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Temporal patterns of antidromic invasion latencies of sympathetic preganglionic neurons related to central inspiratory activity and pulmonary stretch receptor reflex
162
Brain Research, 135 (1977) 162-166 ~) Elsevier/North-HollandBiomedicalPress
Temporal pa#ems of anlidromic invasion latoncies of sympathetic preganglionic neurons related to central inspiratory activity and pulmonary stretch receptor reflex J. LIPSKI, J. H. COOTE** and A. TRZEBSK! Department of Physiology, Institute o f Physiological Sciences, Medical Faculty, 00-927 Warsaw 64 (Poland) (Accepted June 22n~ t977~
Mass activity recorded from t~: white rami and other preganglionic nerves shows a grouping temporally related to the phrenic nerve bursts (e.g. refs. 1 and 11). This inspiration-locked,excitatory input has been also demonstrated in more than half of the 'spontaneously' active single sympathetic preganglionic neurons (SPN) 14,18,19. On the ome.- hand, the reflex influence of lung inflation on SPN firing has been little studied. It has been suggesteds,e, 9 that stretching of the lungs inhibits the activity of SPN, but the effect has not been directly demonstrated. In the two intracelhilar studies of SPlq4,s, the synaptic inputs related to the central inspiratory activity (CIA) and to lung inflation have not been analyzed. Also from the studies it is evident that the intracellular technique may not enable long enough impalement times, necessary to reveal weak and slowly chanf,ing inputs. For this reason an indirect method of estimation of the membra'e potential changes has been adopted in the present investigation. The technique is based on the measurement of small variations in the latency to SD spikes of antidromicallyexcited neurotic, and it has been successfully used for estimation of excitability changes in spinal motoneuronss,~ and respiratory neurons of the brain stemle,xe. The presence of two components in the rising phase of the antidromically evoked action potential in SPN 4.s indicates the presence of a delay between IS and SD spikes, and suggests that this delay can he also influenced by the polarization of the cell bodys. The present study was undertaken to test whether the changes in excitabilityof SPN related to CIA can he detected by measuring variations in antidromic latency. Secondly, we were interested to check if there are any lung inflation-related excitabifity changes in the cell bodies of SPN. Six cats were anesthetized with chloralose (BDH, 70 ms/ks i.v.). The right phrenic nerve root was exposed low in the neck and used for recording. Phrenic activity was rectified, smoothed with an RC integrating circuit (TC 0.1-0.2 sec) and ** Present address: Department of Physiology,The Medical School, Universityof Birmingham, Birmingham BI5 2TJ, England.
163 used to deliver a pulse marking the approximate onset of phrenic discharge. A laminectomy was performed at T~-T3. On the right side the 1"3 white ramus was carefully dissected retropleuraUy after removing the heads of ribs 2 and 3, cut distally and placed on a pair of fine silver wire stimulating electrodes. The animals were placed in a stereotaxic frame and fixed by clamps at the lilac crests, Le, T4 and T1 vertebra. They were paralyzed with Flaxedil (4 mg/kg) and subjected to bilateral pneumothorax. They were artiriciaUy ventilated with a pump TM t~ggered twice in each r,.~v,r,,"~"; °*"-',,,,.y cycle. The strokes of the pump in each cycle were delayed from the onset of phrenic discharge by 0.4 sec and 1.5-2.5 sec using a Digitimer. The tidal volume of the pump was adjusted to keep the end tidal CO9 which was recorded by Beckman LB-2 gas analyzer close to 4%. The rate of lung inflation during each stroke could be independently controlled. Tracheal pressure and arterial blood pressure recorded from the femoral artery were measured by Statham P23Db transducers. Extracellular recordings were made from single neurons in the 1"3 segment with glass microelectrodes rifled with 4 M NaCI with a DC resistance of 4-6 Mf~. Antidromic stimulation of the Ta white ramus was used to identify SPN. Antidromic latency was defined as the time from the onset of the stimulus to the beginning of a pulse triggered by a Schmitt discriminator during the linear portion of the rising phase of the SD spike. The latency was measured with a resolution of 0.01 msec by a digital meter with memory. A digital-to.analog converter was used to obtain a voltage proportional to the latency, and this voltage was fed to the computer (Anops-3) to be averaged usually over 64 respiratory cycles. Details of the technique of averaging of antidromic latency are presented elsewhere18. Along with the antidromic latency, tracheal pressure and integrated phrenic nerve activity were averaged on two other channels of the computer. All recordings were made at a depth of 1-1.5 mm from the dorsal surface of the spinal cord, the micropipette being inserted just lateral to the sulcus dorsolateralis. A total of 27 units responding antidromically to Ta white ramus stimulation (0.1 msec, 3-8 V) were successfully tested. Out of the 27 units, 13 showed no background activity. Antidromic latency of the units ranged from 1.9 to 7.0 msec (mean 4.4 msec) and the calculated axonal conduction velocity was 3.6 m/sec to 12 m/sec (mean 6.2). Although the units could follow antidromic stimulation with a high frequency ( > 100 Hz), a stimulus repetition rate of 3 Hz was routinely used in all experiments. Care was taken to avoid artifactual measurements due to the spike amplitude fluctuation. In all SPN the variations in the antidromic latency were rather small (up to 0.15 msec, Fig. 1) and at first look random. Therefore Merrill's technique of a direct display of the latencyis could not be used for showing rhythmical dependence. In 8 units no antidromic latency variations correlated with CIA and/or lung inflation could be shown in normocapnic conditions even with averaging. However, in the remainder (19 neurons) averaging revealed a clear inspiratk, and/or pump-locked latency shift. In the following account shortening or lengthening refers to the latency shifts with respect to the base.line during the periods between CIA or lung inflation. During CIA, 16 SPN ( ~ 60 %) showed shortening of the antidromic latency up to 0.1 msec. In 3 neurons (11 ~ ) the opposite effect was observed. These responded with a lengthening of the
164
!
I
!
I
Fig. I. Anfidromicinvasionof 4 SPN. In A and C, pulsesfrom a Schmidttrigger are shown,indicating levels of latency discrimination. Amplitude calibration, 500 #V, time calibration, --I reset. latency during CIA, while no response to lung inflation was seen. During the recording from 12 cells which showed the latency shifts related to inspiration, manoeuvres were made which increased the CIA, either by a decrease of tidal volume of the pump (up to 6.5% end tidal CO2) or by ventilating the animals with 7-8% CO~ in oxygen. In all units tested in this way, the magnitude of the latency shift corresponded to the size of CIA (Fig. 2). About half of the total number of SPN (13 out of 27) showed changes in antidromic latency to lung inflation. In 8 an increase of antidromic latency to lung inflation was observed, and in 5 a decrease was seen. The influence ofchanging the rate of lung inflation within each stroke of the pump was also checked. In all 4 neurons tested in this way, the size of the shift in antidromic latency corresponded to the rate of lung inflation. In one of the neurons, which was tested repeatedly and showed a decrease in antidromic latency related to inflation, bilateral vagotomy abolished the pump-related latency shift.
C
D
Fig. 2. Computer displayof temporal patterns of antidromic invasionofSPN. TRP, trachealpressu-e, lung inflation-downwardsdisplacement (calibration, 5 nun Hg); ADL, antidromi¢ latency (shortening of the latencyindicated by downwardsdisplacement);PHR, integrated phrenic nerve activity. A: control record; B: effect of hypoventilationevoked by ttrning down the tidal volume; C: second ¢ontr')!run 0idal volumeback to control); D: effectof increasingthe rate and depth of lung inflations. Each record is averaged over 64 respiratory cycles.
165 The technique used enabled a test of whether or not the neuron was influenced both by CIA and by lung inflation. Different combinations of latency shifts in different neurons could be demonstrated. However, the most common (7 out of 19 neurons) was the one showing a decreased latency during CIA and an increased latency during lung inflation. Both "silent" neurons and 'active" neurons showed this pattern. The temporal pattern of antidromic invasion latencies reflects changes in the membrane potential (refs. 3 and 16; Richter, personal communication). Antidromic latency measured to SD spike is shorter when the cell is depolarized, and longer when the cell is hyperpolarized (or disfacilitated). Apart from the fact that no cell impalement is necessary, another advantage of the technique is that cells with no ongoing activity can also be studied. Our data seem to indicate that the technique, improved by averaging of anfidromic latency, can be useful for evaluating excitability changes of SPN. Shortening of the latency obser~ed in most of the SPN during inspiration is most probably related to an excitatory influence of inspiratory neurons on SPN antecedent neurons situated in the brain stem11,is. In some units no latency variations correlated with CIA cot~ld be demonstrated in normocapnic conditions. This does not mean, however, that these units cannot be influenced by a CIA related input, since Preiss and Polosa 19 demonstrated that SPN which show no sign of inspiratory grouping may reveal inspiratory activity in hypoxic and/or hypercapnic conditions. Three neurons were found which showed an increase in the latency during CIA. This finding may be related to the observation by Preiss et al. TM that some SPN show an expiration-related activity. The other main point is that there was a group of SPN which showed a change in antidromic latency during lung inflation. The response was most probably related to vagal stretch receptor afferent input, since in one neuron we were able to abolish the effect by sectioning both vagus nerves. The excitability changes during lung inflation were not simply due to the interaction of vagal stretch receptor input with CIA, since they have been examined during a second inflation within each respiratory cycle, when no inspiratory activity was present. No evidence concerning the neuronal circuitry underlying this influence is provided, but the hypothesis may be suggested that R/~ neurons localized in the area of the nucleus of tractus solitarii are involved. They are excited by lung inflation and send long axons to the spinal cord (refs. 2, 12 and 17; Merrill, personal communication). Since the recordings were made at "1"8segment, many of the SPN probably have a cardiac destination4,15. However the population studied was rather heterogenous as far as antidromic latency shifts are concerned, so it is possible that some of the neurons projected to other than cardiovascular effectors. This investigation was supported by Grant 10.4.2. of the Polish Academy of ~iences.
166 1 Adrian, E. D., Bronk, D. W. and Ph~iips, G., Disdtarl~ in mammalian sympathetic nerves, J. Physiol. (Lmtd.), 74 (1932) 115-133. 2 B ~ h i , A. L. and Barillot, J. (2., A~ivity of medallary respiratmy neurons during reflexes from the lungs in cats, Resp. Physio/., 25 (1975) 335-352. 3 Brock, L. G., Comnbs, J. S. and Ecgtes, $. C.. Intracgllular recording from antidromically activated motone~omk J. Physiol. (/.and.J, 122 (1953) 429-461. 4 Come, $. and Westbury, D., Propexties of sympathetic preganglionic neurones located in the third t h o r a ~ sqgmem of the spinal cord, in preparation. 5 Daly, M. De B., Hazzledine, J. L. and UnIw, A., The reflex effects of alterations in lung volume on systemic vascular resistance in the dog, J. Physiol. (£ond.), 188 (1967) 331-351. 6 Daly, M. De B. and Robinson, B. H., An analysis of the reflex systemic vasoclilator response. elicited by lung inflation in the dog, J. Physiol, (Lon~), 195 (1968) 387--406. 7 Ecgles, J. C., The central action of antidromic impulses in motor nerv© fibres, P~gers Arch. ges. Physiol., 260 (1955) 385-415. 8 Fernandez De Molina, A., Kuno, M. and Pad, E. IL, Antidromically evoked responses from sympathetic prqgmglionic .,~orous, X. Physiol. (Lond), 180 (1965) 321-335. 9 Glick, G., Wechslcr. A. S., Eps~'.'n, S. E., Lewis, IL M. and McGill, R. D., Reflex cardiovascular depression produc~l by stimulation of pulmonary stretch receptors in the dog, J. din. Invest., 48 (1969) 467-473. 10 Huszczuk, A., A respiratory pump controlled by phrcnic nerve activiW, J. Physiol. (Lond.), 200 (1970) 183--184P. I I Koizumi, K., Seller, H., Kanfman, A. and Brooks, C. Mc~., Pattern of sympathetic dischaq~es and their relation to baroroceptor and rospiratory acth'itie~ Brain Research, 27 (1971) 281-294. 12 L/pski, J., The effect of COz on single respiratory neurons of the nucleus of the tractus solitarii, Bull. F.urop. physiopathol. Resp., 12 (1976) 225P. 13 Lipski, J. and Kedra, J., A method for averaging the response latenc~ patterns of antidromically excited neurons, Acta neurobiol. Exp. in press. 14 Mannard, A. and Polusa, C., Analysis of background firing of single sympathetic presanglionic neurons of cat cervical nerve, J. Ncnrophysiol., 36 (1973) 398-408. 15 Malliani, A., Schwartz, P. J. and Zanchetti, A., A sympathetic reflex elicited by experimental coronary occlusion, Amer. J. Physiol., 217 (1969) 703-709. 16 Merrill, E. G., Finding a respiratory function for the medullary respiratory neurons. In R. Bellairs and E. G. Gray (Eds.), F.asaya on the Nervous Syatem, Claredon Press, Oxford, 1974, pp. 451-486. 17 Nakayama, $. and yon Baumigatlen, R., Iacalisierung absteiilencler Atmunpbahnen in R(lckenmark der Katze mittels antidromer geizung, Pilfers Arch. 8es. Physiol., 281 (1964) 231-244, 18 Preiss, G., Kirchner, F. and Polma, C., Patterning of sympathetic preganglioni¢ neuron firing by the central respiratory drive, Brain Research, 87 (1975) 363-374. 19 Preiss, G. and Polusa, C., The relation between end-tidal COt and discharge patterns of sympathetic preganslionic neurons, Brain Research, 122 (1977) 255-267.
Brain Research, 135 (1977) 162-166 ~) Elsevier/North-HollandBiomedicalPress
Temporal pa#ems of anlidromic invasion latoncies of sympathetic preganglionic neurons related to central inspiratory activity and pulmonary stretch receptor reflex J. LIPSKI, J. H. COOTE** and A. TRZEBSK! Department of Physiology, Institute o f Physiological Sciences, Medical Faculty, 00-927 Warsaw 64 (Poland) (Accepted June 22n~ t977~
Mass activity recorded from t~: white rami and other preganglionic nerves shows a grouping temporally related to the phrenic nerve bursts (e.g. refs. 1 and 11). This inspiration-locked,excitatory input has been also demonstrated in more than half of the 'spontaneously' active single sympathetic preganglionic neurons (SPN) 14,18,19. On the ome.- hand, the reflex influence of lung inflation on SPN firing has been little studied. It has been suggesteds,e, 9 that stretching of the lungs inhibits the activity of SPN, but the effect has not been directly demonstrated. In the two intracelhilar studies of SPlq4,s, the synaptic inputs related to the central inspiratory activity (CIA) and to lung inflation have not been analyzed. Also from the studies it is evident that the intracellular technique may not enable long enough impalement times, necessary to reveal weak and slowly chanf,ing inputs. For this reason an indirect method of estimation of the membra'e potential changes has been adopted in the present investigation. The technique is based on the measurement of small variations in the latency to SD spikes of antidromicallyexcited neurotic, and it has been successfully used for estimation of excitability changes in spinal motoneuronss,~ and respiratory neurons of the brain stemle,xe. The presence of two components in the rising phase of the antidromically evoked action potential in SPN 4.s indicates the presence of a delay between IS and SD spikes, and suggests that this delay can he also influenced by the polarization of the cell bodys. The present study was undertaken to test whether the changes in excitabilityof SPN related to CIA can he detected by measuring variations in antidromic latency. Secondly, we were interested to check if there are any lung inflation-related excitabifity changes in the cell bodies of SPN. Six cats were anesthetized with chloralose (BDH, 70 ms/ks i.v.). The right phrenic nerve root was exposed low in the neck and used for recording. Phrenic activity was rectified, smoothed with an RC integrating circuit (TC 0.1-0.2 sec) and ** Present address: Department of Physiology,The Medical School, Universityof Birmingham, Birmingham BI5 2TJ, England.
163 used to deliver a pulse marking the approximate onset of phrenic discharge. A laminectomy was performed at T~-T3. On the right side the 1"3 white ramus was carefully dissected retropleuraUy after removing the heads of ribs 2 and 3, cut distally and placed on a pair of fine silver wire stimulating electrodes. The animals were placed in a stereotaxic frame and fixed by clamps at the lilac crests, Le, T4 and T1 vertebra. They were paralyzed with Flaxedil (4 mg/kg) and subjected to bilateral pneumothorax. They were artiriciaUy ventilated with a pump TM t~ggered twice in each r,.~v,r,,"~"; °*"-',,,,.y cycle. The strokes of the pump in each cycle were delayed from the onset of phrenic discharge by 0.4 sec and 1.5-2.5 sec using a Digitimer. The tidal volume of the pump was adjusted to keep the end tidal CO9 which was recorded by Beckman LB-2 gas analyzer close to 4%. The rate of lung inflation during each stroke could be independently controlled. Tracheal pressure and arterial blood pressure recorded from the femoral artery were measured by Statham P23Db transducers. Extracellular recordings were made from single neurons in the 1"3 segment with glass microelectrodes rifled with 4 M NaCI with a DC resistance of 4-6 Mf~. Antidromic stimulation of the Ta white ramus was used to identify SPN. Antidromic latency was defined as the time from the onset of the stimulus to the beginning of a pulse triggered by a Schmitt discriminator during the linear portion of the rising phase of the SD spike. The latency was measured with a resolution of 0.01 msec by a digital meter with memory. A digital-to.analog converter was used to obtain a voltage proportional to the latency, and this voltage was fed to the computer (Anops-3) to be averaged usually over 64 respiratory cycles. Details of the technique of averaging of antidromic latency are presented elsewhere18. Along with the antidromic latency, tracheal pressure and integrated phrenic nerve activity were averaged on two other channels of the computer. All recordings were made at a depth of 1-1.5 mm from the dorsal surface of the spinal cord, the micropipette being inserted just lateral to the sulcus dorsolateralis. A total of 27 units responding antidromically to Ta white ramus stimulation (0.1 msec, 3-8 V) were successfully tested. Out of the 27 units, 13 showed no background activity. Antidromic latency of the units ranged from 1.9 to 7.0 msec (mean 4.4 msec) and the calculated axonal conduction velocity was 3.6 m/sec to 12 m/sec (mean 6.2). Although the units could follow antidromic stimulation with a high frequency ( > 100 Hz), a stimulus repetition rate of 3 Hz was routinely used in all experiments. Care was taken to avoid artifactual measurements due to the spike amplitude fluctuation. In all SPN the variations in the antidromic latency were rather small (up to 0.15 msec, Fig. 1) and at first look random. Therefore Merrill's technique of a direct display of the latencyis could not be used for showing rhythmical dependence. In 8 units no antidromic latency variations correlated with CIA and/or lung inflation could be shown in normocapnic conditions even with averaging. However, in the remainder (19 neurons) averaging revealed a clear inspiratk, and/or pump-locked latency shift. In the following account shortening or lengthening refers to the latency shifts with respect to the base.line during the periods between CIA or lung inflation. During CIA, 16 SPN ( ~ 60 %) showed shortening of the antidromic latency up to 0.1 msec. In 3 neurons (11 ~ ) the opposite effect was observed. These responded with a lengthening of the
164
!
I
!
I
Fig. I. Anfidromicinvasionof 4 SPN. In A and C, pulsesfrom a Schmidttrigger are shown,indicating levels of latency discrimination. Amplitude calibration, 500 #V, time calibration, --I reset. latency during CIA, while no response to lung inflation was seen. During the recording from 12 cells which showed the latency shifts related to inspiration, manoeuvres were made which increased the CIA, either by a decrease of tidal volume of the pump (up to 6.5% end tidal CO2) or by ventilating the animals with 7-8% CO~ in oxygen. In all units tested in this way, the magnitude of the latency shift corresponded to the size of CIA (Fig. 2). About half of the total number of SPN (13 out of 27) showed changes in antidromic latency to lung inflation. In 8 an increase of antidromic latency to lung inflation was observed, and in 5 a decrease was seen. The influence ofchanging the rate of lung inflation within each stroke of the pump was also checked. In all 4 neurons tested in this way, the size of the shift in antidromic latency corresponded to the rate of lung inflation. In one of the neurons, which was tested repeatedly and showed a decrease in antidromic latency related to inflation, bilateral vagotomy abolished the pump-related latency shift.
C
D
Fig. 2. Computer displayof temporal patterns of antidromic invasionofSPN. TRP, trachealpressu-e, lung inflation-downwardsdisplacement (calibration, 5 nun Hg); ADL, antidromi¢ latency (shortening of the latencyindicated by downwardsdisplacement);PHR, integrated phrenic nerve activity. A: control record; B: effect of hypoventilationevoked by ttrning down the tidal volume; C: second ¢ontr')!run 0idal volumeback to control); D: effectof increasingthe rate and depth of lung inflations. Each record is averaged over 64 respiratory cycles.
165 The technique used enabled a test of whether or not the neuron was influenced both by CIA and by lung inflation. Different combinations of latency shifts in different neurons could be demonstrated. However, the most common (7 out of 19 neurons) was the one showing a decreased latency during CIA and an increased latency during lung inflation. Both "silent" neurons and 'active" neurons showed this pattern. The temporal pattern of antidromic invasion latencies reflects changes in the membrane potential (refs. 3 and 16; Richter, personal communication). Antidromic latency measured to SD spike is shorter when the cell is depolarized, and longer when the cell is hyperpolarized (or disfacilitated). Apart from the fact that no cell impalement is necessary, another advantage of the technique is that cells with no ongoing activity can also be studied. Our data seem to indicate that the technique, improved by averaging of anfidromic latency, can be useful for evaluating excitability changes of SPN. Shortening of the latency obser~ed in most of the SPN during inspiration is most probably related to an excitatory influence of inspiratory neurons on SPN antecedent neurons situated in the brain stem11,is. In some units no latency variations correlated with CIA cot~ld be demonstrated in normocapnic conditions. This does not mean, however, that these units cannot be influenced by a CIA related input, since Preiss and Polosa 19 demonstrated that SPN which show no sign of inspiratory grouping may reveal inspiratory activity in hypoxic and/or hypercapnic conditions. Three neurons were found which showed an increase in the latency during CIA. This finding may be related to the observation by Preiss et al. TM that some SPN show an expiration-related activity. The other main point is that there was a group of SPN which showed a change in antidromic latency during lung inflation. The response was most probably related to vagal stretch receptor afferent input, since in one neuron we were able to abolish the effect by sectioning both vagus nerves. The excitability changes during lung inflation were not simply due to the interaction of vagal stretch receptor input with CIA, since they have been examined during a second inflation within each respiratory cycle, when no inspiratory activity was present. No evidence concerning the neuronal circuitry underlying this influence is provided, but the hypothesis may be suggested that R/~ neurons localized in the area of the nucleus of tractus solitarii are involved. They are excited by lung inflation and send long axons to the spinal cord (refs. 2, 12 and 17; Merrill, personal communication). Since the recordings were made at "1"8segment, many of the SPN probably have a cardiac destination4,15. However the population studied was rather heterogenous as far as antidromic latency shifts are concerned, so it is possible that some of the neurons projected to other than cardiovascular effectors. This investigation was supported by Grant 10.4.2. of the Polish Academy of ~iences.
166 1 Adrian, E. D., Bronk, D. W. and Ph~iips, G., Disdtarl~ in mammalian sympathetic nerves, J. Physiol. (Lmtd.), 74 (1932) 115-133. 2 B ~ h i , A. L. and Barillot, J. (2., A~ivity of medallary respiratmy neurons during reflexes from the lungs in cats, Resp. Physio/., 25 (1975) 335-352. 3 Brock, L. G., Comnbs, J. S. and Ecgtes, $. C.. Intracgllular recording from antidromically activated motone~omk J. Physiol. (/.and.J, 122 (1953) 429-461. 4 Come, $. and Westbury, D., Propexties of sympathetic preganglionic neurones located in the third t h o r a ~ sqgmem of the spinal cord, in preparation. 5 Daly, M. De B., Hazzledine, J. L. and UnIw, A., The reflex effects of alterations in lung volume on systemic vascular resistance in the dog, J. Physiol. (£ond.), 188 (1967) 331-351. 6 Daly, M. De B. and Robinson, B. H., An analysis of the reflex systemic vasoclilator response. elicited by lung inflation in the dog, J. Physiol, (Lon~), 195 (1968) 387--406. 7 Ecgles, J. C., The central action of antidromic impulses in motor nerv© fibres, P~gers Arch. ges. Physiol., 260 (1955) 385-415. 8 Fernandez De Molina, A., Kuno, M. and Pad, E. IL, Antidromically evoked responses from sympathetic prqgmglionic .,~orous, X. Physiol. (Lond), 180 (1965) 321-335. 9 Glick, G., Wechslcr. A. S., Eps~'.'n, S. E., Lewis, IL M. and McGill, R. D., Reflex cardiovascular depression produc~l by stimulation of pulmonary stretch receptors in the dog, J. din. Invest., 48 (1969) 467-473. 10 Huszczuk, A., A respiratory pump controlled by phrcnic nerve activiW, J. Physiol. (Lond.), 200 (1970) 183--184P. I I Koizumi, K., Seller, H., Kanfman, A. and Brooks, C. Mc~., Pattern of sympathetic dischaq~es and their relation to baroroceptor and rospiratory acth'itie~ Brain Research, 27 (1971) 281-294. 12 L/pski, J., The effect of COz on single respiratory neurons of the nucleus of the tractus solitarii, Bull. F.urop. physiopathol. Resp., 12 (1976) 225P. 13 Lipski, J. and Kedra, J., A method for averaging the response latenc~ patterns of antidromically excited neurons, Acta neurobiol. Exp. in press. 14 Mannard, A. and Polusa, C., Analysis of background firing of single sympathetic presanglionic neurons of cat cervical nerve, J. Ncnrophysiol., 36 (1973) 398-408. 15 Malliani, A., Schwartz, P. J. and Zanchetti, A., A sympathetic reflex elicited by experimental coronary occlusion, Amer. J. Physiol., 217 (1969) 703-709. 16 Merrill, E. G., Finding a respiratory function for the medullary respiratory neurons. In R. Bellairs and E. G. Gray (Eds.), F.asaya on the Nervous Syatem, Claredon Press, Oxford, 1974, pp. 451-486. 17 Nakayama, $. and yon Baumigatlen, R., Iacalisierung absteiilencler Atmunpbahnen in R(lckenmark der Katze mittels antidromer geizung, Pilfers Arch. 8es. Physiol., 281 (1964) 231-244, 18 Preiss, G., Kirchner, F. and Polma, C., Patterning of sympathetic preganglioni¢ neuron firing by the central respiratory drive, Brain Research, 87 (1975) 363-374. 19 Preiss, G. and Polusa, C., The relation between end-tidal COt and discharge patterns of sympathetic preganslionic neurons, Brain Research, 122 (1977) 255-267.