Ventilatory response to alterations of H+ ion concentration in small areas of the ventral medullary surface

Rostral ventrolateral medulla: selective projections to the thoracic autonomic cell column from the region containing C1 adrenaline neurons

Anterograde, retrograde, and combined axonal transport methods were used to describe the descending efferent projections of a region of rostral ventrolateral medullary reticular formation important in cardiovascular control. We have termed this region, which contains C1 adrenaline-synthesizing neurons, the nucleus reticularis rostroventrolateralis (RVL). Efferent projections from the RVL innervate all segmental levels of the thoracic intermediolateral and intermediomedial columns as shown using retrograde transport of lectin-conjugated horseradish peroxidase (HRP) or fast blue dye, and anterograde transport of either HRP or labeled amino acids. The projection is highly specific in that there are no projections to thoracic dorsal or ventral horns. This innervation corresponds to the distribution of preganglionic sympathetic neurons in the intermediolateral column. In particular, terminals surround neurons projecting to the adrenal medulla, as demonstrated by combined anterograde and retrograde transport methods at the light level. Terminals containing phenylethanolamine-N-methyl transferase (PNMT) were mapped using immunocytochemical techniques. PNMT-labeled terminals were present at all levels of thoracic intermediolateral column, in a distribution similar to that of the descending projections from the RVL. We have previously shown using double label techniques (Ross et al., '81-'83), that many of the spinal projections of the RVL originate from C1 neurons. These data support our suggestion that certain bulbospinal neurons within the RVL, in particular the C1 neurons, are crucial for tonic vasomotor control.

Pressor effect of CO2 in the rat: different thresholds of the central cardiovascular and respiratory responses to CO2

Rats were anesthetized with urethane and the vagi, aortic and carotid sinus nerves were sectioned bilaterally. Hypocapnia was induced by artificial hyperventilation with 100% O2. Administration of 5% CO2 in O2, without alteration of the respiratory rate or tidal volume, induced significant increases in mean systemic arterial pressure ( mSAP ) in rats with intact central nervous system (CNS) and after midcollicular section (36 +/- 4 and 34 +/- 2 mm Hg, respectively; mean +/- S.E.). Smaller but significant increases in mSAP (17 +/- 3 mm Hg) were induced by inhalation of 5% CO2 after section of the spinal cord at the C4 level. Ganglionic blockade with hexamethonium completely abolished the pressor response to CO2. In hypocapnic (paCO2 15.5 +/- 0.7 mm Hg) apneic rats with intact CNS, after denervation of the peripheral chemoreceptors, inhalation of 1.5% CO2 in O2 increased paCO2 to 22.3 +/- 1.2 mm Hg and mSAP by 16 +/- 1 mm Hg, but the animals remained apneic for up to 45 min of continuous CO2 administration. Higher FICO2s induced further immediate increases in mSAP and, after delays of up to 6-7 min, also a resumption of central rhythmic respiratory activity (monitored by the intercostal muscles or phrenic nerve electrogram). The paCO2 threshold for this respiratory response was 25 +/- 1 mm Hg. When rhythmic respiratory activity resumed, a further slight increase in mSAP and the appearance of respiratory modulated oscillations of the SAP were observed in most animals. When, after a period of CO2 inhalation, 100% O2 was again administered to the animals mSAP fell immediately, reaching the control level within 20-30 s, while the respiratory activity, if present, disappeared only after 1.5-2 min. From these experiments we conclude that in the hypocapnic rat, after denervation of the peripheral chemoreceptors: (1) CO2 induces a neurogenic hypertensive response even in the absence of rhythmic respiratory activity; (2) the central chemosensitive sites appear to be located in the ponto-medullary region and in the spinal cord; and (3) the central mechanisms responsible for the pressor response have a lower CO2 threshold and a much shorter latency than those responsible for the initiation of the rhythmic respiratory activity.

Effects of respiratory and (isocapnic) metabolic arterial acid-base disturbances on medullary extracellular fluid pH and ventilation in cats

Ventilation is influenced by the brain extracellular fluid (ecf) pH which is sensed by the central chemoreceptors. In the present experiments we have investigated to what extent ventilatory effects of brain ecf pH changes depend on the origin of these pH changes. With this aim we have compared the effects of 'respiratory' (via changes in PaCO2) and 'metabolic' (via isocapnic pHa changes) ecf pH changes on steady state ventilatory activity. Experiments were performed in anaesthetized (both artificially ventilated and spontaneously breathing) cats with cut sinus nerves; medullary surface ecf pH was measured with a glass electrode with a flat pH-sensitive surface. We found that ecf pH changes caused by changes in PaCO2 give rise to greater ventilatory responses than the same ecf pH changes caused by (isocapnic) changes in pHa. Moreover, within the pH ranges measured, isocapnic pHecf-ventilatory response lines at higher PaCO2 are shifted upwards compared with those at lower PaCO2 levels. It was concluded that with the present technique it is impossible to show a unique relation between ecf pH and ventilation.

Dynamics of brain extracellular fluid pH and phrenic nerve activity in cats after end-tidal CO2 forcing

Ventilation is influenced by the interstitial [H+] of the brain. The pHecf, which in turn is determined largely by ventilation (via PaCO2) is sensed by the central chemoreceptors. In order to investigate the dynamics of both pHecf and neural tidal volume, we measured in cats with cut vagi and sinus nerves the dynamic medullary pHecf changes and the associated changes in integrated phrenic nerve activity after end-tidal CO2 forcing. The medullary surface ecf pH was measured with a glass electrode with a flat pH-sensitive surface. After CO2 up-steps, the pHecf changed with a time constant of about 43 sec, after down-steps 30 sec was found. The central time constant of the neural tidal volume response was 50 sec (mode) in both cases, whereas the overall response had a (modal) time constant of 80 sec. The results indicate that pHecf dynamics and the dynamic characteristics of the central neural respiratory organization are about equally important in determining the dynamic neural tidal volume response. It is argued that when PaCO2 changes, the dynamic pHecf change is perfusion limited and macroscopically homogeneous within the brainstem. Therefore, in our view it seems that the location of the central chemoreceptors within the brainstem is of minor importance in determining the dynamic neural tidal volume response to PaCO2 changes.

Central chemosensitivity and the reaction theory

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Morphological observations on superficial medullary CO2--chemosensitive areas

Physiological investigations have indicated that the ventrolateral surface of the medulla oblongata is involved in the chemical drive to respiration. In this investigation, light and electron microscopic investigations of the 3 chemosensitive regions reveal the following. (1) Evaginations of the ventral surface abut the overlying pia mater thereby delimiting discrete compartments; invaginations of the surface delimit wide cisternae lined with basement membrane. Neuronal elements with numerous synapses, were found scattered among astrocytic processes of the marginal glia in intermediate and caudal chemosensitive areas Microvasculature are conspicuously absent from the marginal glia. Intramedullary vessels are surrounded by perivascular spaces and the endothelium shows zonulae occludentes at cell junctions. (2) Horseradish peroxidase (HRP) applied to the ventral surface diffused throughout the interstitial and perivascular compartments, into synaptic clefts and neuronal soma. Diffusion of HRP into blood vessels was blocked at zonulae occludentes. Following intravenous injection of HRP, no reaction product was found outside cerebral vasculature in chemosensitive areas. (3) In spontaneously breathing cats, 2% procaine applied to the caudal chemosensitive area resulted in respiratory depression which began with the second breath. It is proposed, that substances which stimulate or depress respiration, when applied to the ventral medullary surface, produce their effects on superficial neurons located in the intermediate and caudal chemosensitive areas after diffusion through interstitial spaces.

Neurophysiological studies on superficial medullary chemosensitive area for respiration

In cats anesthetized with chloralose-urethane (40 mg/kg chloralose; 200 mg/kg urethane) pH sensitivity of neurons in the caudal chemosensitive area on the ventrolateral surface of the medulla oblongata was examined while monitoring phrenic nerve activity simultaneously. pH was varied by superfusion of the ventral medullary surface with mock cerebrospinal fluid (CSF) of different pH (pH 7.4 control, 7.0 and 7.8). A total of 130 units from 21 cats changed their firing rate in response to CSF-pH changes. These were subdivided into 3 groups. In Group I, 31 respiratory pH sensitive units increased their firing rate in response to decreased mock CSF-pH, noxious pinch, joint movement in contralateral forelimb, and increased inspired CO2. These responses may have originated from respiratory center neurons. Group II consisted of 59 non-respiratory pH sensitive units whose firing rate changed in an inverse manner with CSF-pH changes. Of these, 30 responded to contralateral distal forelimb movement, 15 to hair manipulation, 9 to heavy pressure and 5 to noxious pinch. Increased inspired CO2 (rebreathing) did not modify activity. The response to pH is believed to be from non-specific neurons. Group III consisted of 40 non-respiratory pH sensitive units responding to CSF-pH changes and to increased inspired CO2. The firing rate was irregular, the interval distribution approaching an exponential function. It may be postulated that the impulse frequency of chemosensitive impulses may be irregular at the site of impulse generation, the irregularity decreasing by convergence during transmission to the respiratory centers. The time course of Group III chemosensitive units was similar to phrenic nerve responses.

Effects of medullary area I(s) cooling on respiratory response to chemoreceptor inputs

The effect on respiration, as measured by phrenic nerve activity, of bilateral graded cooling of the intermediate, or I(s), areas of the ventral medulla was determined in anesthetized, vagotomized, glomectomized and paralyzed cats. In addition the effect of cooling the I(s) areas on the responses to central and peripheral chemoreceptor afferent test stimuli were studied. When end-tidal PCO2 was kept constant, graded cooling of the I(s) areas led to graded reductions of phrenic activity and arterial pressure. Furthermore, the respiratory response to test stimuli (carotid sinus nerve or CO2) was decreased progressively during graded cooling of the I(s) areas from 40 degrees C to 20 degrees C. We conclude that area I(s) is part of a common pathway for afferent input from both the central and peripheral chemoreceptors and that it is involved in the initial integration of both inputs.

Morphology of the medullary chemosensitive fields. 1. Mapping of the neuronal matrix by a horseradish peroxidase technique

A modified horseradish peroxidase labelling technique was used to study the distribution pattern of neurons in the central chemosensitive fields of the medulla oblongata of cats. In several cryosectioned medullae a mapping of superficially located HRP-labelled neurons was achieved. The distribution and configuration of the labelled neurons indicate that most of them belong to the nucleus paragigantocellularis lateralis. However, by varying the time of incubation it was possible to identify different types of neurons. On the basis of certain aspects of the HRP incorporation mode and neuronal topography a specific type of small-sized neurons has been identified. Some functional implications of these small neurons with respect to their possible chemosensitive activity are discussed.

Ventilatory responses to hypocapnic vertebral artery perfusion in intact and carotid body denervated dogs

The ventilatory responses to step changes in vertebral artery PCO2 were investigated in intact and carotid body denervated dogs. The steady-state ventilatory responses of the denervated dogs were less than those of intact dogs. However, when expressed as a ratio to the control ventilation there was no difference between the two groups. While the arterial PCO2 was held at 56 mm Hg by adding CO2 to the inspired air the perfusion of the vertebral arteries was switched from the dog's own arterial supply to hypocapnic blood. The ventilation of the denervated dogs decreased at a faster rate (half time = 130 +/- 9 sec) to a level less than the room air control ventilation. The ventilation in the intact dogs decreased at a slower rate (half time = 184 +/- 23 sec) and was maintained above the room air control level after ten minutes of hypocapnic perfusion. Increasing the medullary blood flow, as measured with radiolabeled microspheres, augmented the rate of decline of ventilation in intact dogs. We conclude, (1) the influence of the peripheral chemoreceptors appears to increase as central drive is decreasing, and (2) the remaining time course of the decrease in ventilation is related to the rate of brain stem perfusion.

Is the central inspiratory activity responsible for pCO2-dependent drive of the sympathetic discharge?

Out of 27 cats anesthetized with chloralose-urethane mixture, paralyzed, vagotomized and artificially ventilated, phrenic nerve response to systemic hypercapnia (7-8 vol.% CO2/O2 mixture) was accompanied by an increase in blood pressure and sympathetic discharge in 19 cats. Out of these 19 cats, 12 were totally debuffered and in the remaining 7 cats one carotid sinus nerve was left intact. Single unit activity in the sympathetic cervical nerve and spontaneous mass activity in the cervical, splanchnic, renal sympathetic and phrenic nerves were recorded. Evoked response in the phrenic nerve was produced by electrical stimulation of the descending bulbospinal inspiratory pathways in the midplane area of the medulla or in the ventrolateral cervical spinal cord. Starting from the control mean end-tidal CO2 concentration of 4.7 vol.% (+/- 1.0 S.D.) a progressing hypocapnia was induced by hyperventilation up to the end-tidal CO2 concentration of 1.3-3.2 vol.% (mean 2.4 vol.% +/- 0.5 S.D.) significantly below paCO2 apneic threshold. In chemo- and baroreceptor denervated cats with a pressor and excitatory sympathetic response to hypercapnia, a hypocapnia resulted in a fall of the arterial blood pressure (mean 16.9 mm Hg +/- 7.5 S.D., 2.2 kpa +/- S.D.). With the increasing paCO2 over the period of hypocapnic apnea a pressor and excitatory sympathetic response preceded, in all experiments, the onset of the phrenic nerve rhythmic activity. The difference between paCO2 threshold for the pressor and sympathetic response (35.7 mm Hg +/- 3.6 S.D., 4.7 kpa +/- 0.5 S.D.) and paCO2 threshold for the reappearance of the phrenic nerve rhythmic activity (43.6 mm Hg +/- 2.6 S.D., 5.8 kpa +/- 0.3 S.D.) was highly significant. If apneic hypocapnia was combined with the continuous stimulation of the afferent fibers of the superior laryngeal nerve the CO2 threshold for phrenic rhythmic activity was significantly increased whereas CO2 threshold for the pressor and sympathetic excitatory response remained unchanged. CO2 administration during hypocapnia apnea caused a progressing reduction of the magnitude of the evoked phrenic nerve response. From these findings it is concluded that the central excitatory effect of CO2 on the sympathetic activity may be accomplished in the absence of the rhythmic respiratory activity and independently of the subthreshold tonic inspiratory activity. Pressor and sympathetic excitatory response to CO2 observed during hypocapnic apnea is presumably caused by a neuronal pool different from that responsible for the central inspiratory activity. It is suggested that this CO2 sensitive neuronal mechanism might be involved in the central generation of sympathetic tone.

Cardiovascular control by medullary surface chemoreceptors

The cardiovascular and respiratory effects of superfusion of the ventral surface of the medulla with acid hypercapnic or alkaline hypocapnic solutions have been studied in anaesthetized, paralyzed, artificially ventilated cats. Peripheral chemoreceptor and baroreceptor denervation was achieved by section of carotid sinus, aortic and cervical vagus nerves. Systemic arterial and central venous pressure, hindquarters blood flow, heart rate and phrenic nerve activity were recorded. Acid hypercapnic (pH 6.8, pCO2 85 mm Hg) superfusion caused increases in systemic arterial pressure, phrenic nerve activity and heart rate, and a decrease in hindquarters blood flow. Alkaline hypocapnic (pH u.i, pCO2 less than 10 mmHg) superfusion caused opposite effects. These experiments indicate a significant role of the chemoreceptors of the ventral surface of the medulla in cardiovascular control.

Projections to the spinal cord from neurons close to the ventral surface of the hindbrain in the rat

Neurons located within 100 micron of the ventral surface of the hindbrain were labeled following injections of horseradish peroxidase into the thoracic and lumbar spinal cord in the rat. Labeled neurons were distributed into two groups, one ventrolateral to the inferior olive and ventral to the facial nucleus, and one ventral to the raphe pallidus. The location of the ventrolateral subpial group is very similar to that of the brain stem chemosensitive zones, and suggests that the neurons in this group may be important in cardiovascular regulation.

The ventromedullary hypotensive effect of muscimol in the anaesthetized cat

Muscimol, a rigid analogue of GABA, was injected into the CNS of urethane-anaesthetized, normotensive cats. Bilateral microinjections of a small dose of muscimol (100 ng/kg, 0.5 microliter on each side) in the nucleus reticularis lateralis (NRL) induced hypotension. The marked fall in blood pressure obtained by injecting 1 microgram/kg of muscimol unilaterally into the NRL is completely reversed by subsequent local administration of bicuculline (5 microgram/kg, 0.5 microliter), a specific GABA antagonist. These data confirm that a GABAergic inhibitory system modulates the pressor tonic structures localized in the region of the NRL and that the anteroventral part of the medulla oblongata includes a trigger zone for the hypotensive action of muscimol.

The respiratory role of the ventral surface of the medulla studied in the anaesthetized rat

1. The respiratory role of the ventral surface of the medulla was studied in rats anaesthetized with a urethane-chloralose mixture. 2. In fifty-eight studies on twelve animals, direct superfusion of the medullary surface with artificial c.s.f. made acid by the reduction of bicarbonate content or by the increase of PCO2 produced no significant stimulation of respiration provided that the temperature of the brain surface was unaltered. 3. Superperfusion of the medullary surface with c.s.f. of low bicarbonate content produced an inhibition of respiration in fourteen of thirty-eight experiments. 4. Electrical stimulation on the surface revealed a localized area lateral to the pyramids and rostral to the XIIth nerve where stimulation at low intensity produced an increase in the frequency and depth of respiration. 5. The application of carbachol to a similar region increased both the frequency and amplitude of ventilation at lower concentrations than were required to obtain effects from surrounding areas. 6. Sudden switching between perfusates at different temperatures produced changes of ventilation within 1-2 sec of a change of surface temperature. The Q10 for the ventilation/temperature relationship was approximately 6. 7. The experiments confirm that the ventral surface of the medulla contains neural elements which, at least during urethane-chloralose anaesthesia, have a significant effect on respiration. The stimulus for these effects in the rat does not appear to be a change in H+ concentration. It appears more probable that the primary role of the area lies in the link between thermal and respiratory regulation.

Respiratory effects of gamma-hydroxybutyric acid in anesthetized rats

Rats lightly anesthetized with halothane were treated with graded intraperitoneal doses of gamma-hydroxybutyric acid (GHBA), a GABA analogue. The drug induced a dose dependent decrease in minute ventilation, mainly due to reduced respiratory frequency. A reduced pH in arterial blood was recorded. GHBA also blunted or abolished the respiratory response to CO2 exposure in a dose-related way. Picrotoxin (0.25, 0.5 or 1.0 mg/kg intravenously), a presumed GABA antagonist did not significantly change the respiratory pattern when given alone but clearly antagonized the GHBA-induced respiratory depression. It is concluded that GABA-ergic mechanisms are involved in central respiratory control.

Cardiovascular effects elicited from the ventral surface of medulla oblongata in the cat

Cardiovascular adjustments induced by topical application of drugs on a restricted area on the ventral surface of the medulla oblongata, corresponding to the caudal part of the rostral chemoreceptor area and the intermediate area, have been studied in chloralose-anesthetized cats. Topical application of GABA or glycine on these structures resulted in blood pressure fall, bradycardia, vasodilatation in the kidney and the skeletal muscles and also depression of respiration. Similar responses except for a slight tachycardia occurred with application of physostigmine. Application of GABA resulted in a marked attenuation of the reflex vasoconstrictor responses to removal of arterial baroreceptor restraint (carotid occlusion), particularly in the kidney, and to disappearance of the reflex renal vasodilatation to baroreceptor stimulation. The findings suggest that GABA application leads to a general diminution of the tonic vasomotor neuron activity, and with regard to renal vasomotor neurons a virtual cessation. Atropine methylnitrate application induced blood pressure rise, increased peripheral resistance in both skeletal muscle and kidney and a strongly potentiated renal vasoconstrictor response to carotid occlusion. The results indicate that the studied superficial medullary structures play an important role for the maintenance of tonic vasomotor neuron activity, especially renal.

A cholinergic mechanism involved in the respiratory chemosensitivity of the medulla oblongata in the cat

1. Cholinomimetic and adrenomimetic substances were tested on the chemosensitive zones of the ventral surface of the medulla oblongata using a plexiglas ring method. Tidal volume and respiratory frequency, arterial pressure and heart frequency were observed. 2. The increase of ventilation and the depression of arterial blood pressure by locally applied acetylcholine could be blocked by previous local application of atropine. It is therefore assumed that the acetylcholine receptors have muscarinic properties. 3. Nicotine in a small dose raises arterial pressure and with higher doses a drop is observed. The responses of respiration and of arterial pressure to nicotine were blocked by previous intravenous administration of hexamethonium. 4. Local application of atropine in the caudal (L) and rostral (M) chemosensitive zones reduced resting ventilation and the slope of the ventilatory response to CO2-inhalation. Physostigmine in these areas enhanced resting ventilation leaving unchanged the slope of the ventilatory response to CO2-inhalation. 5. With high concentrations of (L)-noradrenaline and (L)-adrenaline a slight increase of arterial pressure was seen while serotonin caused a drop. 6. These results together with those of Fukuda and Loeschcke (1978) suggest that a cholinergic transmission in the surface layer of the ventral medulla is a component in the respiratory and circulatory control systems.

Topography of the respiratory and circulatory responses to acetylcholine and nicotine on the ventral surface of the medulla oblongata

1. Acetylcholine and nicotine were superfused on the ventral medullary surface between the ponto-medullary border and C1 in anaesthetized cats in order to determine the topical distribution of their actions on respiration and circulation. 2. Acetylcholine (10(-4) g . ml-1 = 5.5 . 10(-4) mMol . ml-1) produced an increase in respiration and a lowering of blood pressure. The magnitude and the time course of the responses varied according to the points of superfusion on the surface. 3. Nicotine (10(-4) g . ml-1 = 6.2 . 10(-4) mMol . ml-1) elicited hyperventilation and more often an increase in arterial pressure on unilateral superfusion of the surface. In some cases, however, a drop in blood pressure was also observed. 4. The responsive regions of the surface on which nicotine acted and elicited hyperventilation, bear a close resemblance to the regions responsive to acetylcholine. 5. The topographical distribution of the respiratory effects elicited by the above-mentioned drugs were similar to the distribution of the responses to changes in pH on the ventral medullary surface or to electrical stimulation. 6. Procaine (2 . 10(-2) g . ml-1 = 7.3 . 10(-2) mMol . ml-1) applied bilaterally in the intermediate zone (S) caused profound inhibition of respiration and of arterial pressure. Procaine at this concentration also inhibited respiratory hyperventilation caused by nicotine (10(-4) g . ml-1 = 6.2 . 10(-4) mMol . ml-1) applied to the caudal and rostral areas.

Elimination of central chemosensitivity by coagulation of a bilateral area on the ventral medullary surface in awake cats

Breathing and respiratory response to CO2 were observed in 6 awake cats and 1 control before and after bilateral coagulation of the formerly described area S (Schläfke and Loeschcke, 1967) on the ventral medullary surface under hyperoxic conditions. Ventilation decreased, PCO2 rose and CO2 response was almost or completely abolished in 4 cats, and moderately reduced in 2 cats. Inhalation of CO2 had an inhibitory effect on ventilation in two cases. In some instances the respiratory frequency was increased by CO2. Periodic breathing as well as spontaneous hyperventilation elicited by 'arousal' indicate parallels to the Pickwickian or Ondine's curse syndrome. No respiratory changes were produced by a lesion on the pyramidal tract medial to the area S. It is concluded that central chemosensitivity can be eliminated within the superficial layer of the area S. The loss of CO2 response seems to be correlated with complete destruction of the superficial nerve cells located within the area S (Petrovický, 1968) and degeneration within the ventral part of the nucleus paragigantocellularis.

Influence of the CSF bicarbonate concentration on the ventilatory response to CO2 in relation to the location of the central chemoreceptors

In anaesthetized cats, in which the cerebrospinal fluid bicarbonate concentration was varied by a ventriculocisternal perfusion technique, the ventilatory response to CO2 during hyperoxia could be satisfactorily described by VE = S(PCSFCO2 -B). Both the slope S and the intercept B were positively and linearly related to the CSF bicarbonate concentration. Assuming that the PCSFCO2 is equal to the PCO2 in extracellular fluid, it can be shown that VE is a linear, but not a unique function of the [H+] at the site of the chemoreceptors; the slope of this relation varies with the bicarbonate concentration at that site, possibly due to chemical complex formation between HCO-3 and Ca2+ or Mg2+. Changes in the B-value were related to the location of the central chemoreceptors with the models of Pappenheimer and Berndt aand their coworkers. It was found that changes in the CSF bicarbonate concentration are reflected for 60 per cent at the site of the central chemoreceptors, and that this was independent of the cerebral perfusion. Using Berndt's model a distance between CSF and central chemoreceptors of approximately 100 micron was found; this calculated distance is relatively insensitive to relationship (logarithmic or not) between ventilation and H+ concentration and to changes in cerebral perfusion, owing to the approximate nature of the diffusion model.

On the transmission of the stimulating effects of carbon dioxide to the muscles of respiration

1. Electromyography was used to measure the response of the diaphragm and intercostal muscles to CO2 in artificially ventilated decerebrate cats. 2. Hypocapnia produced tonic activity in either inspiratory or expiratory muscles or both, according to the preparation. 3. A graded effect of CO2 on both rhythmic and tonic activity was observed and for the latter this could be seen at as low as 10 torr PA,CO2. 4. In one human subject tonic firing of expiratory motoneurones was also induced by hypocapnia and this activity showed a graded increase with increasing (CO2. 5. A saggital incision of the medulla aimed at interrupting inspiratory bulbospinal axons abolished activity in inspiratory muscles and at eupnoeic levels of CO2 converted the activity of expiratory muscles from a periodic to a topic firing pattern. 6. Following such lesions the threshold for rhythmic excitation of expiratory muscles was elevated and this revealed that the graded effect of CO2 on tonic expiratory activity extends to as high as 60 torr. 7. The tonic activation of respiratory muscles in response to CO2 ceased after cervical cord transection or when the saggital incision in the medulla was extended caudally to the first cervical segment. 8. It is concluded that the CO2 dependent activation of spinal respiratory motoneurones is conveyed by bulbospinal axons which decussate in the vicinity of the obex and that this activation can be rhythmic or tonic. 9. It is suggested that the rhythmic excitation of expiratory muscles derives from a periodic inhibition of expiratory bulbospinal neurones which are subjected to a tonic CO2 dependent excitation which is continuously variable over the physiological range.

CO2-dependent component of the neurogenic vascular tone in the cat

Complete vascular isolation of the hindlimbs was performed in vagotomized cats under Sodium Pentobarbital anesthesia. The hindlimbs were perfused at constant flow with blood kept at a constant and physiological PO2, PCO2, and pH values by means of a specially designed pump-oxygenator system. The animals were hyperventilated with different CO2 mixtures (0%, 5%, 7% and 10% in O2) thereby changing blood gases and pH levels in the upper body but not in the hindlimb vascular bed. At body PaCO2 (mm Hg) of 13.7 +/- 1.0 (means +/- SE), 30.6 +/- 1.05, 40,4 +/- 0,9 and 58.4 +/- 2.9 the hindlimb perfusion pressure (mm Hg) was, respectively 124 +/- 7.6, 138 +/- 7.4, 156 +/- 11.9 and 187 +/- 15.1. These changes in perfusion pressure were still present after complete peripheral chemoreceptor denervation but were abolished after section of the spinal cord at the T5 level. Since hindlimb perfusion pressure fell when body PaCO2 was lowered below physiological levels it is concluded that part of the neurogenic vascular tone of the hindlimbs is maintained by a CO2 mediated stimulation of supraspinal structures.

Arterial blood gases during raised intracranial pressure in anesthetized cats under controlled ventilation

✓ In anesthetized paralyzed cats ventilated with air, blood gases were analyzed repeatedly before and during episodes of raised intracranial pressure (ICP). The ICP was raised by infusion via the lumbar subarachnoid or intraventricular route, and increases were maintained for at least 30 minutes. A minor degree of hypoxemia commonly developed, but was always associated with hypercapnia; normoxia was restored by increasing the ventilation sufficiently to restore normocapnia. Relative under-ventilation is thus liable to develop if the minute volume is maintained constant when ICP is raised, probably because of increased metabolic rate which may be associated with a rise in temperature; there is no evidence to implicate more obscure causes of hypoxemia in this circumstance. Pulmonary hemorrhage and edema were found post mortem in nine of 20 animals, but only two of these had developed greater hypoxemia than could be accounted for by under-ventilation. Phrenic electroneurograms were recorded, and respiratory activity was shown to persist during prolonged periods of cerebral ischemia.

Relationship between the ventromedullary clonidine-sensitive area and the posterior hypothalamus

The connections between the areas 'S' which have been previously described as the ventromedullary sites of the action of clonidine and the posterior hypothalamus have been investigated. Superficial electrocoagulation of the left area 'S' suppresses the pressor response to electrical stimulation of the homolateral part of the posterior hypothalamus. Although such medullary lesions cause a significant reduction of the mean arterial pressure, the contralateral hypothalamic stimulation can still increase blood pressure. Clonidine it self applied topically (8 micrograms/kg) to the ventral face of the brain stem decreases the blood pressure response to liminal hypothalamic stimulation. It is concluded that efferent pathways, which are involved in vasomotor regulation, originate in the posterior hypothalamus and run through the ventrolateral part of the brain stem. The mechanism of the blocking effect of clonidine on these pathways is discussed.

Carbon dioxide sensitivity of the central chemosensitive mechanisms. An exploration by direct stimulation in rats

In urethane-anaesthetised adult albino rats ventral surface of the brainstem was stimulated chemically by increasing the local CO2 concentration and electrically. Two areas were demarcated on the ventral surface of the brainstem, one which showed an increase in pulmonary ventilation on chemical and electrical stimulation, and another which showed a decrease in pulmonary ventilation and sometimes even respiratory arrest. EEG activity recorded from the area from where increased pulmonary ventilation was obtained showed a synchronous slow wave activity during chemical stimulation and inhalation of a CO2-air mixture. This area is situated 0.5--1 mm lateral to the mid-line extending up to the rootlets of the VIIth to IXth cranial nerves. The response increased proportionately on increasing the strength of the chemical stimulus, till it reached a plateau. In carotid body denervated and chronic hypoxic animals, the magnitude of the responses was shown to be increased, probably due to increased sensitivity of the central chemosensitive mechanisms.

Metabolic control of respiratory neuronal activity and the accompanying changes in breathing movements of the rabbit. 1. Mainpulation of inspiratory and expiratory-inspiratory neurons

The property of the neuronal membrane to be permeable to metabolic modifiers of two regulatory enzymes has been utilized to manipulate the spike activity of inspiratory (I) and expiratory-inspiratory (EI) neurons of the bulbar respiratory centre. The neurons have been classified according to their response to lung distention or collapse (alpha- or beta-type) and to hyperventilation (tonic firing denoted by "+", cessation of activity by "-"). Using extracellular microelectrodes for single unit recording, the medulla oblongata was superfused with a metabolite-containing CSF. The various neuronal sub-types exhibited a differential activating or inhibitory response to one or several metabolic effectors. For example Ialpha+ units were activated by 5 mM glucose-6-phosphatase (G-6-P) and 3.5 mM 3-phosphoglycerate (3-PGA), which both inhibited Ibeta+ neurons, while 5 mM AMP inhibited Ialpha+ much more strongly than Ibeta+ cells. The spike density of Ialpha- and Ibeta- neurons was increased in the presence of 2.5 mM fructose-6-phosphate and 3.5--5 mM AMP, but became reduced by G-6-P. In contrast, 3 mM fructose-1,6-diphosphate and 5 mM 3-PGA activated the Ialpha- but inhibited the Ibeta- neurons. The EIbeta units were characteristically activated by 10 mM citrate, which inhibited all I-type neurons. Activations of the Ialpha and Ibeta neurons led to an accelerated respiratory rate and a higher tidal volume, while the opposite was true for EIbeta neurons. Intravenous injection of metabolites could not duplicate the striking effects under local applications.

pH sensitivity of cells located at the ventrolateral surface of the cat medulla oblongata in vitro

pH sensitivity of cells located at the ventral surface of the medulla oblongata was examined in a thin brain slice of the cat in vitro and the following results were obtained: (1) Transmembrane potential of the surface cells located in the area medial to the XIIth cranial nerve was reduced slightly by application of low pH solution; (2) In the rostral part of the area medial to the XIIth cranial nerve regular neuronal discharges could be observed extracellularly. The rate of firing of these cells was increased by lowering the external pH. These results were considered to support the idea the H+ receptor cells may exist in the surface layer of the ventral medulla.

Effect of intermittently raised intracranial pressure on breathing pattern, ventilatory response to CO2, and blood gases in anesthetized cats

In anesthetised cats, breathing pattern, blood gases, and ventilatory response to CO2 were recorded before and during intermittent 10-minute episodes of hydrostatically raised intracranial pressure. The first effect on breathing was a stimulation which was followed at higher pressures by irregularity, depression, and periods of apnea; hyperventilation at high intracranial pressure (ICP was rare. Raised ICP did not consistently depress the ventilatory response to CO2 until ventilation during airbreathing was already depressed; therefore, we cannot experimentally justify applying this test clinically to detect incipient ventilatory depression. When hypoxemia developed during raised ICP, it was compatible with the degree of hypoventilation due to central depression of breathing; thus, there was no evidence of a neurally mediated effect on the lungs, causing defective gas exchange.

Role of the ventral surface of the brain stem in the hypotensive action of clonidine

The areas S of the ventral surface of the brain stem and the immediately surrounding zone were superficially destroyed by the means of electro-coagulation, in 14 cats. This destruction produced a drop in blood pressure, which was transient in 9 and definitive in 4 animals; in one cat only the arterial pressure did not change after the destruction. In 6 animals which have been sham-operated, clonidine (15 mug/kg, i.v.) always induced a marked fall in blood pressure whereas in 10 animals which had maintained or recovered a normal blood pressure after the destruction of the area S, clonidine (15 mug/kg) injected intravenously no longer produced any decrease of the arterial pressure. These results suggest that the integrity of the areas S is necessary for the development of the hypotensive action of clonidine. This hypotensive drug may act, at least at the level of the ventral surface of the brain stem, through inhibition of a vasopressive structure.

The control system regulating breathing in man

This review takes the regulation of breathing in man as an example of a highly developed and much studied control system in higher mammals.