635
J. Physiol. (1976), 256, pp. 635-649 With 5 text-ftgure8 Printed in Great Britain
PROLONGED INACTIVATION OF CORTICAL PYRAMIDAL TRACT NEURONES IN CATS BY DISTENSION OF THE CAROTID SINUS
BY HAZEL M. COLERIDGE, JOHN C. G. COLERIDGE AND FRED ROSENTHAL From the Cardiovascular Research Institute, University of California San Francisco, San Francisco, California 94143, U.S.A.
(Received 26 August 1975) SUMMARY
1. We have investigated the effects of stimulating carotid sinus baroreceptors upon the activity of single cortical pyramidal tract cells (PTcells) in anaesthetized cats. 2. Extracellular potentials were recorded from PT-cells, which were driven orthodromically (1/sec) by stimulating thalamic nuclei (N. ventralis lateralis, N. ventralis posterolateralis) or afferent nerves in the contralateral forepaw. Baroreceptors were stimulated by inflating small balloons placed in the bifurcations of one or both common carotid arteries. 3. Distension of the carotid sinus caused a prolonged depression of the orthodromically evoked discharge of eighteen of thirty-two PT-cells, the effect ranging from a 15 % reduction in firing to complete cessation of activity (average reduction, 39 %). The depression of firing outlasted the period of balloon inflation by an average of 85 see; in some experiments inhibition persisted for as long as 2-3 min. 4. Inflation of the balloon caused a reflex fall in arterial pressure (mean decrease, 29 mmHg), pressure reverting to the control level as soon as the balloon was deflated. Single fibre recording from the carotid sinus nerve confirmed that stimulation was confined to baroreceptors and that carotid chemoreceptors were unaffected by balloon distension. 5. Depression of PT-cell activity could not be explained simply by a fall in cerebral blood flow resulting from the reflex fall in arterial blood pressure. When a comparable or greater degree of hypotension was produced by bleeding or peripheral vagal stimulation, PT-cell firing fell in a third of experiments but reverted immediately to the control level when arterial pressure was restored. Thus some factor other than a decrease in
636 H. M. COLERIDGE AND OTHERS cerebral perfusion pressure was responsible for the prolonged inhibition evoked by carotid sinus distension. 6. Our results are consistent with the hypothesis that baroreceptor input to the reticular formation exerts an ascending influence on cortical mechanisms, with prolonged inhibitory effects comparable to those previously demonstrated at spinal level. INTRODUCTION
It has long been known that the reflex effects of stimulating the carotid sinus baroreceptors are not confined to the circulatory and respiratory systems. Thus, distension of the carotid sinus or stimulation of the sinus nerve causes a decrease in muscle tone in anaesthetized animals (Tournade & Malm6jac, 1929), an effect which is probably due to reflex depression of y efferent firing to muscle spindles (Schulte, Henatsch & Busch, 1959b). Distension of the carotid sinus also diminishes the electrical signs of cortical activity in unanaesthetized dogs (Bonvallet, Dell & Hiebel, 1953), it inhibits attacks of sham rage in decorticate cats (Bartorelli, Bizzi, Libretti & Zanchetti, 1960), and even induces the appearance of a sleep-like state in the conscious dog (Koch, 1932). The present study was undertaken to determine whether the inhibitory influence of carotid baroreceptor input extends to pyramidal tract neurones in the motor cortex. METHODS
General. Cats were anaesthetized with chloralose (70 mg/kg, i.v.). During the recording of PT-cell activity, the cats were paralysed with decamethonium bromide (Syncurine, 1 mg iv.). Periodically the effect of the neuromuscular blocking agent was allowed to diminish so that the depth of anaesthesia could be assessed and adjusted. Additional doses of chloralose were given as needed to maintain abolition of the corneal reflex. The trachea was cannulated. In experiments in which PT-cell activity was recorded, the lungs were ventilated with air or 100% 02 by a Starling Ideal pump; end-tidal C02 was monitored continuously with a Beckman LB-1 gas analyser, and was maintained at 5 % by adjusting ventilation. In all 'other experiments, the cats breathed spontaneously. Femoral arterial pressure was measured with a Statham P23Gb strain gauge and was recorded by a Grass Polygraph. In some experiments we also recorded blood pressure in an external carotid artery (see below). Recording of PT-cell activity. The pharynx and upper part of the trachea were reflected, and the basioccipital bone was nibbled away to expose the medullary pyramids. The cat was placed in the prone position and its head was gripped in a conventional stereotaxic frame. The left pericruciate cortex was exposed by removing the calvarium and reflecting the dura. Drying of the exposed cortical surface was prevented by covering it with thin polythene film and by periodically applying warm saline (a solution of NaCl, 0 9 g/100 ml. at 370 C). Cortical pulsations were reduced by light pressure from a small plastic plate, which had a 2 mm hole through
PT-NEURONES AND CAROTID SINUS
637
which the micro-electrode was inserted. The drainage of cerebrospinal fluid that accompanied exposure of the medullary pyramids also helped to reduce pulsation. Activity of single PT-cells in the left pericruciate cortex was recorded extracellularly with glass micro-electrodes filled with 3 M-KC1. The electrodes had tip resistances of 5-25 MQ and tip diameters of 1 ,t or less. The potentials were led through a condenser-coupled amplifier with an over-all low frequency time constant of at least 30 msec. They were displayed on a Tektronix 565 oscilloscope and were photographed. PT-cells were identified by their antidromic response to single square-wave pulses (0.01-0.03 msec duration) applied through bipolar silver wire electrodes to the exposed ipsilateral bulbar pyramid. A unit was classified as a PT-cell if it responded to threshold stimulation with a single spike having an invariant latency of less than 10 msec, and if it faithfully followed repetitive stimulation at rates of 100/sec or greater (Patton, Towe & Kennedy, 1962). When a PT-cell had been identified, we examined the effect of various procedures (see below) upon its response to orthodromic stimulation. The cell was driven orthodromically by single square-wave pulses (0-1-0-3 msec duration) applied 1/sec (or occasionally 2/sec) through needle electrodes in the skin of the contralateral forepaw, or through bipolar electrodes placed stereotaxically in thalamic nuclei (N. ventralis lateralis and N. ventralis posterolateralis) with the aid of established coordinates (Jasper & Ajmone-Marsan, 1954). Distension of carotid sinus. It was our original intention to stimulate carotid baroreceptors by distending the vascularly isolated sinus, having restored continuity of blood flow to the external carotid artery. But because cortical bleeding made unit recording difficult after anticoagulant had been administered, we distended the sinus with a small balloon (Biscoe & Sampson 1970a). The common carotid artery was opened and the balloon on the end of a catheter (Fogarty arterial embolectomy catheter, size 4 F, Edwards Laboratories) was placed in the carotid bifurcation (see below). Inflating the balloon with 0-8 ml. air stretched the wall of the sinus and evoked a reflex fall in arterial pressure (Fig. 2). In some experiments a balloon was placed in the right carotid sinus, the left common carotid artery being left intact; in others, balloons were placed in both carotid arteries. We examined the effects upon PT-cell firing of withdrawing blood (4-15 ml.) from a femoral artery (into a syringe containing heparin) to produce a decrease in arterial pressure comparable in magnitude and duration to that evoked reflexly by distending the carotid sinus. We also mimicked the hypotensive effect of carotid sinus distension by cutting the right cervical vagus nerve and stimulating the caudal stump (20/sec, 0-1 msec pulse duration), increasing the voltage until arterial pressure had fallen to the desired level. Validation of the methods Effects of obstructing the common carotid arteries. In the cat, the cerebral hemispheres and thalamus are largely supplied by the carotid arteries, and the cerebellum, pons and medulla by the vertebral arteries (Holmes, Newman & Wolstencroft, 1958; Reneman, Wellens, Jageneau & Stynen, 1974). In some cats the vertebral arteries also contribute to the blood supply of the cerebral cortex (Reneman et al. 1974). The important question in our experiments was whether vertebral blood could adequately supply the cerebral hemispheres when one or both carotid arteries were obstructed by insertion of the balloons. Occlusion of one carotid artery causes negligible changes in regional cerebral blood flow and spontaneous cortical activity in human patients with good collateral circulation (Trojaborg & Boysen, 1973) and no change in cerebral blood flow or the e.e.g. in baboons (Eklof & Schwartz, 1969). We have been unable to find any
H. M. COLERIDGE AND OTHERS 638 description of the effects of carotid occlusion on cerebral bloodflow in cats. The cerebral hemisphere in cats is largely supplied by the ipsilateral carotid artery (Holmes et al. 1958; Reneman et al. 1974), and in our experiments, involving distension of one carotid sinus, the balloon was always placed on the side opposite to that chosen for cortical recording. Obstruction of the contralateral carotid artery caused no obvious reduction in the number of PT-cells encountered during exploration of the cortex, nor had it any discernible effect on unit responses to orthodromic and antidromic stimulation. Obstruction of both carotid arteries undoubtedly impaired cerebral blood flow in some cats; the cortex was pale, and few or no PT-cells were identified. In other cats, however, the cortex was pink, active PT-cells were encountered as frequently as with both carotid arteries intact, and stable recordings were as readily maintained. These findings probably reflect individual variations in the extent to which other channels provided alternative sources of cerebral blood flow. When the carotid arteries are occluded, the 'stump' pressure above the occlusion provides a measure of the collateral circulation (Trojaborg & Boysen, 1973). In a preliminary series of experiments on seven cats, we divided the right common carotid artery and measured the pressure in the rostral stump. Mean 'stump' pressure ranged from 75 to 102 mmHg. Occlusion of the left carotid artery caused right 'stump' pressure to fall to 29-57 mmHg. In four cats, however, 'stump' pressure then gradually increased to 78-131 mmHg (mean 99 mmHg), pointing to the development of a considerable backflow of blood into the external carotid artery from the vertebral and other arteries. In the remaining three cats, 'stump' pressure remained low (2237 mmHg), suggesting that backflow was relatively small. It seems likely that these different effects accounted for the wide variation in cortical activity observed in our experiments. Whatever the explanation, it was clearly possible in some cats with both carotid arteries obstructed to obtain recordings from single PT-cells whose general behaviour was indistinguishable from that of PT-cells in cats with the cerebral circulation intact (Tow e, Patton & Kennedy, 1963; Rosenthal, 1967, 1971). Afferent fibres stimulated by balloon distension. Our aim was to place the balloon so that it would stretch the wall of the carotid sinus and stimulate baroreceptors, but not obstruct the occipital artery which supplies the carotid body. We made several trial inflations at the beginning of each experiment while the cat was breathing spontaneously, to determine the optimal position of the balloon and to confirm that the effects of inflation were consistent with stimulation of baroreceptors only. The possibility remained, however, that repeated inflations might displace the balloon, obstructing the occipital artery and stimulating the carotid chemoreceptors. Since the cats were artificially ventilated during PT-cell recording, this might be overlooked. We therefore examined the reflex effects of inflating the balloons for 20-30 sec at 10 min intervals over a period of 3-4 hr in nine cats breathing spontaneously. Breathing was recorded with a stethograph connected to a strain gauge. In seven cats, distension of one or both balloons invariably produced a reflex fall in arterial pressure; there was either no change in breathing or a trivial reduction in rate and depth. In the remaining two cats effects were more variable. After several inflations, which produced changes consistent with baroreceptor stimulation, the pattern of response changed. Breathing now increased and, after a brief initial fall, arterial pressure rose above control level. The effects persisted until the balloons were deflated. The response to inflation reverted to its original pattern when the balloon was withdrawn a few millimetres. Balloon inflation never evoked an increase in arterial pressure in any of the experiments in which PT-cell activity was recorded, suggesting that the balloons had not been displaced.
PT-NEURONES AND CAROTID SINUS
639
In four cats, we also examined the effect of inflating the balloon upon impulse activity in baroreceptor and chemoreceptor fibres dissected from the sinus nerve. Chemoreceptors were identified by ventilating the lungs with 5 % 02 in N2. With the balloon in situ but uninflated, firing in fifteen baroreceptor fibres ranged from o to 66 impulses/sec (mean 21 impulses/sec), increasing to 29-238 impulses/sec (mean 124 impulses/sec) when the sinus was distended. Some adaptation occurred, so that after inflation had been maintained for 30 sec, firing frequency on average was only 42 % of the initial peak value. The adaptation probably contributed to the partial recovery of arterial pressure during prolonged inflation of the balloon (Results). Firing fell immediately to the control level or below when the balloon was deflated. Balloon inflation had little effect on the firing of carotid chemoreceptors. In twelve chemoreceptor fibres the discharge before inflation ranged from 0 3 to 3-4 impulses/sec (mean 1-7 impulses/sec); after 30 sec of inflation it was 0 1-4 4 impulses/sec (mean 1-8 impulses/sec).
RESULTS
Distension of carotid sinus. We investigated the effect of distending one or both carotid sinuses on thirty two PT-cells in fifteen cats. The cells were driven orthodromically by single pulses delivered at 1 see intervals. Each stimulus evoked a short train of 1-7 spikes, and changes in PT-cell firing were assessed by counting the number of spikes in each 5 see period throughout the experiment (Fig. 1). In eighteen of these cells, firing decreased when the balloons were inflated, inhibition occurring in four out of ten experiments in which one carotid sinus was distended and in fourteen out of twenty-two when both sinuses were distended. Inflation was maintained for 10-65 sec, and invariably produced a reflex fall in arterial blood pressure (Figs. 2 and 5A; see below). PT-cell firing tended to change progressively during and after balloon distension, decreasing to a minimum and then returning gradually to the control level, so that the duration of the inhibitory effect was easy to assess (Figs. 1, 2 and 5A). Measured at the point of maximum effect, the number of spikes decreased on average by 39 % (s.E. of mean + 5-1 %), the effect varying from a 15 % reduction in firing to complete cessation of activity (Fig. 2A). The reduction in PT-cell firing began within 5-20 see of balloon inflation, measurements being made to the nearest 5 sec, and in most experiments the maximum effect did not occur until after the balloons were deflated (Figs. 1, 2A and 5A). Inhibition of PT-cell firing outlasted the period of carotid sinus distension by 85 + 116 see (mean+ s.E.), indeed in five cells the control level of firing was not regained until 120-210 see after the balloons were deflated and arterial blood pressure had returned to its original level. Inhibition of firing was sometimes followed by a brief increase in activity above the control level. Inflation of the balloons after they had been withdrawn
H. M. COLERIDGE AND OTHERS
640
20
21
is
16
14
22
20
15
20
40
60
80
120
145
10 mV 5 msec
B 22
0
Ln -
0
00
V
20
-0
0.
0
0
18
0
0..
...*.... A,@
---
0-4
0
0
-
o
0
0
U
.M =u
E
16
0
00 0 00
14
1
0
2-
40
00 00
0
0
0
80 Time (sec)
120
Fig. 1. Effect upon the evoked responses of a PT-cell of distending both carotid sinuses. The cell was driven 1/sec by single shocks to the thalamus (N. entralis posterolateralis). A, each of the thirty-five panels represents the response to one stimulation (note stimulus artifact followed by brief train of two to five spikes). Each column of five panels comprises (from above downwards) the responses to five consecutive stimulations. The columns show representative recordings made during the experiment, the number at the bottom of each column indicating the time in seconds at which the last panel in that column was recorded. The numbers at the top indicate the total number of spikes evoked in each 5 sec period. B, plot of PT-cell activity (spikes/5 see). The dashed line indicates mean PT-cell activity during the control period. The balloons were inflated at 1 and deflated at 2.
into the common carotid arteries had no effect on the firing of any of the PT-cells examined. In experiments on two of the remaining fourteen PT-cells, the evoked response increased when the balloons were inflated, firing increasing by approximately 50 % and remaining above the control level for about 50 sec after the balloons had been deflated (Fig. 3). Both of these cells
PT-NE URONES AND CAROTID SINUS A 12 0° 0 0 I00 000 0 81s s ~~~~00 4 -0 o0
#A
00
641 0
000
._
........
120 co
100*
J. Physiol. (1976), 256, pp. 635-649 With 5 text-ftgure8 Printed in Great Britain
PROLONGED INACTIVATION OF CORTICAL PYRAMIDAL TRACT NEURONES IN CATS BY DISTENSION OF THE CAROTID SINUS
BY HAZEL M. COLERIDGE, JOHN C. G. COLERIDGE AND FRED ROSENTHAL From the Cardiovascular Research Institute, University of California San Francisco, San Francisco, California 94143, U.S.A.
(Received 26 August 1975) SUMMARY
1. We have investigated the effects of stimulating carotid sinus baroreceptors upon the activity of single cortical pyramidal tract cells (PTcells) in anaesthetized cats. 2. Extracellular potentials were recorded from PT-cells, which were driven orthodromically (1/sec) by stimulating thalamic nuclei (N. ventralis lateralis, N. ventralis posterolateralis) or afferent nerves in the contralateral forepaw. Baroreceptors were stimulated by inflating small balloons placed in the bifurcations of one or both common carotid arteries. 3. Distension of the carotid sinus caused a prolonged depression of the orthodromically evoked discharge of eighteen of thirty-two PT-cells, the effect ranging from a 15 % reduction in firing to complete cessation of activity (average reduction, 39 %). The depression of firing outlasted the period of balloon inflation by an average of 85 see; in some experiments inhibition persisted for as long as 2-3 min. 4. Inflation of the balloon caused a reflex fall in arterial pressure (mean decrease, 29 mmHg), pressure reverting to the control level as soon as the balloon was deflated. Single fibre recording from the carotid sinus nerve confirmed that stimulation was confined to baroreceptors and that carotid chemoreceptors were unaffected by balloon distension. 5. Depression of PT-cell activity could not be explained simply by a fall in cerebral blood flow resulting from the reflex fall in arterial blood pressure. When a comparable or greater degree of hypotension was produced by bleeding or peripheral vagal stimulation, PT-cell firing fell in a third of experiments but reverted immediately to the control level when arterial pressure was restored. Thus some factor other than a decrease in
636 H. M. COLERIDGE AND OTHERS cerebral perfusion pressure was responsible for the prolonged inhibition evoked by carotid sinus distension. 6. Our results are consistent with the hypothesis that baroreceptor input to the reticular formation exerts an ascending influence on cortical mechanisms, with prolonged inhibitory effects comparable to those previously demonstrated at spinal level. INTRODUCTION
It has long been known that the reflex effects of stimulating the carotid sinus baroreceptors are not confined to the circulatory and respiratory systems. Thus, distension of the carotid sinus or stimulation of the sinus nerve causes a decrease in muscle tone in anaesthetized animals (Tournade & Malm6jac, 1929), an effect which is probably due to reflex depression of y efferent firing to muscle spindles (Schulte, Henatsch & Busch, 1959b). Distension of the carotid sinus also diminishes the electrical signs of cortical activity in unanaesthetized dogs (Bonvallet, Dell & Hiebel, 1953), it inhibits attacks of sham rage in decorticate cats (Bartorelli, Bizzi, Libretti & Zanchetti, 1960), and even induces the appearance of a sleep-like state in the conscious dog (Koch, 1932). The present study was undertaken to determine whether the inhibitory influence of carotid baroreceptor input extends to pyramidal tract neurones in the motor cortex. METHODS
General. Cats were anaesthetized with chloralose (70 mg/kg, i.v.). During the recording of PT-cell activity, the cats were paralysed with decamethonium bromide (Syncurine, 1 mg iv.). Periodically the effect of the neuromuscular blocking agent was allowed to diminish so that the depth of anaesthesia could be assessed and adjusted. Additional doses of chloralose were given as needed to maintain abolition of the corneal reflex. The trachea was cannulated. In experiments in which PT-cell activity was recorded, the lungs were ventilated with air or 100% 02 by a Starling Ideal pump; end-tidal C02 was monitored continuously with a Beckman LB-1 gas analyser, and was maintained at 5 % by adjusting ventilation. In all 'other experiments, the cats breathed spontaneously. Femoral arterial pressure was measured with a Statham P23Gb strain gauge and was recorded by a Grass Polygraph. In some experiments we also recorded blood pressure in an external carotid artery (see below). Recording of PT-cell activity. The pharynx and upper part of the trachea were reflected, and the basioccipital bone was nibbled away to expose the medullary pyramids. The cat was placed in the prone position and its head was gripped in a conventional stereotaxic frame. The left pericruciate cortex was exposed by removing the calvarium and reflecting the dura. Drying of the exposed cortical surface was prevented by covering it with thin polythene film and by periodically applying warm saline (a solution of NaCl, 0 9 g/100 ml. at 370 C). Cortical pulsations were reduced by light pressure from a small plastic plate, which had a 2 mm hole through
PT-NEURONES AND CAROTID SINUS
637
which the micro-electrode was inserted. The drainage of cerebrospinal fluid that accompanied exposure of the medullary pyramids also helped to reduce pulsation. Activity of single PT-cells in the left pericruciate cortex was recorded extracellularly with glass micro-electrodes filled with 3 M-KC1. The electrodes had tip resistances of 5-25 MQ and tip diameters of 1 ,t or less. The potentials were led through a condenser-coupled amplifier with an over-all low frequency time constant of at least 30 msec. They were displayed on a Tektronix 565 oscilloscope and were photographed. PT-cells were identified by their antidromic response to single square-wave pulses (0.01-0.03 msec duration) applied through bipolar silver wire electrodes to the exposed ipsilateral bulbar pyramid. A unit was classified as a PT-cell if it responded to threshold stimulation with a single spike having an invariant latency of less than 10 msec, and if it faithfully followed repetitive stimulation at rates of 100/sec or greater (Patton, Towe & Kennedy, 1962). When a PT-cell had been identified, we examined the effect of various procedures (see below) upon its response to orthodromic stimulation. The cell was driven orthodromically by single square-wave pulses (0-1-0-3 msec duration) applied 1/sec (or occasionally 2/sec) through needle electrodes in the skin of the contralateral forepaw, or through bipolar electrodes placed stereotaxically in thalamic nuclei (N. ventralis lateralis and N. ventralis posterolateralis) with the aid of established coordinates (Jasper & Ajmone-Marsan, 1954). Distension of carotid sinus. It was our original intention to stimulate carotid baroreceptors by distending the vascularly isolated sinus, having restored continuity of blood flow to the external carotid artery. But because cortical bleeding made unit recording difficult after anticoagulant had been administered, we distended the sinus with a small balloon (Biscoe & Sampson 1970a). The common carotid artery was opened and the balloon on the end of a catheter (Fogarty arterial embolectomy catheter, size 4 F, Edwards Laboratories) was placed in the carotid bifurcation (see below). Inflating the balloon with 0-8 ml. air stretched the wall of the sinus and evoked a reflex fall in arterial pressure (Fig. 2). In some experiments a balloon was placed in the right carotid sinus, the left common carotid artery being left intact; in others, balloons were placed in both carotid arteries. We examined the effects upon PT-cell firing of withdrawing blood (4-15 ml.) from a femoral artery (into a syringe containing heparin) to produce a decrease in arterial pressure comparable in magnitude and duration to that evoked reflexly by distending the carotid sinus. We also mimicked the hypotensive effect of carotid sinus distension by cutting the right cervical vagus nerve and stimulating the caudal stump (20/sec, 0-1 msec pulse duration), increasing the voltage until arterial pressure had fallen to the desired level. Validation of the methods Effects of obstructing the common carotid arteries. In the cat, the cerebral hemispheres and thalamus are largely supplied by the carotid arteries, and the cerebellum, pons and medulla by the vertebral arteries (Holmes, Newman & Wolstencroft, 1958; Reneman, Wellens, Jageneau & Stynen, 1974). In some cats the vertebral arteries also contribute to the blood supply of the cerebral cortex (Reneman et al. 1974). The important question in our experiments was whether vertebral blood could adequately supply the cerebral hemispheres when one or both carotid arteries were obstructed by insertion of the balloons. Occlusion of one carotid artery causes negligible changes in regional cerebral blood flow and spontaneous cortical activity in human patients with good collateral circulation (Trojaborg & Boysen, 1973) and no change in cerebral blood flow or the e.e.g. in baboons (Eklof & Schwartz, 1969). We have been unable to find any
H. M. COLERIDGE AND OTHERS 638 description of the effects of carotid occlusion on cerebral bloodflow in cats. The cerebral hemisphere in cats is largely supplied by the ipsilateral carotid artery (Holmes et al. 1958; Reneman et al. 1974), and in our experiments, involving distension of one carotid sinus, the balloon was always placed on the side opposite to that chosen for cortical recording. Obstruction of the contralateral carotid artery caused no obvious reduction in the number of PT-cells encountered during exploration of the cortex, nor had it any discernible effect on unit responses to orthodromic and antidromic stimulation. Obstruction of both carotid arteries undoubtedly impaired cerebral blood flow in some cats; the cortex was pale, and few or no PT-cells were identified. In other cats, however, the cortex was pink, active PT-cells were encountered as frequently as with both carotid arteries intact, and stable recordings were as readily maintained. These findings probably reflect individual variations in the extent to which other channels provided alternative sources of cerebral blood flow. When the carotid arteries are occluded, the 'stump' pressure above the occlusion provides a measure of the collateral circulation (Trojaborg & Boysen, 1973). In a preliminary series of experiments on seven cats, we divided the right common carotid artery and measured the pressure in the rostral stump. Mean 'stump' pressure ranged from 75 to 102 mmHg. Occlusion of the left carotid artery caused right 'stump' pressure to fall to 29-57 mmHg. In four cats, however, 'stump' pressure then gradually increased to 78-131 mmHg (mean 99 mmHg), pointing to the development of a considerable backflow of blood into the external carotid artery from the vertebral and other arteries. In the remaining three cats, 'stump' pressure remained low (2237 mmHg), suggesting that backflow was relatively small. It seems likely that these different effects accounted for the wide variation in cortical activity observed in our experiments. Whatever the explanation, it was clearly possible in some cats with both carotid arteries obstructed to obtain recordings from single PT-cells whose general behaviour was indistinguishable from that of PT-cells in cats with the cerebral circulation intact (Tow e, Patton & Kennedy, 1963; Rosenthal, 1967, 1971). Afferent fibres stimulated by balloon distension. Our aim was to place the balloon so that it would stretch the wall of the carotid sinus and stimulate baroreceptors, but not obstruct the occipital artery which supplies the carotid body. We made several trial inflations at the beginning of each experiment while the cat was breathing spontaneously, to determine the optimal position of the balloon and to confirm that the effects of inflation were consistent with stimulation of baroreceptors only. The possibility remained, however, that repeated inflations might displace the balloon, obstructing the occipital artery and stimulating the carotid chemoreceptors. Since the cats were artificially ventilated during PT-cell recording, this might be overlooked. We therefore examined the reflex effects of inflating the balloons for 20-30 sec at 10 min intervals over a period of 3-4 hr in nine cats breathing spontaneously. Breathing was recorded with a stethograph connected to a strain gauge. In seven cats, distension of one or both balloons invariably produced a reflex fall in arterial pressure; there was either no change in breathing or a trivial reduction in rate and depth. In the remaining two cats effects were more variable. After several inflations, which produced changes consistent with baroreceptor stimulation, the pattern of response changed. Breathing now increased and, after a brief initial fall, arterial pressure rose above control level. The effects persisted until the balloons were deflated. The response to inflation reverted to its original pattern when the balloon was withdrawn a few millimetres. Balloon inflation never evoked an increase in arterial pressure in any of the experiments in which PT-cell activity was recorded, suggesting that the balloons had not been displaced.
PT-NEURONES AND CAROTID SINUS
639
In four cats, we also examined the effect of inflating the balloon upon impulse activity in baroreceptor and chemoreceptor fibres dissected from the sinus nerve. Chemoreceptors were identified by ventilating the lungs with 5 % 02 in N2. With the balloon in situ but uninflated, firing in fifteen baroreceptor fibres ranged from o to 66 impulses/sec (mean 21 impulses/sec), increasing to 29-238 impulses/sec (mean 124 impulses/sec) when the sinus was distended. Some adaptation occurred, so that after inflation had been maintained for 30 sec, firing frequency on average was only 42 % of the initial peak value. The adaptation probably contributed to the partial recovery of arterial pressure during prolonged inflation of the balloon (Results). Firing fell immediately to the control level or below when the balloon was deflated. Balloon inflation had little effect on the firing of carotid chemoreceptors. In twelve chemoreceptor fibres the discharge before inflation ranged from 0 3 to 3-4 impulses/sec (mean 1-7 impulses/sec); after 30 sec of inflation it was 0 1-4 4 impulses/sec (mean 1-8 impulses/sec).
RESULTS
Distension of carotid sinus. We investigated the effect of distending one or both carotid sinuses on thirty two PT-cells in fifteen cats. The cells were driven orthodromically by single pulses delivered at 1 see intervals. Each stimulus evoked a short train of 1-7 spikes, and changes in PT-cell firing were assessed by counting the number of spikes in each 5 see period throughout the experiment (Fig. 1). In eighteen of these cells, firing decreased when the balloons were inflated, inhibition occurring in four out of ten experiments in which one carotid sinus was distended and in fourteen out of twenty-two when both sinuses were distended. Inflation was maintained for 10-65 sec, and invariably produced a reflex fall in arterial blood pressure (Figs. 2 and 5A; see below). PT-cell firing tended to change progressively during and after balloon distension, decreasing to a minimum and then returning gradually to the control level, so that the duration of the inhibitory effect was easy to assess (Figs. 1, 2 and 5A). Measured at the point of maximum effect, the number of spikes decreased on average by 39 % (s.E. of mean + 5-1 %), the effect varying from a 15 % reduction in firing to complete cessation of activity (Fig. 2A). The reduction in PT-cell firing began within 5-20 see of balloon inflation, measurements being made to the nearest 5 sec, and in most experiments the maximum effect did not occur until after the balloons were deflated (Figs. 1, 2A and 5A). Inhibition of PT-cell firing outlasted the period of carotid sinus distension by 85 + 116 see (mean+ s.E.), indeed in five cells the control level of firing was not regained until 120-210 see after the balloons were deflated and arterial blood pressure had returned to its original level. Inhibition of firing was sometimes followed by a brief increase in activity above the control level. Inflation of the balloons after they had been withdrawn
H. M. COLERIDGE AND OTHERS
640
20
21
is
16
14
22
20
15
20
40
60
80
120
145
10 mV 5 msec
B 22
0
Ln -
0
00
V
20
-0
0.
0
0
18
0
0..
...*.... A,@
---
0-4
0
0
-
o
0
0
U
.M =u
E
16
0
00 0 00
14
1
0
2-
40
00 00
0
0
0
80 Time (sec)
120
Fig. 1. Effect upon the evoked responses of a PT-cell of distending both carotid sinuses. The cell was driven 1/sec by single shocks to the thalamus (N. entralis posterolateralis). A, each of the thirty-five panels represents the response to one stimulation (note stimulus artifact followed by brief train of two to five spikes). Each column of five panels comprises (from above downwards) the responses to five consecutive stimulations. The columns show representative recordings made during the experiment, the number at the bottom of each column indicating the time in seconds at which the last panel in that column was recorded. The numbers at the top indicate the total number of spikes evoked in each 5 sec period. B, plot of PT-cell activity (spikes/5 see). The dashed line indicates mean PT-cell activity during the control period. The balloons were inflated at 1 and deflated at 2.
into the common carotid arteries had no effect on the firing of any of the PT-cells examined. In experiments on two of the remaining fourteen PT-cells, the evoked response increased when the balloons were inflated, firing increasing by approximately 50 % and remaining above the control level for about 50 sec after the balloons had been deflated (Fig. 3). Both of these cells
PT-NE URONES AND CAROTID SINUS A 12 0° 0 0 I00 000 0 81s s ~~~~00 4 -0 o0
#A
00
641 0
000
._
........
120 co
100*
