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HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



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FIG. 8. Inhibition of reticular unit discharges by stimulation 

 of the cerebellum and augmentation of reticular unit dis- 

 charges by sensory stimulation. Encfphale isote preparation, 

 without cential anesthesia. For each strip, the top line registers 

 an electroencephalographic tracing recorded from the fronto- 

 temporal regions of a hemisphere; the lower line indicates unitary 

 spike discharges picked up from the median bulbar reticular 

 formation by a microelectrode. A. .Activated electroencephalo- 

 gram and continuous high frequency discharge of the reticular 

 unit. B. Total inhibition of the unit by positive polarization of 

 the anterior vermis of the cerebellum (0.5 ma); EEG trace not 

 modified. C. Immediately after discontinuation of the cere- 

 bellar polarization. D. Same as C a few seconds later. Tlie 

 reticular unit reappears (C) and progressively increases in 

 discharge frequency (D); during the early part of this period, 

 slow high-amplitude waves appear in the EEG. E. Some 

 minutes later, an intense tactile stimulation (brisk rubbing of 

 the bridge of the nose) causes the appearance of a multiple 

 reticular discharge (including recruitment of new units), and 

 an increase in frequency of EEG waves. [From Mollica et at. 

 (6.).] 



is subject to some degree of interference according to 

 the state of activity in other parts of the nervous 

 system. Now, is it possible to define somewhat more 

 specifically the relationship between the classical 

 sensory paths and these other parts? No final inter- 

 pretations are warranted since the data are as yet 

 incomplete for any one sensory system. Nonetheless, 

 in each sensory system there can be identified certain 

 centrifugal sensory control mechanisms which bear 

 close analogy with structural or functional aspects of 

 one or another of the other sensory systems. Generali- 

 zations that might not be permitted for one system 

 alone seem to gain in strength when all of them are 

 examined together. 



Paralleling the classical succession of ascending 

 neurons appears a descending system which links the 



same nuclear relay stations from above downward. 

 Although analogous centrifugal projections have been 

 identified anatomically for many individual parts of 

 other sensory systems, the auditory pathway probably 

 po.ssesses the most completely documented succession 

 of descending fibers. These pursue a course in reverse 

 direction that roughly parallels the ascending; audi- 

 tory pathway. They pass step-by-step downward from 

 the auditory cortex to the medial geniculate body and 

 inferior colliculus, thence to the lateral lemniscus and 

 trapezoid body and to the superior olive where they 

 are succeeded by the olivocochlear efferent bundle. 

 As Galambos says, "It is unlikeh that these descend- 

 ing fiber .systems — some reasonably powerful, some 

 weak — perform no function in audition. What this 

 function might be will unfortunately continue to re- 

 main entireK' speculati\e until more anatomical and 

 physiological data become available. One can hazard 

 a guess, however, that the solution of certain problems 

 of hearing resides as much in the understanding of 

 the function of these descending pathways as in the 

 knowledge of the ascending ones" (25, p. 503). Pre- 

 sumably centrifugal fiber projections which belong to 

 the visual, somesthetic and olfactorv svstems mia;ht 



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FIG. 9. Influence of hypothalamic and toe pad stimulation 

 on sciatic nerve responses elicited within the brain-stem reticu- 

 lar formation. Recording from the right side of the midbrain 

 reticular formation in a curarized cat without central anes- 

 thesia. A. Sciatic responses, (/) before, (.?) during, (3) 8 sec. 

 after and (^) 20 sec. after repetitive stimulation in the right 

 hypothalamic region (50 per sec. for 3 sec). B. Sciatic response 

 (/) before, (s) during and (3) 10 sec. after pinching the toe pads 

 of right hind limb (ipsilateral to reticular recording site). 

 [From Hernandez-Peon & Hagbarth (36).] 



