THE RETICULAR FORMATION 



I28-, 



Single pulse excitation of a stimulus source would 

 usually suffice to elicit a response, although increas- 

 ing numbers of units were recruited following itera- 

 tive pulses (ill). Convergence of impulses upon a 

 single unit initiated by stimulating several sources 

 (9, 241, 270) was comparable to that demonstrated 

 by macroelectrode techniques. This convergence was 

 prominent but not absolute (10, iii, 241). These re- 

 sults confirm evidence which indicates that the reticu- 

 lar formation is a system of neurons responsive to di- 

 verse rather than specific stimuli. They suggest, 

 moreover, that some organization of response to com- 

 plex stimuli may occur in the RAS as well as in pri- 

 mary systems. 



CEPHALIC CONDUCTION OF RAS INFLUENCE. Thc reticular 

 activating system consists of a caudally-located reticu- 

 lar portion and a rostral thalamic portion. While EEG 

 and behavioral arousal elicited from rostral and 

 caudal RAS appear identical, the two zones exhibit 

 differences in responsiveness to slow frequency stim- 

 ulation, divergent sensitivity to drugs and contrasting 

 influences upon spinal structures. The thalamic seg- 

 ment will be discussed in detail in a separate review 

 and will be mentioned here only briefly. 



Anatomical evidence has indicated that cells in the 

 reticular formation connect with mid-line and intra- 

 laminar nuclei of the thalamus which suggests that 

 reticular influence upon the cortex is mediated at 

 least in part by a thalamic relay (42, 285). Reticular 

 neurons which bv-pass the thalamus also have been 

 demonstrated, thus an additional or alternative route 

 through which reticular influences could reach the 

 cortex appears to be at least potentially available 

 (202, 239, 285). The existence of these dual pathways 

 has been verified physiologically (263), although it 

 is not possible at present to a.ssess the degree of inde- 

 pendence each is capable of displaying in mediating 

 arousal. 



Single-shock pulses applied to both caudal and 

 cephalic portions of the RAS evoke widespread corti- 

 cal responses which have similar wave forms and long 

 time courses. Such responses appear to be comparable 

 to 'synaptic' potentials in the spinal cord (69), hence 

 represent fluctuation in the excitatory state of large 

 neuron populations (52, 54, 154, 157, 220). Cephali- 

 cally oriented influences of reticulothalamic structures, 

 therefore, seem to be comparable in nature to cau- 

 dally-directed effects where similar slow potential 

 shifts are known to modify unit discharges in motor 

 neurons (150, 151, 266). 



Wakefulness and Sleep 



As its structural organization would indicate, the 

 RAS, when stimulated directly or through any of its 

 principle inputs, is capable of initiating widespread 

 alteration of electrical activity in cephalically-oriented 

 cerebral structures. These modifications in the EEG 

 clearly have functional implications, the most impor- 

 tant of which relates to the state of awareness exhibited 

 by the subject. Slow wave activity such as spindle- 

 bursting and recruiting have features resembling the 

 tracing of sleep, while low-voltage fast activity in- 

 duced by repetitive reticular excitation parallels the 

 appearance of the wakeful tracing. It becomes neces- 

 sary, therefore, to examine behavioral responses which 

 result from experimental stimulations of the RAS and 

 relate them to electrical alterations. (Chapter LXIV 

 by Lindsley in this Handbook may also be consulted 

 concerning consciousness and sleep.) 



.AROUSAL RESPONSE. The Concept of a mesodien- 

 cephalic sleep regulating mechanism is an old one 

 (175). Early experimental observations by Hess sug- 

 gested that a sleep center existed. He was able to 

 induce delayed somnolence in chronically prepared 

 animals by prolonged excitation in the region of the 

 mid-line thalamus, presumably the diffusely project- 

 ing thalamic nuclei. Subsequently, a contrasting pro- 

 posal was made which suggested that initiation of 

 wakefulness, not sleep, was the function of the brain- 

 stem centers (191). The electrocortical events char- 

 acterizing wakefulness were elicited by hypothalamic 

 excitation (199), and behavioral changes suggesting 

 arousal (251) were induced by cortical stimulation. 

 However, Moruzzi & Magoun (198) first recognized 

 that the process of arousal depended upon excitation 

 of the central brain stem and elaborated the physio- 

 logical mechanism responsible for it. 



It has subsequently been demonstrated that EEG 

 arousal results from excitation of the RA.S or any of 

 its principle inputs, and rapidly repetitive pulses are 

 found to be most effective for this purpose. EEG de- 

 synchronization has been induced by appropriate 

 excitation of the same nerves or receptors which, when 

 stimulated with single shocks, induce responses in the 

 reticular formation. More specifically, EEG arousal 

 in animals immobilized by cervical cord section 

 {encephale isole of Bremer) or curare has been induced 

 by stimulation of peripheral nerves (85, 263), auditory 

 receptors (85, 105, 263), olfactory system (34, 59, 

 105), sympathetic nerves (85) and vagi (59, 286). 

 Comparable effects can also be elicited by stimula- 



