1 140 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY II 



O-" . • . CONTROL 

 O^ - \ ^ ' • AMPLIFI. I 



SECTION X I 

 AMPLIFI. I 



go- SECTION I 



- - ylMPLIFI. 2 J 



SECTION n 

 AMPLIFI i 



SECTION H 

 AMPLIFI. 12 S 



SECTION HE 

 AMPLIFI. i 



SECTION HI 

 AMPLIFI 12.5 



accelerator tonic discharge in the inferior cardiac 

 nerves (fig. 3,-l, Section II). 



Alexander further showed that the depressor area, 

 too, probably is tonically active, exhibiting a con- 

 tinuous inhibitory influence on the spinal cardio- 

 vascular neurons. Transection at the level of C i , 

 secondary to the above-mentioned transection just 

 above the obex, resulted in an increased tonic dis- 

 charge in the inferior cardiac fibers, indicating a 

 release from a tonic inhibitory influence (fig. 3.'!, 

 Section III). 



If Alexander's observations are correctly inter- 

 preted, we should tiius have a steady stream of ex- 

 citatory impulses from the pressor area and inhibitory 

 impulses from the depressor area, bomljarding the 

 spinal vasoconstrictor neurons. The intensity of the 

 spinal cardioxascular tonic discharge would accord- 

 ingly be the result of the balance between these ex- 

 citatory and inhibitory medullary discharges project- 

 ing on the final common path — the preganglionic 

 \ asoconstrictor neuron. This hypothesis has the 

 advantage of being rather simple and therefore at- 

 tractive, but it needs experimental confirmation. 



According to Lim el al. (148) the influences of the 

 pressor and depressor areas are not confined solely 

 to the spinal vasomotor neurons. They speak of a 

 myelencephalic excitatory and a myelencephalic in- 

 hibitory center with excitatory and inhibitory in- 

 fluences on all spinal sympathetic preganglionic 

 neurons [Suh et al. (200), Chen et al. (55-58), Lim 

 et al. (14B), Harrison et al. (113), Wang & Ranson 

 (218, 219), Alexander (7)]. 



FIG. 3. A {upper): Artfiial pressure and tonic activity in the 

 inferior cardiac nerve of an unanesthetized decerebrate and 

 decerebellate cat. Sections indicated refer to transections of 

 the brain stem at the levels shown in part D of fig. 3B. Amount 

 of amplification of nerve potentials indicated relative to con- 

 trol level. Scales at left give arterial pressure in mm Hg. Time 

 signal in all recordings gives 0.2 sec. intervals. [From Alexander 

 (7).] B (lower) : Localization of pressor and depressor centers 

 in the brain stem of the cat. Pressor regions indicated by cross 

 halcliing; depressor regions by horizontal ruling, .i to C: Cross 

 sections through the medulla at levels indicated by guide lines 

 (I to ///) in D. D: Semidiagrammatic projection of pressor and 

 depressor regions onto the dorsal surface of the brain stem 

 viewed with the cerebellar peduncles cut across and the cere- 

 bellum removed. AT, auditory tubercle; BC, brachium con- 

 junctivum; BP, brachium pontis; Ci, first cervical nerve; CV, 

 cuneate nucleus; FG, facial genu; GN, gracile nucleus; IC, in- 

 ferior colliculus; 10, inferior olivary nucleus; L.N, lateral 

 reticular nucleus; RB, restiform body; SO, superior olivary 

 nucleus; SPV, spinal trigeminal tract; TB, trapezoid body; 

 TC, tubcrculum cinereuni; T.S, tractus solitarius; V, 17, VII, 

 and .V, corresponding cranial nerves; /, //, and ///, levels of 

 tran.section. [From .Alexander (7).] 



MEDULL.^RY c.ARDi.'^c CENTERS. Ever siiicc the pioneer- 

 ing investigations of Hunt (130), the heart rate has 

 been considered to be controlled by the tonic and 

 reciprocal actions of two medullary centers, one ac- 

 celeratory and the other inhibitory. The acceleratory 

 center is thought to be located in those reticular struc- 

 tures that include the pressor center, but its exact 

 localization is unknown. The inhibitory center is 

 believed to lie in communication with the \agal 

 nucleus or amygdaloid nucleus. The efferent ac- 

 celerator impulses run in the sympathetic outflow, the 

 inhibitory impulses in \'agal fibers. In principle, 

 the medullary control of cardiac activity is con- 

 sidered to be organized like the v-asomotor control, 

 although with the difference that the cardiac control 

 possesses an efferent parasympatiielic inhibitory path- 

 way, the vagus. 



Investigations of recent years have established that 

 parallel with accelerating fibers there are fillers in the 



