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HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY 1 



hemiplegia and in those who had tumors. This ex- 

 tensive bilateral cortical representation for facial 

 sensation including pain is consonant with the well- 

 known similar motor representation. On the average 

 the tumor patients showed the greater deficit, the 

 lesion having been present a shorter time in a more 

 adult brain. The contralateral parts showed an im- 

 pairment of appreciation and localization of a sharp 

 point, which increased progres><i\ely in the following 

 sequence: face, trunk, thigh, upper arm, leg, fore- 

 arm, foot and hand. A delay in the appreciation of 

 all stimuli on the paralyzed side was short in the 

 trigeminal area and longest in the distal parts of 

 the limbs. Pain elicited from the abnormal side was 

 more disagreeable than that from the normal side. 

 Cold and hot stimuli were painful and could not be 

 differentiated. When a pin was applied simultane- 

 ously to similar sites on the two sides, there was 

 consistent extinction only below the elbow and knee 

 on the paretic side. 



Evoked Potentials 



The somatic sensory areas I and II of the cerebral 

 cortex of many mammals including monkeys have 

 been outlined on the Ijasis of cortical electrical 

 potentials evoked by tactile stimuli to the body 

 surface (311). There has i^een very little work to 

 determine in animals the cerebral representation for 

 painful stimuli; but in continuation of our assump- 

 tion that 'A' gamma-delta impulses may be asso- 

 ciated with pain, we shall summarize the work of 

 Amassian (5, 6) on the cortical respon.ses evoked 

 from such fibers in the splanchnic nerves of car- 

 nivores. From tiny areas on the cortex at the junction 

 of leg and arm representation in both sen.sory areas 

 I and II in the dog and cat, he found brief initially 

 surface-positive waves. These were obtained from 

 both sensory areas contralaterally and from the 

 ipsilateral area II in the cat when only splanchnic 

 'A' beta fibers were excited, as shown in figure 11. 

 At much higher voltage, when 'A' gamma-delta 

 activation was also visible in the record from the 

 splanchnic nerve, a small deflection appeared on the 

 returning limb of the primary response in area I 

 as the only early cortical evidence of presumed 

 activity of pain fibers. There was no change in the 

 primary response from area II. Activation of the 

 gamma-delta fibers also evoked a late secondary 

 response generalized over the cortex. In the monkey, 

 Ruch et al. (232) were able to find a splanchnic 

 representation only in cortical area I. 



0.13v. 



3.8 V. 



FIG. 1 1 . Splanchnic A gamma-delta fibers and cerebral 

 evoked potentials. Stimulating electrodes on splanchnic nerve 

 distally. Upper records obtained with stimulus 0.13 v.; there is a 

 maximal primary response from cortical area 1 Qupper /efl~) with 

 no A gamma-delta discharge on the neurogram of the sym- 

 pathetic trunk O'Pper right^. Lower records obtained with stim- 

 ulus 3.8 V. and pulse duration i msec, the A gamma-delta 

 group is active (^second wave, lower right'), but the only cortical 

 correlate therewith is a small deflection on the returning limb 

 of the primary response (lower lejl). [From Amassian (5).] 



A somewhat similar type of study has been carried 

 out in the cat by Mountcastle et al. (196) working 

 with nerves to muscle. They monitored oscilloscopi- 

 cally the ventral root instead of the stimulated 

 exposed nerve and took as evidence of excitation of 

 the Group III fibers, i.e. "the delta pile,' the ap- 

 pearance of a late polysynaptic spinal cord reflex in 

 the ventral roots. Only at stimuli .sufficiently intense 

 to excite these fibers did they evoke potentials in 

 the contralateral cerebral cortex. Responses were 

 seen in both somatic areas I and II; they were of 

 higher amplitude in area II. Their latency at 18 to 

 19 msec, was about twice that of cortical potentials 

 seen upon stimulation of adjacent cutaneous nerves. 

 Because the small myelinated fibers in these muscle 

 nerves are from i to 8 /x in diameter and conduct 

 up to 40 m per sec, it was thought that their im- 

 pulses might include some of nociceptive character. 



Second Sensory Area in Man 



Penfield & Rasmussen (211) have shown in man 

 that sensation can be evoked from the secondary 

 sensory area at the lowest part of the postcentral 

 gyrus extending into the superior lip of the Sylvian 

 fissure to include part of the parietal operculum. 



