CENTRAL CARDIOVASCULAR CONTROL 



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FIG. 5. The impulse activity from a pure carotid sinus baro- 

 ceptor preparation of the cat. A: Control, artificial respiration 

 with 100 per cent O2. B: Control, artificial respiration with 

 4.5 per cent O: in N3. C to H: Records of the impulse discharge 

 during a slow pressure decrease (2 mm Hg per sec). Note the 

 minimum impulse frequency between 60 and 50 mm Hg and 

 the increased discharge below 50 mm Hg. Time: 0.04 sec. 

 [From Landgrcn ( 144).] 



recordings were made from the inferior cardiac 

 nerves and in the cervical sympathetics of the cat. 

 Electrical stimulation of the aortic nerve inhibited 

 continuous firing of the sympathetic. Afferent im- 

 pulses in the aortic nerve with a frequency of 2 to 1 5 

 per sec. caused grouping of the sympathetic dis- 

 charge (fig. 6). Higher frequencies of the afferent 

 impulses led to a temporary total inhibition of the 

 tonic discharge, followed by escape from the inhibi- 

 tion. Distention of the carotid sinus had the same 

 eff'ect (fig. 7). 



Dontas (66) observed that normal splanchnic ac- 

 tivity appeared as groups of slowly conducted im- 

 pulses at the end of each carotid pressor receptor 

 discharge; continuous activity was found less fre- 

 quently. Splanchnic activity was enhanced by a de- 

 crease of pressor receptor inflow. 



The observation that afferent inhiljitory firing with 

 a suitable frequency is able to produce grouping of 

 the cardiovascular efferent impulses indicates that 

 the normal rhythmic activity of the cardiovascular 

 centers is due to pulsatile volleys of inhibitory im- 

 pulses originating from, among otiier sources, the 

 baroceptor areas. 



h) Chemoceptors and cliemoceptive fibers. The influence 

 of asphyxia on impulse traflic in the sinus nerve was 

 first reported by Heymans & f^ijiniii ( i-'i ). Quantita- 



FIG. 6. Rhythmic waves of activity (grouped impulses) re- 

 corded in sympathetic cardiac nerve and refiexly driven by 

 stimulating central end of aortic nerve at times indicated by 

 arrows. Each wave is associated with the previous stimulus 

 (interval between stimulus and the associated wave, 0.3 sec). 

 [From Bronk d al. (39).] 



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FIG. 7. Inhibition of efferent sympathetic impulses to the 

 heart by distension of the carotid sinus. Upper record: Pressure 

 in the sinus. Lower record: Sympathetic impulses. Time: 0.2 

 sec. [From Bronk et at. (40).] 



tive studies of the relation between oxygen tension, 

 carbon dioxide tension and pH, on the one hand, 

 and the chemoceptor activity on the other, were 

 soon conducted by Bouge & Stella (37), Samaan & 

 Stella (187), Zotterman (231) and others, partly with 

 a technique which allowed perfusion of the carotid 

 sinus under constant pressure but with changes of 

 the chemical composition. It was observed that the 

 chemoceptor discharges in a single fiber increased 

 with an increase of the carbon dioxide tension, with 

 a decrease of the pH and especially with a lowered 

 oxygen tension. Zotterman drew attention to the 

 relatively small height of the chemocepti\e spikes, in- 

 dicating that the cliemoceptive fibers ijelong to the 

 5 group with a transmission rate of 20 to 40 m per sec. 



Samaan & Stella (187) observed a chemoceptor 

 discharge also under normal respiratory conditions. 

 It did not disappear until the arterial carbon dioxide 

 tension was 32 to 35 mm Hg or less, von Euler et al. 

 (213) also observed chemoceptor impulses under rest- 

 ing conditions in chloralosed cats. The chemoceptors 

 first responded when hemoglobin saturation fell below 

 96 per cent. The impulse frequency then increased 

 approximately in proportion to the degree of oxygen 

 deficit in the blood. Carijon dioxide was found to 

 elicit chemoceptor activity even at alveolar tensions 

 below 30 mm Hg. 



These observations indicate that there may be a 

 chemoceptor bombardment of medullary structures 

 under pinsiologic conditions. However, since the 

 obser\ations were made on anesthetized animals. 



