822 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY II 



through the cerebral peduncle but deviate before 

 reaching the pyramids. 



EFFECT OF SECTION OF MEDULLARY PYRAMID. Section 



of the medullary pyramid of monkeys and chim- 

 panzees is followed by a flaccid paralysis, and the 

 muscles show conspicuous atrophy from disuse (289, 

 422, 423). Tower (422, 423) has interpreted these 

 results as attributable to a deficient function, rather 

 than to a release phenomenon, and concludes that 

 there is no evidence of an inhibitory action by the 

 pyramidal system. Walshe (463) does not accept this 

 conclusion, contending that the prominent Babinski 

 response following pyramidal section in the chim- 

 panzee should be regarded as a release phenomenon. 

 Denny-Brown (117) characterizes Tower"s findings 

 on pyramidal section as "the final blow to the clinical 

 conception of disorder of the pyramidal system," 

 but Walshe considers this premature. 



TERMINATION OF PYRAMIDAL TRACT FIBERS IN MEDULLA. 



It has long been known that the fiber content of the 

 pyramid at the lower end of the medulla is signifi- 

 cantly less than at the upper end. Some of these 

 fibers may end in relation to neurons of the reticular 

 systems of the central zones of the medulla, but there 

 is evidence that many terminate more dorsally 

 among the neurons of sensory nuclei and may, thus, 

 participate more directly in modulating the sensory 

 influx essential to motor mechanisms (85, 243). 

 These investigations have indicated that following 

 various lesions in the cat, including hemidecortication, 

 frontal decortication and selective lesions of either 

 leg, arm or face areas of the motor cortex, degen- 

 erating recurrent as well as transtegmental fibers can 

 be traced from the p\ramidal tract to the region 

 along the ventral aspect and into the hilus of the 

 cuneate and gracile nuclei. Other fibers can be traced 

 to the medial part of the spinal trigeminal nucleus 

 and adjoining lateral parts of the tegmentum. This 

 cortical projection to the rostral part of the spinal 

 trigeminal nucleus and its vicinity originates mainly 

 in the face area, whereas the projections to the cuneate 

 and gracile nuclei have their main origin in the fore- 

 limbs and hind-limbs areas, respectively. 



The question of the presence of ascending fibers 

 in the pyramid requires confirmation (cf. Chapter 

 XXXIV by Patton & Amassian in this work), but 

 suggestive evidence has been obtained in the cat by 

 Brodal & Walberg (62) who concluded that they 

 arise in the spinal cord and also in the cuneate and 

 gracile nuclei. Nathan & Smith (345) have described 



ascending fibers in the pyramid after lateral spinal 

 tractotomy in man, and these fillers were traced 

 thnniijh the pxraniidal decussation into the pons, 

 cerebral peduncle and internal capsule. Phvsiological 

 esidence concerning ascending fibers in the pyramid 

 is inconclusi\e. 



EFFECTS OF CEREBR.-kL LESIONS IN INFANCY ON RESIDUAL 



p\'RAMiDAL FUNCTIONS. Ipsilateral control in infantile 

 hemiplegics is often associated with hypertrophy of 

 the ipsilateral pyramidal tract, von Monakow (447) 

 described a compensatory hypertrophy of one cortico- 

 spinal tract following degeneration of the other in 

 infancy. Reports of similar findings may be seen in 

 the older literature (84, 115, 286). This liypertrophy 

 of the remaining pyramid may double its \olume and 

 is then associated with a large uncrossed lateral and 

 %entral corticospinal tract from the healthy hem- 

 isphere. Verhaart (432) believes that this is not due 

 to an increased number of fibers but simply to an 

 abnormal number of thick fibers with a diameter 

 greater than 3 ju, whereas the number of smaller 

 fibers is within normal limits. 



SENSORIMOTOR INTEGRATION IN PERFORMANCE 

 OF MOTOR .ACTIVITIES 



Reflex acti\ation of pyramidal neurons by im- 

 pulses reaching the cortex along afferent pathways 

 has been seen in a variety of experimental conditions. 

 Stimulation of relay or diffusely projecting thalamic 

 relay nuclei (23, 65, 351 ), as well as peripheral somatic 

 (12, 25), visual (25, 460) and acoustic (25) stimulation, 

 has been shown to induce changes in the excitability 

 of the motor area, or to produce a discharge in the 

 pyramidal tract. Amantea (19, 20) found that fol- 

 lowing strvchninization of the motor representation 

 of one limb, stimulation of the skin of the same limb 

 intensified the clonus produced by the strychnine 

 until ultimately a generalized seizure might follow. 

 A seizure can also be induced by strychninization of 

 cortical sensory projection areas and stimulation of 

 the corresponding receptor organs, as shown by 

 Clementi for the visual (98) and acoustic (97) areas. 

 The cortical origin of the motor activities in both 

 Amantea's and dementi's experiments is proved 

 by the fact that ablation of the motor cortex prevents 

 the appearance of the epileptic attack (50, 159)- 

 An electrophvsiological analysis of dementi's photic 

 epilepsy (417) has led to the conclusion that interareal 

 connections are inxolved in the spread to the motor 



