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Biomedical subjects

F Shima

Publications and source records attributed to F Shima.

59 records · Page 4Linked to original sources
PubMedDate not supplied

Origin and distribution of thalamic somatosensory evoked potentials in humans.

The distribution and generator sources of somatosensory evoked potentials (SEPs) in the thalamus and subthalamic area were studied, using a 'semi-microelectrode' during stereotaxic surgery on 34 patients with involuntary movements or intractable pain. Electrical stimulation was given to the median nerve at the wrist. Two distinct SEPs were evoked by contralateral stimulation. A high voltage (160 microV) positive SEP with a peak latency of 15.5 msec was strictly confined to the ventral part of the sensory relay nucleus (nucleus ventro-caudalis, V.c). A much lower voltage, positive-negative-positive triphasic SEP showed peak latencies of the initial positivity and the major negativity of 13.3 msec and 16.0 msec, respectively, and had maximal voltage (16 microV) in the ventralmost parts of the nucleus ventro-intermedius (V.im) and radiatio praelemniscalis (Ra.prl), and substantial potentials in the lemniscus medialis (L.m) and nucleus ventro-oralis posterior (V.o.p). The potential field of the triphasic SEP spread farther across the different thalamic nuclei and subthalamic region with identical configurations and peak latencies, but with decreasing amplitude. These findings suggest that the high voltage positive SEP reflects a postsynaptic potential generated by the V.c neurons, and the smaller triphasic SEP a presynaptic axonal potential generated in the rostral part of the lemniscal pathway, extending by means of volume conduction.

Adolescent↗
PubMedDate not supplied

Origin of scalp far-field N18 of SSEPs in response to median nerve stimulation.

To identify the origin of scalp-recorded far-field negativity of short-latency somatosensory evoked potentials to median nerve stimulation (designated N18), direct records were made from the thalamus and ventricular system during 4 stereotaxic and 3 posterior fossa operations. In the thalamus a negative potential with almost the same latency as the scalp N18 was restricted to the Vim nucleus, but there was a large positive potential in the VC nucleus and medial lemniscus. Vim negativity increased in amplitude when high frequency stimulation was given to the median nerve, indicative of a facilitation effect. In contrast, the amplitude of scalp N18 decreased at high frequency stimulus. Direct recordings made through the medulla oblongata to the mid-brain showed a negative potential with gradually increasing latency. Above the upper pons, there was stationary negativity with no latency shift. The similarity between this negative potential and N18 is shown by their having the same latency and same response to the amplitude reduction and latency prolongation produced by high frequency stimulus. Our data suggest that scalp N18 comes from brain-stem activity between the upper pons and the mid-brain rather than from the thalamus.

Adult↗
PubMedDate not supplied

Direct recording of somatosensory evoked potentials in the vicinity of the dorsal column nuclei in man: their generator mechanisms and contribution to the scalp far-field potentials.

Somatosensory evoked potentials (SEPs) in the vicinity of the dorsal column nuclei in response to electrical stimulation of the median nerve (MN) and posterior tibial nerve (PTN) were studied by analyzing the wave forms, topographical distribution, effects of higher rates of stimulation and correlation with components of the scalp-recorded SEPs. Recordings were done on 4 patients with spasmodic torticollis during neurosurgical operations for microvascular decompression of the eleventh nerve. The dorsal column SEPs to MN stimulation (MN-SEPs) were characterized by a major negative wave (N1; 13 msec in mean latency), preceded by a small positivity (P1) and followed by a large positive wave (P2). Similar wave forms (P1'-N1'-P2') were obtained with stimulation of PTN (PTN-SEPs), with a mean latency of N1' being 28 msec. Maximal potentials of MN-SEPs and PTN-SEPs were located in the vicinity of the ipsilateral cuneate and gracile nuclei, respectively, at a level slightly caudal to the nuclei. The latencies of P1 and N1 increased progressively at more rostral cervical cord segments and medulla, but that of P2 did not. A higher rate of stimulation (16 Hz) caused no effects on P1 and N1, while it markedly attenuated the P2 component. These findings suggest that P1 and N1 of MN-SEPs, as well as P1' and N1' of PTN-SEPs, are generated by the dorsal column fibers, and P2 and P2' are possibly of postsynaptic origin in the respective dorsal column nuclei.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗