1950 



HANDBOOK OF 1'HYSIuLOGY 



NEUROPHYSIOLOGY III 



cortical and deep neurons, previously engaged, now 



remain silent. Similarly, habituation is associated 

 with a general decrease in response to the familiar 



stimulus (149). Knots are associated with reap- 

 pearance of electrical activity and, when different 

 flicker frequencies signify plus or minus reinforcement, 

 with different response rates in the cortex and sub- 

 cortex (152). Learning, thus, involves the inhibition 

 of neurons that were earlier engaged while an ap- 

 propriate response was being established; but dimi- 

 nution of irradiation as effective channels arc formed 

 1 sec below) may also be involved. In much the same 

 way, a computer that can 'learn' will first explore all 

 possible actions, as the next move in a chess game, 

 but later will not bother with those that have regularly 

 proved worthless. 



Whether inborn or learned, automatic reflex 

 behavior cares for the routine adjustments of living 

 and is attended by little or no immediate conscious- 

 ness. One scratches an itch — as one walks with no 

 attention to the act. When, however, the routine 

 response does not eliminate the disturbing stimulus, 

 messages continue to pour into the nervous system, 

 summate at the early junctions and irradiate to new 

 ones outside the usual pathways (Gastaut & Fischer- 

 Williams, [51). The longer the input continues, the 

 greater the irradiation, until such emergency struc- 

 tures as the diencephalic autonomic centers and the 

 reticular formation are thrown into activity perhaps 

 via the amygdala (Gloor; 195, 196). The former, 

 through liberation of epinephrine or related sub- 

 Stances, directly lowers the thresholds of neurons,' 

 and the latter, by increasing the activating bom- 

 bardment, indirectly does likewise. The result is a 



'The- inhibitors anion ol epinephrine lias been recognized 

 loi some time 'hi-'. Hi) 1 , the excitatory action, only more 

 i'. •nils, although a rebound from inhibition was noted by 

 Marrazzi. Besides evidence summarized elsewhere (114), the 

 following points are made in this volume on epinephrine activa- 

 tion li prolongs pentylenetetrazol convulsions (Gastaut & 

 Fischer-Williams), increases receptor excitability and the 

 size ol iIh Pacinian generatoi potential (Gray), facilitates the 

 a. Hon of acetylcholine on the muscle end plate 1 Fan I, increases 



In. nn oxygen consumption (Sokolotl l, and stimulates neurons 

 in the hypothalamus and the upper midbrain reticular forma- 

 tion (Ingram, Jasper] French regards the exciting action on 



the nervous system in general as se. Iar\ to the peripheral 



action ol epinephrine in enhancing proprioceptive input, but 

 this is excluded b) the findings of Sigg tt al. (363) Moreover, 

 the original value of such an inhibitor) action, as a negative 



1 ii.i mi 11 autonomic ganglia (193), is reduced 



by the evidence thai norepinephrine rather than epinephrine 

 is the orthosympathetii transmitter (von Bulei . and even us 

 inhibitor] a< tion mi su< h lia is disputed (Hillarp). 



still greater irradiation and probably a much in- 

 creased reverberation. New responses appear and 

 follow one another until a successful behavior Anally 

 eliminates the disturbing situation. 



This is innovative or creative behavior, replacing 

 the routine reflex behavior, and with it appears 

 increasingly intense consciousness. [Yet much imagi- 

 native creation proceeds unconsciously (94)]. In 

 parallel with the idea of a wakefulness of necessity 

 (159; see 136), one might speak of a consciousness ol 

 necessity. This is presumably first a simple awareness, 

 involving midbrain and hypothalamic structures 

 (Penfield, Ingram), especially the reticular for- 

 mation arousal system. Later it increases to attention 

 and then to a high level of vigilance (French), as in 

 the cat watching a mouse hole or the sheep dog 

 quiveringly alert to his master's command and his 

 charges' movements. Affective responses enter as the 

 hypothalamic and limbic systems are aroused (Mac- 

 Lean), with pain or displeasure, and with increasing 

 anxiety, as the functional neuron reserve of the cortex 

 is engaged. Perhaps the role of the periodic inhibitory 

 discharges of sleep is to break the continuing re- 

 verberations, and so to prevent excessive fixation 

 and to release reserve neurons. (A comparable 

 suggestion is made by Walter regarding delta waves; 

 indeed, as noted, descending inhibitory impulses are 

 probably essential to prevent explosive avalanche 

 conduction up the cord, as reported by Battle) 1 The 

 block of inhibition by strychnine (Grundfest) is 

 similarly explosive. Besides the general level of 



consciousness and mass arousal effects, the content 



of consciousness is no less important. This depends on 

 the particular activity patterns, determined in turn 

 by the structured residues of past experience and I >\ 

 the organization of contemporary input. As more of 

 such neuron assemblies enter into activitv, reason and 

 imagination appear in consciousness, and in- 

 sightful problem-solving behavior replaces routine or 

 random actions. 



DSNSION. There is much evidence that even brief 

 inputs continue to reverberate in closed loops, or sel 

 up continued synchronous aeiiviiv, rather than 

 simplv pass unidireet ionallv through the nervous 

 svstem. The effects of such stimuli far outlast any 

 possible 1 1 ii li- 1 throughput time. Evidence has 

 already been presented that activity continues for 

 minutes and hours, during which a transient ex- 

 perience produces an enduring memory trace. 

 Similarly, unfinished business, the "unconscious 

 work' of anxiety, dreams, unsatisfied drives and 



