THE RETICULAR FORMATION 



1301 



rhinencephalic structures. While the reticular forma- 

 tion has not yet been implicated directly in connection 

 with all these peripheral receptors, its participation in 

 the process of receptor sensitivity has been so well 

 established as to indicate that it contributes a measure 

 of control to all such systems. 



.Similar influence, as so far investigated largely 

 inhibitory in nature, is known to be exerted by the 

 reticular formation upon spinal and brain-stem 

 sensory relays. Hagbarth & Kerr (107) found that 

 responses evoked by peripheral excitation were inhib- 

 ited by reticular formation stimulation in the poste- 

 rior column, both the dorsal root reflex aitd dorsal 

 column relay being involved. .Scherrer & Hernandez- 

 Peon (242) reported that similar stimulation inhibited 

 conduction in the nucleus gracilis, and Hernandez- 

 Peon & Hagbarth (in) noted comparable suppres- 

 sion in the sensory nucleus of the fifth nerve. Even 

 transmission in the ventroposterolateral nucleus of 

 the thalamus was seen to be modified by King el al. 

 (141) following reticular excitation. 



There is reason to believe that reticular modulation 

 of sensory conduction is exerted tonically, at least 

 afferent responses in the spinal cord are enhanced fol- 

 lowing high spinal transection or anesthesia (107, 155) 

 or by destroying the reticular formation (112). In 

 spite of such tonic discharge, however, it is evident 

 that reticulopetal inputs contribute additionally to 

 sensory modulation. 



Hagbarth & Kerr (107) were able to elicit inhibi- 

 tion of conduction in the spinal cord by excitation of 

 the sensorimotor, parietal and cingulate cortex as 

 well as from the anterior \ermis of the cerebellum. 



This inhibition due to cortical excitation was compar- 

 able to that elicited by stimulation of the bulbar and 

 mesencephalic reticular formation. Kerr & Hagbarth 

 (137) induced olfactory bulb inhibition by rhinen- 

 cephalic excitation. Granit et al. (loi) reported that 

 excision of the anterior lobe of the cerebellum resulted 

 in paralysis of gamma efferent mechanisms. Adey el 

 al. {3) found that conduction within the reticular 

 formation could be facilitated or inhibited by stimula- 

 tion of all activating cortical loci including the 

 rhinencephalon. Thus, as in other functions mediated 

 by the reticular formation, that involved with control 

 of sensory inputs is subject to modification from 

 higher structures. 



The implications and significance of these physio- 

 logical observations is being explored currently and 

 extensively by observations made on chronically 

 prepared or conditioned animals. These observations 

 are discussed elsewhere, and brief comment is made 

 here only to suggest that assessment of the role served 

 by the reticular formation in these studies will be of 

 the utmost importance. Green & Arduini (105) were 

 among the early observers to implicate the reticular 

 formation in the phenomenon that animals adapt to 

 repeated stimuli and are aroused most effectively by 

 new and strange stimuli. Subsequently, great interest 

 has been expressed in the electrophysiological corre- 

 lates of habituation to monotonously repeated stimuli 

 (89, III, 112, 113) and conditioning (89, 113, 130). 

 Already it has been possible to observe inhibition of 

 photically-evoked potentials in the visual system 

 during alerting elicited by acoustic or olfactory stimuli 

 and stimulation of the reticular formation (114). 



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