THE PYRAMIDAL TRACT: ITS EXCITATION AND FUNCTIONS 



857 



CONTRA 



IPSI 



■ AFFERENT WAVE 



"""•">— I —CORTICOFUGAL l^' 



FIG. 19. Bulbar responses to forepaw and cortical stimulation 

 in cats anesthetized with chloralose. Upper. Responses to contra- 

 lateral and ipsilateral forepaw stimulation recorded at the 

 points indicated in the drawing. Depth from surface of pyramid 

 indicated by numbers at /(//. Note different locations for maximal 

 recording of afferent wave and corticofugal wave. Lower. Left 

 column shows D-I complexes evoked by stimulating the ipsilat- 

 eral area I; right column, responses to contralateral forepaw 

 stimulation. Depths indicated by numbers. Note different 

 locations for maximal recording of afferent wave and D-I 

 complex (corticofugal wave is small because of cortical expo- 

 sure). 



TOPOGRAPHICAL ORGANIZATION AND COURSE 

 OF PYRAMIDAL TRACT 



It has been pointed out previously that the cortical 

 origins of the pyramidal fibers destined for the cervical 

 and lumbar spinal segments are recognizably distinct 

 (although overlapping at the borders) and correspond 

 to the classical arm and leg motor subdivisions as 



PYRAMID—" 



-\ 



— AFFERENT WAVE 



— D WAVE 



1.0 2.0 1.0 2.0 



MM. FROM SURFACE 



FIG. 20. Graphic representation of data from fig. 19. Ordinate, 

 size of response; abscissa, depth of recording electrode in milli- 

 meters from the pyramidal surface. 



determined by mapping foci for movement.'" Indeed, 

 such mapping experiments indicate an overlapping 

 mosaic of cortical foci controlling individual muscles 

 (26). Unfortunately, the attractive and likely hypoth- 

 esis that pyramidal origins are similarly discretely 

 localized cannot be tested by experiments in which 

 both pyramidal and extrapyramidal origins are ex- 

 cited. 



To what extent is the cortically established separa- 

 tion of arm and leg pyramidal P.bers maintained in the 

 descending course of the tract? According to Barnard 

 & Woolsey (6), who traced Marchi degeneration in 

 monkeys following carefulK' controlled discrete corti- 

 cal lesions, some degree of topographical organization 

 is seen in the internal capsule but with considerable 

 overlap. Leg fibers are situated most caudally in the 

 posterior limb of the capsule, with fibers for distal 

 arm and proximal arm located more rostrally in that 

 order. At the midbrain and pontine levels, the separa- 

 tion becomes less distinct; and at the bulbar and 

 spinal levels, fibers destined for the arm and leg seg- 

 ments are thoroughly mixed. Both Tower (103) and 

 Mettler & Mettler (77) observed that deficits resulting 

 from partial pyramidal lesions were equally prominent 

 in forelimb and hindlimb. 



'" This fact has been questioned by Glees & Cole (40) on the 

 basis of finding 17 degenerated fibers (Marchi stain) in the 

 lumbar lateral column of monkey following a cortical lesion 

 said to be restricted to the arm and face motor disisions. Even 

 discounting the capricioiisness of the method and the oppor- 

 tunity for injury to cortical regions other than those intention- 

 ally ablated, this finding appears an inadequate challenge to 

 the doctrine of topographical organization; no one seriously 

 questions some overlap of cortical topographical regions. That 

 pyramidal fibers destined for lower cord segments do not give 

 off a significant number of excitatory collaterals in the cervical 

 region was first shown by Frohlich & Sherrington (36) who 

 noticed that antidromic corticospinal stimulation in the oral 

 end of the severed thoracic cord produced no forelimb move- 

 ment. 



