THE PYRAMIDAL TRACT: ITS EXCITATION AND FUNCTIONS 



847 



FIG. 8. Simultaneously recorded pyramidal responses from 

 the lateral column between Ci and C2 (upper traces) and the 

 lateral column between L2 and L3 (lower traces) in the monkey 

 (Dial anesthesia). Stimulus foci in precentral gyrus shown in 

 drawing above and enlarged sketch below. Note that stimulation 

 of foci below the superior precentral dimple yields only I waves 

 in lumbar lead. Picture above 10 msec, time scale shows re- 

 sponses to single shock on right; to two shocks on left. Lumbar 

 test response is obliterated; cer\ical test response reduced to D 

 component. 



were found in the precentral gyrus and the rostral 

 part of the postcentral gyrus, but small deflections 

 were recorded from the entire parietal lobe and from 

 the cortex rostral to area 4. The reasons for the dis- 

 crepancy between the results obtained by the two 

 methods is not clear, but the complexity of the anti- 

 dromic potential wave and the possible errors de- 

 scribed by Landau (55) indicate the need for further 

 work. 



ACTIVATION OF PYRAMIDAL TRACT BY 

 CORTICOPET.AL AFFERENT SYSTEMS 



Direct cortical stimulation is an unnatural means 

 of exciting the pyramidal tract. A more nearly natural 

 approach is to excite Betz cells via corticopetal 

 (thalamocortical, callosal, etc.) pathways. As first 

 shown by Adrian & Moruzzi (i), in cats anesthetized 

 with chloralose, a volley fired into the ascending 

 systems, evoked by a shock to a peripheral nerve or to 



skin, generates a corticofugal discharge which can be 

 recorded from the bulbar pyramid. The input to the 

 cortex may be monitored by lemniscal, thalamocorti- 

 cal or surface cortical recordings; the cortex may thus 

 be analyzed as a reflex center. Figure iiA shows the 

 responses to superficial radial nerve stimulation re- 

 corded by an electrode inserted to the positions shown 

 in figure iiB and C. When the electrode is in the 

 pyramid, the response usually consists of a simple 

 positive or positive-negative wave of smooth contours 

 suggesting asynchronous firing of pyramidal axons 

 over a period of 15 msec, or more. Each pyramid 

 discharges to a stimulus delivered to either side of the 

 body, but the ipsilateral response is usually more 

 labile and of longer latency (15 to 20 msec, for fore- 

 limbj than the contralateral response (10 to 12 

 msec). Both ipsilateral and contralateral responses 

 are readily abolished by asphyxia, barbiturate injec- 

 tion or an undercutting of the sensorimotor cortex 

 ipsilateral to the recording site (fig. 12). Volleys 

 originating in the hind limb, the forelimb, or in cu- 

 taneous or muscle branches of nerve trunks are 

 equally effective in provoking the discharge. 



When the electrode is inserted dorsal to the pyra- 

 mid, the corticofugal discharge is smaller, but an 

 early deflection appears about 5 msec, earlier (figs. 

 II, 19). Sometimes this early deflection is recorded 

 when the electrode tip is in the pyramid (particularly 

 if a deep puncture has previously been made), but it 

 is always of maximal amplitude when the electrode 

 is about 1.5 to 2.0 mm dorsal to the pyramidal sur- 

 face. Since this is the location of the medial lemniscus, 

 the early deflection is reasonably ascribed to ascending 

 lemniscal impulses rather than to a 'pyramidal aflfer- 

 ent system,' as described by Brodal & Kaada (16). 

 In barbiturate-anesthetized or decorticated prepara- 

 tions in which the corticofugal discharge is lacking, the 

 lemniscal discharge may be recorded in isolation 

 (fig. 12). 



The pathways for the ipsilateral corticofugal dis- 

 charge are of interest because the afferent pathways 

 to the sensory cortex are thought to be crossed for the 

 most part. Chronic ablation of somatosensory area II, 

 which receives an uncrossed input, does not abolish 

 the pyramidal discharge to ipsilateral sensory stimu- 

 lation. Although a callosal volley is capable of initiat- 

 ing a pyramidal discharge, the ipsilateral response to 

 forelimb afferent stimulation is independent of the 

 contralateral hemisphere because ipsilateral responses 

 persist in chronically hemidecorticate cats. The ipsi- 

 lateral response must therefore depend on an un- 

 crossed afferent pathway to the cortex. That the 

 uncrossed pathway is capable of exciting cells in soma- 



