THE RETICULAR FORMATION 



1299 



Ingram in Chapter XXXVII, a review of the physio- 

 logical characteristics of the reticular system would be 

 incomplete without brief comment concerning its 

 contributions to viscera! phenomena. There is abun- 

 dant evidence which indicates that visceral mecha- 

 nisms, residing in the brain stem, are subject to 

 inhibitory and facilitatory influences from the senses, 

 higher neural structures and products of metabolism 

 in a way comparable to, if not identical with, those 

 exerted upon nonvisceral mechanisms. It is becoming 

 increasingly clear, in fact, that modifications in 

 somatic function induced by physiological or experi- 

 mental stimuli never occur without appropriate 

 adjustments in visceral response (64). 



Interrelationships between somatic and visceral 

 mechanisms are indicated by the fact that stimulation 

 by a single electrode placed within the reticular 

 formation can rarely, if ever, be accomplished with- 

 out inducing a variety of effects (17). The basic reports 

 of Pitts et al. (213) and of Pitts (212) described two 

 discrete areas ventrally and dorsally located in the 

 medullary reticular formation which when stimulated 

 elicited inspiratory and expiratory responses, re- 

 spectively, in the respiratory rhythm of cats. Similarly, 

 Wang & Ranson (278) were able to elicit pressor 

 responses by stimulating the lateral medullary reticu- 

 lar formation and depressor changes by moving the 

 excitatory electrode to more ventromedial loci in the 

 bulb. Later, Bach (17) was able to confirm in a 

 general way the existence of discrete loci in the reticu- 

 lar formation which were capable of inducing, when 

 stimulated, appropriate modification of respirations 

 or of arterial pressure. 



It is tempting to conclude that medullary centers 

 for enhancing or suppressing visceral activity function 

 much as do comparable, even partly coextensive, 

 brain-stem loci which influence somatic movement, 

 and such a proposal has been made (17, 120). Recent 

 investigations, however, do not entirely support con- 

 cepts which assign exclusively facilitatory or inhibi- 

 tory properties to specific brain-stem loci. Bach (17), 

 studying the effects of stimulation applied to tlie med- 

 ullary reticular formation simultaneously upon respira- 

 tion, arterial pressure and induced reflex activity, found 

 that usually all three processes were modified by excita- 

 tion of each stimulus point. Furthermore, responses 

 were not parallel in all instances; in fact, he found 

 that simultaneous facilitation occurred in reflex re- 

 sponse, respiration and arterial pressure in only g per 

 cent of the stimulation points tested. 



While emphasis here has been placed upon respira- 

 tory and vasomotor responses to stimulation of the 



reticular formation, examples abound of influences 

 this brain-stem region exerts upon other autonomic 

 mechanisms. The observations of Hemingway et al. 

 (no) clearly indicate that mechanisms contributing 

 to regulation of temperature control course from the 

 preoptic area through the brain-stem tegmentum and 

 enter in the lateral funiculus of the spinal cord with 

 other reticulospinal fibers to terminate upon seg- 

 mental neurons. Inhibition of shivering can be 

 elicited by repetitive stimulation of loci along the 

 extent of this axis in the reticular formation. Further- 

 more, structures contributing to mechanisms medi- 

 ating gastrointestinal function (73), vomiting (25), 

 the galvanic skin reflex (276) and a variety of other 

 autonomic functions, reside in the same extent of 

 brain stem reticular formation. 



Inputs to autonomic centers in the brain stem paral- 

 lel in many ways similar connections with the reticular 

 formation which are known to function in control of 

 somatic structures. No attempt will be made here to 

 review specific sensory inputs to brain-stem 'centers' 

 regulating respiration, vasomotor control, gastro- 

 intestinal activity and other autonomic functions for 

 which the appropriate chapters in this Handbook 

 should be consulted. In addition to specific excitation 

 mechanisms, however, brain-stem regions implicated 

 in autonomic regulation are susceptible to non- 

 specific impulses converging upon them from most, if 

 not all, cranial and segmental sensory systems. 

 Through such contacts, reflex responses — marked 

 alteration in respiration following application of a 

 painful stimulus to the skin is an example — are made 

 possible. Thus, contributions made by somatic affer- 

 ent systems to brain-stem autonomic control are 

 comparable in every way to identical inputs which 

 modify reticular formation participation in such of 

 its other diverse functions as arousal, somatomotor 

 control and sensory modulation. 



Inputs from the cortex to the reticular formation 

 are known also to exert important influences over 

 visceral mechanisms mediated by the brain stem. In 

 a previous portion of this review, evidence was pre- 

 sented which served to connect functionally discrete 

 cortical loci residing in the frontal oculomotor, sen- 

 sorimotor, cingulate, orbital, temporal, paraoccipital 

 and rhinal areas with the reticular formation. These 

 loci, known to be capable of inducing arousal and 

 influencing somatic motor movement, when stimu- 

 lated also elicit facilitatory and inhibitory effects upon 

 visceral function (11, 132, 133, 215). For example, 

 essentially all the cortical areas listed above have been 

 implicated in gastrointestinal function {82). Paral- 



