SENSORIMOTOR CORTICAL ACTIVITIES 



817 



We will consider here only intracortical and inter- 

 cortical connections. Studies with local strychniniza- 

 tion (131, 295) have brought only limited knowledge. 

 Significant difficulties beset this method (99, 155, 

 ■299). Onlv the anatomical investigations appear to 

 fulfill the essential criteria of accuracy, but even here 

 discrepancies arise. 



Area 4 is extensively connected within itself (cf. 

 240, 321-324) and also receives abundant fibers froixi 

 postcentral areas i, 2 and 3 (240, 307, 321-324, 353J, 

 as well as from area 5 and 7 (cf. 353). The supple- 

 mentary motor area also sends fibers to area 4 ( 121 ). 

 Connections seem to e.xist between the paracentral 

 lobule and the cingulate cortex (240). In turn, area 4 

 projects to area 6 and to more rostral regions (cf. 321- 

 324) as well as to the parietal con\olutions 5 and 7 

 (353). Numerous association fibers go to the post- 

 central gyrus (298; cf. 240, 321-324) and form with 

 those running in the opposite direction a large U- 

 shapcd bundle (438, 440). In addition to intra-areal 

 connections (240), area 6 receives fibers from area 4 

 and also from temporal (302, 468) and parietal areas 

 (314, 353). Other connections to area 6 have been 

 described with strychninography (468). Area 6 sends 

 fibers to the postcentral and parietal areas (321-324) 

 as well as to area 4 (240, 307). 



There are extensive connections within the parietal 

 areas including those between i, 2, 3, 5 and 7, with 

 area 3 sending fibers to area 5, and areas i and 2 to 

 area 7 (301, 314, 321-324, 353). In turn, area 5 

 projects to area 2 to a greater extent than to areas 

 3 and I . Areas 5 and 7 are abundantly interconnected, 

 and area 7 sends fibers to i and 2. Fibers froin parietal 

 areas have been described as reaching the superior 

 temporal gyrus and the occipital lobe (353)- Histo- 

 logical and strychninography studies, .giving a more 

 detailed description of the connections of precentral 

 and postcentral areas and their subdivisions, may be 

 consulted (36, 37, 135, 23B, 239, 294, 295, 300, 415). 



Calloiol Interhemispheric Connections 



Precentral and postcentral motor and sensory cor- 

 tex receive through the corpus callosum numerous 

 fibers from the cortex of the opposite hemisphere. 

 This commissure, mainly concerned in cortical inter- 

 hemispheric connections, is seen only in placental 

 mammals and not in the monotreines or marsupials. 

 In the latter, however, motor and sensory cortical 

 areas are closely interconnected through the un- 

 usually large anterior commissure (Adey, VV. R., 

 unpublished observations). 



Most authors agree that callosal fibers arise from 

 small and medium-sized pyramidal cells, the majority 

 of which are located in the fifth and sixth layers. 

 Callosal fibers terminate in fine filaments in the super- 

 ficial layers of the corte.x, mostly superficial to layer 4 

 (cf. 58 for references). The existence (435-436) of an 

 increased number of terminal arborizations at the 

 cortical level at which the large pyramidal neurons 

 are located is disputed (86), but physiologicallv there 

 is clear exidence that callosal \'olleys will activate 

 neurons which send their axons into the pyramidal 

 tract (175). (See also Chapter XXXI\' b\- Patton & 

 Ama.ssian in this work.) Electrical stimulation (87, 

 107, 108, 260, 352; cf. 55, 58) or strychninization 

 (34, 167, 181; cf. 55, 58) of restricted points on the 

 cortex of one hemisphere results in contralateral 

 cortical responses which are usually maximal in the 

 homotopic point but may also be distributed in a 

 heterotopic fashion. These investigations show 

 inequalities in the density of callosal connections 

 between homologous areas. In the Rolandic motor 

 area, face, neck and trunk subdivisions have a dense 

 callosal interconnection (34, 108, 167). These areas 

 are involved in motor functions usually requiring 

 synergism between the two sides of the body. These 

 findings contrast sharply with the arrangement in leg 

 and arm areas where no callosal fibers are present. 

 While these data are in agreement with anatomical 

 observations (239, 240), in other aspects discrepancies 

 exist. Histological data may be regarded as more 

 reliable. Numerous callosal fibers were found by 

 Minkowski (321-324) and confirmed by Milch (314) 

 and Peele (353) interconnecting the pre- and post- 

 central gyri of the two hemispheres but have not been 

 seen by Krieg (240). Extensive connections between 

 area 6 in the two hemispheres have been described 

 (240, 307). In the human brain many callosal fibers 

 emanate from area 6 (319). Connections between 

 area 5 and contralateral areas 5, 3, i and 2 have 

 been described, and also a few with 4 (239, 314, 353). 

 Peele (353) has observed projections from area 7 to 

 contralateral areas 7, 5, 2 and i in decreasing order 

 of significance. 



The study of the functions of these callosal inter- 

 connections probably began in 1879 with Brown- 

 Sequard (6g). Bremer et al. (58) have reviewed the 

 literature in this field. In 1939 Moruzzi demonstrated 

 in the rabbit that subliminal stimulation of the corti- 

 cal area from which masticatory movements can be 

 obtained reduces the threshold of the homologous 

 area of the opposite hemisphere to electrical stimula- 

 tion (330). In the cat, a sustained facilitation of the 



