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HANDBOOK OF I'HVSIOLOCY 



NEUROPHYSIOLOGY I 



or rhombencephalic animal should only be tonic; 

 they may be accompanied by a few clonic jerks with 

 brief seizure discharges, but there is never a clonic 

 phase for this only represents the rhythmic inhibition 

 of the tonic spasm. Finally, it explains the erroneous 

 interpretation given by Ziehen (200) and Bechterew 

 (16) who, on the basis of the results of decortication, 

 related the tonic component in grand mal to brain 

 stem structures and the clonic component to the 

 cerebral cortex. 



One may conclude therefore that neuronal fatigue 

 and exhaustion are responsible for the progressive 

 slowing of the cortical and muscular discharge in 

 grand mal during its tonic phase, whereas the thal- 

 amocaudate inhil)itory system is responsible for the 

 periods of relaxation in the clonic pha.se and for the 

 episodes of cortical electrical silence or slow ' ijraking' 

 waves which are the EEG accompaniment of the re- 

 laxation periods. 



MYOCLONUS OF PETIT M.\L. This disorder may be con- 

 sidered as a miniature or extremely short grand mal 

 seizure (54, 59, 149). Numerous arguments may be 

 adduced in favor of this concept. 



a) The etiology is often the same. Myoclonus is 

 frequently associated with a;rand mal fits, and precedes 

 the majority of spontaneous grand mal attacks (52, 

 149, 170) or of attacks precipitated by pentylene- 

 tetrazol (Feuillet). In most cases the myoclonic jerks 

 which precede grand mal are repeated at shorter and 

 shorter intervals until their fusion constitutes the be- 

 ginning of the tonic phase (Ribot, Muskens). 



6) Electrographically, the form, frequency and 

 topography of the discharges are the same in grand 

 mal as in myoclonic petit mal; the multiple spikes of 

 myoclonus appear like a i^urst of spikes in the clonic 

 phase of grand mal or, even more, like the discharge 

 just at the onset of the tonic phase. 



c) Clinically, the peripheral manifestations are 

 similar in the two types, generalized increase in 

 muscle tone masking the fact that other effectors are 

 brought into play.* 



d) Finally, experimental studies furnish the most 

 important arguments. Myoclonus is provoked experi- 

 mentally by the same procedures as grand mal fits 

 and is accompanied by a similar thalamocortical dis- 

 charge. This lilDcrates the facilitating reticulospinal 



"* The myoclonic discharge is obviously too brief to cause 

 glandular secretion, but it is however able to bring about a 

 slight alteration in arterial pressure (Morin >& Roger, unpub- 

 lished observations). 



system responsible for the momentary tonic reinforce- 

 ment which we call "myoclonus". 



The one feature that difTerentiates myoclonus from 

 a grand mal seizure is its duration, and therefore only 

 the abrupt and premature ending of the myoclonic 

 discharge remains to be explained. The sustained 

 frequency testifies to the fact that it is not terminated 

 by exhaustion and that a process of active inhibition, 

 like that already envisaged in regard to grand mal 

 fics, is probably involved. 



Ii may i^e concluded that patients suffering from 

 myoclonic petit mal possess a more active inhibitory 

 system than those with grand mal, and that this 

 system is thrown into action from the start of the 

 thalamocortical discharge, thus bringing about an 

 almost immediate interruption of the seizure. This ex- 

 plains why the generalized muscular contraction is 

 only momentary and why the EEG expression is 

 limited to a few spikes which are isolated or followed 

 by one or several ' braking' slow waves. 



PETIT M.AL '.\bsence'. Petit mal 'absence' may be in- 

 terpreted on the basis of the same hypothesis as 

 myoclonic petit mal. It may be considered as a 

 thalamic discharge occurring in a subject with a very 

 effective inhibitory mechanism. Because of this, the 

 discharge is inhibited almost immediately after it 

 has been fired and a slow ' braking' wave appears in 

 the thalamus immediately after the development of 

 a single spike. The rhythmic repetition of the phe- 

 nomenon may be explained on the basis that the 

 termination of each wave of inhibition allows the 

 thalamic discharge to reappear, provoking a spike and 

 a new inhibitory wave. 



The spikes and slow waves are transmitted to the 

 cortex by the system of diffuse projection and furnish 

 the classical spike-and-wave recorded on the EEG 

 during the 'absence'. Relative independence may 

 exist between the two mechanisms generating the 

 spike and the wave so that they function separately 

 for a certain length of time; this may explain the 

 numerous cases of atypical spike-and-wave, and par- 

 ticularly those cases in which the spike disappears and 

 leaves only the slow waves at the end of a clinical 

 'absence'. This independence also helps to explain 

 the observations made in man by Williams (197) and 

 in animals ijy Ralston & Ajmone-Marsan (163) who 

 dissociated the spike and the slow wave in the thala- 

 mus and on the cortex. 



Most of the features of petit mal and notably those 

 which distinguish it from, or even oppose it to, grand 

 mal may be interpreted on the basis of the pre- 



