742 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



of the nervous system. This interference inxoKes an 

 active process that is usually inhibitory. In the waking 

 state, the sensory pathways seem ordinarily to be 

 under a tonic inhibitory influence; evidenth a good 

 deal of the potential content of sensory experience is 

 being continuously reduced or eliminated within the 

 initial stages of sen.sory integration. Inasmuch as 

 activity along sensory pathways appears to be modifi- 

 able to some extent according to an animal's en\iron- 

 mental experience and according to its overtly ex- 

 pressed direction of attention, the interference with 

 sensory transmission appears to be regulatory and to 

 constitute a goal-seeking physiological mechanism. 



These findings call for some adjustment of current 

 physiological, psychological and philosophical con- 

 cepts relating to perception. Most such concepts have 

 been based upon a) physiological findings deri\ed 

 from an examination of anesthetized animals, findings 

 which usually reveal the activities of only a few parts 

 of the nervous system at a time, and A) behavioral 

 evidence obtained with waking unanesthetized ani- 

 mals in which the nervous system is treated as a whole. 

 Some degree of closure between these two experi- 

 mental realms of .science was apparently achieved 30 

 years ago. Adrian and other physiologists discovered 

 that the strength of a stimulus necessary to elicit 

 action currents in peripheral sensory nerves of anes- 

 thetized animals was approximately equal to that 

 found by psychologists for threshold perception in 

 attentive human subjects (4, chapter VI}. Compa- 

 rable stimuli, again in anesthetized animals, were then 

 found to yield evoked cortical responses that were 

 localized to certain 'sen.sory receiving' areas of the 

 cortical mantle (59). Detailed analysis in anesthetized 

 animals of activity taking place within various relay 

 stations between the peripheral nerves and the cortex 

 re\-ealecl that the spinal (80), brain-stem (60) and 

 thalamic synaptic relays (69} were quite reliable in 

 their transmission of evoked signals. 



Naturally such findings led to an interpretation 

 that the sen.sory nerves and the central ascending 

 paths reliably convey to the cortex whatever messages 

 are generated by the sensory end organs. It was argued 

 that only when the impulses reach the cortex are they 

 then accessible to such p.sychological factors as habit- 

 uation, focus of attention, suggestion, etc., long known 

 to intervene in sen.sory perception. The cortex was 

 believed to be only the first stage in the integration 

 of sensation from sense data (7, pp. 39, 40, 62). This 

 view fitted well with the traditional conception of 

 hierarchical supremacy of the cortex — notions de- 

 ri\ecl partly from the recognition of its topmost loca- 



tion, enormous areal extent, anatomical complexitv, 

 phylogenetic recency, etc., and partly from the mo- 

 mentum of theoretical conceptions of Pa\lo\- and 

 others who assigned most psychological functions to 

 the cortex (64). 



^ et for more than 50 years anatomists have recog- 

 nized that certain nervous pathways enter sensory 

 nuclei and relay stations from above, and that nearly 

 all .sensory systems have eff"erent fibers passing from 

 the neuraxis to receptor organs. When the indi\idual 

 anatomical features of these centrifugal projections 

 are grouped together, they appear to constitute a 

 series of descending neuronal cascades which con- 

 ceivably might have an influence upon ascending 

 sensorN impul.ses. These descending and efl"erent sen- 

 sory projections ha\e usually been considered piece- 

 meal and few conceptual generalizations are available. 

 Perhaps the most prophetic of these appears in an 

 interpretive discussion of neuropathology by Brouwer 

 in 1933: ". . . We accept that there is also a cen- 

 trifugal side in the process of sensation, of \ision, of 

 hearing, and so on. I believe that a further anahsis 

 of these descending tracts to pure sensory centers will 

 also help physiologists and psychologists to under- 

 stand some of their experiences" (10, p. 627). 



CONTROL OF RECEPTOR .ACTIVITY' 



Sympalhetif Iiifluemt' nii Touch Receptors 



Single touch receptor activity in isolated skin areas 

 of the frog can be facilitated in stimulation of the 

 sympathetic nerve supply to that region (56). Activity 

 in these receptors can also be facilitated by the local 

 application of epinephrine or norepinephrine, or by 

 introducing these hormones into the circulation. 

 Thus, individual receptors are evidently subject to 

 generalized as well as local sympathetic influences. 

 Sympathetic ner\e influences have alreadv been 

 shown to be facilitatory to transmission across the 

 neuromuscular junction (see 56 for references); their 

 effects on touch receptor acti\it\- therefore appear to 

 be parallel and to place the peripheral sensory as well 

 as peripheral motor portions of the reflex arc under 

 some degree of central control. By \irtue of these in- 

 fluences, the reflex arcs relating to touch should no 

 longer be considered such simple units of neuro- 

 physiological and behasioral systems. Since appar- 

 ently all sensory receptors receive sympathetic fibers, 

 it is perhaps not too extra\agant a generalization to 

 suppo.se that all of them may be found susceptible to 

 this kind of central interference. 



