Physiologic basis for focal motor seizures and the Jacksonian "March" phenomena.

Abstract

SUMMARY:Mechanisms underlying focal motor seizures and the Jacksonian “march” have been explored on the basis of recent physiologic data concerning the “nested ring” spatial and functional organization of motor cortex. Focal motor seizures can be understood in terms of the focal representation at motor cortex of elements controlling movement of a limb part about a single joint. The preponderance of three foci of origin of motor seizures, first reported by Hughlings Jackson, are related to lower thresholds of excitability at these loci, as demonstrated for forelimbs in the case of subhuman primates, and to the resultant bias of peripheral and central input to activate these loci. The “march” phenomenon which typically involves specific patterns of spread in the forearm, is explained in terms of the recently discovered nested ring organization. Thus the centrifugal spread of excitation from a particular locus in motor cortex would demand the pattern of spread of convulsive movements generally observed in clinical situations. The latter involves spread from face or leg to shoulder, forearm, then to hand, or conversely.The physiological basis of activation and/or inhibition of focal motor epilepsy by peripheral stimuli is also now available on the basis of current experimental evidence. Thus the point to point coupling of periphery and motor cortex as defined in terms of control of limb parts about single forelimb joints, provides a substantial understanding of many of the clinical events sometimes termed “reflex” epilepsy.