SENSORIMOTOR CORTICAL ACTIVITIES 82 1 



in the fusiform layer. In Peele's view, this parietal 

 projection system may form a mechanism for sensiti- 

 zation of sensory neurons. Confirmatory evidence of 

 the existence of parietospinal projections in the 

 monkey comes from the finding of Levin & Bradford 

 (265) that degeneration of cells in areas 3, i, 2 and 



5 follows hemisection of the cord at the fourth cervical 

 segment. 



By contrast with tliese findings in the primate, 

 Chambers & Liu (85) found no evidence in the cat 

 for an origin of the pyramidal tract from parietal 

 areas, nor from occipital and temporal areas. They 

 concluded that it arises only in the sigmoid, coronal 

 and anterior ectosylvian gyri. 



EVIDENCE CONCERNING PYRAMIDAL TRACT FIBERS 



ARISING IN TEMPORAL AND OCCIPITAL LOBES. Ap- 

 proximately one-third of the fibers of the pyramidal 

 tract remain after complete frontal lobotomy in the 

 monkey. All are fine fibers and all are of cortical 

 origin (257). Many of the remainder may take origin 

 in the parietal lobe, but some at least may arise in 

 temporal and occipital areas. Evidence for a temporal 

 and occipital origin for these fibers is scanty. Walberg 



6 Brodal (452), using the Glees method of silver 

 impregnation in the cat, have traced degenerating 

 fiijers via the internal capsule, cerebral peduncle, 

 longitudinal bundles of the pons and medullary 

 pyramid to the lumbar le\el of the cord. .Some of 

 these fibers passed via the corpus callosum to the 

 contralateral internal capsule to descend in the 

 contralateral cerebral peduncle and pyramid. Many 

 of these fibers crossed in the pyramidal decussation 

 to descend in the opposite lateral corticospinal tract, 

 but the ipsilateral lateral corticospinal tract and both 

 ventral corticospinal tracts all sliowed degenerating 

 fiijcrs. 



While the precise extent to which cortical regions 

 outside the precentral motor area contribute to the 

 pyramidal tract appears to vary in different species, 

 and confirmatory evidence is required concerning 

 the contributions from such regions as the temporal 

 and occipital lobes, there is no evidence that any of 

 the fibers of the tract have a subcortical origin (257, 

 265). 



Descending Connections of Pxramidal System 



While the spinal connections of the pyramidal 

 tract are described in detail in Chapter XXXI\" by 

 Patton & Amassian in this work, attention may be 

 directed here to those pvramidal connections which 



may be particularly concerned in sensorimotor in- 

 tegration. 



EFFECTS OF STIMULATION AND SECTION OF CEREBRAL 



PEDUNCLE. Section of the peduncle has attracted 

 attention as a means of relieving involuntary move- 

 ment (cf 259). Stimulation with a 60 cycle sine wave 

 current gives rise to movement in the human only 

 from the intermediate portions of the peduncle, 

 with movements of the lower limb evoked from more 

 lateral zones than for the upper limb. Section of the 

 peduncle to a depth of 7 mm (probably involving the 

 substantia nigra) in the lateral half to four-fifths 

 produced only a transient weakness disappearing in a 

 few months in many cases. The relatively small 

 amount of paresis seen from such a section may be 

 due to the cut not involving the anticipated portions 

 of the peduncle by reason of the small size of the 

 peduncle in the.se cases or by the displacement of 

 an abnormal peduncle. Where excitation rostral 

 to the cut was continued during sectioning until 

 movements were practically abolished, the ensuing 

 hemiparesis was then usually moderate and long- 

 lasting (cf. 259). It is suggested from these results 

 that damage to these fibers at their cortical origin 

 gives rise to much more severe and lasting paresis 

 than interruption of the fibers at any other point in 

 their course. This may be explained in terms of 

 'pyratnidal' and "extrapyramidal" cells in the cortex 

 being capable of excitation of spinal motor units via 

 other pathways. The lack of paresis seen here following 

 lesions of the pyramidal tract does not support the 

 notion of a precise somatotopic cortical representa- 

 tion. There is some evidence of somatotopic locali- 

 zation in the internal capsule, cerebral peduncle and 

 pons of the monkey and rat, but the overlap is con- 

 siderable and increases from above downwards (39). 

 In similar studies of the effect of section of the basis 

 pedunculi in inonke\s (76), the paralysis was inter- 

 mediate between a spastic and a liypotonic paresis, 

 and was characterized Idv a hypotonicity of all 

 mu.scle groups excepting the extensors of the digits, 

 by hyperactive deep reflexes and by absence of clonus. 

 It is assumed from these results that inhibitory path- 

 ways descending from the cereljral corte.x do not 

 course exclusi\ely within the basis pedunculi. The 

 majority of the fibers, the interruption of which leads 

 to hypertonicity, and also those mediating clonus 

 would appear to have deviated from the corticospinal 

 projection prior to reaching the cerebral peduncle. 

 Those, the interruption of which leads to hyper- 

 reflexia, accompany the corticospinal projection 



