CENTRAL CARDIOVASCULAR CONTROL 



II4I 



sympathetic outflow that selectively influence the 

 contractile force of the heart muscle. Stimulation of 

 sucli fibers increases the contractile force without 

 necessarily increasing the heart rate [Anzola & 

 Rushmer (18), Randall & Rohse (179), Rushmer 

 (185)]. The central control of these inotropic fibers 

 to the heart has not yet been subjected to detailed 

 studies, but their significance in the regulation of 

 cardiac activity is evident. Interesting papers are to 

 be expected in this new experimental field. 



FREQUENCY AND RHVTHMICITV OF MEDULLARY DIS- 

 CHARGE. The tonic discharge in the thoracic cardio- 

 vascular outflow was found to have a frequency of 

 about 2 to 3 per sec. [Bronk et al. (39)]. During as- 

 phyxia the frequency was observed to increase up to 

 10 to 15 per sec, a frequency which allows a maximal 

 vasoconstrictor and cardioaccelerator response (see 

 p. 1 146). 



The preganglionic — and consequently the post- 

 ganglionic — firing showed a marked grouping in 

 volleys. Since the volleys were bilaterally synchronous, 

 they must have been due to a rhythmic outburst of 

 impulses from the centers. The rhythm frequently 

 was synchronous with the pulse and respiration. The 

 latter two forms of rhythmic cellular activity were 

 shown to be due largely to bursts of impulses from the 

 baroceptors in the carotid sinus and the aortic area, 

 initiated by the systolic rises in pressure, or to impulses 

 from distention receptors in the lungs. It was possible 

 experimentally to drive the cardiovascular centers by 

 repetitive stimulation of the central ends of the carotid 

 sinus or aortic nerves, thus causing the vasomotor 

 nerve cells to discharge periodically with the fre- 

 quency of the afferent stimulation. Section of the 

 sinus and depressor nerves abolished or reduced 

 markedly the grouped activity of the efiferent vaso- 

 motor discharge. Recording the splanchnic discharge 

 in cats, Dontas (66) observed rhythmical outbursts 

 of impulses with their maximum occurring after the 

 end of the depressor volleys in the sinus nerve. 



The integrity of the medullary vasomotor area is 

 essential for the pressor and depressor reflexes elicited 

 by the specific receptors in the carotid sinus and the 

 aortic arch, and also for similar reflexes elicited bv 

 stimulating various afferent nerves [Ranson & 

 Billingsley (180), Alexander (7)]. These va,somotor 

 reflexes as well as the influence of supramedullary 

 regions on the medullary vasomotor centers will be 

 dealt with below. 



MEDULLOSPINAL VASOMOTOR PATHWAYS. The cxcitatory 

 fibers from the pressor region pass to the spinal vaso- 

 motor neurons in the ventrolateral column [Lim et 

 al. (148), Wang & Ranson (218, 219), Alexander 

 (7)]. Stimulation in the pressor area causes a bi- 

 lateral increase of the firing in the inferior cardiac 

 nerves but only an ipsilateral increase of the firing 

 in the cervical sympathetics. In fact, there is a contra- 

 lateral inhibition of the activity in the cervical .sympa- 

 thetics [Alexander (7)]. Since hemisection of the spinal 

 cord considerably reduces but does not abolish pressor 

 responses resulting in ipsilateral hypothalamic or 

 medullary stimulation, partial crossing of the ex- 

 citatory fibers seems to occur at spinal levels [Harrison 

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



The inhibitory fibers from the depressor region pass 

 in the dorsolateral column [Alexander (7)]. These 

 fibers are supposed to cross extensively in the medulla 

 oblongata [Wang & Ranson (218, 219)]. However, 

 further electrophysiologic recordings of the cardio- 

 vasomotor discharge and observations on the pe- 

 ripheral vasomotor responses will be necessary to 

 elucidate the functional organization of the bulbo- 

 spinal excitatoiy and inhibitory outflows. 



MEDULL.ARY v.iiSOMOTOR REFLEXES. In the Carotid 

 sinus and the aortic arch there are localized spe- 

 cialized receptors sensitive to mechanical and chemi- 

 cal stimuli The former receptors, the baroceptors, 

 are stimulated by stretch and are situated in the 

 vascular wall around the bifurcation of the common 

 carotid and in the aortic arch. They react to changes 

 in arterial pressure. The chemoceptors are epithelioid 

 cells concentrated in specialized structures, the carotid 

 and the aortic bodies, which are sensitive to the 

 chemical composition of the arterial blood. Activa- 

 tion of the baro- and chemoceptors elicits depressor 

 and pressor reflexes, respectively. 



A detailed discussion of the morphologic and 

 functional properties of the baro- and chemoceptors 

 is beyond the scope of this article. The reader is re- 

 ferred to the numerous reviews on these subjects 

 [Heymans et al. (120), Heymans & Bouckaert (118)]. 

 It must suffice here to deal with questions relevant 

 to the medullospinal cardiovascular tonic discharge. 



Pressor and depressor reflexes can also be elicited 

 by stimulation of various afferent somatic nerves. 

 These reflexes are mediated, at least in part, via the 

 medulla oblongata. 



A substantial flow of afferent impulses from cardiac 

 and pulmonary receptors has been observed in the 

 vagus nerves. These impulses are considered to influ- 



