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HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY III 



manner are illustrated by the arrows directed to all 

 parts of the cerebral cortex. 



Beginning in the medulla, the reticular formation 

 extends upward through the central core of the brain 

 stem, through the region of the pons, midbrain, 

 hypothalamus, subthalamic and thalamus. Some of 

 its ascending pathways appear to reach the cortex 

 and other forward structures via the internal capsule; 

 others go to the thalamus, especially to the reticular, 

 intrathalamic and other unspecific nuclei. The extra- 

 thalamic route appears to provide the ARAS with 

 direct access to widespread cortical areas, whereas 

 the thalamic route brings the ARAS into relation 

 with the DTPS and possibly with the specific relay 

 nuclei as well. Thus the more direct pathways from 

 the reticular formation to the cortex via the internal 

 capsule constitute a possible mechanism subserving 

 preliminary arousal and general alerting of the cortex 

 to impending messages in specific sensory systems. 

 The thalamic component of the ARAS, in conjunc- 

 tion with the DTPS, may provide a kind of scanning 

 and screening mechanism capable of modifying or 

 regulating the influx of messages to the cortex via the 

 specific thalamic relay systems. It may also aid in 

 controlling the distribution and integration of the 

 messages upon arrival at the cortex. As such it may 

 constitute a specific alerting mechanism capable of 

 sharpening and shifting the focus of attention upon a 

 given sense modality or within a modality. Such 

 possibilities will be further discussed in the next 

 section which deals with the DTPS. 



To return to the means of excitation of the ARAS, 

 note in figure i that the arrows branching off from tin- 

 classical sensory pathways symbolize collaterals to 

 the reticular formation. Evidence now indicates that 

 even sense modalitv , apart from its specific or primary 

 message-carrying function, also has connections with 

 some part of the reticular formation and is capable of 

 exciting this structure, thus gk ing rive to an unspecific 

 or secondary sensory influence as manifested in the 

 activity of the ARAS. Moruzzi & Magoun (iHi) 

 demonstrated not only that electrical stimulation of 

 the midbrain reticular formation was capable of 

 arousing behaviorally and electrocortically a drowsy, 

 sleeping or lightly anesthetized cat, but that natural 

 sensory stimuli of all types would produce a similar 

 effect. Furthermore in the waking cat the same 

 stimuli led to cortical activation and behavioral 

 alerting, I ile< trical recording directly from the reticu- 

 lar formation has demonstrated that natural sensory 

 stimuli of all types evoke potentials there. These 

 functional demonstrations of the influence of col- 



laterals from sensory pathways to the reticular forma- 

 tion are in accord with previous neuroanatomical 

 knowledge which has recently been further elaborated 

 by Olszewski (187 1, Brodal (40), Rossi & Brodal (203) 

 and the Scheibels 1206). 



Still another mode of excitation of the reticular 

 formation has been demonstrated which is of particu- 

 lar importance to the topics under consideration 

 here. Several investigators (2, 3, 38, 104, 120, 185) 

 have shown that stimulation of various parts of the 

 cortex gives rise to potentials in the brain-stem reticu- 

 lar formation, but in particular the work of French 

 et al. (77) may be referred to here. Figure 2 shows 

 schematically both the corticifugal pathways and the 

 collaterals of classical afferent pathways converging 

 upon the reticular formation of the lower brain stem. 

 These of course are by no means the mjIc afferents to 

 this structure, for others arise in the cerebellum, 

 basal ganglia, thalamus, hypothalamus and rhinen- 

 cephalon. For an excellent review of the anatomy and 

 physiology of the reticular formation and its afferent 

 and efferent connections, an extensive article by 

 Rossi & Zanchetti (204) should be consulted. Other 

 valuable surveys of the reticular formation are those 

 by Segundo (211), O'Leary & Coben (186) and 

 French in Chapter FI1 of this Handbook. 





in. 2 Corticifugal pathways and collaterals "t classical 

 ill, n in pathways convcrijini; on the reticular formation of the 

 lower brain stem. Stimulation of widespread cortical areas 

 gives rise to electric potentials in the reticular formation, hence 

 functional connection l>\ assumed corticorenculai paths. Af- 



ferent impulses from all s us and impulses originating in the 



\ are capable of exciting the ARAS, which in turn main- 

 tains the cortex and behavior in a state "l arousal and alert- 

 ness, and perhaps selectivel) controls attention. From French 

 tl al. I 



