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Biomedical subjects

M J Purves

Publications and source records attributed to M J Purves.

18 recordsLinked to original sources
PubMed1979

The cholinergic pathway to cerebral blood vessels. I. Morphological studies.

The application of cobalt chloride to the peripheral cut end of the greater superficial petrosal nerve (g.s.p.n.) in rats revealed that only a few fibers in the plexus of nerves on the adventitial surface of the internal carotid artery were in axonal continuity with the g.s.p.n. A similarly small contribution of cholinergic fibers to cerebral blood vessels from this nerve was suggested by the observation that section of the g.s.p.n. resulted in an insignificant reduction in the density of the AChE-staining plexus in the internal carotid and cerebral arteries and in the incidence of at most 2% degenerate terminals of those observed on the middle cerebral artery. Alternative explanations of the results are discussed: that the AChE-staining fibers are postganglionic, that the time course for degeneration is unusually slow and that non-cholinergic fibers stain non-specifically for AChE. It is concluded that a cholinergic dilator pathway is most probably carried by the g.s.p.n. but that it is not unique.

Acetylcholinesterase↗
PubMed1979

The cholinergic pathway to cerebral blood vessels. II. Physiological studies.

The effect of stimulating the greater superficial petrosal nerve (g.s.p.n.) upon retroglenoid venous blood flow has been tested in anaesthetized, paralysed and artificially ventilated rats. In 11 out of 15 tests, blood flow increased by an average of 25% with a time to peak response of 28 s. This response was abolished with the injection of atropine 0.1 mg kg-1 injected intra-arterially. With both petrosal nerves intact, the administration of 6-7% CO2 in air or 15% O2 in N2 caused average increases in blood flow of 105% and 45% respectively. These responses were not affected by bilateral section of the g.s.p.n. Similar experiments were carried out in 5 anaesthetized, spontaneously breathing rabbits in which, in addition to PaCO2 and PaO2, PO2, PCO2 and blood flow in the caudate nucleus were measured continuously using chronically implanted mass spectrometer catheters and heated thermistors. Caudate nucleus blood flow increased in response to hypoxia and hypercapnia and this response was not significantly affected by section of one or both g.s.p.n., sinus or vagus nerves. With section of sinus and vagus nerves, blood flow changed passively with arterial pressure.

Animals↗
PubMed1979

A structure in the wall of the internal carotid artery in the rat.

A structure has been found consistently in the wall of the internal carotid artery of the rat as it passes through the carotid canal. It consists of modified smooth muscle cells arranged longitudinally which exhibit abundant pinocytosis and are located close to endothelial cells, with which they make contact. These cells do not appear to be innervated. The closeness of this structure to the arterial lumen, into which it may actually protrude, suggests that it may be sensitive to vaso-active substances in arterial blood. If so, it is well place to affect cerebral blood flow.

Animals↗
PubMed1978

Carbon dioxide and venous return and their interaction as stimuli to ventilation in the cat.

1. Respiratory responses were measured in forty-seven cats, made decerebrate or anaesthetized with pentobarbitone or chloralose-urethane, to changes in the level of CO(2) infused into the inferior vena cava via an external oxygenator circuit or to changes in the volume of venous return or to the inhalation of CO(2).2. No consistent difference was found between the respiratory response to the increase in the level of CO(2) infused or CO(2) inhaled provided that the volume of venous return during both sets of tests was held constant at normal levels.3. If the volume of venous return was increased and the level of CO(2) infused maintained at levels such that V(CO2) did not increase, ventilation increased with a fall in P(a, CO2), a response lying approximately on the isometabolic curve.4. If the volume of venous return and the level of CO(2) infusion were raised together, a spectrum of intermediate respiratory responses was obtained which reproduced all those seen in earlier papers, in muscular exercise or other hypermetabolic states.5. None of the steady-state respiratory responses was significantly affected by bilateral vagotomy or section of the sinus nerves, though sinus nerve section slowed the responses to infused or inhaled CO(2) and they were then less precisely controlled.6. Additional experiments indicated that where CO(2) was infused or inhaled, the effective stimulus to respiration was an increase in mean P(a, CO2) in proportion to the CO(2) added and that the respiratory response to CO(2) was enhanced by reduced blood volume. How the changes in venous return were sensed and affected respiration, remains unclear.7. These results may explain why previous workers have obtained exaggerated respiratory responses to the infusion of CO(2) and why respiration increases rapidly at the start of exercise and is then maintained at high levels without discernible change in the chemical stimulus in arterial blood.

Animals↗
PubMed1977

The pH of brain extracellular fluid in the cat.

1. The blood supply to the medulla was determined by the injection of indian ink via the vertebral arteries. Virtually the whole medulla was supplied by penetrating vessels from the ventral surface. The highest density of small arterioles and venules was found close to the roots of XII and on the ventrolateral surface.2. The pH of extracellular fluid (pH(e.c.f.)) was measured with pH microelectrodes of tip size 1-3 mum in cortex and medulla in seventeen cats, anaesthetized with pentobarbitone or a chloralose-urethane mixture. Parallel measurements were made of the pH of c.s.f. and plasma, the DC potential between plasma and brain and ventilation or phrenic nerve discharge.3. In the majority of tests under steady conditions, the pH of e.c.f. was found to be lower than that of c.s.f. by between 0.03 and 0.08 units. No systematic pH gradients could be found to a depth of 5 mm beneath the surface of either medulla or cortex.4. When plasma P(CO2) was altered, pH(e.c.f.) changed with a latent period and speed of response related to the density of blood vessels. In vascular areas of the medulla and in the cortex, the latent period of 4 sec and the change of pH(e.c.f.) coincided with changes in ventilation. Changes in pH(c.s.f.) over the same areas were invariably slower.5. CO(2) buffering capacities were in the order plasma > e.c.f. > c.s.f. Typical values were respectively, -2.2, -2.1 and -1.6.6. The pH of e.c.f. was unaffected by the intravenous injection of H+ and only slowly by the injection of HCO(3)-. Only up to a depth of 1 mm beneath the surface was pH(e.c.f.) affected by superfusion of mock c.s.f. in the range 6.8-8.0 units. This response had a latent period of 2-3 min and was complete in 15 min.7. The pH of e.c.f. fell with hypoxia after a latent period of > 1 min and if all vasosensory nerves had been cut, pH(e.c.f.) was markedly affected by changes of blood pressure.8. These results indicate that even under steady conditions, the pH of e.c.f. and c.s.f. is not identical, that pH(e.c.f.) is more obviously affected by changes in P(a, CO2) than pH(c.s.f.) and that putative H+ sensors which drive respiratory neurones are likely to be similarly affected.

Animals↗
PubMed1975

Central and peripheral neural respiratory activity in the mature sheep foetus and newborn lamb.

Activity from respiratory neurones in the medulla, the phrenic and intercostal nerves was recorded in 25 foetal sheep, exteriorized a few days before term from ewes given a spinal anaesthetic, and from nine newborn lambs, anaesthetized with an allobarbitone-urethane mixture. In 12 foetuses, there was little or no sustained respiratory activity, central activity consisting of tonically discharging expiratory and other neurones and silent inspiratory neurones. In the remaining 13 foetuses, respiratory activity was periodic or continuous and it was possible to confirm that the motor component of the respiratory reflex was mature, that apnoea was not due to medullary depression, that foetal respiration was unaffected by chemoreceptor stimulation, by noise, by light, by electrical stimulation of the sciatic nerve and only slightly by inflation or deflation of the lungs. All these stimuli were effective shortly after birth. Occlusion of the umbilical cord caused poorly sustained gasps in the "non-breathing" foetuses and in the "breathing" foetuses, abolition of inspiratory and expiratory activity in the medulla and the onset of gasps and flattening of the electro-corticogram. Rhythmic respiration resumed after release of the cord with a latency which varied with the duration and severity of the asphyxia. This type of respiratory depression was not reflex but due to a direct, central action of hypoxia. The sequence of respiratory events at birth is discussed.

Animals↗