958 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY II 



FIG. 2. Reconstruction of single neurons in the reticular 

 formation, showing the extensive ramifications of cell processes. 

 The axons send inajor branches toward the cerebrmn and also 

 caudally into the spinal cord. [Figure by A. Scheibel and M. 

 Scheibel, fi-om French (55).] 



and .serve as the final common pathways for these 

 important autonomic activities, it is not appropriate 

 to designate any of them as the so-called 'centers of 

 vital activity' of the brain stem, since 'centers' may 

 greatly transcend any restricted locality. Nevertheless 

 in the lower part of the brain stem, in such a small 

 area as the medulla, we ha\e a fairly compact group- 

 ing of the neural arrangements which make possible 

 integration of some very important vital regulations. 



ciRCUL.^TORY REGULATION. According to Mitchell, the 

 functions of the medulla oblongata in regulation of the 

 heart first became known about 1845, and in 1873 

 Dittmar (45) showed that if the brain stem is gradually 

 sliced away from above downwards, a fall in arterial 

 pressure is observed after transection in the middle of 

 the pons. Progressive transections then cause greater 



and greater falls until the upper part of the medulla 

 is reached. These findings were held to indicate that a 

 center for arterial pressure regulation, perhaps con- 

 trolling vasoconstriction, was located in the upper 

 part of the medulla. No further progress was made 

 imtil igi6 when Ranson & Billingsley (138) showed 

 that both pressor and depressor responses could be 

 elicited by stimulation of the floor of the fourth 

 ventricle in cats. Stimulation in the posterior part of 

 the fourth ventricle just lateral to the obex beneath 

 the area postrema produced decreases in arterial 

 pressure. Stimulation in the inferior fovea at the apex 

 of the ala cinerea gave increases in pressure (fig. 3). 

 Ranson's observations were extended, among others, 

 by Alexander (3) in 1946, who found by the method 

 of stereota.xic exploration that a pres.sor center occu- 

 pies an extensise region in the lateral reticular forma- 

 tion and the rostral two thirds of the medulla, while 

 the depressor center includes a greater part of the 

 medial reticular formation in the caudal half of the 

 medulla. This center was shown to be functionally 

 significant because of its capacity for tonic inhibition 

 of the spinal cardioxascular mechanisms. The neces- 

 sity of these structures for the occurrence of cardio- 

 vascular responses produced by stimulating somatic 



FIG. 3. Pressor and depressor areas of the Hoor of the fourth 

 ventricle of the cat. F.i., fovea inferior, site of the pressor area; 

 d.p., depressor point; /.i., fovea superior; c.f., facial colliculus; 

 c, clava; o., obex; t.c, tuberculum cinereum. [From Ranson & 

 Billingsley (138).] 



