846 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY II 



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FIG. 7. Effect of prccentral ablation on coitically evoked pyramidal discharges in monkey (Dial 

 anesthesia). Recordings from contralateral spinal cord (Ci). Upper set of traces shows responses to 

 stimulating si.\ postcentral and seven precentral foci shown in inset at right. Lower set of traces shows 

 responses from stimulating same postcentral foci after ablation of cortex enclosed by dotted line in 

 inset. Right lower trace is recording following stimulus to precentral white matter exposed by abla- 

 tion. 



physical spread, neural spread, as indicated in figures 

 6 and 7, is extensive. Figures 8 and 9 show that even 

 under barbiturate anesthesia such neuronal spread 

 may cross functional as well as anatomical boundaries; 

 in this experiment, the leg area was mapped while 

 the recording electrode was installed in the lumbar 

 lateral column. Lumbar D waves were recorded onlv 

 following stimulation of the medially situated leg sub- 

 division, but shocks in the arm subdivision caused ap- 

 preciable indirect firing of Betz cells having impulses 

 which were destined for lumbar segments. 



It seems likely that such interareal spread accounts 

 for some of the confusion concerning the topographical 

 organization of the motor cortex (69, 70). Strong 

 stimulation and light anesthesia mask discrete cortical 

 representation by favoring interareal spread. In un- 

 anesthetized monkeys (an ideal preparation for inter- 

 areal spread), Lilly (71) evoked movements by stimu- 

 lating virtually any part of the cortex, including visual 

 and auditory receiving areas. While some of the 

 explored regions may provide sufficient 'extrapyrami- 

 dal' projections to account for the observed movement, 

 it is also likely that interareal connections with 

 pyramidal projection zones participate. If this be true, 

 the wisdoin of designating such areas 'motor' may be 

 questioned, for they are really afferent to the projec- 

 tion areas. 



Another method of delineating the pyramidal pro- 

 jection areas of the cortex consists of mapping the 

 cortical responses to antidromic stimulation of the 

 bulbar pyramid (fig. 10). This method was first used 

 by Woolsey & Chang (112) and has been used 

 repeatedly by others (24, 51, 55). In the cat, the maps 

 agree fairly well with those obtained by orthodromic 

 stimulation. Landau (55) questions the extent of the 

 projection zone into the postcruciate cortex, ascribing 

 the potentials recorded in somatosensory area I to a 

 combination of volume pickup from precruciate pro- 

 jections (this is diminished by differential recording 

 between the cortical surface and white matter) and 

 stimulus spread to the medial lemniscus. While both 

 of these factors are undoubtedly likely to complicate 

 antidromic potential configurations, there is no doubt 

 that somatosensory area I contributes a significant 

 number of axons to the pyramid in the cat. Using 

 microelectrode unitary recording techniques, Patton 

 & Towe (unpublished observations) found that of 310 

 cortical units isolated within the arm subdivision of 

 area I, 79 could lie fired antidromically by pyramidal 

 stimulation. 



In the monkey, maps of pyramidal projections 

 (fig. 10) derived from antidromic stimulation reveal a 

 more extensive pattern than that indicated by map- 

 ping D wave sources (figs. 6, 7). The largest potentials 



