THE RETICULAR FORMATION 



1285 



to llie variability in responses exhibited. There is re- 

 cent indication that the 'significance' of the aflferent 

 signal may influence its tendency to evoke reticular 

 discharge (John, R., personal communication). 



EVOKED POTENTIALS: CORTICI FUG AL CONNECTIONS. Fol- 

 lowing early descriptions by Bremer & Terzuolo (39), it 

 was found that potentials also can be evoked through- 

 out the extent of RAS by single shocks applied to each 

 of a number of loci in the cortex (79, 127). These loci 

 in the monkey are found to occupy discrete, circum- 

 scribed cortical areas located in the frontal oculo- 

 motor fields, sensorimotor cortex, cingulate gyrus, 

 orbitofrontal surface, temporal tip, first temporal 

 gyrus and the paraoccipital region (79). Recently, 

 Adey et al. (4) have demonstrated that potentials can 

 be evoked in the RAS also by stimulation of the 

 entorhinal cortex; Green & Adey (104) recorded 

 similar responses from excitation of the hippocampus. 

 Potentials evoked by cortical excitation exhibited the 

 same high degree of conv-ergence which characterized 

 responses of peripheral sensory origin (39, 79). More- 

 over, convergence within the RAS was extended to in- 

 clude corticifugal as well as sensory inputs, for the 

 same electrode placed in the reticular activating 

 system recorded potentials elicited by pulses applied 

 to aflferent conducting systems and active cortical 

 loci alike. 



Many of the active cortical loci described were 

 found to funnel into a common pathway by which 

 they are connected to the RAS, as Gunn et al. (106) 

 and Eliasson (72) were able to record evoked poten- 

 tials in the septal region from excitation of many 

 activating cortical zones. Alternati\ely, some cortical 

 loci exhibit individual connecting routes with the 

 RAS. For example, neurons from the central and 

 premotor gyrus were found to accompany the py- 

 ramidal tract (172, 177) to bulbar levels where they 

 entered the RA.S and the entorhinal cortex con- 

 nected with the thalamic and reticular brain stem via 

 the stria medularis (4). 



EVOKED POTENTIALS: CEREBELLUM AND BASAL GANGLIA. 



Evoked potential studies show that the RAS receives 

 important connections from the cerebellum (195, 

 252, 255). Additionally, central brain-stem responses 

 can be recorded from shocks applied to the basal 

 ganglia (258). 



EVOKED potentials: NEURONOGRAPHY. Connections 

 between cerebral structures and the RAS were thought 

 to be pauci- or, perhaps, monosynaptic (79) as tetanus 



waves elicited in many of the activ-e cortical loci bv 

 the local application of strychnine solution were re- 

 corded in the central brain stem (physiological neuro- 

 nography) (67). Such connections were reported from 

 prefrontal (283), cingulate (204, 280), orbital (235), 

 precentral (177), parietal (Kaufman, A., D. Hansen 

 & T. Shaw, unpublished observations), occipital 

 (178) and temporal (5) regions. It probably is not 

 essential for structures capable of energizing the RAS 

 to display as intimate a relationship with the brain 

 stem as indicated by these neuronographic studies, 

 however. At least, important influences are known 

 to be exerted by the multisynaptic spinoreticular 

 system, and the rhinencephalon may well have devious 

 as well as direct contacts with the RAS (2). 



EVOKED potentials: CONDUCTION RATES. Potentials 

 evoked in the central brain stem by peripheral nerve 

 or receptor excitation display latencies which are 

 somewhat longer than are those recorded more 

 laterally in primary conductive systems (83). Re- 

 sponses evoked by sciatic stimulation exhibited laten- 

 cies of 12 to 18 msec, in the RAS as compared with 6 

 to 9 msec, in the medial lemniscus at the same level. 

 Auditory potentials were even more slowly conducted, 

 requiring an elapsed time of 9 to 11 msec, to traverse 

 the distance from receptor to lateral lemniscus and 

 16 to 20 msec, to reach the RAS. Cortically-elicited 

 responses exhibited comparable or even longer con- 

 duction times. Latencies from frontal oculomotor 

 fields were found to be 6 to 12 msec, and similar re- 

 sults were obtained from other neocortical (79) as 

 well as rhinencephalic (4) structures. Impulses are 

 transported, therefore, from the sciatic to the RAS at 

 the rate of about 1 7 to 30 m per sec. and from the 

 cortex to the RAS with a speed of 3.5 to 6.5 m per sec. 

 Conduction within the reticular formation itself is 

 even slower, occurring at a rate of only i .5 to 3 m per 

 sec (83). Such slow transport of potentials within the 

 RAS has led to the conclusion that short chain neuron 

 systems must be involved in reticular conductioii (169). 



Conduction time from the sciatic and other periph- 

 eral stimulus sites did not vary significantly when 

 measured to caudal or thalamic portions of the 

 reticular systems (83). Similarly, little variation was 

 noted between the cortex and various recording sites 

 throughout the extent of the RAS (79). These ob- 

 servations prompted the conclusion that the principle 

 delay in reticular as compared to lemniscal potentials 

 occurred in the centrally located structure itself. 



Rapid conduction occurred from the sciatic to the 

 brain stem where discharges entered the reticular 



