CENTRAL CARDIOVASCULAR CONTROL I I 57 



ever, this 'vasodilator band' forms the bulbar part of 

 that sympathetic vasodilator outflow to the muscles 

 which, originating in the motor cortex, passes to the 

 spinal cord. There are no experimental data to sup- 

 port the opinion that it is an integrative vasodilator 

 area. 



The tonically active vasomotor areas of the medulla, 

 the pressor and the depressor areas, thus modulate 

 \ascular tone solely by \'arying the vasoconstrictor dis- 

 charge, as schematically outlined in figure 14. 



VASOCONSTRICTOR INHIBITION AND VASODILATOR 

 ACTIVATION : TWO FUNCTIONALLY SEPARATE 

 VASODILATOR MECHANISMS 



The vasodilatation associated with medullary vaso- 

 depressor reflexes, as pointed out above, is due to a 

 reduction of vasoconstrictor tone. This vasodilata- 

 tion is totally abolished by sympathectomy but is 

 quite unaffected by sensory denervation. Further, it 

 is abolished by sympatholytic drugs which block the 

 effect of the vasoconstrictor transmitter. Vasodilata- 

 tion due to inhibition of vasoconstrictor tone is not 

 influenced by atropine. 



In skeletal muscles vasodilatation can be produced 

 by intracerebral stimulation of sympathetic vaso- 

 dilator nerves. Atropine, as has been repeatedly 

 pointed out, completely abolishes these vasodilator 

 eff"ects since the sympathetic vasodilator fibers are 

 cholinergic. The complete subsidence of vasodilata- 

 tion shows that it is not due to inhibition of vaso- 

 constrictor tone. 



There are, accordingly, two vasodilator mecha- 

 nisms, effected via sympathetic nerves: a) inhibition 

 of vasoconstrictor tone and b) initiation of vasodilator 

 activity. Available evidence suggests that these two 

 vasodilator mechanisms are functionally separate. 

 Destruction of the depressor region in the medulla 

 oblongata abolishes the medullary vasodepressor 

 reflexes [Scott (192), Lindgren & Uvnas (153, 154)] 

 but has not the slightest effect on the sympathetic 

 vasodilator outflow in the medulla oblongata. Hypo- 

 thalamic stimulation, for instance, is still able to pro- 

 duce va.sodilatation in the skeletal muscles [Lindgren 

 & Uvnas (153, 1 54)]- The two vasodilator mecha- 

 nisins, vasodilator activation and vasoconstrictor in- 

 hibition, have never been experimentally observed 

 to be elicited simultaneously. 



Other vasodilator nerves, the vasodilator to the 



TRUNCUS SYMPATICUS 



NERVUS SPINALIS 



FIG. 14. Schematic drawing showing suprameduUary and 

 afferent influences on the medullary pressor and depressor 

 areas, and the projection of excitatory and inhibitory impulses 

 on the spinal vasomotor neurons. Sympathetic vaso- 

 constrictor fibers, .... sympathetic \asodilator fibers; Y 

 medullary pressor area, © medullary depressor area. 



tongue and to the erectile tissues of the genitals take 

 part in the local regulation of blood flow according to 

 the needs of the local activity. 



The vasodilator outflows seem to be devoid of tonic 

 activity and there is no interaction between the vaso- 

 constrictor and the vasodilator outflows. However, all 

 experiments designed to elucidate the organization 

 of the central vasomotor control have been performed 

 on narcotized animals. Anesthesia, without doubt, 

 has a strong depressive action on suprameduUary 

 synapses and neurons. The possibility' is not excluded, 

 therefore, that in the nonanesthetized animal the 



