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The inhibitory effect of somatic inputs on the excitatory responses of vagal cardiomotor neurones to stimulation of the nucleus tractus solitarius in rabbits
Brain Research, 439 (1988) 350-353 Elsevier
350 BRE 22691
The inhibitory effect of somatic inputs on the excitatory responses of vagal cardiomotor neurones to stimulation of the nucleus tractus solitarius in rabbits Qiang Wang, Xue-Qin Guo and Peng Li Department of Physiology, Shanghai Medical University, Shanghai (People's Republic of Chh~a) (Accepted 29 September 1987)
Key words: Somatic input; Vagal preganglionic ueuron; Dorsal vagal nucleus; Nucleus ambiguus; Nucleus tractus solitarius
Experiments were done in 41 rabbits anaesthetized with urethane and chloralose, paralyzed with Flaxedil and ventilated artificially. Extracellular recordings of 142 units were made in the dorsal vagal nucleus (DVN) and the nucleus ambiguus (NA), identified by antidromic response to stimulation of the cervical vagus nerve. In total 63.5% of them exhibited spontaneous activity and 22 units (17 in DVN and 5 in NA) showed a cardiac rhythm; their ant'd'gmic conduction velocity was 3.7-12.5 m/s, which suggests their having axons in the range of B fibres. These neurones were classified as vagal cardiomotor neurones. A total of 16 DVN and 4 NA vagai cardiomotor neurones were excited orthodromically by electrical stimulation of the contralateral nucleus tractus solitarius (NTS). Electrical stimulation of the superficial peroneal nerve (SP) with low intensity or the deep peroneal nerve (DP) with high intensity which activated C fibres inhibited excitatory responses of 16 neurones (14 in DVN and 2 in NA). The other 4 neurones were unaffected by SP inputs. These results provide electrophysiological evidence for the inhibitory effect of somatic inputs on the evoked discharges of vagal cardiomotor neurones in the DVN and the NA. Quest and Gebber have reported in cats, that vagal bradycardia evoked by stimulation of both the nucleus tractus solitarius (NTS) and carotid sinus nerve (CSN) was blocked by sciatic nerve stimulation I°. The same blocking effect was also observed recently in this laboratory by prolonged stimulation of either the superficial peroneal nerve (SP) with low intensity (0.1-0.3 mA) or the deep peroneal nerve (DP) with high intensity (0.6-0.8 mA) 1~. This blocking effect is likely to occur in the central nervous system, for the SP stimulation only blocked the bradycardia evoked by NTS or CSN stimulation, but failed to affect those evoked by stimulation of the peripheral end of cut cervical vagus !1j.11. The present investigation was designed to observe the effect of somatic inputs on the vagal cardiomotor neurones within the medulla with an electrophysiological technique. A total of 41 rabbits (2.0-3.0 kg) were anaesthetized with urethane and chloralose (700 mg/kg and 35 mg/kg, i.v. respectively), paralyzed with Flaxedil (4 mg/kg, as needed) and ventilated artificially. The left
femoral artery was cannulated to measure the blood pressure. The rectal temperature was maintained at 37-38 °C. The animal's head was mounted on a stereotaxic frame in a prone position and flexed at an angle of 45 ° . The caudal portion of the floor of the fourth ventricle was exposed. A single-barrelled glass microelectrode filled with Pontamine sky blue in 0.5 M sodium ~cetate (tip diamer 1/~m, impedance 6-10 Mr2 at 1 kHz) was inserted into the right dorsal vagai nucleus (DVN) area (1 mm caudal to 2 mm rostral to the obex arid within 2 mm from the midline) or nucleus ambiguus (NA) area (1 mm caudal to 3 mm rostral to the obex and 3 - 4 mm la,eral to the midline) to record single units extiacellularly. Unit activity was amplified through a differential preamplifier and displayed on an oscilloscope for monitoring and photographing. Another unipolar stimulating electrode, consisting of an insulated stainless steel wire (0.1 mm in diameter, bared only at the tip), was inserted into the contralateral NTS area (1 mm caudal to 2 mm rostral to the obex, 0 . 5 - 2 . 5 m m lateral to the mid-
Correspondence: P. Li, Department of Physiology, Shanghai Medical University, Shanghai 200032, People's Republic of China. 0006-8993/88/$03.5(I © 1988 Elsevier Science Publishers B.V. (Biomedical Division)
351 line). Pulse-triggered histograms of the spontaneously firing neurones and post-stimulus time histograms of neurones responding to NTS stimulation (50-150 /~A, 0.2-0.3 ms and 2 Hz) were obtained by a Z-8088 processor (Shanghai Medical University) and recorded on a SJ-42 polygraph (Shanghai Medical instrumental factory). The right cervical vagus was stimulated by a bipolar electrode (0.2 ms, 50-800 ~A at 2 Hz delivered by a constant current stimulator). Either the SP or the DP was stimulated for 10-15 min through a bipolar electrode with repetitive pulses (0.1-0.3 mA or 0.6-0.8 mA, 0.5 ms and 10 Hz). In some animals, the action potentials of sciatic nerve elicited by deep and superficial peroneal nerve stimulations were analysed by a medical data processing computer (Type TQ 19, China). The recording sites in DVN or NA and stimulating sites in the NTS were marked by the injection of Pontamine sky blue dye, and the deposition of iron from the tip of the stimulating electrode~ respectively. The brain was fixed with a mixture of 1% potassium
200
ferrocyanide in 10% formaline for 4 - 7 days, 50-um frozen sections were taken and stained with Neutral red. The identification of blue spots was made with reference to Meessen's atlas ~. A total of 142 antidromically firing neurones in DVN (119) and NA (23) were recorded by electrical stimulation of the ipsilateral cervical vagus. Antidromically evoked responses were identified by (1) the "all or none" nature of the responses: (2) the constant latency at a specific frequency of stimulation: (3) its ability to follow high frequency (up to 150 Hz) stimulation and (4) cancellation of the evoked response by collision with a spontaneous spike. However, some neurones did not present spontaneous activity and could not be tested by the collision method. Of all neurones tested, 63.5% units (n = 93) exhibited spontaneous activity: 22 neurones (17 in the DVN and 5 in the NA) showed discharges synchronous with cardiac rhythm (Fig. 1). On the basis of axonal conduction velocity calculated from the latency of the recorded antidromic irapulse and the
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Fig. 1. A: the .:ardiac rhythm of presumed cardiomotor neurones in the DVN. Upper trace: Femoral arterial blood pressure. Lower trace: pulse-triggered histogram of cardiomotor neuronal activity; 297 cycles. 5 ms bin width. B: cancellation of antidromic responsc by collision with spontaneous spike in lower trace. A total of 10 superimposed sweeps in the upper trace and a single sweep in thc middle and lower traces. Arrow indicates time of stimulation.
352 straight line horizontal distance between the recording and the stimulating electrodes, 22 units showed cardiac modulation and had axons in the range of B fibres referring to conduction velocity 3.7-12.5 m/s in the DVN and 4.1-11.6 m/s in the NA. These neurones were classified as vagal cardiomotor neurones. Most vagal cardiomotor neurones (16 in the DVN and 4 in the NA) were excited orthodromically by the contralateral NTS area stimulation with single or double shocks. Bradycardia and depressor response could be induced if the NTS area was stimulated with a repetitive pulse (<150 pA, 0.2 ms at 20 Hz). The onset latency of excitatory responses ranged from 1.1 to 15.2 (5.2 + 0.8) ms in the DVN and 1.3-11.7 (3.8 + 1.9) ms in the NA. The evoked discharges of 13 DVN and 3 NA units induced by the NTS stimulation with 1.5-2 times threshold intensity, determined by a decrease in heart rate (5-10 beats/min), were significantly inhibited during prolonged electrical stimulation of the SP with low intensity for 10 min (0.1-0.3 mA, Fig. 2). In 2 DVN neurones, the evoked discharges were completely abolished. The inhibitory effect often took place in 1-6 min and still persisted even more than 10 min after cessation of stimulation. In contrast, the evoked discharges of 3 DVN neurones and 1 NA neuron were unaffected following the SP stimulation. In 6 vagal cardiomotor neurones (4 DVN and 2 NA units), the same results were obtained by DP stimulati::)n with high intensity (0.6-0.8 mA), but not with low intensity (0.1-0.3 mA, Fig. 2). In another group of ai~tidromically activated neurones (12 DVN and 2 NA units), which also fired spontaneously but without cardiac rhythm, the evoked discharges were unaffected by stimulation of either the SP with low intensity or the DP with high intensity. Measurements of the conduction velocity of peroneal nerve fibres revealed that the A~ (96-100 m/s), A/3 (40-44 m/s) and A,5 (18-22 m/s) afferents were activated by stimulation of DP with low intensity, but the C-fibres (< 1 m/s) were only activated by stimulation of SP with low intensity or DP with high intensity. The vagal cardiomotor neurones described in the present study were not only classified by antidromic activation of cervical vagus stimulation, they had just the properties similar to those previously described
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Fig. 2. Post-stimulus time histograms (50 sweeps and 1 ms/bin) to show the inhibitory effect of somatic inputs on the excitatory response of a vagal cardiomotor neurone in the DVN to NTS stimulation. Traces from top to bottom: before SP stimulation, 4 min during SP stimulation, 10 min after SP stimulation, 5 min during DP stimulation, 8 min after DP stimulation. Open columns indicate time of stimulation.
in the cat 1"2, dog ~' and rabbit ~. They received excitatory inputs from the contralateral NTS area, from where marked cardiovascular responses were evoked, and exhibited spontaneous activities which had a direct correlation with the heart rate. Evidence also came from the fact that they have axons in the range of B-fibres, consistent with previous observations in the cat TM, dog 3 and rabbit 4.
353 The activation of C-fibres of somatic nerve is likely to play an important role in inhibitory processes, for nerve stimulation with only C-fibre activation exerted an inhibitory effect on the excitatory responses of neurones and NTS stimulation-evoked bradycardia. The actual central pathway by which the somatic inputs exert an inhibitory effect on the excitatory responses of vagal c a r d i o m o t o r neurones to NTS stimulation is unknown. Jordan and co-workers reported that the excitatory responses of vagal neurones to aortic nerve stimulation were suppressed by a conditioning stimulus delivered to the h y p o t h a l a m u s in the cat and rabbit 5. In recent experiments (Wang, unpublished observations), we have found in rabbits that the blocking effect of somatic inputs on bradycardiac responses to NTS or aortic nerve stimulation
was greatly reduced after either electrolytic lesion or microinjection of procaine into the rostrai ventrolateral medulla. Moreover, the excitatory responses of the vagal cardiac preganglionic neurones to aortic nerve stimulation were also suppressed by a conditioning stimulus delivered to the rostral ventrolateral medulla. These results suggest that both the hypothalamus and the ventrolateral medulla are important regions in the inhibitory action. In addition, the time course of inhibitory effects observed in the present study suggests some humoral link which should be investigated further.
1 Ciriello, J, and Calaresu, F.R., Medullary origin of vagal preganglionic axons to the heart of the cat, J. Auton. Nerv. Syst., 5 (1982) 9-22. 2 Ciriello, J. and Calaresu, F.R., Distribution of vagal cardioinhibitory neurons in the medulla of the cat, Am. J. Physiol., 238 (1980) R57-R64. 3 Jewett, D.L.. Activity in single efferent fibres in the cervical vagus of the dog with special reference to possible cardioinhibitory fibres, J. Physiol., 175 (1964) 321-357. 4 Jordan, D., Khalid, M.E.M., Schneiderman, N. and Spyer, K.M., The location and properties of preganglionic vagal cardiomotor neurones in the rabbit, Pflug. Arch., 395 (1982) 244-250. 5 Jordan, D., Khalid, M.E.M.. Schneiderman, N. and Spyer, K.M., The inhibitory control of vagal cardiomotor neurones, J. Physiol., 301 (1980)54P-55P. 6 Katona, P.G.. Poitras. J., Barnett, O. and Terry, B., Car-
diac vagal efferent activity and heart period in the carotid sinus reflex, Am. J. Physiol., 218 (1970) 1030-1037. 7 Kunze, D.V., Reflex discharge patterns of cardiac vagal effe,,ent fibres, J. Physiol., 222 (1972) 1-15. 8 McAllen, R.M. and Spyer, K.M., Two types of vagal preganglionic motoneurones projecting to the heart and lungs, J. Physiol., 282 (1978) 353-364. 9 Meessen, H. and OIszenski, J., A Cytoarchitectonic Atlas of the Rhombencephalon of the Rabbit, Karger, Basel, 1949. 10 Quest, J.A. and Gebber. L.G., Modulation of baroreceptor reflexes by somatic afferent nerve stimulation, Am. J. Physioi., 222 (1972) 1251-1259. I 1 Wang, Q.. Guo, X.-Q. and Li, P., The blocking effect of somatic afferent inputs on bradycardia induced by stimulation the nucleus tractus solitarius in the rabbit. Chin. J. Physiol. Sci.. In press.
The excellent technical assistance of Mrs. G u Huizhen, Mr. Tong Chuang and Miss Z h u W e n h e is gratefully acknowledged.
350 BRE 22691
The inhibitory effect of somatic inputs on the excitatory responses of vagal cardiomotor neurones to stimulation of the nucleus tractus solitarius in rabbits Qiang Wang, Xue-Qin Guo and Peng Li Department of Physiology, Shanghai Medical University, Shanghai (People's Republic of Chh~a) (Accepted 29 September 1987)
Key words: Somatic input; Vagal preganglionic ueuron; Dorsal vagal nucleus; Nucleus ambiguus; Nucleus tractus solitarius
Experiments were done in 41 rabbits anaesthetized with urethane and chloralose, paralyzed with Flaxedil and ventilated artificially. Extracellular recordings of 142 units were made in the dorsal vagal nucleus (DVN) and the nucleus ambiguus (NA), identified by antidromic response to stimulation of the cervical vagus nerve. In total 63.5% of them exhibited spontaneous activity and 22 units (17 in DVN and 5 in NA) showed a cardiac rhythm; their ant'd'gmic conduction velocity was 3.7-12.5 m/s, which suggests their having axons in the range of B fibres. These neurones were classified as vagal cardiomotor neurones. A total of 16 DVN and 4 NA vagai cardiomotor neurones were excited orthodromically by electrical stimulation of the contralateral nucleus tractus solitarius (NTS). Electrical stimulation of the superficial peroneal nerve (SP) with low intensity or the deep peroneal nerve (DP) with high intensity which activated C fibres inhibited excitatory responses of 16 neurones (14 in DVN and 2 in NA). The other 4 neurones were unaffected by SP inputs. These results provide electrophysiological evidence for the inhibitory effect of somatic inputs on the evoked discharges of vagal cardiomotor neurones in the DVN and the NA. Quest and Gebber have reported in cats, that vagal bradycardia evoked by stimulation of both the nucleus tractus solitarius (NTS) and carotid sinus nerve (CSN) was blocked by sciatic nerve stimulation I°. The same blocking effect was also observed recently in this laboratory by prolonged stimulation of either the superficial peroneal nerve (SP) with low intensity (0.1-0.3 mA) or the deep peroneal nerve (DP) with high intensity (0.6-0.8 mA) 1~. This blocking effect is likely to occur in the central nervous system, for the SP stimulation only blocked the bradycardia evoked by NTS or CSN stimulation, but failed to affect those evoked by stimulation of the peripheral end of cut cervical vagus !1j.11. The present investigation was designed to observe the effect of somatic inputs on the vagal cardiomotor neurones within the medulla with an electrophysiological technique. A total of 41 rabbits (2.0-3.0 kg) were anaesthetized with urethane and chloralose (700 mg/kg and 35 mg/kg, i.v. respectively), paralyzed with Flaxedil (4 mg/kg, as needed) and ventilated artificially. The left
femoral artery was cannulated to measure the blood pressure. The rectal temperature was maintained at 37-38 °C. The animal's head was mounted on a stereotaxic frame in a prone position and flexed at an angle of 45 ° . The caudal portion of the floor of the fourth ventricle was exposed. A single-barrelled glass microelectrode filled with Pontamine sky blue in 0.5 M sodium ~cetate (tip diamer 1/~m, impedance 6-10 Mr2 at 1 kHz) was inserted into the right dorsal vagai nucleus (DVN) area (1 mm caudal to 2 mm rostral to the obex arid within 2 mm from the midline) or nucleus ambiguus (NA) area (1 mm caudal to 3 mm rostral to the obex and 3 - 4 mm la,eral to the midline) to record single units extiacellularly. Unit activity was amplified through a differential preamplifier and displayed on an oscilloscope for monitoring and photographing. Another unipolar stimulating electrode, consisting of an insulated stainless steel wire (0.1 mm in diameter, bared only at the tip), was inserted into the contralateral NTS area (1 mm caudal to 2 mm rostral to the obex, 0 . 5 - 2 . 5 m m lateral to the mid-
Correspondence: P. Li, Department of Physiology, Shanghai Medical University, Shanghai 200032, People's Republic of China. 0006-8993/88/$03.5(I © 1988 Elsevier Science Publishers B.V. (Biomedical Division)
351 line). Pulse-triggered histograms of the spontaneously firing neurones and post-stimulus time histograms of neurones responding to NTS stimulation (50-150 /~A, 0.2-0.3 ms and 2 Hz) were obtained by a Z-8088 processor (Shanghai Medical University) and recorded on a SJ-42 polygraph (Shanghai Medical instrumental factory). The right cervical vagus was stimulated by a bipolar electrode (0.2 ms, 50-800 ~A at 2 Hz delivered by a constant current stimulator). Either the SP or the DP was stimulated for 10-15 min through a bipolar electrode with repetitive pulses (0.1-0.3 mA or 0.6-0.8 mA, 0.5 ms and 10 Hz). In some animals, the action potentials of sciatic nerve elicited by deep and superficial peroneal nerve stimulations were analysed by a medical data processing computer (Type TQ 19, China). The recording sites in DVN or NA and stimulating sites in the NTS were marked by the injection of Pontamine sky blue dye, and the deposition of iron from the tip of the stimulating electrode~ respectively. The brain was fixed with a mixture of 1% potassium
200
ferrocyanide in 10% formaline for 4 - 7 days, 50-um frozen sections were taken and stained with Neutral red. The identification of blue spots was made with reference to Meessen's atlas ~. A total of 142 antidromically firing neurones in DVN (119) and NA (23) were recorded by electrical stimulation of the ipsilateral cervical vagus. Antidromically evoked responses were identified by (1) the "all or none" nature of the responses: (2) the constant latency at a specific frequency of stimulation: (3) its ability to follow high frequency (up to 150 Hz) stimulation and (4) cancellation of the evoked response by collision with a spontaneous spike. However, some neurones did not present spontaneous activity and could not be tested by the collision method. Of all neurones tested, 63.5% units (n = 93) exhibited spontaneous activity: 22 neurones (17 in the DVN and 5 in the NA) showed discharges synchronous with cardiac rhythm (Fig. 1). On the basis of axonal conduction velocity calculated from the latency of the recorded antidromic irapulse and the
,.-
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5 ms
Fig. 1. A: the .:ardiac rhythm of presumed cardiomotor neurones in the DVN. Upper trace: Femoral arterial blood pressure. Lower trace: pulse-triggered histogram of cardiomotor neuronal activity; 297 cycles. 5 ms bin width. B: cancellation of antidromic responsc by collision with spontaneous spike in lower trace. A total of 10 superimposed sweeps in the upper trace and a single sweep in thc middle and lower traces. Arrow indicates time of stimulation.
352 straight line horizontal distance between the recording and the stimulating electrodes, 22 units showed cardiac modulation and had axons in the range of B fibres referring to conduction velocity 3.7-12.5 m/s in the DVN and 4.1-11.6 m/s in the NA. These neurones were classified as vagal cardiomotor neurones. Most vagal cardiomotor neurones (16 in the DVN and 4 in the NA) were excited orthodromically by the contralateral NTS area stimulation with single or double shocks. Bradycardia and depressor response could be induced if the NTS area was stimulated with a repetitive pulse (<150 pA, 0.2 ms at 20 Hz). The onset latency of excitatory responses ranged from 1.1 to 15.2 (5.2 + 0.8) ms in the DVN and 1.3-11.7 (3.8 + 1.9) ms in the NA. The evoked discharges of 13 DVN and 3 NA units induced by the NTS stimulation with 1.5-2 times threshold intensity, determined by a decrease in heart rate (5-10 beats/min), were significantly inhibited during prolonged electrical stimulation of the SP with low intensity for 10 min (0.1-0.3 mA, Fig. 2). In 2 DVN neurones, the evoked discharges were completely abolished. The inhibitory effect often took place in 1-6 min and still persisted even more than 10 min after cessation of stimulation. In contrast, the evoked discharges of 3 DVN neurones and 1 NA neuron were unaffected following the SP stimulation. In 6 vagal cardiomotor neurones (4 DVN and 2 NA units), the same results were obtained by DP stimulati::)n with high intensity (0.6-0.8 mA), but not with low intensity (0.1-0.3 mA, Fig. 2). In another group of ai~tidromically activated neurones (12 DVN and 2 NA units), which also fired spontaneously but without cardiac rhythm, the evoked discharges were unaffected by stimulation of either the SP with low intensity or the DP with high intensity. Measurements of the conduction velocity of peroneal nerve fibres revealed that the A~ (96-100 m/s), A/3 (40-44 m/s) and A,5 (18-22 m/s) afferents were activated by stimulation of DP with low intensity, but the C-fibres (< 1 m/s) were only activated by stimulation of SP with low intensity or DP with high intensity. The vagal cardiomotor neurones described in the present study were not only classified by antidromic activation of cervical vagus stimulation, they had just the properties similar to those previously described
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Fig. 2. Post-stimulus time histograms (50 sweeps and 1 ms/bin) to show the inhibitory effect of somatic inputs on the excitatory response of a vagal cardiomotor neurone in the DVN to NTS stimulation. Traces from top to bottom: before SP stimulation, 4 min during SP stimulation, 10 min after SP stimulation, 5 min during DP stimulation, 8 min after DP stimulation. Open columns indicate time of stimulation.
in the cat 1"2, dog ~' and rabbit ~. They received excitatory inputs from the contralateral NTS area, from where marked cardiovascular responses were evoked, and exhibited spontaneous activities which had a direct correlation with the heart rate. Evidence also came from the fact that they have axons in the range of B-fibres, consistent with previous observations in the cat TM, dog 3 and rabbit 4.
353 The activation of C-fibres of somatic nerve is likely to play an important role in inhibitory processes, for nerve stimulation with only C-fibre activation exerted an inhibitory effect on the excitatory responses of neurones and NTS stimulation-evoked bradycardia. The actual central pathway by which the somatic inputs exert an inhibitory effect on the excitatory responses of vagal c a r d i o m o t o r neurones to NTS stimulation is unknown. Jordan and co-workers reported that the excitatory responses of vagal neurones to aortic nerve stimulation were suppressed by a conditioning stimulus delivered to the h y p o t h a l a m u s in the cat and rabbit 5. In recent experiments (Wang, unpublished observations), we have found in rabbits that the blocking effect of somatic inputs on bradycardiac responses to NTS or aortic nerve stimulation
was greatly reduced after either electrolytic lesion or microinjection of procaine into the rostrai ventrolateral medulla. Moreover, the excitatory responses of the vagal cardiac preganglionic neurones to aortic nerve stimulation were also suppressed by a conditioning stimulus delivered to the rostral ventrolateral medulla. These results suggest that both the hypothalamus and the ventrolateral medulla are important regions in the inhibitory action. In addition, the time course of inhibitory effects observed in the present study suggests some humoral link which should be investigated further.
1 Ciriello, J, and Calaresu, F.R., Medullary origin of vagal preganglionic axons to the heart of the cat, J. Auton. Nerv. Syst., 5 (1982) 9-22. 2 Ciriello, J. and Calaresu, F.R., Distribution of vagal cardioinhibitory neurons in the medulla of the cat, Am. J. Physiol., 238 (1980) R57-R64. 3 Jewett, D.L.. Activity in single efferent fibres in the cervical vagus of the dog with special reference to possible cardioinhibitory fibres, J. Physiol., 175 (1964) 321-357. 4 Jordan, D., Khalid, M.E.M., Schneiderman, N. and Spyer, K.M., The location and properties of preganglionic vagal cardiomotor neurones in the rabbit, Pflug. Arch., 395 (1982) 244-250. 5 Jordan, D., Khalid, M.E.M.. Schneiderman, N. and Spyer, K.M., The inhibitory control of vagal cardiomotor neurones, J. Physiol., 301 (1980)54P-55P. 6 Katona, P.G.. Poitras. J., Barnett, O. and Terry, B., Car-
diac vagal efferent activity and heart period in the carotid sinus reflex, Am. J. Physiol., 218 (1970) 1030-1037. 7 Kunze, D.V., Reflex discharge patterns of cardiac vagal effe,,ent fibres, J. Physiol., 222 (1972) 1-15. 8 McAllen, R.M. and Spyer, K.M., Two types of vagal preganglionic motoneurones projecting to the heart and lungs, J. Physiol., 282 (1978) 353-364. 9 Meessen, H. and OIszenski, J., A Cytoarchitectonic Atlas of the Rhombencephalon of the Rabbit, Karger, Basel, 1949. 10 Quest, J.A. and Gebber. L.G., Modulation of baroreceptor reflexes by somatic afferent nerve stimulation, Am. J. Physioi., 222 (1972) 1251-1259. I 1 Wang, Q.. Guo, X.-Q. and Li, P., The blocking effect of somatic afferent inputs on bradycardia induced by stimulation the nucleus tractus solitarius in the rabbit. Chin. J. Physiol. Sci.. In press.
The excellent technical assistance of Mrs. G u Huizhen, Mr. Tong Chuang and Miss Z h u W e n h e is gratefully acknowledged.