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HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY 11 



formation throughout its extent by means of a brush- 

 work of collateral fibers. Subsequently, repeated 

 synaptic delays intervened to retard the conduction 

 of the impulse markedly. A comparable time-se- 

 quence was encountered in studying other reticular 

 inputs. 



All stimulus transmission within the RAS is not 

 conducted at such slow rates, however, as occasionally 

 fairly short latency responses are evoked in the cephalic 

 brain stem on stimulation of the medullary reticular 

 formation. It is probable that such responses are re- 

 corded in fiber collections such as the central teg- 

 mental tract (162), hence that rapid as well as slow- 

 cephalic transportation of impulses is characteristic of 

 the RAS. 



Another distinction between potentials e\oked in 

 'primary' and in 'secondary' .sensory systems relates 

 to the wave form exhibited by each. Lemniscal re- 

 sponses as recorded upon a cathode ray oscilloscope 

 exhibit a sharp, spike-like initial deflection lasting 8 

 to 1 2 msec, followed by a wave of longer duration 

 and frequently opposite polarity (83). By contrast, in 

 centrally-evoked responses, the initial short-duration 

 spike-like discharge is lacking, the potential appear- 

 ing as a high-amplitude wave of long duration. Poten- 

 tials evoked in the RA.S from cortical stimulation ex- 

 hibited comparable appearances to these discharges of 

 peripheral origin. The wave-like form and long time 

 course of evoked reticular activity suggests, in the 

 light of recent work, that dendrite potentials may be 

 prorninently involved (54, 220). The absence of re- 

 cruitment is not in favor of this possibility, however. 



EVOKED pOTENTL\Ls: REPETITIVE STIMULI. Significant 

 differences between the electrophysiological char- 

 acteristics of RAS and primary sensory pathways was 

 exhibited by responses resulting from multiple shock 

 stimuli. Paired or multiple shocks applied to periph- 

 eral nerve or receptor systems at 37 msec, intervals 

 evoked serial volleys in the appropriate primary 

 brain-stem pathway for as long as the pulses continued, 

 the second and succeeding responses being only 

 modestly attenuated. By contrast, in the RAS the 

 second pulse was diminished in amplitude and suc- 

 ceeding responses eliminated when stimuli were sep- 

 arated by as long as 92 msec. (83). Comparable at- 

 tenuation was encountered when the second of a pair 

 of shocks was applied to a source different from the 

 first (38, 39, 83). For example, a click response was 

 occluded if elicitation was attempted 13 msec, or a 

 cortical response 10 msec, after a potential was 

 evoked in RAS by sciatic stimulation {79, 83). In 



addition, Bremer & Terzuolo (38) first were able to 

 demonstrate facilitatory as well as blocking interaction 

 between potentials evoked by stimulation of two re- 

 ceptors in rapid succession. The long recovery time 

 appears to be a function of the multisynaptic organi- 

 zation of the RAS. Moreover, the reticular system 

 cells will react to a great variety of inputs, whereas 

 specific conduction pathways and nuclei are sensi- 

 tive only to signals of one modality. 



MicROELECTRODE STUDIES. Knowledge concerning the 

 function of the reticular forination has been greatly 

 advanced in recent years by the wealth of information 

 which has become available concerning the responses 

 by single cells within it (9, 10, 90, iii, 192, 241, 

 270-272). This material has been beautifully epito- 

 mized and interpreted by Moruzzi in symposia on the 

 subject (196, 197). In general, the results of these in- 

 vestigations can be said to confirm and extend those 

 of earlier macroelectrode studies. 



A variety of resting rhythms in reticular cells have 

 been described which are characterized by low fre- 

 quency firing (2 to 5 per sec), more rapid sustained 

 discharge (50 to 100 per sec.) or intermittent spiking 

 {10, 197). These characteristic discharges have been 

 shown to be altered by potentials reaching the unit 

 from a variety of stimulus sites. The firing rate was 

 augmented on some occasions while at other times it 

 became inhibited (10, 241). That existing conditions 

 of excitability in stimulus and recording site influenced 

 the response to some degree was indicated by the fact 

 that, at different times, a stimulus induced both aug- 

 mentory and inhibitory responses in a single unit 

 (272). However, true and consistent reversal in reac- 

 tion of a single cell was demonstrated by Gernandt & 

 Thulin (95). They found that firing of a unit was 

 augmented by vestibular stimulation induced by 

 turning the head in one direction while inhibition fol- 

 lowed reverse vestibular excitation. Also, a single stim- 

 ulus was found to increase the activity of some units 

 and decrease the firing of others (241). Responses 

 such as these appeared to be quite consistent, suggest- 

 ing to Scheibel et al. (241) that characteristic patterns 

 of firing were exhifjited by individual neurons of the 

 RAS. 



Responses ha\e been elicited in reticular cells by 

 stimuli applied to nerves or receptors of somatic sen- 

 sory (to, 90, III, 241, 270), vestibular (95), auditory 

 (ill, 241), visual (75), neocortical (usually in or near 

 motor cortex) (i 1 1, 272) and cerebellar (45, 90, 272) 

 structures. The cerebellar vermis was more responsive 

 to positive polarization than to single shocks (45). 



