1284 



HANDBOOK OF PHVSIOLOGV 



NEUROPHYSIOLOGY II 



Reticulofiigal Projections 



It is not clear anatomically how influences from 

 the thalamic reticular system reach the cortex, for es- 

 sentially complete decortication results in no retro- 

 grade change in the intralaminar nuclei (217, 218, 

 274). Contrastingly, cells in the reticular nuclei of the 

 thalamus (along with appropriate relay and associa- 

 tion nuclei) do degenerate after cortical resection (53, 

 228, 229), and one proposal holds that reticulo- 

 thalamic influences upon the cortex are mediated 

 through the reticular nuclei of the thalamus (125, 

 126). In addition to a thalamic route, fibers have been 

 described recently which course cephalically from 

 the reticular formation in a region ventrolateral to the 

 thalamus (42, 202, 239, 295), hence it is likely that 

 direct reticulocortical connections exist. 



It has been demonstrated that mid-line and intra- 

 laminar nuclei degenerate if certain telencephalic 

 structures are destroyed. Rose & Woolsey (229) 

 found intralaminar degeneration only if they de- 

 stroyed the rhinencephalon, striatum and amygdala 

 in the rabbit, and Powell & Cowan (218) reported 

 similar results by ablating the lateral preoptic area 

 and adjacent parts of the striatum and pallidum. It 

 appears, therefore, that an intimate anatomical rela- 

 tionship exists between the central brain stem and 

 either the paleocortex or perhaps, the basal ganglia 

 or both, as suggested by Droogleever-Fortuyn (65). 

 Connections with the neocortex are somewhat less 

 direct. 



Another important projection system of the reticular 

 formation is directed into the cerebellum. According 

 to Brodal (41) and Brodal & Torvik (43), a large 

 collection of cells in the medullary reticular formation 

 (called the paramedian reticular nucleus) send fibers 

 to the anterior lobe, pyramis, uvula and possibly the 

 fastigial nucleus. 



ASCENDING INFLUENCES 



In 1935 Bremer noted that transection of the 

 brain stem of animals at the collicular level {cerveau 

 isole) resulted in behavioral and EEG manifestations 

 of sleep attributed, he correctly suggested, to de- 

 aflPerentation of the cortex (33). Later developments 

 indicated, however, that the coma exhibited did not 

 depend upon the total elimination of sensory informa- 

 tion from cerebral structures, for it was known that 

 responses could be recorded in primary receptive 

 areas of the cortex even when animals were anes- 



thetized. The observations of Moruzzi & Magoim 

 (198) clarified the matter by showing that afferent 

 influx transported centrally through the brain stem 

 was implicated in the arousal mechanism. Clearly, 

 therefore, the coma of cerveau isole animals was shown 

 to depend upon exclusion from higher structures of 

 'activating' stimuli conducted selectively throus;ii 

 this median zone rather than upon the blockade of 

 primary .sensory signals transported to the cortex 

 through the lateral brain stem. The area implicated 

 in conveying such "arousing' information to the brain 

 was occupied principally by the reticular formation 

 and related tiialamic nuclei; hence these structures 

 became known as the 'reticular activating system' or 

 RAS. 



Electrophxsiological Characteristics 



EVOKED POTENTI.^LS: SENSORY CONNECTIONS. In 1 936 



Derbyshire et al. (63) called attention to impulses 

 with long latency elicited by stimulation of the 

 sciatic nerve which could be recorded in surface areas 

 remote from the sensory cortex and hence were dis- 

 tinct from signals conducted in the primary sen.sory 

 pathways. Sul)sequently, Dempsey et al. (62) deter- 

 mined that this response, called by Forbes & Morison 

 (77) the 'secondary' as opposed to the 'primary' 

 lemniscal response, was conducted through tiic brain 

 stem in an area central to the medial lemniscus. Since 

 that time, evoked potentials have been recorded in 

 the reticular formation from excitation of somatic 

 sensory (85, 263), proprioceptive (61), sympathetic 

 (85), vagal (59), auditory (85, 263), visual (59, 85) 

 and olfactory (59) receptive or conductive system. 



These potentials have been recorded throughout 

 the reticular formation and areas related to it, such 

 as the central grey substance, sub- and hypothalamus, 

 and medial and intralaminar thalamic nuclei (85, 263). 

 More recently sensory signals have been recorded even 

 farther forward in the septal region (2). In general, 

 the extent of the central brain stem over which po- 

 tentials of each of the aforementioned modalities of 

 sensation can be evoked is common for all (85, 263). 

 Thus, a single electrode placed within the reticular 

 formation can record responses from stimuli applied to 

 a variety of conductor or receptor systems, for exam- 

 ple, from stimulation of the sciatic, the sympatiietic 

 and the radial nerves as well as from an audible click. 

 While this conxergence is extensive it is not absolute, 

 for in practice potentials may be evoked more readily 

 from excitation of some conductive systems than 

 others, and species differences may contribute as well 



