THE PYRAMIDAL TRACT: ITS EXCITATION AND FUNCTIONS 845 



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FIG. 6. Map of pyramidal responses evoked by stimulating different cortical foci in Dial-anesthe- 

 tized monkey. Responses recorded from contralateral spinal cord iC;) are superimposed on stimulus 

 foci. Pulse below each trace is stimulus current, which was kept constant ie-\cept in one of the 

 lower traces) at 6 ma (duration, o.i msec). Major cortical markings from left to right: intraparietal 

 sulcus, central sulcus, arcuate sulcus. 



face receiving zone. Direct pyramidal responses from 

 the face corte.x are presumably excluded by bulbar 

 recording. The data agree approximately with those 

 of Lance & Manning (51). 



Surrounding the regions from which D activity can 

 be elicited is a fringe, variable in extent, from which 

 only I activity inay be evoked. Presumably these 

 regions have few or no pyramidal projections but are 

 connected with projection areas through cortical 

 interneurons. The size of the T fringe' is variable but 

 is most extensive under light anesthesia. 



In the monkey (81), the major projection zone is 

 precentral (fig. 6); stimulation on the postcentral 

 gyrus and the parietal cortex elicits a pyramidal 

 response which is dominated by I acti\ity, and which 

 is largely abolished by removal of the precentral 

 gyrus (fig. 7). This is surprising in view of the ana- 

 tomical claim that the postcentral and parietal cortex 

 contribute axons to the pyramid; it is possible that 



such contributions are derived from small cells not 

 easily excited by the rather weak stimuli employed in 

 the mapping experiments. 



A feature of interest is the \'ery extensive I fringe in 

 the lightly anesthetized monkey (figs. 6, 7, g). Even 

 with weak stimulation (far below the threshold for 

 movement), pyramidal projection cells are excited by 

 shocks applied to cortical areas remote from the main 

 projection areas. Such connections between remote 

 zones and motor projection areas, although not sur- 

 prising, raise problems in the interpretation of map- 

 ping experiments in which muscle contraction is used 

 as an end point. Firstly, the strength of stimulus re- 

 quired to produce movement incurs extensive physical 

 spread of stimulating current; indeed, Phillips (86) 

 found that single shocks in the motor cortex of the 

 cat sufficient to produce movement (10 msec, pulse, 

 740 amp.) caused direct firing of Betz cells virtually 

 throughout the motor cortex. Quite apart from 



