THE PYRAMIDAL TRACT: ITS EXCITATION AND FUNCTIONS 



841 



Although the physiological role of the pyramidal 

 tract remains problematical, the results of pyramid- 

 otomy and of pyramidal stimulation permit some 

 general conclusions. First, it appears clear that 

 voluntary movement is less dependent on pyramidal 

 function than is commonly supposed; the special 

 contribution of pyramidal impulses appears to be fine, 

 precise control, particularly of the distal musculature. 

 Secondly, apart from reciprocal effects, the prepon- 

 derant spinal action of impulses traversing the pyra- 

 mids appears to be e.xcitatory. There remains the 

 distinct possibility, however, that supraljulbar col- 

 laterals of descending excitatory axons may feed into 

 descending systems which have an opposite effect on 

 the excitability of spinal motoneurons. 



CORTICAL EXCITATION OF PYRAMIDAL TRACT 



FIG. 2. Pyramidal tract responses to stimulation of the motor 

 cortex and white matter in monkey (Dial anesthesia). Re- 

 cording electrode in lateral column at C|. Downward deflection 

 in this and all subsequent hgures indicates positivity at the 

 exploring electrode. Left, stimulus to the contralateral motor 

 cortex; D and I wa\'es are labelled. Right, stimulus to white 

 matter after ablation of motor cortex, only the D wave persists. 

 Time, i msec. 



Although direct cortical stimulation has been ex- 

 tensively used to study motor areas since 1870, record- 

 ing from the pyramidal tract was not attempted until 

 the pioneering studies by .\drian & Moruzzi ( i ) in 

 1939. Unfortunately, they chose to record from the 

 region of pyramidal decussation and, consequently, 

 their recordings were contaminated with activity of 

 the liulbar reticular units through which the decussat- 

 ing pyramidal fibers pass (80). Electrodes placed in 

 the bulbar pyramid or the lateral column of the cord'^ 

 record a characteristic configuration when a single 

 shock of short duration (o.i msec.) is applied to the 

 cortex (80). The initial deflection is a stable positive 

 wave (fig. 2) with a latency of about 0.7 msec, at the 

 bulbar level and about i msec, at cord segment Ci; 

 this is followed, some 2.0 to 2.5 msec, later, by a 

 series of variable positive deflections which recur at 

 intervals of about 2.0 to 2.5 msec. The initial deflec- 

 tion occurs after a latency too brief to allow both 

 conduction and synaptic transfer, readily follows 

 stimulus repetition rates up to 400 to 500 per sec, 

 and can be elicited by stimulation of white matter 

 after removal of the cortex (fig. 2). This deflection 

 therefore represents activity directly initiated in Betz 

 cells' by the cortical shock and accordingly is called 



^ Cord recording also carries the hazard of contamination 

 of response. .All critical observations reported here have been 

 carefully checked by bulbar recording. 



' It is unfortunate that there is no acceptable term for corti- 

 cal cells projecting into the pyramid; the term 'Betz cell' pre- 

 sumably applies only to the largest of such cells, although the 

 size limit has never been clearly defined (106). In any case, 

 cells with cross-sectional areas of goo to 1400 ^i^ account for 

 only a small percentage of the pyramidal axons; hence, many 



the D wa\'e. The deflections following the D wave 

 vary in amplitude, fail to follow high stimulus repeti- 

 tion rates, and are more susceptible than the D waves 

 to asphyxia, anesthesia and cortical injury. All these 

 properties suggest activity resulting from synaptically 

 relayed excitation of Betz cells, and the late waves 

 are therefore referred to as I (for indirect) waves. 



In our original analysis we suggested that the I 

 waves were not significantly contaminated by activity 

 of directly excited, slowly conducting fibers because 

 the pyramidal response to white matter stiinulation 

 failed to reveal much late activity (figs. 2, 3). Never- 

 theless, both anatomical (20, 62) and functional (13) 

 studies indicate the presence of many slowly-conduct- 

 ing fibers in the pyramids. Moreover, Patton c& Towe 

 (unpublished observations) have isolated a few cortical 

 units capaljle of following antidromic pyramidal stim- 

 ulation rates up to 100 per sec, which have bulb-to- 

 cortex conduction times up to 7 to 8 msec. Directly- 

 evoked activity in such slowly-conducting units may 

 well reach the pyramid simultaneously with I activity 

 of more rapidly-conducting units. However, it is 

 quite clear that directly-excited fibers of slow conduc- 

 tion rates can account for only a small amount of the 

 I activity. 



The cellular elements that bombard the Betz cells 

 to produce I discharges are located in the cortex, for 

 there are no I waves in the pyramidal discharge 

 evoked bv stimulation of the white matter after the 



smaller cells must contribute. In this discussion, the term 'Betz 

 cell' has been used to indicate any cortical cell, irrespective of 

 size, with the axon transversing the pyramid. 



