'34 



HANDBOOK OF PIIVSKJLOGV 



NEUROPHYSIOLOGY II 



. MAX CONSTRICTION 



%ioor 



PHYSIO- DISCHARGE RANGE 



FIG. 2. The correlation between stimulation rate and con- 

 strictor response in about 40 experiments. The spread between 

 the different experiments indicated by the traced surface. A 

 represents the aserage of 10 experiments where the constrictor 

 responses were especially marked. B indicates how the correla- 

 tion between constrictor response and stimulation rate is 

 changed when \ascular tone is reduced by vasodilator drugs. 

 [From Folkow (80).] 



innervation i.s further discussed by Gregg (109), 

 Folkow et al. (81) and von Euler (210). 



PHYSIOLOGIC PROPERTIES AND IMPULSE FREqUENCY. 



The preganghonic fibers are regarded as myehnated 

 B fibers, and the postganglionic as nonmedulated C 

 fibers. According to Maltesos & Schneider (160) 

 there is a not inconsiderable variation in fiber di- 

 ameters. On stimulation of \asoconstrictor fibers 

 in the sympathetic chain of dogs, the distribution of 

 the threshold values showed a tendency to accumu- 

 late around certain values suggestive of a grouping 

 around certain fiber diameters. 



Since the vasomotor fibers are of B and C type, 

 they might be expected a priori to show a low firing 

 frequency under physiologic conditions. Available 

 experimental data confirm that this is the case. The 

 series of beautiful experiments conducted by Bronk 

 and co-workers in the 1930's is still the most important 

 contribution that has been made to electrophysiologic 

 data on the central cardiovascular control. Bronk 

 et al. (39) determined the transmission rate in post- 

 ganglionic cardiac fibers to range from 0.6 to 1.5 m 

 per sec. The recorded action potentials varied in 

 magnitude, but not infrequently amounted to 50 

 ixv, a fact indicating a synchronous activity in groups 

 of postganglionic fibers. This grouped activity is dis- 

 cussed in further detail on page 1141. In single fiber 

 preparations from the cervical sympathetic trunk, 

 the authors observed impulse frequencies in pre- 

 ganglionic fibers ranging from i to 2 up to i o to 1 5 



per sec, a frequency tiiat was seldom exceeded. 

 Folkow (80), Girling (looj, Celander & Folkow (53), 

 Celander (51) and Folkow et al. (86) have indirectly 

 reached similar conclusions. 



Girling stimulated the cervical sympathetic trunk 

 in rabbits and found that the degree of vasoconstric- 

 tion in the ear was a function of the frequency of the 

 stimulation. The effective range of frequency was from 

 0.5 to 60 stimuli per sec, with approximately 25 per 

 sec. giving the maximum effect. The relation of re- 

 sistance to flow was such that a small change of fre- 

 quency could produce a marked change in the re- 

 sistance to flow. Girling also noted that the higher 

 the frequency of stimulation and the more pronounced 

 the vasoconstriction, the higher was the critical 

 closing pressure. 



Folkow studied the correlation between frequency 

 of stimulation and vasoconstrictor response on stim- 

 ulating the lumbar sympathetic chain and recording 

 the blood flow in the skeletal muscles of a hind limb 

 in the cat. An increase in frequency of the stimuli from 

 o to 6 impulses per sec. resulted in an almost linear 

 rise of the peripheral resistance and a vasoconstriction 

 amounting to 80 to 85 per cent of the maximal effect. 

 Virtually maximal vasoconstriction was obtained at 

 a frequency of stimuli of 10 impulses per sec (fig. 2). 

 Celander & Folkow (53) observed in cutaneous vessels 

 of the cat that 2 stimuli per sec. already increased 

 the peripheral resistance 15- to 20-fold; between 6 

 and 10 stimuli per sec. there was a steep rise of the 

 peripheral resistance; the curve then flattened and 

 maximal values of i oo-fold were found at a frequency 

 of 15 to 25 per sec. Folkow and co-workers (86) re- 

 ported a similar hyperbolic relation between the 

 frequency of stimuli and the degree of effector response 

 on stimulation of accelerator fibers to the heart, go 

 per cent of the maximal acceleration effect having 

 appeared at 8 to 10 irnpulses per sec. 



Folkow (80) further reported that a reflex vaso- 

 constrictor response elicited by occluding the common 

 carotids in the cat disappeared within 4.5 to 5 sec. 

 after release of the vessels. Vasoconstriction produced 

 by stimuli with frequencies below 6 to 8 per sec. 

 similarly disappeared within 4.5 to 5.5 sec. after 

 cessation of the stimulation. On stimulation with 

 frequencies exceeding 6 to 8 per sec, the latent period 

 for abolition of the vasoconstriction increased; Folkow 

 attributed this to a local accumulation of abnormally 

 large amounts of vasoconstrictor transmitter substance 

 which took longer to be eliminated at the nerve ter- 

 minals at the.se excessively high impulse frequencies. 

 Since an impulse frequency of 6 to 8 per sec. is the 



