CENTRAL CARDIOVASCULAR CONTROL 



II47 



the medulla and hypothalamus may be increased in 

 anoxia {35, 95). However, further experiments with 

 techniques permittino localization of the oxygen 

 deficiency in the central nervous system specifically 

 are needed. 



Mesencephalon 



We possess little knowledge of the significance of 

 the mesencephalon in vasoconstrictor nervous activity. 

 Both early investigators, using somewhat primitive 

 techniques, and more recent workers employing the 

 Horsley-Clarke technique have, it is true, observed 

 pressor and depressor responses to stimulation of 

 difTerent mesencephalic regions (12, 116, 136, 202). 

 However, it has often been impossible to decide 

 whether neurons originating in the mesencephalon, 

 or merely fibers passing through it, ha\e been stimu- 

 lated. 



Lindgren (149) published an extensive study on the 

 significance of the mesencephalon in vasomotor con- 

 trol. He found that vasoconstrictor neurons origi- 

 nating from the hypothalamus passed immediately 

 beneath the superior colliculus and probably had junc- 

 tions there with new mesencephalospinal neurons. 

 This outflow contains vasoconstrictor fibers to the 

 intestines and to the skin, and passes in close anatomic 

 relation to sympathetic vasodilator fibers to the 

 skeletal muscles and to fibers capable of activating 

 the catechol secretion of the adrenals. The fact that 

 vasomotor neurons running directly to spinal levels 

 emanate from the mesencephalon is interesting in 

 principle. It indicates that the mesencephalon is the 

 most caudal integrating relay station for this vaso- 

 motor outflow and that it may constitute a previ- 

 ously overlooked integrative region for central vaso- 

 motor control. 



In the opinion of Ranson and his associates (136, 

 159, 218, 219), the hypothalamic-medullary efTerents 

 have a somewhat dififuse extension in the lateral por- 

 tions of the tegmentuin and in the substantia grisea 

 centralis around the aqueduct. These workers dis- 

 puted the view that the medial longitudinal bundle, 

 the medial reticular formation and the corticospinal 

 tract were of paramount importance for the trans- 

 mission of sympathetic impulses from the hypo- 

 thalamus — a view which was asserted bv Beattie ct al. 

 (30). 



even to the cardiovascular apparatus. Pressor and de- 

 pressor responses to hypothalamic stimulation sug- 

 gest, it is true, that vasomotor structures are scattered 

 diffusely in the hypothalamic regions [for reviews see 

 Karplus (137), He.ss (116, 117), Ranson & Magoun 

 (181)], but they shed little light on the function of 

 these structures. 



An integrative function requires neuron synapses 

 in the integrative area. Persisting vasomotor responses 

 to hypothalamic stimulation in chronic decorticate 

 animals show that va.somotor neurons originate in the 

 hypothalamus — a fact that was already observed by 

 Karplus (137) in association with Kreidl. 



INFLUENCE ON CARDIOV.\SCULAR DISCHARGE. PittS, 



Bronk and Larrabee (i 75, i 76) reported an exemplary 

 study of the hypothalamic influence upon the tonic 

 cardiovascular discharge. They found that the fre- 

 quency of discharge in the cardiovascular neurons — 

 recorded in the inferior cardiac nerve and in the 

 cervical sympathetic — was linearly proportional to 

 the frequency of hypothalamic stimulation (fig. 8). 

 The preganglionic discharge amounted to about one 

 impulse per 20 to 25 hypothalamic stimulatory im- 

 pulses. Unilateral stimulation of the hypothalamus 

 produced bilateral sympathetic discharge. On simul- 

 taneous bilateral stimulation of the hypothalamus, an 

 almost arithmetical summation of the peripheral dis- 

 charge was obtained, the occlusion being minimal. 



Hypothalainui 



The integrative activity of the hypothalamus on 

 somatic and vegetative functions is considered to apply 



FIG. 8. The relation between frequency of discharge of 

 impulses in a single fiber of the cervical sympathetic nerve and 

 frequency of hypothalamic stimulation. Intensity of stimulation 

 constant. [From Pitts U/^J.] 



