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17-13 DISSOCIATION OF CYTOARCHITECTONIC AND METABOLIC BARRELS IN THE PRENATALLY X-IRRADIATED RAT NEOCORTEX. MUNEYUKI ITO, Department of Physiology, Aichi Prefectural

Colony, 713-8 Kamiya-cho, Kasugai 480-03, Japan

'The rodent's neocortex is characterized by the presence of unique groups of neurons in

layer IV, called barrels, which represent individual vibrissae. Where such cytoarchitectonic:

barrels are found, aggregates of thalamocortical axon terminals also are normally found in an overlapping fashion. Rats were exposed to X-irradiation (200 r) through the body wall of the mother on the embryonic day 17 and examined in the aulthood. Cells that would have occupied Layer IV were killed and cytoarchitectonic barrels as examined by Nissl stain failed to develop. However, histochemical reaction for cytochrome oxidase (mitochondrial enzyme) detected barrels in surviving layer Vb, probably reflecting thalamic afferent terminal aggregation. Curren.1 source density analysis of vibrissa-evoked field potentials located the primary sink in this

layer. Further, physiologically identified vibrissa-sensitive neurons of this layer were stained with intracellular horseradish peroxidase injection. They were of bizarre morphology, namely with a large soma and a bifurcating apical dendritic shaft.

17-14 TOPOGRAPHICANDSUCCESSIVEZRE%'~ENTATION OFGENERATORSOFSOMATOSENSORYEVOKED POTENTIALS IN MONKEYS. Ne’. HISAO NISHIJO. MASAJI FUKUDA. SABURO

HOMMA2 AND? Tovama 93041. Dept. Phvsiol.. 2Meiii Colleee of Oriental Medcine. Kvoto 629-03.

Dipole tracing (DT) which can transpose surface distribution of potentials to an equivalent dipole can be used for non-invasive investigation of animal and human brain functions. Generators of somatosensory evoked potentials (SEPs) elicited by electrical stimulation of the right median nerve in 3 anesthetized monkeys (Macaca fuscata) were investigated by DT. We related the dipoles to a 3 dimensional reconstruction of a monkey brain by recording SEPs with 21-27 electrodes placed on the dura mater to reduce the influence of the skull, which has a much lower electrical conductivity than the brain tissue. Loci and latencies of the generators estimated by DT were confirmed by recording SEPs directly from the surface of the sensorimotor cortex (cortical surface recording) and multiple unit activity (MUA) from the parietal cortices of the same monkeys investigated by DT. Loci of generators estimated by DT for each SEP component were: P7, thalamus; PlO and NlO, area 3b; P12, area 1 and 2; P18, area 5. These generators estimated by DT corresponded to the topographical mapping created from the cortical surface recording. MUA from areas 3b. 1 and 2, and 5 increased in correspondence to PlO and NlO, P12, and Pl8, respectively. The results indicate that generators of SEPs progressed from the thalamus to area 5 via the primary somatosensory area (areas 3b, 1, and 2). This is consistent with the hierarchical sequence of somatosensory processes. The present results show that DT can accurately identify localization of current source generators in the brain, and trace their successive movement.

17- 15 DIFFERENT INTERACTION MECHANISM OF TWO COMPONENTS OF SOMATOSENSORY EVOKED F’OTENTIALS (SEPs) IN THE HUMAN CUNEATE NUCLEUS

7 MI0 SHIMA ment of Clinical Neurouhvsiolonv. Neuroloeical Institute. Kvushu University 60.3-1-l Maidashi. Hiaashi-ku. Fukuoka 812. Ianan

SEPs to second finger stimulation were recorded from the ipsilateral cuneate nucleus (CN) in 6 patients with spasmodic torticollis during cervical rhizotomy, and were investigated for their generator sources and interaction mechanism using vibratory sensation given in the fingers. The SEPs were characterized by a negative potential (N wave) with a mean latency of 13 msec, that was superimposed by three sharp notching potentials, proceeding to a prolonged positive potential (P wave) with a mean peak latency of 35 msec. A higher rate of stimulation at 20 Hz or vibratory sensation given on the second finger markedly decreased the amplitude both of N and P waves, while the notching potentials remained unchanged. When vibration was given on the first or fifth finger, N wave was attenuated almost equally to the effects of vibration given on the second finger, while P wave was decreased less than the vibratory effect of the second finger. These findings suggest that the notching potentials are generated by the dorsal column fibers, and N wave is of postsynaptic origin in the CN. P wave closely resembles the prolonged positive potential found in the cat CN, which was postulated to be due to the prolonged depolarization of the presynaptic terminals of the cuneate tract fibers elicited by descending cortical volleys (Andersen et al., 1963). The somatotopical manner of the vibratory effects on P wave may reflect the cortical participation, whereas N wave is attenuated equally by giving vibration on any finger.