$8 ~-] Abnormal excitability of the corticospinal pathway in patients with amyotrophic lateral sclerosis: a single motor unit study using transcranial magnetic stimulation Nobuo Kohara 1, Ryuji Kaji l, Yasuhiro Kojima 1, Kerry R. Mills 2, Hiroshi Fujii l, Toshiaki Hamano 1, Jun Kimura i. I Department of Neurology, Kyoto University Hospital, Kyoto, Japan; ~ University Department of Clinical Neurology, The Radcliffe Infirmary, Oxford The pathophysiology of corticospinal tract degeneration in amy- otrophic lateral sclerosis (ALS) was investigated by studying the effect of transcranial magnetic stimulation on discharge character- istics of single motor units during voluntary activation. The motor units were recorded from the first dorsal interosseus muscles of 12 patients with ALS, 14 healthy subjects, 12 patients with upper motor neuron lesion and 9 with pure lower motor neuron dis- eases. More than 100 magnetic stimuli were delivered over the scalp during minimal muscle contraction. The occurrence of mo- tor unit discharges was plotted in a peristimulus time histogram (PSTH). An increase in discharge probability at latencies of 20- 30 msec, that represents monosynaptic activation (primary peak) was found in normal units. Motor units from ALS patients with short disease durations had significantly increased discharge prob- abilities in the primary peak (p < 0.01). Motor units from 4 ALS patients with upper motor neuron sign showed double primary peaks; an initial synchronized peak followed by a dispersed peak. The latter was ascribed to a polysynaptic corticospinal pathway, which remains undetected or is functionally insignificant under normal conditions. We conclude that the excitabilities of the sur- viving corticospinal tract pathways are abnormally increased in ALS, especially in the early stage. ~-~The plateau potentials and their role in regulating motoneuronal firing Hans Hultborn. Department of Medical Physiology, University of Copenhagen, Denmark Long-lasting excitability increase in motoneurones (with self- sustained firing) may, under certain circumstances, be initiated by short-lasting synaptic excitation and terminated by short-lasting synaptic inhibition. This behaviour is intrinsic to the motoneu- rones as depolarizing and hyperpolarizing current pulses through the recording microelectrode can produce the same excitability changes. These motoneuronal properties are contingent upon ac- tive innervation of serotonergic or noradrenergic pathways. The plateau potentials, which are responsible for the maintained ex- citability increase, are due to a voltage dependent non-inactivating nifedipin-sensitive Ca++-conductance. The monoamines act by decreasing a K + conductance 'uncovering' the plateau. From ex- periments on freely moving rats, it was suggested that plateau potentials contribute to postural tone. It is not yet known to what extent they contribute during phasic movements as, for example, locomotion. In chronic spinal cats there is preliminary evidence for plateau potentials reappearing together with the development of spastic stretch reflexes. Plateau potentials have also been sug- gested as a pathogenetic mechanism in some instances of muscle cramps. (Baldissera et al. Brain 1994, 117, 929-939). [1] Eken, T., Hultborn, H. & Kiehn, O. (1989). In Progress in Brain Research, Vol. 80, eds. Allum, J.H.J. & Hulliger, M. pp. 257-267. Elsevier Science Publishers B.V. Symposium 2. Cortical motor control S-2. CORTICAL MOTOR CONTROL ~-~ Anatomo-functional organization of the agranular frontal cortex in primates Massimo Matelli, Giacomo Rizzolatti. Institute of Human Physiology, University of Parrna, 43100 Parma, Italy Classically, the agranular frontal cortex of the monkey and man has been considered to be formed by two motor areas: the primary motor area and the supplementary motor area (SMA). Evidence will be presented that this subdivision is inadequate and that the agranular frontal cortex rostral to the precentral cortex (F1, area 4) is constituted of a series of three caudo-rostral sectors: inferior area 6, superior area 6 and mesial area 6. Each sector can be fur- ther subdivided into two parts. Inferior area 6 is constituted by F4 caudally and F5 rostrally, superior area 6 is formed by F2 caudally and F7 rostrally and mesial area 6 includes F3 caudally and F6 ros- trally. A clear complete somatotopic parcellation is present only in F3 (SMA-proper). Leg and arm fields are found in F2. Mouth and arm representations are found in F4 and in F5. Essentially a single representation is present in the two other agranular areas: mostly an arm field in F6 (pre-SMA) and an eye field in the medial part of F7 (Supplementary Eye Field). Thus, if the arm is taken in consideration, there are at least 5 independent arm fields in the frontal agranular cortex rostral to F1. This pattern of multiple arm representations fits well with the notion that the extent of the cortical representation of a single body part is proportional to its functional importance. It raises, however, the problem of the role played by each cortical area in motor control. Possible different functional role of the various agranular areas will be discussed on the basis of recent physiological and anatomical data. • 7 Organization of medial frontal motor areas J. Tanji. Department of Physiology, Tohoku University School of Medicine, Sendai, 980, Japan As a result of recent development of studies on motor areas in the medial frontal cortex, a novel concept on their organization has been proposed. The traditionally defined supplementary motor area is now divided into two areas, the pre-SMA (or F6) ros- trally and the SMA proper (or F3) caudally. Rostrolateral to the pre-SMA lies an oculomotor field called as the supplementary eye field (SEF). In addition, two new motor areas are found in the banks of the cingulate sulcus, that is, rostral and caudal cin- gulate motor areas (CMAr and CMAc). The definition of each area is based on constellation of morphological and functional findings. I would first discuss about the basis for delineating the SMA, pre-SMA, and SEE Thereafter, I would present experimen- tal findings on subhuman primates concerning cellular activities that characterize each of these areas. In relation to such simple motor tasks as in the reaction time task, the cellular activity imme- diately preceding movement-initiation is found in both the SMA and pre-SMA, but smaller in magnitude in the pre-SMA. When motor tasks are more complicated, cells in the pre-SMA exhibit characteristic properties. For instance, in relation to redirection of planned reaching movements, pre-SMA cells show prominent ac- tivity selectively related to changes in planning directions of future movements. When motor tasks are increasingly more demanding to subjects, the amount of cellular activity appears in the order of pre-SMA > SMA > MI. Cellular activity in the SEF is primarily related to eye movements, though influenced by accompanying arm movements.