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HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY II 



ever, they number only 25,000 in each hemisphere, 

 and since there are approximately one million fibers 

 in the human pyramidal tract in the upper medulla, 

 many of these fibers take origin in other cells, von 

 Monakow (450) pointed out that these cells could not 

 be the only source of pyramidal fibers since very few 

 Betz cells occur in the main face and arm motor 

 strips. This view was supported by Mettler's (304) 

 finding that progressive impairment of movement, 

 rather than progressive paralysis, follows removal of 

 increasing portions of one hemisphere in the monkey. 



EXTENT OF FRONTAL LOBE CONTRIBUTIONS TO PYRAM- 

 IDAL TRACT. On the basis of regeneration studies, 

 Flechsig (146) concluded that the greater part of the 

 pyramidal tract in man comes from the precentral 

 gyrus. Degeneration is severe when the lesion involves 

 the upper and middle thirds of this gyrus and the para- 

 central lobule; but after lesions affecting the lower 

 third, the number of degenerating fibers in the pyra- 

 mid is so small that usually a distinct area of degen- 

 eration cannot be found in the pyramids. Mettler 

 (305) removed area 4 in the macaque and found that 

 the myelinated fibers in the medullary pyramid were 

 completely degenerated. Lassek (256) found in the 

 macaque that extirpation of area 4 causes degenera- 

 tion of one-third of the pyramidal fibers just rostral 

 to the decussation and that these are the largest 

 fibers. 



The extent to which the premotor cortex, lying 

 anterior to the precentral gyrus, contributes to the 

 pyramidal tract is still debated. Bilateral excision 

 of area 6 in a human patient with parkinsonism leads 

 to degenerating axons in the pyramidal tract (319). 

 Minkowski (321-324) traced degenerating fibers in 

 the pvramidal tract of the macaque down to the 

 lumbar segments, following lesions in that part oi 

 the frontal lobe lying immediately anterior to the 

 precentral gyrus. Both Hoff (209) and Kennard 

 (229) reported that in the monkey, fibers arising in 

 area 6 could be traced in the pyramidal tract as far 

 as the second sacral segment, and Kennard found 

 that they were intermingled with those from area 4 

 at all levels of the tract. Both these workers found 

 fibers from area 6 running bilaterally in the lateral 

 pyramidal tract. Levin (264) and Levin & Bradford 

 (265) criticized these findings on the basis that the 

 degeneration observed by Kennard and Hofl" arose 

 from incidental damage to area 4. Verhaart & 

 Kennard (433) subsequently repeated these experi- 

 ments with more limited lesions in area 6 and noted 

 a much reduced degree of degeneration in the pyr- 



amid. There is general agreement that in the monkey 

 no region rostral to area 6 contributes to the pyram- 

 idal tract (229, 307). 



Dejerine (114) concluded that the frontopontine 

 bundle joined the corticospinal tract somewhere 

 between the peduncle and the lower pons. This 

 opinion was supported by Verhaart (432) from a 

 study of two gibbons which indicated that some 

 small fibers running with the frontopontine tract 

 leave this tract in the pons and continue caudally 

 with the corticospinal tract. However, Meyer et al. 

 (310) and Beck (42) have found no evidence that 

 Arnold's bundle, which is formed of fibers originating 

 from granular frontal cortex, excluding area 8, 

 sends significant nunii)ers of fibers into the cortico- 

 spinal tract. 



CONTRIBUTION OF POSTCENTRAL CORTEX TO PYRAMIDAL 



TRACT. Melius in 1894 removed a small part of the 

 postcentral thumb area in the monkey and observed 

 degenerating fibers bilaterally in the pyramidal 

 tract (297). On the basis of both animal studies 

 and human pathological material, von Monakow 

 (450) concluded that a small part of the pyramidal 

 tract arises in the parietal lobe. Extensive subcortical 

 connections from areas 5 and 7 were seen by CHark & 

 Boggon (96) and Mettler (301), including fibers 

 running into the peduncle, pons and pretectal region, 

 and into or through the nucleus ventralis postero- 

 lateralis, in a monkey which also had some injury 

 to areas 18 and 19. Biemond (49), Sakuma (389) 

 and Uesugi (429) described similar projections in 

 the macaque, including ipsilateral and contralateral 

 projections into the pyramidal tracts. Sunderland 

 (416) traced a large parietal component through the 

 lateral half of the cerebral peduncle into the pontine 

 gray where most of the fibers terminated but some 

 entered the medullary pyramid. 



Peele (353) has provided an extensive review of the 

 parietal contributions to the pyramidal tract in the 

 monkey. In addition to a parietospinal component, 

 all parietal areas send fibers to the thalamic nuclei, 

 including ventralis posterolateralis and medialis. 

 Fibers from rostral parietal areas terminate rostrally 

 in the nuclei. All parietal areas send fibers to pontine 

 nuclei of the same side, and all areas send fibers 

 through the pyramid to the spinal cord, hi the cord 

 these fibers run in the lateral pyramidal tracts of 

 both sides, the majority running contralaterally. 

 Peele concluded that the parietospinal fibers arise 

 from cells located in the external and internal pyram- 

 idal laminae, whereas parietothalamic fibers arise 



