CENTRAL CARDIOVASCULAR CONTROL 



I I 39 



The examples adduced abo\e show that the spinal 

 cardiovascular neurons are capable of maintaining 

 a tonic discharge even when all communications with 

 supramedullary regions have been severed and of 

 responding to afferent impulses with segmental and 

 intrasegmental adjustments of the vasoconstrictor 

 tone. 



The extent to which spinal vasomotor reflexes are 

 active within the intact organism, with its dominant 

 supramedullary influences, is unknown. However, 

 spinal viscerocutaneous reflexes have been observed 

 both in intact animals and intact human beings. 

 Kuntz (142) observed vasodilatation and vasocon- 

 striction, respectively, in the small intestine of un- 

 anesthetized white rats on heating and cooling of the 

 caudal half of the thoracic region. Viscerocutaneous 

 vasoconstrictor reflexes can be elicited in man [Adams- 

 Ray et al. (1-5)]. Restricted pallor of the abdominal 

 skin occurred on distention of the urinary bladder in 

 cholecystitis, pancreatitis and other infections. Stiirup 

 (199) observed localized cutaneous pallor on disten- 

 tion of the esophagus. 



DESCENDING SPINAL VASO.MOTOR PATHWAYS. Excitatory 

 impulses pass from the medulla oblongata to the 

 spinal vasomotor neurons in the ventrolateral column, 

 inhibitory impul.ses in the dorsolateral coluinn [Suh 

 et al. (200), Chen et al. (58), Lim et al. (148), Harrison 

 et al. (113), Wang & Ranson (218, 219J, Alexander 

 (7)]. On the spinal vasomotor neurons there also 

 converge excitatory and inhibitory pathways from 

 suprabulbar levels, including the mesencephalon and 

 the hypothalamus. Such pathways can pass through 

 the medulla oblongata without having synapses there 

 (seep. 1 147). The excitatory and inhibitory influences 

 of medullary and supramedullary regions on the 

 spinal vasomotor neurons will be discussed in greater 

 detail on page 1141. 



Medulla Oblongata 



HISTORY AND NOMENCLATURE. As was first recognizee! 

 by Dittmar (64), Owsjannikow (169) and other 

 members of the Ludwig school in Germany, the in- 

 tegrity of medullary structures is essential for the 

 maintenance of normal cardiovascular tone. As early 

 as 1 90 1, Bayliss (26-28) introduced his dualistic vaso- 

 motor center theory, according to which the vaso- 

 motor tone was governed by the activity of two medul- 

 lary centers, a vasoconstrictor and a vasodilator 

 center, acting reciprocally. These two centers were 

 thought to be tonically active, the former governing 



a vasoconstrictor and the latter a vasodilator nerve 

 outflow, Bayliss' theory, which was further elaborated 

 in a monograph of 1923 (29), dominated scientific 

 discussion for several decades. Even though Bayliss' 

 argumentation has been fully confuted by later ex- 

 perimental observations, several authors of textbooks 

 and manuals still subscribe to his theory. 



In the second and third decades of this century a 

 number of investigators Ijy exploratory topical stim- 

 ulation of the medulla oblongata charted some more 

 or less limited regions at the base of the fourth ven- 

 tricle which were found to be more responsive than 

 other portions of this structure. Regions which on 

 stimulation produced rises or falls of arterial pressure 

 were assumed, in conformity with Bayliss' theory, to 

 constitute the postulated vasoconstrictor and vaso- 

 dilator centers [Ranson & Billingsley (180), Scott & 

 Roberts (193) and several others]. 



Experimental evidence available today, as will be 

 discussed in the following, suggests that the medullary 

 control of vasomotor tone is effected solely via vari- 

 ations in the vasoconstrictor discharge and not via 

 alterations in vasodilator nerve activity. Modern 

 authors speak of medullary pressor and depressor re- 

 gions in order to make clear that they affect the vaso- 

 constrictor tone and hence the arterial pressure by 

 excitation and inhibition, respectively, of the spinal 

 vasoconstrictor neurons. 



LOC.'KTION OF PRESSOR AND DEPRESSOR REGIONS. Ex- 

 ploratory stimulation of the medulla oblongata witii 

 the Horsley-Clarke technique has shown that the 

 pressor region comprises an extensive zone within the 

 lateral reticular formation, with its principal extension 

 in the rostral two thirds of the bulb. A depres.sor area 

 is localized to the medial reticular formation and ex- 

 tends chiefly into the caudal third of the bulb [Suh 

 et al. (200), Chen et al. (58), Monnier (164), Wang & 

 Ran.son (218, 219), Alexander (7), Bach (20)]. 



TONIC ACTIVITY OF PRESSOR AND DEPRESSOR REGIONS. 



The importance of the medullary pressor area for 

 tonic cardiovascular discharge was demonstrated 

 with an electrophysiologic technique b\' Alexander 

 (7). In chloralosed or decerebrate cats he recorded 

 a continuous discharge in the inferior cardiac nerves 

 and in the cervical s\inpathetic (fig. 3.-!, Section I). 

 Transection of the medulla oblongata just rostral to 

 the obex, a procedure which cut off most of the pressor 

 area froin its descending connections but left the de- 

 pressor area more or less intact, produced an equiva- 

 lent reduction in arterial pressure and in cardio- 



