CHAPTER XXXIV 



The pyramidal tract: its excitation and functions' 



HARRY D. PATTON 



V A H E E. A M A S S I A N^ 



University of Washington School of Medicine, Seattle, Waslinigton 



CHAPTER CONTENTS 



Anatomical Features 



Pyramidal Deficit; Section of Pyramid 



Stimulation of the Pyramids 



Cortical Excitation of Pyramidal Tract 



Origin of Pyramidal Tract 



Activation of Pyramidal Tract by Corticopetal Afferent Systems 



Timing of Betz Cell Discharge 



Betz Cell Spike 



Recurrent Axon Collaterals of Betz Cells 



Pyramidal Fiber Sizes and Conduction Velocities 



Afferent Fibers in Pyramids 



Topographical Organization and Course of Pyramidal Tract 



Spinal Mechanism of Pyramidal Tract 



ANATOMICAL FEATURES 



THE PYRAMIDAL TRACT is primarily an anatomical 

 rather than a physiological entity. Strictly defined, 

 it coinprises those neurons with descending axons' 

 which traverse longitudinally the bulbar pyramids 

 (fig. i). Excluded are the thin strands of external 

 arcuate fibers which stream transversely over the sur- 

 face or, sometimes, through the substance of the 

 pyramid (fig. i); this exclusion is appropriate physio- 

 logically as well as anatomically. Inappropriately ex- 

 cluded by strict anatomical definition, however, are 



' Previously impublished work of the authors of this chapter 

 was supported in part by a research grant (B395) from the 

 National Institute of Nem'ological Diseases and Blindness of the 

 National Institutes of Health, Bethesda, Maryland. 



^John and Mary R. Markle .Scholar in Medical Science. 

 Now at the Department of Physiology, Albert Einstein College 

 of Medicine, Yeshiva University, New York City. 



■' The question of ascending fibers in the pyramid is discussed 

 below. 



the corticofugal fibers which supply cranial motor 

 nuclei. These fibers ("aberrant pyramidal bundles'), 

 although presumably bearing the same functional re- 

 lationship to cranial motor nuclei that the cortico- 

 spinal fibers bear to spinal motor nuclei, depart from 

 the main tract at the level of the pons and do not 

 traverse the bulbar pyramid (iio, p. 10). Inappro- 

 priately included are corticobulbar fibers which de- 

 part from the pyramid to terminate in the overlying 

 reticular formation (95, 96), and hence are at least 

 potentially pathways of the 'extrapyramidal' type. 

 Thus, even at the bulbar level, where the tract is 

 purest, it is both contaminated and incomplete. 

 Figure i shows the relationship of the cat pyramid to 

 other bulbar structures at the level of the inferior 

 olive. The right pyramid is degenerated owing to 

 ablation of the ipsilateral cortex one month previ- 

 ously. The histological change is evident, but the 

 tract has lost little bulk, being about i mm thick; 

 with a longer degeneration period, the pyramid 

 shrinks grossly. The undegenerated fibers dorsal to 

 the pyramid, and interposed between it and the olive, 

 constitute the medial lemniscus. The proximity of the 

 two tracts persists throughout the bulbar extent of the 

 pyramid (41). The picture emphasizes the difficulty 

 of either selective stimulation or selective section of 

 the pyramid. Moreover, the proxiinity of the pyramid 

 to such active structures as the lemniscus and the 

 reticular formation (the entire dorsoventral extent of 

 the bulb is only 4 to 5 mm), all immersed in an ex- 

 cellent conductor, indicates the necessity for careful 

 depth measurements and differential recording in 

 studies designed to measure electrical activity of the 

 pyrainids. With the exception of Lloyd's study (72) 

 these pitfalls have rarely been taken fully into ac- 



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