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Biomedical subjects

S J Larson

Publications and source records attributed to S J Larson.

At least 127 records · Page 7Linked to original sources
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Evoked potentials in experimental myelopathy.

Evoked potentials recorded from the cerebral cortex and spinal cord secondary to peripheral stimulation are reversibly reduced in amplitude by both pathologic distraction and pathologic flexion of the vertebral column. While the cerebral responses are lost within two minutes after complete occlusion of the ascending aorta, the responses recorded from the spinal cord persist without change for approximately ten minutes and then gradually disappear. During the first few minutes after aortic occlusion, changes produced by spinal distraction and spinal flexion are indistinguishable from those produced when the same maneuvers are made with the aorta patent. The responses mediated by dorsal columns and corticospinal tracts are affected in the same way by flexion and distraction, suggesting that somatosensory evoked potential recordings should be a reliable means of detecting spinal cord dysfunction during surgical procedures affecting the spinal cord. It may also be possible to differentiate a mechanical from a vascular insult by the time required for the evoked potential to become abnormal following a particular surgical maneuver.

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Physiological and histological effects of cerebellar stimulation.

Cerebellar implants have been placed in 62 patients with postoperative follow-up of 4 months to 3 years. Initially currents were applied through electrodes of alternate polarity on the superior surface of the cerebellar hemispheres and subsequently through negative electrodes on the superior surface to positive electrodes on the posterior surface. The amount of current required for clinical improvement was approximately the same as that required to significantly reduce the amplitude of the somatosensory evoked potential. The clinical and electrophysiological effects were proportional to the intensity of current and to the number of electrodes through which the currents were applied. Currents applied through the cerebellum were more effective than those confined near the cerebellar surface. Histological examination of the cerebellum from the chimpanzees and from 1 patient who died of causes unrelated to stimulation failed to demonstrate any evidence of neuronal damage related to application of current.

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PubMedDate not supplied

Evaluation of electrode configurations in cerebellar implants.

Capacitively coupled currents of 100 Hz, 0.25 msec duration, were applied to multielectrode arrays implanted upon the superior and posterior surfaces of the chimpanzee cerebellum. The current required for 90% reduction in the amplitude of the evoked potential was inversely proportional to the number of electrodes upon the cerebellar surface. A study of various waveforms showed that z Hz, 0.25 msec pulse duration is near optimal for reduction of amplitude of the somatosensory evoked potential. The current densities per electrode were 5--11 mA/cm2 with a charge per pulse of 0.04--0.08 muC in humans with 15--20 electrodes on each superior surface and 10 electrodes on each posterior cerebellar surface.

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PubMedDate not supplied

Innovations in neurologic implant systems.

Physiologic findings and clinical observations with several new implant systems are discussed. One system studied is designed with electrodes implanted over the anterior region of the spinal cord. Another system is designed with an anterior-posterior spinal cord electrode passing current across the spinal cord. Also given are observation and description of cortical and cerebellar implant systems. Current-density plots of the electrode arrays are included.

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