CENTRAL AUTONOMIC MECHANISMS 



961 



responses from the central gray. This region contains 

 the dorsal longitudinal fasciculus whicli is considered 

 to be one of the descending pathways for impulses 

 froin the hypothalamus. It is well-known that stimu- 

 lation of the perifornical region of the hypothalamus 

 produces strong autonomic discharges coupled with 

 affective types of beha\'ior. Hunsperger (82, 83) found 

 that lesions of the periaqueductal gray prevented 

 the elicitation of visceral and behavioral responses 

 upon stimulation of the hypothalamus. Skultety 

 (Skultety, F.M., unpublished observations) has re- 

 cently re\iewed the structure and function of the 

 periaqueductal area and has carried out some critical 

 experiments in which he was unable to produce any 

 changes in arterial pressure, gastrointestinal activity 

 or blood sugar by lesions of this area. The principal 

 observed effect seems to be a tendency to diminished 

 aflfcctive reactivity and greatly diminished socal 

 activity. The weight of evidence indicates that this 

 portion of the midbrain is probablv in the main a 

 pathway for impulses which sui)serve autonomic 

 effects. 



CONTROL OF URINARY BLADDER. It lias been held that 

 the midbrain e.xerts a control upon the tonus of the 

 urinary bladder. This idea goes back to the experi- 

 ments of Harrington (16) in 1925, in which jjilateral 

 lesions just ventral to the superior cerebellar peduncle 

 were followed by a permanent inability to empty the 

 bladder. Lesions lateral to the posterior end of the 

 aqueduct appeared to yield permanent loss of con- 

 sciousness of the need to micturate or defecate, al- 

 though these functions persisted at a spinal level. 

 More extensive lesions sometimes produced increased 

 frequency of involuntarv micturition. Tang & Ruch 

 (158, 159) have attempted to determine regions of 

 the brain stem concerned with controlling the mic- 

 turition refle.x in cats. Thev conclude, "At least four 

 levels of the neural axis . . . influence profoundly the 

 excitability of the sacral micturition reflex, namely, 

 (i) a cerebral inhiijitory region, (2) a posterior 

 hypothalainic facilitatory area, (3) a mesencephalic 

 inhibitory area, (4) an anterior pontine facilitatory 

 area (see fig. 18, right, of Chapter XLVIII by Ruch, 

 in this volume). The influence of these areas can be 

 removed successively by the following transections of 

 the neural axis: transhypothalamic decerebration, 

 supercollicular decerebration, intercollicular decere- 

 bration and subcollicular decerebration or spinal 

 transection." By using the method of cystometry and 

 experimental lesions after suitable tran.sections, evi- 

 dence was advanced that a facilitatory area for 



micturition is located in the mammillary region of 

 the liypothalamus. A bilateral inhibitory area is 

 located in the midbrain tegmentum, just lateral to 

 the central gray, at the level of the superior colliculus; 

 Harrington's pontine facilitatory area is bilaterally 

 located in the dorsal tegmentum at the level of the 

 isthmus just ventral to the lateral angles of the peri- 

 ventricular gray. 



We have just seen an exainple of a system so organ- 

 ized as to exert inhibitory and facilitatory influences on 

 refle.xes which can be mediated by a much lower level 

 of the nervous system. It is well-known that the 

 reticular formation contains inhibitory, or suppressor, 

 and excitatory, or facilitatory, mechanisms for so- 

 matic reflex action. These are so organized as to make 

 possible the regulation of .skeletal muscle tonus and 

 the prevention or occurrence of decerebrate rigidity. 

 We find that the same principle holds for visceral 

 functions of the nervous system. Wang and his co- 

 workers (166-169) have recently carried out a series 

 of experiments on these types of influences as they 

 affect the galvanic skin reflex in cats. They ad\anced 

 evidence that these facilitatory and inhibitory func- 

 tions are located at different le\els so that the reticular 

 substance of the lower part of the brain stem, which 

 is left after intercollicular decerebration, contains 

 mechanisms for inhibitory influences. These, however, 

 are subject to control from higher levels. Thus, after 

 removal of the telencephalon the galvanic skin reflex 

 is augmented in intensity. This is true also after re- 

 moval of the forebrain and the thalamus. However, 

 after total removal of the diencephalon there is a 

 slow decline in the reflex with eventual abolition. 

 After intercollicular decereijration there is a sharp 

 fall in intensity and eventual complete loss of the 

 reflex. This is interpreted to mean that the telen- 

 cephalon normally inhibits the reflex. After thalamic 

 removal, there is an increase in facilitatory influence 

 perhaps from damaged neurons; and, as remarked 

 before, after intercollicular decerebration inhibitory 

 influences are unchecked. In acute decerebrate cats 

 it was found that cooling the medulla or anesthetizing 

 the ventromedial reticular formation restores the 

 galvanic skin reflex which had been abolished by the 

 decerebration; then as the anesthesia wears off, there- 

 flex disappears again as an inhibitory mechanism 

 comes back into activity. It is of interest to note that 

 in a normal animal acute spinal transection at Ci 

 remo\es excitatory influences and decreases the 

 galvanic skin reflex. In the decerebrate cat, however, 

 this procedure eliminates inhibitory influences and 

 restores the reflex. Turning now to the method of di- 



