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Biomedical subjects

V E Amassian

Publications and source records attributed to V E Amassian.

At least 55 records · Page 3Linked to original sources
PubMed1989

Focal stimulation of human peripheral nerve with the magnetic coil: a comparison with electrical stimulation.

The hypothenar compound motor action potential (CMAP) response to ulnar nerve stimulation at the elbow was used as a test system in the human to compare excitations by a round magnetic coil (MC), 92 mm in outer diameter, and by electrical bipolar or tripolar stimulation. Optimal focality of excitation was obtained with the MC at 90 degrees to the extended arm and its plane parallel to the arm, i.e., an orthogonal-longitudinal orientation. Tangential orientation of the MC on the arm, i.e., laying it flat on the arm, powerfully excited additional structures. As expected from classical axonology, orthogonal-transverse orientations were the least effective. With orthogonal-longitudinal orientation and submaximal stimulation, the spread of excitation lateral to the median nerve at the wrist was of the order of 10-15 mm, the thickness of the MC being 12 mm. With the same orientation, the site of origin of the distally propagating impulse was estimated by comparing CMAP latencies to bipolar electrical (with cathode distal) and MC stimulation. Tripolar stimulation (with cathode intermediate) had no advantage over bipolar stimulation. The impulse originated 13-22 mm from the midpoint of the contacting edge. Rotating the MC through 180 degrees and thus reversing the field polarity did not significantly change the CMAP latency, indicating that the effective cathode and anode lay within a few mm of each other. Stimulating with a tilted MC resulted in a maximum CMAP when the orthogonal-longitudinal orientation failed to do so. A simple volume conductor model yielded a potential gradient of the right order of magnitude (35 mV/mm) when the MC stimulator output was 25% of maximum, i.e., a little above threshold for exciting ulnar motor axons.

Arm↗
PubMed1988

Intracranial stimulation of facial nerve in humans with the magnetic coil.

Using ourselves as subjects, maximal compound motor action potentials (CMAPs) were evoked in ipsilateral nasal and orbicularis oculi muscles (onset latency 4.9-5.4 msec) by a magnetic coil (MC) tangentially oriented over parieto-occipital scalp. The facial nerve was also electrically stimulated sequentially at the posterior tragus near the stylomastoid foramen, anterior tragus and 3 cm more distally. Onset latency of the CMAP elicited at posterior tragus ranged from 1.0 to 1.3 msec less than that elicited by the MC over scalp. Because the measured distal facial nerve motor conduction velocity was 50-60 m/sec, the locus of impulse generation induced by magnetic coil stimulation was estimated to be approximately 6.5 cm proximal to the site of electrical stimulation at the posterior tragus, i.e., closer to the exit of the facial nerve from the brain-stem than to its entrance into the internal auditory meatus. This non-invasive technique should be useful in evaluating patients with peripheral facial nerve disorders including Bell's palsy.

Action Potentials↗
PubMed1988

An analysis of peripheral motor nerve stimulation in humans using the magnetic coil.

We compared conventional electrical and magnetic coil (MC) stimulation of distal median nerve in 10 normal subjects and 1 patient. Orthogonal (90 degrees to volar forearm)-longitudinal (the plane of the MC aligned with the long axis of nerve or wire), tilted (to 45 degrees) longitudinal, and tangential edge orientations elicited maximal or near maximal compound motor axon potentials (CMAPs) without simultaneous co-activation of ulnar nerve. Transverse and symmetrical tangential orientations were inefficient. A simulation study of an ideal volume conductor confirmed these findings by predicting that the maximum current density was near the outer edge of the MC and not at the center where the magnetic flux intensity is maximal. An orthogonal-longitudinal MC induces a current in the adjacent volume conductor (for example elbow or wrist), which flows in the same circular direction as in the MC. This differs from a tangentially orientated MC which classically elicits current flow in the volume conductor opposite in circular direction to that in the MC. Amplitude and latency of the CMAP were both altered, but not identically, by changing the intensity of MC and cathodal stimuli. Rotating an orthogonal-longitudinal MC through 180 degrees, thus reversing the direction of current flow, elicited single fiber muscle action potentials whose peak latencies differed at most by 100 microseconds. Thus, the (virtual) cathode and anode are significantly closer (i.e., 5-6 mm) with MC than with electrical stimulation where they are at least 20 mm apart. A disadvantage of MC stimulation is the imprecision in defining exactly where the distally propagating nerve impulse originates. In different subjects, using maximum output and orthogonal or tilted (to 45 degrees) longitudinal orientations, the calculated site of excitation in the median nerve varied 2-15 mm distal to the midpoint of the contacting edge of the MC. This limits the usefulness of the MC in its current configuration for determining distal motor latencies. Future advances in MC design may overcome these difficulties.

Action Potentials↗
PubMed1987

Physiological basis of motor effects of a transient stimulus to cerebral cortex.

This contribution includes a selective review of previously published material, findings from some new experiments, and discussion of some relationships between animal and recent human data. The major questions are: What descends from the cerebral cortex after a brief surface stimulus? What explains the various components of the corticofugal discharge? What are the motor consequences of the corticofugal discharge, and what are the effects of lesions on both? The focus is on the corticospinal system, which through its monosynaptic connection with alpha motoneurons of distal muscles accounts for the short latency movements after a transient cortical stimulus. The pyramidal and lateral corticospinal tract response in monkey or cat to a surface stimulus applied to area 4 is a direct (D) wave conducted in fast axons followed by several indirect (I) waves with a period of greater than 1 ms. Although computer summing reveals, at increasing amplitudes, D and I waves in recordings from nuchal skin, vertebra, and surface of the spinal cord, "killed end" recording is essential to reveal the true extent of I relative to D waves. The D wave might result from excitation of: the initial segment (IS), i.e., the classical spike trigger zone; the first or deeper nodes in white matter; or arborizations of the axon collaterals in gray matter. Under different circumstances, each of these modes of excitation can be effective. Thus, with threshold stimulation through separated bipolar electrodes, intracellular recording from pyramidal tract (PT) and uninvaded motor cortical neurons shows that D activation usually occurs when the membrane potential immediately before the stimulus is relatively depolarized, implying excitation of the IS region, i.e., close to the site of synaptic transfer. A monopolar surface (+) stimulus at the appropriate focus usually generates a D wave at weaker intensity than does a surface (-) stimulus. However, if a little above threshold, stimuli of either polarity generate both D and I waves, but the ratio of D:I amplitude is usually greater with surface (+) stimulation. A theoretical estimate of the depth of excitation by a surface (+) stimulus was consistent with threshold excitation occurring at the first node. Slow PT neurons are excited by surface stimulation, but trivially contribute to population PT or corticospinal recordings. Intracellular recording from PT neurons identifies a monosynaptic excitatory postsynaptic potential as the cause of the first I wave, the period between successive I waves reflecting single delays for synaptic discharge.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1987

Human cerebral cortical responses to contralateral transcranial stimulation.

We wished to develop a noninvasive electrophysiological measure of functioning of human cerebral cortex. The response chosen was that elicited by a corticocortical afferent input, specifically the transcallosal response (TCR). We used ourselves as subjects. The large shock artifacts associated with passing large currents through the head were reduced by: stimulating percutaneously with a focal anode (5 cm2) situated midway between F4 and C4 (i.e., frontal region, standard 10-20 nomenclature) and a medially located, semicircular grounded cathode (26 cm2) no closer than 3 to 4 cm away; and recording with the focal scalp electrode over homologous left frontal cortex vs. two or more reference electrodes interconnected through variable resistors and at least 6 cm away. Transcranial stimulation with pulses of 80 to 140 mA and lasting 100 microseconds elicited a brief diphasic deflection (latency, 2 to 4 ms), followed by an initially positive, often bifid wave with a latency of 9 to 14 ms, lasting 18 to 44 ms and reaching 5 to 10 microV. The initial diphasic deflection greatly increased when the stimulating anode was moved laterally over temporalis muscle, indicating an electromyographic (EMG) origin. The EMG, but not the positive wave, was elicited when the interpolar distance was reduced to 1 cm, which greatly reduced the fraction of current traversing the cortex (epicranial stimulation); furthermore, occluding the circulation to the scalp for 30 minutes by rubber tubing above the zygomas and brows dulled sensation, reduced the EMG, but did not alter the latency or rising phase of the positive wave. Early somatosensory components, if present, were small compared with the positive wave.(ABSTRACT TRUNCATED AT 250 WORDS)

Afferent Pathways↗
PubMed1986

A dynamic role of rubral neurons in contact placing by the adult cat.

The potential dynamic role of red nucleus in contact placing (CP) was investigated in intact awake cats. The activity of individual rubral neurons was recorded and the timing of their discharges was compared with electrical activity in selected contralateral forelimb muscles and with associated changes in joint angles characteristic of CP of the forelimb or hindlimb. The series included 273 rubral neurons (RN) that were recorded extracellularly in five cats; 62% were identified as rubrospinal tract neurons (RTN). Latencies of antidromic invasion following stimulation in the caudal contralateral medulla ranged from 0.25 to 3.3 ms. In some units, invasion occurred intermittently regardless of the stimulus strength above threshold. Most RN discharged tonically in the absence of obvious movement at, e.g., rates of 5-60 spikes/s. Spike train analyses were performed on 21 RN with stable resting discharge. Although the interspike interval distribution usually resembled that of a gamma process, the discharges were nonrandom. Usually, successive intervals were not independent, long and short intervals tending to be followed by intervals longer and shorter, respectively, than the mean interval. The sequence of intervals often shows episodes of relatively periodic discharge with mean interval differing between episodes. Among 120 RN that altered in firing rate during the first lifting-withdrawal phase of CP, 97 (81%) initially accelerated in rate. Variability in the delay between forepaw contact and the initial RN discharge permitted the distinction between contact-locked responses (53%), which began with a short latency, e.g., 20 ms, after contact and peaked usually within 50 ms. When CP failed to occur to a trial, contact-locked responses were further subdivided into a noncontingent type in which the RN response occurred regardless of whether CP occurred or not (type A) and a contingent type in which the RN response was significantly reduced or even absent when CP failed to occur (type B); delayed movement-locked responses (47%), which could be identified only when CP movements were delayed after contact. Delayed movement-locked responses were either types A or B, according to whether the RN response preceded (e.g., by 40-160 ms), or started during the electromyogram (EMG) response or the movement, respectively. Thus response types 1B and 2A can predict the success or failure of subsequent CP. Among an additional 17 RN initially responding after the paw cleared the top corner of the placing apparatus, the firing rate accelerated.(ABSTRACT TRUNCATED AT 400 WORDS)

Action Potentials↗
PubMed1980

Response of forelimb guard hair afferent units to air-jet stimulation of entire receptive field.

1. In anesthetized, immobilized cats, individual axonal responses to movement of forelimb G hairs were recorded in the ipsilateral cuneate fasciculus at C1-C2 with a glass-insulated tungsten microelectrode. Several test criteria were used to identify primary afferent axons among the recorded neurons. 2. The output discharge in the parent sensory axon during continuous air-jet stimulation of the entire receptive field was irregular and was usually maintained. Criteria for statistical stability were introduced, permitting an analysis of the response as a stable point process. 3. The main interspike interval during the response to maximal air-jet stimulation ranged in different units from 4 to 100 ms, with a transition zone near 20 ms (50/s) between short and long mean interval units. Long and short mean interval units tended to be related to G2- and G1-type hair receptors, respectively. 4. In both short and long mean interval units, the interspike-interval distribution (IID) had a dead time (usually 2-4 ms), a sigmoidal rising limb, a single mode, and a monotonic falling limb, a single mode, and a monotonic falling limb. In short mean interval units, the decrement of the falling limb was described by a single exponential. 5. The value of the standard deviation of the IID was a little less than that of the mean interval minus dead time, the inequality probably resulting from the finite duration of the rising limb and, in long mean interval units, the nonexponential falling limb. 6. Joint interval analysis revealed that successive intervals were independent except in short mean interval units, where the shortest intervals tended to be followed by intervals shorter than the mean. 7. The expectation density function did not reveal any periods in the output discharge. In short mean interval units, a constant mean (planteau) level was attained after a finite rise time and usually one or more overshooting-undershooting sequences. In long mean interval units, the expectation density function monotonically rose to the plateau level. 8. Reducing the air-jet intensity caused an increase in mean interval of discharge and in standard deviation of the IID and slowed the rise time of the expectation density function to the plateau level. 9. Similarities and differences between the whole-field response to air-jet stimulation and the Poisson process are discussed.

Action Potentials↗
PubMed1978

Developing role of sensorimotor cortex and pyramidal tract neurons in contact placing in kittens.

Unilateral or bilateral removal of kitten sensorimotor (SM) cortex prior to the fourth postnatal week was followed by recovery of contact placing (CP) to stimulation of any of the four forepaw cutaneous fields. However, immediately after a removal under ethyl chloride, CP was depressed contralaterally, especially to lateral stimulation (lateral CP). When the operated kitten aged to 5--9 weeks, the likelihood of CP secondarily declined contralateral to the SM cortical extirpation, but was restored by administration of d-amphetamine. An initial extirpation after the fifth postnatal week was followed by a severe deficit in CP. 2. Cooling the SM cortex, internal capsule or bulbar pyramid during the 2nd and 3rd week was followed by reversible, contralateral loss of lateral CP, extension of the forelimb and, later, by reduced movements. Lateral CPs were often hypermetric prior to loss. By contrast, cooling SM cortex in the first week was usually ineffective. Thus, during the 2nd week, lateral CP developed a dependence on SM cortex, and in particular on PT neurons. 3. After a delay usually of 1 or more min, cooling the SM cortex reduced the resting discharge of individual neurons in buried motor cortex; the antidromic conduction time in PT neurons was significantly increased and the spike height was reduced.

Animals↗