Donald H. York

Motor Evoked Potentials from Transcranial Stimulation of the Motor Cortex in Humans

Electrical testing of central nervous system pathways is assuming increasing importance in clinical medicine. However, there is no direct monitor of the motor system. We previously reported using a motor evoked potential created by direct excitation of the spinal cord, placing a stimulating electrode over the corticospinal tract area. To produce a less invasive test, we now use direct transcranial stimulation of the motor cortex through the scalp or direct stimulation of the motor cortex itself during operation. A descending signal can be recorded over the spinal cord and in the peripheral nerves where no retrograde sensory signals should be able to descend. This motor cortex stimulation produces contralateral limb movements and selective activation of the peripheral nerves of a limb. The characteristics of this signal are similar to those described in the neurophysiological literature for a descending motor signal. With a depth electrode, it was found that the signal was strongest in the spinal cord near the corticospinal tracts and in the anterior horn cell area. A set of lesioning studies showed that most of the signal travels in the area of the corticospinal tract, with some traveling in the ventral portion of the spinal cord, perhaps in the anterior corticospinal tract. Section of the pyramid essentially abolishes the signal, but lesioning of the red nucleus does not. This test offers an electrical assessment of the motor system that can be useful in experimental work on spinal cord and brain function. It has potential clinical applicability in humans.

Response variability of somatosensory evoked potentials during scoliosis surgery.

Somatosensory evoked potentials (SSEP) were recorded from the scalp for intraoperative monitoring of patients undergoing surgical correction of spine deformities or spine fractures. Alterations in the SSEP with distraction, spine manipulation, anesthesia, hypotension, and other intraoperative variables are described. When loss of the SSEP occurred and a waiting period was undertaken until it returned, all patients with an SSEP present upon closing, which was within ±2 SD of their anesthetized control values, had no neurologic complications. Alterations in SSEP consisting of increases in latency of 15% and decreases in amplitude of 50% were not associated with any postoperative neurologic deficits.

Utilization of somatosensory evoked cortical potentials in spinal cord injury. Prognostic limitations.

The prognostic value of somatosensory evoked cortical potentials (SECP) for clinical recovery was studied in 71 patients with complete (28) and incomplete (43) spinal injuries. While the absence of an SECP was associated with no clinical recovery, the presence of an SECP was of little value in predicting the clinical state at the time of examination or the potential for recovery

Effects of chronic intraventricular administration of angiotensin II on drinking behavior and blood pressure

Angiotensin II was continuously infused into the lateral cerebral ventricle of rats, and the effects on daily food and water consumption, urine volume, and aortic blood pressure were studied. All was infused at a rate of 10 ng/hr for seven days, using subcutaneously implanted osmotic minipumps. An intraventricular (IVT) control group was infused with only the saline vehicle, while a third group received AII subcutaneously. IVT AII rats showed a four-fold increase in water consumption, to a mean of 171 ml/day during Days 2-4 of infusion, whereas water intake of the other groups did not change from preinfusion levels. Urine volume showed a similar pattern to water intake, increasing five-fold in the IVT AII group during Days 2-4. These measures declined during the final three days of AII infusion, but significant tolerance was not observed. Food intake decreased markedly in both saline and IVT AII groups after implantation of the pumps, but the latter resumed normal food intake more slowly than the former, and body weight remained below preinfusion levels throughout the AII period. Aortic blood pressure of the IVT AII rats showed a slight, but progressive, rise during the infusion period, but it did not significantly exceed that of the saline rats. These results indicate that continuous, low-level, intraventricular infusion of AII may markedly increase water intake without significantly increasing fluid retention or blood pressure.

Further Studies with a Noninvasive Method of Intracranial Pressure Estimation

A linear relationship between elevated intracranial pressure in patients with hydrocephalus and with cerebral edema and a latency shift of the N2 wave of the flash-evoked cortical potential is demonstrated. Although the relationship is good at pressures of 200 to 300 mm H2O, it is excellent at pressures above 300 mm H2O. This relationship establishes a reliable noninvasive method of estimating intracranial pressure.

Routine, direct measurement of aortic pressure in the conscious rabbit

A method for preparation and application of an indwelling catheter for measurement of aortic pressure in conscious rabbits is described. The catheter is generally reliable for periods longer than one month and requires little interference with the normal circulation. The system utilizes coiled polyethylene tubing which has a tapered tip covered with silicone rubber tubing and, at its exterior end, a Velcro matrix for skin healing. The catheter tip is pulled into the abdominal aorta with a trocar, directed caudally. The exterior end is run subcutaneously to the nape of the neck. In 19 rabbits prepared by this method, mean pressure was determined to be 68 +/- 2 mm Hg and pulse pressure was 29 +/- 1 mm Hg two weeks after implantation. The catheters remained patent in 17 rabbits at the time of sacrifice, 12 to 120 days later. In an in vitro sweep-frequency test, maximal resonance of the catheter system was found to lie between 12-14 Hz, at which point the measured pressure amplitude exceeded control by 12.5%.

Involvement of both adrenergic and cholinergic receptors in the cardiovascular effects of naloxone during hemorrhagic hypotension in the conscious rabbit

Opiate receptor blockade with naloxone reverses the hypotension associated with severe hemorrhage in a variety of animal models. In the present study, we examined the mechanisms of naloxones actions in conscious rabbits made hypotensive by hemorrhage. This was accomplished through pharmacological blockade of the efferent limbs of the sympathetic or parasympathetic nervous systems prior to naloxone injection. In addition, we examined the effects of naltrexone in the same model. Naloxone treatment in hypotensive-hypovolemic, conscious rabbits results in an increase in mean arterial blood pressure (BP) and a decrease in heart rate (HR). The bradycardia appears to be due to a reduction in beta-adrenergic and an increase in muscarinic-cholinergic activity. The pressor effect is apparently due to increased alpha-adrenergic receptor activation, and is accompanied by an increase in cardiac output, stroke volume, and total peripheral resistance. Naltrexone did not significantly affect BP but it did reduce HR. The results from the present study suggest that naloxones effects are mediated by an integrated response of the sympathetic and parasympathetic nervous systems. The actions of naloxone may be mediated through antagonism of endogenous opiates.

Correlation between a unilateral midbrain-pontine lesion and abnormalities of brain-stem auditory evoked potential

Brain-stem auditory evoked potentials (BAEPs) were evaluated over a 39 day period in a patient with a unilateral pontine-midbrain lesion verified by CAT scan and at autopsy. Waves I, II and III were present on the side of lesion, whereas all 5 waves were present on the side opposite the lesion. The findings suggest that the BAEPs may be obtained with only an intact ipsilateral auditory pathway. Crossing fibers in the trapezoid body also appear to make contributions to the normal generation of wave V.

Nonpyramidal motor activation produced by stimulation of the cerebellum, direct or transcranial: a cerebellar evoked potential.

There is a need to monitor the functional status of the motor pathways well enough to predict the state of that function during operations and in injured or diseased patients. We previously reported that a motor evoked potential (MEP) can be produced by direct or transcranial stimulation of the motor cortex in both cats and humans. This signal descends through both the dorsolateral and ventral spinal cord and is primarily localized in the pyramidal tracts, producing a peripheral nerve signal and an electromyogram (EMG) response. It is more sensitive to injury than the somatosensory evoked potential (SEP). We report here that one can stimulate the cerebellar cortex, either directly or transcranially, and produce a descending signal in the spinal cord that has different characteristics from the MEP. The cerebellar evoked potential (CEP), located in the dorsolateral and the ventral cord, has an earlier latency and a faster conduction velocity than the MEP. It is predominantly ipsilateral with some contralateral components and also produces EMG responses. In the peripheral nerves, the CEP often produces a pattern of several waves that is different from the one or two predominant contralateral waves of the MEP. The CEP is not diminished by pyramidotomy. It arises from two sites on the cerebellar cortex, medial and lateral. The pathways activated may be the vestibulospinal, rubrospinal, reticulospinal, and fastigiospinal systems. This test seems to offer a monitor of selected motor pathways in the spinal cord largely separate from and complementary to the MEP. The ventral pathways activated probably include those demonstrated to be most essential to basic ambulation after spinal cord injury in primates. Also of importance, one type of evoked potential can facilitate another, which provides additional diagnostic tests. The CEP should be of investigative and clinical value.

Model for the study of individual mammalian axons in vivo, with anatomical continuity and function maintained.

Methods for the study of axons involve whole nerve preparations, teased preparations of axons that are excised from their proximal and distal connections, and tissue culture models. As a complement to these, it would be advantageous to study separated, isolated axons in vivo, still in continuity with the end organ distally and the spinal cord central nervous system neuron proximally. This would allow the study of axon function, normal or pathological, in a close relationship to its biological environment. To achieve this, we have passed the surgically isolated sciatic nerve of a rat through a chamber specially designed for enzymatic dissociation. This was based on principles derived from a prior in vitro method for dissociating nerve into axons. The chamber has controlled temperature and flow and is on an inverted microscope stage, allowing observation of the process. We perfused the chamber with a calcium-free solution followed by a series of enzymes: collagenase, trypsin, and hyaluronidase. This dissociates that part of the extracellular matrix external to the Schwann cells, leaving free, myelinated axons with their Schwann cells. In this acute preparation, the axons continue to conduct action potentials for at least 8 hours. Furthermore, an in vitro study of the axon after the in vivo dissociation demonstrated that axonal transport was maintained in over 90% of the axons, directly visualized on an AVEC-DIC type of microscope system. Properties of axonal transport or active spike propagation can thus be studied individually in an in vivo axon preparation.(ABSTRACT TRUNCATED AT 250 WORDS)