Channing L. Ewing

Early somatosensory evoked potentials

The early somatosensory evoked potential secondary to median nerve stimulation in the human had an onset latency of 9--12 msec when recorded from scalp electrodes at vertex-to-mastoid, vertex-to-inion or at the base of the skull. Similar latencies were observed from responses recorded over the cervical dorsal columns during neurologic surgery. A latency difference of 1.5 msec was observed between the early response and the responses recorded from the junction of medial lemniscus and nucleus ventralis posterior lateralis of the thalamus during human stereotaxic surgery. Cervical cord transections and transection at the midpontine levels of the monkey showed that the evoked potential was due to generators between these levels. Depth recording of the monkey indicate that the early evoked potential originates in the region of dorsal column nuclei, while the later components are secondary to generators in cerebral cortex.

Experimental Studies of Brain and Neck Injury

Static and dynamic axial tension loads were applied to the intact and isolated cervical column of the monkey and human cadaver. Radioactive microspheres were used to evaluate brain and spinal cord perfusion in the monkey. To determine neural pathway damage, somatosensory evoked potentials were recorded with stimulation of sensorimotor cortex, and in spinal cord with stimulation of cauda equina. The evoked potential amplitude decreased prior to heart rate and blood pressure changes presumably due to brainstem distention. The preliminary studies show, 1) the brain and spinal cord were well perfused as measured with the microspheres when the evoked potentials decreased, 2) the cervical isolated cadaveric monkey spinal column ligaments failed statically at approximately 1/2 to 1/3 the force required for dynamic disruption, 3) in the intact monkey, the cervical ligaments failed statically at approximately 1/2 the dynamic failure force, 4) the isolated human cervical ligaments failed at loads approximately three times those observed in the isolated monkey cervical column.

THE EVOKED POTENTIAL: AN EXPERIMENTAL METHOD FOR BIOMECHANICAL ANALYSIS OF BRAIN AND SPINAL INJURY

Abstract : Axial forces were applied between the shoulders and skull of eight male Macaca mulatta monkeys. Forces from 556 to 1444 Newtons produced marked changes in blood pressure, heart rate and distraction of the cervical spinal column with minimal ligamentous disruption. Somatosensory evoked potentials recorded at the cortical and thalamic levels following dorsal column or peripheral nerve stimulation were altered prior to or during changes in heart rate or blood pressure. Similar findings were observed in the efferent responses recorded from electrodes placed on the thoracic spinal cord following stimulation of sensorimotor cortex. Studies in four monkey cadaveric isolated cervical column preparations indicate that disruption occurs with axial loads which are approximately one-third of the maximum used in the in vivo studies.

EXPERIMENTAL SPINAL TRAUMA STUDIES IN THE HUMAN AND MONKEY CADAVER

Compression studies were conducted on the ligamentous thoracolumbar spines of fresh human male cadavers. For comparison, forces were applied to the posterior upper thoracic region of intact seated cadavers. Since thoracolumbar flexion injury routinely involves ligament failure and vertebral body wedge compression fractures, studies were conducted on single vertebral bodies and isolated ligaments. Similar studies were conducted in isolated monkey ligaments. The intact and ligamentous thoracolumbar spines failed predominantly in the region of the thoracolumbar junction at forces from 1113-5110 N. For both the human and monkey cadavers, the anterior longitudinal ligament was the strongest. The human ligaments were 2-5 times stronger than those of the monkey. For the covering abstract of the conference see HS-036 716. (Author/TRRL)