Afferent impulses in the carotid sinus nerve (nerve of hering) during asphyxia and anoxæmia

102

SUMMARY1. Impulses were recorded in single fibres of aortic chemoreceptors of cats anaesthetized with chloralose. There was no demonstrable difference between the responses of the endings of medullated and non-medullated fibres respectively to any of the natural stimuli, such as hypoxia, reduction in blood pressure, or reduction in 02 content. This indicates that the generator processes are qualitatively and quantitatively identical at the endings of both types of fibres.2. Most of the endings were practically silent while ventilating the lungs with air. The maximum frequency of discharge averaged over 4-10 sec while ventilating the lungs with 4 % 02 ranged from 1.5 to 24 impulses/sec; in most fibres (twenty-one out of twenty-six endings) it was less than 12 impulses/sec.3. All the chemoreceptors tested were considerably stimulated following administration of 0-2 or 2 % CO at a time when the 02 content was greater than 4 ml./100 ml.4. All the chemoreceptors were markedly and rapidly stimulated following circulatory arrest while the cat was ventilated with air. This stimulation fell considerably within 3 min of circulatory arrest. Very little or no excitation followed circulatory arrest while ventilating the cat with pure N2. These results suggest that excitation following circulatory arrest is not produced by a metabolite.5. There was a remarkable difference between the sensitivities of endings of medullated and non-medullated fibres to drugs. The former were either unaffected by relatively large doses of ACh (100-200 ,ug) or phenyl diguanide, or if they were stimulated, the excitation so produced was much less than that produced in endings of non-medullated fibres. This supports the hypothesis that drugs produce their effects by an action at the regenerative regions of the endings, i.e. regions where the nerve impulse is initiated (Paintal, 1964). It also indicates that ACh is not likely to be a transmitter in the normal processes of excitation of chemoreceptors.6. A mechanism of stimulation of chemoreceptors not involving metabolites is presented.

Section: Resultscontrasting

confidence: 54%

Mechanism of stimulation of aortic chemoreceptors by natural stimuli and chemical substances

Paintal¹

1967

102

“…During periods of low arterial blood pressures the number of these impulses could be reduced by administering 100% oxygen to the animal; when an animal was bled out the discharge of small amplitude potentials continued for many minutes after death. These results are similar to those obtained from the chemoreceptors of the carotid bodies (Bogue & Stella, 1935;Landgren & Neil, 1951).…”

Section: Discussionsupporting

confidence: 90%

Chemoreceptor activity in the aortic bodies of the cat

Diamond¹,

Howe²

1956

The aortic bodies are discrete cell masses occurring in the aortic arch region; their structure closely resembles that of the carotid bodies. It is well established that the latter are chemosensory organs, the chemoreceptors and their nerves forming the afferent arc of respiratory and circulatory reflexes; similar reflexes can be initiated from the aortic arch area.The evidence assigning a chemosensory function to the aortic bodies is satisfactory however for only one cell group, that situated below the aortic arch. Indeed some workers have concluded, on the basis of pharmacological tests, that the remaining aortic bodies may not function as chemosensory structures (Comroe, 1939;Gernandt, 1946). If true, this would be surprising since all aortic bodies have essentially the same histological appearance and are supplied with nerves and a patent system of arteries and veins (Howe, 1956).The present investigation was carried out to discover whether or not the application of a suitable physiological stimulus resulted in the appearance of afferent impulses in the nerve supplying the aortic bodies located above the aortic arch on the left side. The results obtained are offered as evidence that these particular aortic bodies contain functional chemoreceptors, and that these may normally contribute to chemoreflexes in the cat. A preliminary report of this work has been published (Diamond & Howe, 1955). METHODSSix adult cats were used and were anaesthetized with chloralose (0.08-1.0 g/kg) after induction with ethyl chloride and ether. A window was made in the chest by removing part of the clavicle and the first four ribs on the left side, the animal being maintained under positive pressure artificial ventilation. The superior lobe of the left lung was also removed to allow a larger field for dissection.The cell groups investigated were those situated around the root of the left subclavian artery (Fig. 1). The bodies, which form groups 2 and 3 as described by Howe (1956), are supplied by * Present address: Department of Physiology, Queen's College, Dundee, University of St Andrews.

“…In certain respects, the direct effect of hypoxia on the foetal adrenal medulla can be compared with that on chemoreceptor tissues such as the glomus cells of the carotid body. Both types of cell contain similar, but not identical, granules (Lever, Lewis & Boyd, 1959); the chemoreceptors also respond to a lowering of oxygen tension and to cyanide (Heymans & Neil, 1958) and continue to discharge over long periods of anoxia (Bogue & Stella, 1935). Both types of cell, therefore, share many common properties, but further examination shows great differences in their reactions.…”

Section: Discussionmentioning

confidence: 99%

Factors responsible for the stimulation of the adrenal medulla during asphyxia in the foetal lamb

Comline¹,

Silver²,

Silver³

1965

138

Previous investigations on the release of catecholamines from the adrenal medulla of the foetal lamb during asphyxia showed that certain characteristic changes occurred during the development of the foetus (Comline & Silver, 1961). From relatively early in pregnancy, 80-90 days (full term 147 days), the adrenal medulla appeared to react directly to asphyxia with a secretion which contained a high proportion of noradrenaline. This response remained unaffected by section of the splanchnic nerves, destruction of the spinal cord, or by hexamethonium and it appeared to be due to the direct effect of asphyxia on the medullary cells. About 15-20 days before the end of gestation a nervous component was demonstrable, which was dependent upon the integrity of the splanchnic nerves, and the secretion then contained large amounts of both adrenaline and noradrenaline. The mechanism for this response, which at the end of pregnancy superseded the direct effect, appeared to lie in the spinal cord, since section at C 1 made little difference to the secretion.In all these experiments (Comline & Silver, 1961) asphyxia was induced by ligation of the umbilical cord, and at that time it was not practicable either to identify the exact nature of the stimuli responsible for the two types of discharge from the adrenal gland, or to determine whether a relation existed between the level of secretion and the severity of asphyxia in these foetuses. The present investigations have been concerned largely with these problems. The changes of Po2, PC02, pH, lactic acid and glucose in the foetal blood during asphyxia which accompany the discharge from the gland have been examined in lambs at different stages of gestation, and the possible factors involved in the response studied under more carefully controlled conditions. It was soon apparent that the output of catecholamines from the adrenal medulla was primarily affected by a fall in foetal arterial Po2; changes in arterial PC02, pH and lactic acid had little effect. Accurate and rapid measurement of both foetal and maternal