimulation of the right vagus nerve with a intensity at a frequency of 100 imp/s elicited a prolongation of TE (Fig. 3A). Simultaneous stimulation of the left vagus nerve at the a + 6 intensity and a frequency of 10 imp/s reduced this response (Fig. 3B). This brings additional evidence that during a + 6 stimulation fibres other than thick myelinated ones are excited, which transmit information stimulating respiratory rhythm generation. The observed effects are a sum of responses elicited by stimulation of thick and thin 1 In the further part of this work stimulation intensity causing prolongation of TE is called a, while the intensity at which TE is shortened is called a+6.

6 Fig. la, Sequential histogram of inspiratory time (TI) and expiratory time (TE) during stimulation (40 impls) of myelinated thick fibres during expiration. The horizontal lines above the x axis designate, successively, stimulation of the left (210 mv), right (180 mv) and both vagus nerves simultaneously. The duration of inspiration and expiration is counted from zero, the inspiratory time is marked with shorter vertical lines filling the space between two successive lines marking the expiratory time. N, number of successive respiratory cycle. B, Sequential histogram of TI and TE during stimulation (8 impls) of thick and thin myelinated fibres during expiration: successively - the left (500 mv), right (300 mv) and both vagus nerves simultaneously. Exp. 32, temp. 38 C; end-tidal C02, 3.5O/0; BP, 85 mm Hg.

7 Fig. 2. Sequential histogram illustrating the magnitude of respiratory responses (cycle time - T) induced by stimulation of thick (rr) and thin ( 4-6 ) fibres of vagus nerve. Stimulation in expiration: first series of stimulation (first horizon30, 40, 50, 60, 70, 80, 90, and 100 impls; tal line): stimulation cr-frequency: second series of stimulation: stimulation a+6 - frequency 30, 40, 50, 60, 70 impls (minimal effect at 70 impls). Stimulation in inspiration: third series of stimula60, 80, 100, 30 impls; fourth series of stimulation: tion: stimulation a+&--50, 100 impls. Exp. 23, temp. 38 C; COz, 4.5O/o; BP, stimulation a impls, u+695 mm Hg. A BP 1 min

8 Fig. 3A, Slow changes of respiratory pattern during and after stimulation of thick myelinated fibres of the right vagus nerve in expiration - stim. freq. 100 imp/s. Records from top to bottom: BP, arterial blood pressure, signal of stimulation of the left (1.v.) and right (r.v.) vagus nerve. Integrated phrenic nerve activity (INT. PHR.). B, Additionally included left vagus nerve stimuiation in expiration stimulating also thin myelinated fibres (stim. freq imp/s). C, as above - different time scale. myelinated fibres (Fig. 30). The observed slow changes of respiratory response during stimulation and after its discontinuation (Fig. 3) are discussed in section 3 and in greater detail in papers (22-24). Figure

9 ELECTRICAL STIMULATION OF THE VAGUS NERVE (409) applied IN INSPIRATION 7 IN EXPIRATION 17-12, n-17 1 I- cont'nuws n: T~ PHR Fig. 4. Changes of respiratory pattern parameters during afferent vagal stimulation. u, stimulation of thick myelinated fibres; cr+fi, stimulation of thin myelinated fibres; 10O0/o, control value; TI inspiratory time; TE, expiratory time; PHR, amplitude of integrated phrenic nerve activity; N, number of experiments. 4 shows changes of respiratory pattern parameters during stimulation u and a f 6. In both cases threshold stimulations were applied, and the frequency of 40 impls is not one of those frequencies which produce maximal reactions. The term "threshold stimulation", which is a function also of stimulation fre'quency, should be regarded as an arbitrary one - it denotes the intensity at which stimulation at frequencies 10 to 40 imp/s induces a response greater than the scatter around the average control value. During stimulation u + 6 the appearance of phrenic nerve activity was observed frequently during expiration, and this activity could be increased by increasing further the intensity of stimulation (or reduction of PaC02). On the records of phrenic nerve integrated activity this was visible as an elevation of the baseline during expiration (Fig. 5). The analysis of the activity of single phrenic motoneurones demonstrated that the observed effect was elicited by extension of the activity of early recruited inspiratory units to the phase of expiration. A rise in the respiratory resistance of the larynx was observed also for the same values of a f 6 stimulation in a series of experiments in which this resistance was determined (Fig. 6).

10 Changes in the activity of phrenic nerve 'motoneurones during vagus nerve stimulation It is known that the pool of phrenic nerve motoneurones contains early recruited inspiratory (low threshold units) and late recruited inspiratory (high threshold units) motoneurones (4, 18, 27). This clas- Fig. 5. Changes in the activity of phrenic nerve motoneurones during electrical stimulation of the vagus nerve. Traces from top to bottom-activity of single phrenic motoneurone, stimulation signal, integrated phrenic nerve activity. A, induction of phrenic nerve activity during expiration (supraliminal stimulation intensity 4-6); B, decreased maximal inspiratory activity of a low-threshold neurone (P) and extinction of high-threshold motoneurone activity; C, increased activity of a motoneurone during stimulation under hypocapnia; x, onset of activity of a late motoneurone (time lapsed from the start of inspiration); y, insp. time. Exp. 14, temp. 36 C; end-tidal COz, 3.6O/0 (A, B); l.oo/o (C); BP, 80 mm Hg, stim. freq., 40 impls.

11 sification is based on the time of inspiration in which recruitment of the activity of a given unit (Fig. 5) is starting. The instantaneous frequency of the early mruited inspiratory units show a similar pattern as that of the signal of the integrated phre'nic nerve activity. On the other hand, late recruited inspiratory units show considerable chan- Fig. 6. Effect of continuous stimulation intensity (40 impls) on changes in the respiratory resistance of the larynx and respiratory rhythm changes. Traces from top to bottom: stimulation signal (duration of stimulation marked with arrows), integrated phrenic nerve activity, endotracheal pressure, arterial blood pressure. Four values of stimulation intensity were used: from top to bottom, 0.13, 0.16, 0.21, 0.28 V; Exp. 4, temp. 3g C; end-tidal C02, 3.0 /o.

12 :oo- 90. %% 100- A *. ' 80- ** ' ' Lo zo- lo- 0 1 o i u L O Y ) 6 0? o m Fig. 7A, Changes in the activity of phrenic nerve motoneurones during stimulation of thick and thin myelinated fibres of the vagus nerve as a function of their threshold (Polo). f(s), frequency of motoneurone discharges in the 5th respiration after starting stimulation; f(c), control value. B, the higher the motoneurone threshold (Polo) the lower is the stimulation frequency inhibiting completely the activity of a given motoneurone stim. freq. o A A ;o o ; ;O 810 wi*j P [ole] Fig. 8. Distribution of the number of motoneurones as a function of their threshold (Polo) before (A) and during (B) stimulation of myelinated vagus nerve fibres. Stim. freq., 40 impls; N, number of motoneurones. Measurements were made always during the 5th respiration after starting stimulation.

13 ges in the rate of rise of their activity and the maximal rate of discharges during vagal stimulation (Fig. 7 A and B) (28). The definition of motoneurone threshold in Fig. 5 which is used in this work is a descriptive one and, due to this fact, the range of its applicability is very limited (see also Karczewski et al. 23). The distribution of the number of motoneurones in the histogram prepared in relation to motoneurone threshold is changed during stimulation as compared with the control one (Fig. 8). This is due to a right-shift of the threshold of units as a result of stimulation with suppression of a part of motoneurones having the highest level (28). It was found that a total inhibition of motoneurone activity is obtained the easier the higher is its threshold (Fig. 7B). No qualitative difference was observed between the effects of a and a 6 stimulations during inspiration; a partial suppression of inspiratory activity was always obtained. Only in the case of stimulation during expiration the duration of the inspiratory activity was 6 longer than the control one for a stimulation and shorter for a stimulation. It must be stressed that with the stimulation method used in this work all myelinated fibres were stimulated in a given phase of the respiratory cycle with the same frequency of impulses. The activity of phrenic motoneurones was investigated at the level of their efferent fibres. + + Fig. 9. Sequential histogram of cycle duration. T changes in relation to stimulation frequency and stimulation intensity. Stimulation in expiration: successively 80 impls-80 mv; 80 imp/s mv; 20 impls- 130 mv; 50 imp/s- 130 mv; 80 mv-a; 130 mv-rrf 8. Exp. 30, temp. 3'i C; end-tidal C02, 4.O0/o; BP, 100 mm Hg. N, number of successive respiration cycles.

14 Breath-by-breath changes in the parameters of the respiratory pattern during and after stimulation During repeated stimulation of Au fibres with high frequency impulses (about 100 impls) in expiration a slow (exponential) return of respiratory pattern parameters to the control values is observed. Sometimes, after several tens of seconds of stimulation, when the initial reaction was not high, the response may disappear completely (24). This has not been observed with low frequency stimulation (10-60 impls). When high frequency stimulation has been applied the obtaining of maximal reactions became again possible after a pause in stimulation of a similar length as the preceding stimulation (see Fig. 3). On the other hand, the control values of cycle duration were obtained again immediately after discontinuing stimulation or after several breaths (Fig. 9), always to values higher than the control ones. With u $ 6 stimulation the situation was different. During this stimulation the parameters of the respiratory cycle changed from breath to breath from the control values to a new steady state. As the frequency of stimulation was increased (in range from 1 to 60 imp/s) the cycle duration became progressively shorter and the steady state was reached sooner. After discontinuing stimulation the control values were reached after several (up to twenty) respiratory cycles, always from the values lower than the control ones (Fig. 9) (22, 24). The return to the initial (control) state was the more rapid the higher was the level of PaC02 and body temperature (30). Breath-by-breath changes in the parameters of the respiratory pattern after starting and discontinuation of stimulation a and u + 6 at low frequencies indicate that in the case of processing the information transmitted along the thick myelinated fibres, the time constant of central summation was comparable to the duration of inspiration. This result has been confirmed by the investigations in which the effect of lung inflation on phrenic output was studied in non-vagotomized animals (9, 31, 39). It is worth also to pay attention to a different order of the value of stimulation frequency which can evoke the first reaction imp/s in the case of a and about 1 imp/s in the case of a + 6 stimulation. DISCUSSION Electrical stimulation of the vagus nerve, similarly as stimulation of every mixed nerve, causes many difficulties in the interpretation of obtained results. From the time when the first reports had been issued from this laboratory on the central integration of respiratory informa-

15 tion transmitted by the vagus nerve (8, 21, 22), the interpretation of obtained results underwent changes parallel to the advance of investigations of the effect of pulmonary receptors activity on respiratory rhythm regulati~n. The recently obtained results permit now a more general insight into this problem, because the results of the investigations carried out independentaly by different methods Show many intriguing similarities. The possibility of using SO2 as a factor inhibiting selectively the activity of most pulmonary stretch receptors (11, 14) and thus making possible the investigations of effects of irritant receptor activity on respiratory rhythm has provided many new and interesting data, especially one fundamental observation, that stimulation of stretch receptors during expiration prolongs its duration, while stimulation of irritant receptors during expiration reduces the TE (11, 25). It has been suggested initially that the regulation of expiratory time is mainly the "task" of irritant receptors, while the activity of pulmonary stretch receptors control the inspiratory time (10). It was also observed that the activity of irritant receptors summated in time (12, 32) may be responsible for spontaneous deep breaths. Similarly, the respiratory effects produced by stimulation of irritant receptors last about 30 s (13). The a + 6 stimulation used in the present work, in which most myelinated fibres are stimulated, can be compared - as it was done by Wyss (38) -to the situation in which one lung is inflated and the other deflated. Because of that, electrical stimulation a + 6 cannot be compared directly to the results of experiments in which irritant receptors were stimulated during SOz block of pulmonary stretch receptors, but it is worth to call attention to certain analogies. Alfa-stimulation (stimulation of thick myelinated fibres of the vagus nerve) during expiration never reduced the expiratory time, and the effects were either absence of changes at low frequencies (about 10 imp/s), or lenlgthening of expiration proportional to the frequency of stimulating impulses. Comparing the effects of a stimulation with the effects produced by stimulation of pulmonary stretch receptors it may be said that low frequencies of discharges of pulmonary stretch receptors during expiration do not accelerate the respiratory rhythm, but inhibit it to a smaller extent. Thus, respiratory rhythm acceleration connected with FRC fall below the control value (2), or, being a result of inhibition of the activity of pulmonary stretch receptors, e.g. by administration of COz or SO2 (19, 36), would be associated with reducing their inhibitory effect on respiratory rhythm. It is worth stressing that changes of TE response during increased stimulation intensity stepped up from a to a + 6 cannot be explained by an increase in the number of sti-

16 mulated fibres of the same type, since maximal a stimulation of both vagus nerves simultaneously is a sum of the effects obtained by stimulation of each of these nerves separately. It has been observed additionally, that the effect produced by increased stimulation intensity in the a range at a steady stimulus frequency could be evoked also by increasing the frequency at the same stimulus intensity (30). During a + 6 stimulation no significant differences were observed between stimulation effects when the frequency of impulses was increased at the same intensity and those obtained with changing stimulation intensity at a steady frequency of impulses. In the correlation Getween the response and the intensity of stimulation no new extreme values or inflexion points were apparent. These results would suggest that even if some C fibres were excited during a + 6 stimulation, the information transmitted by them either would be ignored, or would have comparable effects to stimulation of thin myelinated fibres. During developing inspiratory activity the activity transmitted by the myelinated fibres of the vagus nerve has an inhibitory effect, as evidenced by changes in the integrated activity of the phrenic nerve, and even more by changes in the activity of single motoneurones of the phrenic nerve. The results of the present work indicate that late inspiratory motoneurones are most sensitive to changes in the vagal input. Because of that, investigations of the subtle effects of vagal afferent input on the rate of rise of inspiratory activity should be carried out by controlling the discharge of late inspiratory motoneurones (28). In the present study no positive feedback was observed between the vagal input and the phrenic output (3), and no partly reversible inhibition of phrenic nerve activity (39) was noticed, probably because the applied stimulus was unchanged throughout the whole inspiratory phase in the frequency range identical for both types of fibres. It is not clear whether in the presence of activities transmitted by thick myelinated fibres the information passing along thin myelinated fibres would have a stimulating effect on the generation of inspiratory activity during inspiration. During a + 6 stimulation a greater number of fibres Aa are stimulated than during a stimulation (in view of some overlapping of the diameters of fibres Aa and A6). Because of that, qualitatively identical inhibitory efects observed in inspiration during a and a + 6 stimulation could be mainly due to stimulation of thick fibres, (the inspiratory inhibition was, however, quantitatively bigger during a f 6 stimulation - see also 15). Attention should be called to the fact that during apnoea after hyperventilation a + 6 stimulation has an excitatory effect on the generation of inspiratory activity (similarly as that applied during expiration). It

17 is worth recalling that in the paper by Bradley (5) stimulation of Aa fibres somewhat reduced the inspiratory flow, while additional stimulation of A6 fibres had an opposite effect (in cats). The inspiratoryexcitatory effect of irritant receptors is supported also by the results of experiments (14) with SOz blockade and pulmonary deflation. Excitation of irritant receptors elicits an increase in laryngeal resistance (Widdicombe, personal communication). The fact that in our experiments such an increase followed only a + 6 stimulation additionally supports the view that thin myelinated fibres were involved. Slow changes in respiratory rhythm during a stimulation which have some features of habituation, had been observed also by Stanley at al. (33) during PSR stimulation, and were described also by Karczewski and Romaniuk (24). A reverse effect was observed in the records of integrated phrenic nerve activity in the paper by Bartoli et al. (3) and in the activity of pontine respiratory neurones in the paper by Kahn and Wang (20), dedicated also to the integrative mechanisms in Hering-Breuer reflex (6). Changes in the respiratory rhythm during u + 6 stimulation showed the features of temporal summation with a time constant of the order of 10 s. In summary, it was found in this study that the activities transmitted by thick myelinated fibres of the vagus nerve have an inhibitory effect on the generation of inspiratory activity. The impulses transmitted along thin myelinated fibres accelerated the respiratory rhythm. The central mechanisms processing the stimuli passing along myelinated fibres of the vagus nerve are based on temporal and spatial summation of different time constants. Thanks are due to dr M. Szereda-Przestaszewska and Mrs. E. Jqdrychowska for their help in experiments in which laryngeal resistance was measured (as suggested by Prof. J. G. Widdicombe). We are indebted to Mrs. T. Warnawin for gasometric estimations and to Mrs. B. Sudziarska for preparing the manuscript. The investigation was supported by Project of the Polish Academy of Sciences. REFERENCES 1. ADRIAN, E. D Afferent impulses in the vagus and their effect on respiration. J. Physiol. 79: BARTOLI, A., BYSTRZYCKA, E., GUZ A., JAIN, S. K., NOBLE, M. I. M. and TRENCHARD, D Studies on the pulmonary vagal control of central respiratory rhythm in the absence of breathing movements. J. Physiol. 230: Acta Neurobiol. Exp. 3/80

18 3. BARTOLI, A,, CROSS. B. A., GUZ, A,, HUSZCZUK, A. and JEFFERIES, R The effect of varying tidal volume on the associated phrenic motoneurone output: studies of vagal and chemical feedback. Respir. Physiol. 25: BERGER, A. J Phrenic motoneurons in the cat: subpopulations and nature of respiratory drive potentials. J. Physiol. 42: BRADLEY, G. W The effect of CO,, body temperature and anaesthesia on the response to vagal stimulation. In B. Duron (ed.), Colloque: Respiratory centres and afferent systems. INSERM 4--6 March, Vol. 59, p BREUER, J Self-steering of respiration through the nervus vagus. In R. Porter (ed.), Ciba Foundation Hering-Breuer Symposium. Breathing. London 1970, p BOYD, T. E. and MAASKE, C. A Vagal inhibition of inspiration, and accompanying changes of respiratory rhythm. J. Neurophysiol. 2: BYSTRZYCKA, E., GROMYSZ, H. and HUSZCZUK, A Studies on the Hering-Breuer inflation and deflation reflexes in rabbits. Acta Physiol. Pol. 23: CROSS, B. A. and GUZ, A The effect of changing the rate of inflation of the lung on the associated phrenic motoneurone output. In B. Duron (ed.), Colloque: Respiratory centres and afferent systems. INSERM 4--6 March, p DAVIES, A The effects of irritant receptor activity on expiratory time. J. Physiol. 275: 39P. 11. DAVIES, A., DIXON, M., CALLANAN, D., HUSZCZUK, A., WIDDICOMBE. J. G. and WISE, J. C. M Lung reflexes in rabbits during pulmonary stretch receptor block by sulfur dioxide. Respir. Physiol. 34: DAVIES, 4. and ROUMY, M The inspiratory augmenting effects of lung irritant receptor activity. J. Physiol. 275: 14 P. 13. DAVIES, A., ROUMY, M., WIDDICOMBE, J. G. and WISE, J. C. M Effects of brief changes in lung volume on pattern of breathing in rabbits. Proc. 27 Int. Congr. Physiological Sciences, Paris, Vol. 13, 163 p. 14. EULER, von C., GEOGOWSKA, M. and HOMMA, I Inspiratory facilitator~ reflexes provoked by rapid inflation and deflation in cats. Acta Physiol. Scand. (in press). 15. GROMYSZ, H., KARCZEWSKI, W. A., NASLONSKA, E. and ROMANIUK, J. R The role of timing and magnitude of the vagal input in controlling the phrenic output in rabbits and baboons. Acta Neurobiol. EXP. 40: HAMMOUDA, M. and WILSON, W. H The vagus influence giving rise to the phenomena accompanying expansion and collapse of the lungs. J. Physiol. 74: HAMMOUDA, M. and WILSON, W. H The presence in the vagus of fibres transmitting impulse augmenting the frequency of respiration. J. Physiol. 83: HILAIRE, G., MONTEAU, R. and DUSSARDIER, M Modalites du recrutement des motoneurones phreniques. J. Physiol. (Paris) 64: HUSZCZUK, A., KULESZA, J. and RYBA, M Dependence of total

19 stretch receptor activity on airway COz. Bull. Eur. Physiopathol. Respir. 12: 228 p. 20. KAHN, N. and WANG, S. C Electrophysiologic basis for pontine apneustic center and its role in integration of Hering-Breuer reflex. J. Neurophysiol. 30: KARCZEWSKI, W. A A model of proprioceptive information from the lungs. Proc. V Int. Conf. Med. Electronics, Liege. 22. KARCZEWSKI, W. A., BUDZIRSKA, K., GROMYSZ, H., HERCZYNSKI, R. and ROMANIUK, J. R Some response of the respiratory complex to stimulation of its vagal and mesencephalic inputs. In B. Duron (ed.), Colloque: Respiratory centres and afferent systems. INSERM, Vol. 59 p KARCZEWSKI, W. A., NASEONSKA, E. and ROMANIUK, J. R Inspiratory facilitatory and inhibitory vagal influences during apnoea in rabbits. Acta Neurobiol. Exp. 40: KARCZEWSKI, W. A. and ROMANIUK, J. R Neural control of breathing and central nervous system plasticity. Acta Physiol. Pol. (in press). 25. KNOX, C. K Characteristics of inflation and deflation reflexes during expiration in the cat. J. Neurophysiol. 36: MILLS, J. E., SELLICK, H. and WIDDICO,MBE, J. G Epithelial irritant receptors in the lungs. In R. Porter (ed.), Ciba Foundation Hering-Breuer Centenary Symposium, Breathing. London, p NAIL, B. S., STERLING, G. M. and WIDDICOMBE, J. G Some properties of single phrenic motoneurones. J. Physiol. (London) 200: NASEONSKA, E The correlation between integrated phrenic nerve activity and discharge of single phrenic units in different experimental conditions (in Polish) M. Sc. Thesis, Warsaw. 29. PAINTAL, A. S Vagal sensory receptors and their reflex effects. Physiol. Rev. 53: ROMANIUK, J. R Central summation of respiratory information from the lungs (in Polish). Ph. D. Thesis, Warsaw. 31. ROMANIUK, J. R., RYBA, M. and KULESZA, J The effect of volume and duration of lung inflation on the parameters of respiratory rhythm. Acta Physiol. Pol. 27: ROUMY, M The structure of the augmented breath. J. Physiol. 257: STANLEY, N. N., ALTOSE, M. D., CHERNIACK, N. S. and FISHMAN, A. P Changes in strength of lung inflation reflex during prolonged inflation. J. Appl. Physiol. 38: STRANSKY, A., SZEREDA-PRZESTASZEWSKA, M. and WIDDICOMBE, J. G The effects af lung reflexes on laryngeal resistance and motoneurone discharge. J. Physiol. 231: TRENCHARD, D Neurophysiological studies on afferent information from the lungs of man and animals in normal and pathological circumstances. Ph. D. Thesis, London University, London. 36. TRENCHARD, D., BRADLEY, G. and NOBLE, M. I. M Evidence that activity of pulmonary stretch receptors is responsible for the changes in the frequency of breathing produced by increasing the lung C02 concentration in dogs on cardiopulmonary bypass. Bull. Eur. Physiopathol. Respir. 12:

20 37. WIDDICOMBE, J. G. and GEOGOWSKA, M Relative values of irritant, type-j and pulmonary stretch receptors in lung reflexes. Acta Neurobiol. Exp. 33: WYSS, 0. A. M The part played by the lungs in the reflex control of breathing. Helv. Physiol. Acta 12, Suppl. X: YOUNES, M., BAKER, J. P., POLACHECK, J. and REMMERS, J. E Termination of respiration through graded inhibition of inspiratory activity. In R. S. Fitzgerald, H. Gautier and S. Lahiri (ed.). The regulation of respiration during sleep and anesthesia. Advancesin experimental medicine and biology. Plenum Press, New York, 99: Accepted 9 February 1980 W. A. KARCZEWKI, 9. NASLORSKA and J. R. ROMANIUK, Medical Research Centre Polish Academy of Sciences, Dworkowa 3, Warsaw, Poland.