I 132 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY 11 



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FIG. I . Diagram of sympathetic vasomotor pathways to 

 limbs. [From Richards (183).] 



most of these fibers which makes their isolation and 

 identification very difiicuh. This deficiency in electro- 

 physiological knowledge is particularly to be re- 

 gretted since recording of cardiovascular responses 

 alone provides a very imperfect picture of the be- 

 havior of the neurons evoking them. Such responses, 

 because of their sluggishne.ss, do not reflect the eff'ect 

 of individual impulses in vasomotor nerves. In a 

 vasoconstrictor fiber an impulse frequency as low 

 as I to 2 per sec. is sufficient to produce a tonic 

 contraction of the vessel wall. 



It should be emphasized that variations in arterial 

 pressure provide only a very limited picture of the 

 peripheral vasomotor pattern. As Rein (182) often 

 pointed out, major variations in the distribution of 

 blood flow occur without any change in the arterial 

 pressure level. Thus, stimulation of the hypothalamus 

 activates the sympathetic vasodilator outflow causing 

 a manifold increase in muscle blood flow. Since cu- 



taneous and splanchnic constriction is also evoked, 

 the total peripheral resistance and the arterial pres- 

 sure are not altered (75). 



Another limitation in much experimental work 

 has been the use of anesthetized animals in which 

 results probably are distorted by the absence or dis- 

 tortion of nervous compensatory mechanisms pre- 

 sumably active in conscious animals. Supplementary 

 experiments in the absence of anesthesia are badly 

 needed. 



Thus, broadly speaking, the experimental basis o 

 our knowledge of central cardiovascular control is 

 weak in that it derives to a considerable extent from 

 indirect and often inaccurate methods which have 

 seldom lent themselves to quantitative evaluation of 

 the nervous processes mediating this control. 



EFFERENT P.^THW.AVS 



Symjiatluiu- I 'osncnnstruior .\crres 



PERiPHER.^L DISTRIBUTION. The thoracolumbar sympa- 

 thetic outflow distributes vasoconstrictor fibers to 

 blood vessels throughout the body, both on the ar- 

 terial and on the venous side. Only the capillary bed, 

 the area between the precapillary sphincters and the 

 venules, is considered to be devoid of an efferent in- 

 nervation (and also contractile elements). 



The pre-postganglionic relay stations for the bulk 

 of the preganglionic outflow to the skeletal muscles 

 and skin are in the paravertebral ganglia. The post- 

 ganglionic fibers pass in the grey rami communicantes 

 to the spinal nerves and ramify with the latter to 

 their peripheral destinations. The diagram in figure 

 I shows schematically the course of the vasoconstrictor 

 outflow to the extremities. The intracerebral relay 

 stations are not completely indicated. The proximal 

 portions of the blood vessels of the extremities, the 

 visceral vessels in the abdomen and the cerebral 

 vessels receive the main part of their vasoconstrictor 

 innervation directly from the postganglionic fibers 

 running along the vessels. For particulars of the 

 segmental distribution of the vasomotor innervation, 

 reference should be made to manuals such as that of 

 McDowall (162, 163). 



Considerable interest attaches to the repeated re- 

 ports, in recent years, of 'intermediary' sympathetic 

 ganglia. Sympathetic ganglion cells have been histo- 

 logically demonstrated in close connection with the 

 ventral roots. Postganglionic fibers from such aberrant 

 ganglion cells do not necessarily pass via the sympa- 



