I 142 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY II 



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FIG. 4. Afferent impulses tiom a single pressure receptor m 

 the carotid sinus. The discharge increased when an injection 

 of epinephrine elevated the mean arterial pressure from 55 mm 

 Hg in A to 135 mm Hg in B. Time: 0.2 sec. [From Bronk & 

 Stella (41).] 



cnce the spinal cardiovascular discharge via the 

 medulla oblongata, but the physiologic significance of 

 the observations that have been made is still ob,scure. 



a) Baroceptors and haroceplive fibers. The impulse 

 traffic in single baroceptor fibers was first recorded 

 by Bronk & Stella (41, 42) (fig. 4). They observed, 

 at normal arterial pressure levels, a rhythmic impulse 

 discharge with a ma.ximum frequency at the start of 

 systole. The frequency rapidly decreased in the in- 

 dividual \-olley of impulses and a long resting interval 

 occurred during diastole. As the mean pressure rose, 

 so did the number of active receptors increase, simul- 

 taneously with a rise in the frequency of discharge in 

 the indixidual fiiaer and a prolongation of the dis- 

 charge period. At high arterial pressures the receptor 

 discharge was continuous and the rhythmicity less 

 evident. The frequency of discharge rose from a 

 normal level of about 60 impulses to a maximum of 

 120 to 140 im|5ulses per sec. The baroceptors showed 

 onl\' very slight adaptation to pressure and were in- 

 susceptible to variations in the carbon dioxide and 

 oxygen contents of the blood. 



References to baroceptor impulses in the sinus 

 nerve usually imply the large spikes that have been 

 recorded from thick afferent fibers, von Euler and 

 co-workers (214) observed in electroneurograms from 

 the sinus nerve not only the large spikes but a number 

 of smaller ones, the frequency of which could be corre- 



lated with the pressure in the carotid sinus. By hyper- 

 ventilation of the experimental animal with pure 

 oxvgen, all chemoceptor activity could be made to 

 disappear, but small baroceptor spikes persisted. For 

 quantitati\'e reasons — the small spikes being many 

 times more numerous than the large ones — these 

 authors considered it probable that baroceptor fibers 

 of smaller diameters were of greater significance than 

 the thicker fibers for the sinus depressor reflexes. 



Landgren (144, 145) made a more detailed study 

 of the baroceptor potentials in the sinus nerve of the 

 cat and found that the potentials occurred in groups 

 with 10, 40, 50 and 100 per cent of the maximum 

 amplitude. It may be mentioned, by way of compari- 

 son, that de Castro (61, 62) found 650 to 700 fibers, all 

 of A t\pe, in sinus nerves from the cat. Of these 3.5 

 per cent had diameters exceeding 6 to 8 /j; the main 

 group of 79 per cent, diameters of 3 to 5 fi; and 17.5 

 per cent diameters of 1.5 to 2.8 fi. 



Landgren observed that the different receptor fibers 

 had .somewhat differing response ranges. The thick 

 fibers were activated by perfusion pressures of be- 

 tween 30 and 200 mm Hg, with a threshold value of 

 80 to 120 mm Hg for continuous activity. The fine 

 fibers had a somewhat larger response range, with a 

 threshold value of 120 to 150 mm Hg for continuous 

 discharge. The large baroceptor spikes are considered 

 to be produced in stretch receptors acting parallel 

 with the contractile elements, the small ones in re- 

 ceptors which may be in series with these elements. 

 Alternatively the small spikes may be elicited in 

 nerve endings squeezed between the smooth mu.scle 

 fibers in the media during distention of the wall as 

 well as active contraction of the muscle fibers. For 

 further discussion of this question, reference should 

 be made to the papers of Landgren (144, 145). Figure 

 5 shows the dependence of the spike heights in a 

 baroceptor preparation on the intrasinusal pressure. 



Aortic baroceptive fibers closely resemble in their 

 general properties those of carotid pressure receptors. 

 Their conduction rate is 33 ± 1 1 m per sec. [as shown 

 by Paintal (170)]. According to Douglas, Ritchie and 

 .Schaumann (68-70), not only A fibers but also C 

 fibers occur in the sinus and aortic nerves of the cat 

 and rabbit. Both types of fibers when stimulated pro- 

 duce depressor effects, but the functional significance 

 of the fine depressor fibers is not known; their di- 

 ameters are too small to permit recording from single 

 fibers. 



Bronk and co-workers (39) analyzed the effect of 

 depressor impulses on the frequency of discharge in 

 the svmpathetic cardiovascular outflow. Electrical 



