O.L.d 



THE PHVSIOPATHOLOGY OF EPILEPTIC SEIZURES 345 



■ ■ ■:^^y■ .■ 



strychnine -iO 9«c. 



E cardiazof 2 mm 



OPCg 



G£.5.g 



GLg 



ThaLg- ■ 

 F.PM.g.' 



F.RMd 



-• — ■■ — ■■^^"-4^^l;«l ,^ , 



B 



■ J.^ lll Mj l'— U f 



FIG. 5. Experimental evidence for a thalamic inhibitory system responsible for the interruption of 

 the tonic convulsion of reticular origin and for the tonic-clonic course of generalized epileptic 

 seizures. The first fi\e tracings are cortical (right lateral gyrus, left precruciate gyrus, left ectosylvian 

 gyrus, left lateral gyrus and left suprasylvian gyrus). The sixth tracing is from the lett thalamus. The 

 last three tracings are reticular (right and left mesencephalic reticular formation, below the plane 

 of the red nucleus, and the bulbar reticular formation). A. The cat which has received a weak dose 

 of strychnine 40 sec. previously begins a typical hypersynchronous discharge after an auditory stimu- 

 lus which provokes an evoked potential in the ectosylvian gyrus, in the thalamus and especially in the 

 bulbar reticular formation. Note the exclusively reticular level of the hypersynchronous strychnine 

 discharge and its bulbar predominance, while all the rest of the brain shows merely desynchroniza- 

 tion. B. When the action of the strychnine starts to show a decline, so that the reticular formation 

 discharge is of decreased amplitude and regularity and the peripheral tetanus is less intense, a 

 strong dose of pentylenetetrazol is given. This provokes first several spike discharges appearing inde- 

 pendently in the bulbar reticular formation and in all the rest of the brain, and then an intense 

 rhythmical discharge at the thalamocortical as well as the mesencephalic levels but sparing the 

 bulbar reticular formation where the strychnine discharge persists. This episode evidently corresponds 

 to a tonic seizure caused by excitation of the bulbar reticular formation by the strychnine and by 

 'liberation" of this structure under the influence of the pentylenetetrazol discharge which results in 

 what may be a type of functional exclusion of the thalamocortical level. C. Eventually there devel- 

 ops in the thalamus a rhythm of slow waves of large amplitude and progressively slower frequency. 

 During each of these slow waves there is electrical silence in all the leads, particularly in the bulbar 

 reticular formation where each slow wave interferes with the hypersynchronous discharge of the 

 strychnine type. This corresponds to active inhibition, since the tonic seizure stops for the same 

 time during each interruption of the bulbar discharge, thus making possible the rhythmic relaxa- 

 tions which characterize the clonic phase of the seizure. D. Finally all parts of the thalamocortical 

 system are exhausted (extinction), while the strychnine discharge continues unchanged in the bulbar 

 and even the mesencephalic reticular formation. 



