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HANDBOOK OF PHVSIOLOGV 



NEUROPHYSIOLOGY II 



cally activate collateral pathways. For example, such 

 stimulation evokes in the bulbar reticular formation 

 and the cervical \agus nerve electrical activity which, 

 on the basis of latency and failure to follow high 

 stimulus repetition rates, traverses at least one synapse 

 (Mahnke, Nelson & Patton, unpublished oljserva- 

 tions). However, it is difiicult to eliminate the possi- 

 bility of stimulus spread to the adjacent lemniscus and 

 reticular formation and thus clearly to implicate 

 pyramidal collaterals. 



STIMULATION OF THE PYRAMIDS 



Since the discovery of the motor cortex by Fritsch 

 & Hitzig (35), the movements resulting from stimula- 

 tion of cortical motor foci have been repeatedly 

 studied and mapped (see the preceding chapter). 

 Valuaiile as such studies are, they do not give clear 

 information (as is commonly erroneously supposed) 

 on the role of corticospinal function. Cortical stimula- 

 tion obviously excites 'extrapyramidal' as well as 

 pyramidal pathways (80), and strong participation by 

 the former in initiating movement is indicated by the 

 fact that cortical stimulation provokes movement 

 after chronic pyramidotomy (100). To study the 

 pyramidal contribution to movement requires stimu- 

 lation at the bulbar level where the tract is uncon- 

 taminated. Movement patterns resulting from bulbar 

 pyramidal stimulation were analyzed by Brookhart 

 (18) and by Landau (52), using multiple electro- 

 myography in monkeys anesthetized with barbitu- 

 rates and in decerebrate cats. Such preparations have 

 the ad\'antage that descending pathways activated by 

 antidromic invasion of pyramidal collaterals in the 

 pons are not excluded but present the disadvantage 

 that the hazard of stimulus spread to adjacent struc- 

 tures is uncontrolled. This difficulty can only be cir- 

 cumvented by stimulation rostral to a bulbar transec- 

 tion sparing only the pyramids (72) and sacrificing 

 collateral pathways. 



A striking characteristic of pyramid-evoked move- 

 ment is the need for temporal summation; electro- 

 myographic signs of contraction occurred only when 

 the pyramid was repetitively shocked. In the monkey 

 (18), the duration of a train of given frequency and 

 pulse duration required to produce contraction is 

 characteristic for a particular muscle and varies for 

 different muscles; reducing the threshold train by a 

 single pulse prevents onset of contraction. For all 

 muscles, increasing the frequency of stimulation de- 

 creases the train duration needed to produce contrac- 



tion. Increased pulse duration markedly shortens the 

 train-duration threshold for proximal muscles but is 

 much less effective in shortening the train-duration 

 threshold for distal musculature, e.g. hand and finger. 

 Direct recording from the pyramid indicates that the 

 greater effectiveness of long (2.0 msec.) compared to 

 short (o.i msec.) pulses is largely attributable to re- 

 cruitment of slowly conducting (hence presumably 

 small-diameter) fibers by the longer pulses. It may 

 thus be argued that the larger fibers are primarily 

 concerned with the control of distal musculature and 

 the more numerous, small fibers with the larger 

 proximal muscles (18). 



Landau (52) stresses the variability of movement 

 patterns elicited by pyramidal stimulation in decere- 

 brate cats. In different animals and from time to time 

 in the same preparation, the sequence of muscle 

 activation varied independently of such controllable 

 factors as anesthesia, posture, and locus and parame- 

 ters of stimulation. Landau ascribes this variability 

 to the spinal internuncial system on which the pyram- 

 idal efferents play. Although the internuncials inter- 

 posed between pyramidal endings and motoneurons 

 undoubtedly modulate the transfer of impulses (72), 

 some of the variability of Landau's experiments is 

 more readily explained by assuming varying spread 

 of stimulus to adjacent structures. For example, the 

 organized movements ('walking, batting, digging, 

 scratching') that occurred resembled more the reflex 

 results of afferent stimulation than the response to 

 stimulation of an efferent pathway. The latter possi- 

 bility is particularly unlikely because, although there 

 is evidence that individual pyramidal fibers arising 

 from different topographically organized cortical 

 areas do not overlap extensively in their spinal distri- 

 bution (26), these fibers are thoroughly mixed at the 

 bulbar lev-el (6, 77, 103). Thus, the chance that 

 stimulation at this level would excite fibers selectively 

 and in the proper temporal sequence to produce 

 complex, organized movements appears remote. On 

 the other hand, stimulus spread to the lemniscus, with 

 collateral activation of the reticular formation, might 

 well generate such reflex patterns. 



In addition to contraction of skeletal muscle, py- 

 ramidal stimulation is reported to cause changes in 

 autonomic effectors (53), including sweating (galvanic 

 skin response), piloerection, pupillary dilation, and 

 alterations in arterial pressure, heart rate, intravesical 

 pressure and gastric rhythms. Although the responses 

 were said to be abolished by pyramidotomy below 

 the point of stimulation, the variability in respon.se 

 raises again the question of stimulus spread. 



