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Biomedical subjects

A Starr

Publications and source records attributed to A Starr.

At least 19 recordsLinked to original sources
PubMed1992

The H-reflex to magnetic stimulation of lower-limb nerves.

We elicited H-reflexes by magnetic and electrical stimulation of several different nerves in 10 healthy subjects and two patients with S-1 radiculopathy. The posterior tibial nerve at the popliteal fossa and the femoral nerve at the inguinal ligament were tested with both electrical and magnetic stimulation; the proximal sciatic nerve was tested only with magnetic stimulation. Muscle activity was recorded from the soleus muscle for posterior tibial and sciatic nerve stimulation and from the vastus medialis muscle for femoral nerve stimulation. No significant difference was found between the latency of H-reflexes evoked by magnetic or electrical stimulation. With magnetic stimulation, the mean (+/- SD) Ia sensory fiber conduction velocity in the proximal segment of the sciatic nerve was 72.4 +/- 3.3 m/s, while the motor nerve fiber conduction velocity in the same portion of the nerve was significantly slower, at 60.6 +/- 2.0 m/s. In two patients with unilateral S-1 radiculopathy, the latency of the H-reflex from the soleus muscle to both magnetic and electrical stimulation of the posterior tibial nerve was absent or prolonged on the affected side. Magnetic stimulation can be used to study the H-reflex and Ia fiber conduction velocity and is particularly advantageous when testing deeply located nerve trunks.

Adult↗
PubMed1992

'Fatigue' in patients with multiple sclerosis. Motor pathway conduction and event-related potentials.

Ten patients with a definite diagnosis of multiple sclerosis and complaints of "fatigue" were studied using (1) reaction times and event-related potentials accompanying the performance of auditory memory tasks (target detection, verbal short-term memory) and (2) motor conduction velocities of the pyramidal tract elicited by cerebral and cervical magnetic stimulation. Patients were studied when "rested" and when fatigued. Reaction times of the patients when rested were significantly delayed in the short-term memory but not the target-detection tasks when compared with normal controls. When patients were fatigued, their reaction times became significantly longer in all tasks compared with when they were rested. Event-related potentials in these tasks consist of N1/P2 sensory components and P3a and P3b cognitive components. The N1 component latency was longer and P3a and P3b amplitudes were reduced in patients compared with controls. Fatigue in patients with multiple sclerosis was accompanied by a shortening of P3a latency and an increase in P3a and P3b amplitudes compared with these measures when patients were rested. Pyramidal tract conduction velocities did not differ between rested and fatigued conditions. Thus, fatigue in patients with multiple sclerosis was associated with a slowing of performance (reaction time) on memory tasks, whereas brain potentials reflecting neural events of stimulus encoding and classification were either unchanged or paradoxically speeded up in latency in the fatigued compared with the rested conditions. We postulate that, in patients with multiple sclerosis, fatigue affects neural processes acting after stimulus evaluation but before activation of the primary motor pathways.

Adult↗
PubMed1992

Cortical DC potential shifts accompanying auditory and visual short-term memory.

Negative DC potential shifts appeared over the scalp during the performance of verbal and non-verbal short-term memory tasks. Three items were successively presented (presentation of memory items) and then had to be retained in memory for 3 sec (memory retention) before being compared to a probe which was either a member (in set) or not a member (out of set) of the memory set. Verbal items (the digits "1" through "9") were tested in the auditory and visual modality and non-verbal items (musical notes) were tested in the auditory modality. Stimulus modality had a significant effect on DC potential shifts during both presentation of memory items and memory retention. There was a sustained negative shift during these periods which was larger over frontal regions with auditory than with visual material whereas the negative shift was larger over posterior temporal regions with visual than with auditory material. Out of 21 subjects who participated in the study, 9 reported the use of visual images in the auditory task, 5 used subvocal auditory rehearsal in the visual task and 7 used imagery concordant with the stimulus modality being memorized. These different strategies had a significant effect on the amplitudes and distribution of the DC potential shifts. The speed of response affected the amplitude of the DC potential shifts in the frontal regions, being larger with fast RTs than with slow RTs but only when verbal items were being processed. These results indicate that stimulus modality, modality of mental imagery, and speed of scanning of the memory store affect DC potential shifts during a 3 sec period of memory retention.

Acoustic Stimulation↗
PubMed1992

Inadequate internal mammary artery graft as a cause of postoperative ischemia: incidence, diagnosis and management.

Inadequate left internal mammary artery (LIMA) graft to the left anterior descending artery (LAD) was encountered in 10 of 3,076 patients between 1984 and July 1990. The mean number of bypass grafts was 2.9 per patient. All patients with inadequate LIMA grafts were stable preoperatively with normal to moderately reduced left ventricular function. No technical difficulties were encountered during surgery. All patients were weaned off cardiopulmonary bypass with minimal or no inotropic support. Each patient developed myocardial ischemia of the LAD territory and/or circulatory collapse or recurrent ventricular dysrhythmia during the first 24 h postoperatively. Six patients, who were immediately re-operated on and had an additional saphenous graft to the LAD, recovered with no infarction and good functional results. Four patients, who were medically treated, developed myocardial infarction. In cases of refractory circulatory collapse and/or ventricular dysrhythmia, inadequate LIMA flow should be suspected. We recommend urgent re-operation with additional saphenous vein graft to the LAD.

Aged↗
PubMed1992

Alternatives to randomization in surgical studies.

The purpose of randomization is to provide unbiased estimates of treatment effects and valid probability statements for hypothesis tests in comparative studies. Difficulties with randomized allocation of patients in surgical studies include: patient selection--due to protocol limitations, possible changes in referral patterns, the need for patient consent, and physician co-operation; disruption of the patient-physician relationship; static protocol not responsive to changes as surgical skill evolves and experience regarding patient selection is acquired; the larger number of patients required causes a longer study and more patients receiving what may be a worse therapy. Moreover, for evaluating a new heart valve, the primary purpose should be estimation rather than comparison with a current device. This can be accomplished faster, in a more generalizable way and with more concern for patient care by using non-randomized comparison groups.

Ethics, Medical↗
PubMed1991

Localization of brain activity during auditory verbal short-term memory derived from magnetic recordings.

We have studied magnetic and electrical fields of the brain in normal subjects during the performance of an auditory verbal short-term memory task. On each trial 3 digits, selected from the numbers 'one' through 'nine', were presented for memorization followed by a probe number which could or could not be a member of the preceding memory set. The subject pressed an appropriate response button and accuracy and reaction time were measured. Magnetic fields recorded from up to 63 sites over both hemispheres revealed a transient field at 110 ms to both the memory item and the probe consistent with a dipole source in Heschl's gyrus; a sustained magnetic field between 300 and 800 ms to just the memory items localized to the temporal lobe slightly deeper and posterior to Heschl's gyri; and a sustained magnetic field between 300 and 800 ms to just the probes localized bilaterally to the medio-basal temporal lobes. These results are related to clinical disorders of short-term memory in man.

Adult↗
PubMed1991

Identification of mitochondrial and non-mitochondrial glutaminase within select neurons and glia of rat forebrain by electron microscopic immunocytochemistry.

Antibodies against the mitochondrial enzyme glutaminase (EC 3.5.1.2), have been used in previous immunocytochemical studies to help identify glutamate-releasing neurons among all glutamate-containing neurons. The studies were based on the idea that glutaminase is enriched within the releasable "transmitter" pools of glutamate. However, evidence is also available to suggest that the enzyme does not occur exclusively within glutamate-releasing neurons. Thus we sought to determine whether glutaminase was immunocytochemically detectable within presynaptic terminals forming asymmetric (putatively excitatory) synapses or, alternatively, occurs in association with mitochondria throughout the cell. For this purpose, we examined the cellular and subcellular distribution of glutaminase- immunoreactivity in neocortical (visual and somatosensory) areas known to contain glutamatergic perikarya. This localization was compared with the distribution in striatal (caudate-putamen and nucleus accumbens) regions recognized to contain high densities of glutamatergic terminals but fewer, if any, glutamatergic perikarya. Glutaminase-immunoreactive perikarya were numerous within the infragranular laminae of neocortex (approximately 1 per 1,000 microns 2 tissue area) but sparse within the caudate-putamen nuclei and accumbens nuclei (less than 1 per 20,000 microns 2.). In addition, heterogeneous distribution of small (less than 1 microns) punctate immunoreactive structures was notable. Relatively high densities of these punctate structures occurred within the supragranular laminae of neocortex, dorsolateral quadrant of the caudate-putamen nuclei, and surrounding certain groups of myelinated fiber bundles throughout the striatum. Electron microscopy revealed diffusely distributed peroxidase immunoreactivity in a select population of dendritic spines, glial processes, and axons. Eight percent of all synapses within the supra-granular laminae were formed by terminals labeled for glutaminase. These principally formed asymmetric junctions on spiny processes. When tissue was incubated with the antibody in the presence of a permeabilizing agent, Photo-flo, high levels of glutaminase immunoreactivity was detectable by electron microscopy within select mitochondria of neocortical (4%) and striatal (8%) perikarya and dendrites, while the diffuse distribution of immunoreactivity within axons and glia was greatly diminished. The differential ultrastructural conditions provide direct demonstration that glutaminase in brain occurs in at least two forms discriminable by their diffuse distribution within non-mitochondrial cytoplasm versus discrete localization within mitochondria. The morphological characteristics of synapses formed by axons exhibiting diffuse distributions of glutaminase immunoreactivity are consistent with the idea that glutaminase-enriched terminals mediate excitatory chemical transmission via the release of glutamate. Because glia containing glutaminase occur juxtaposed to the asymmetric junctions, the glia may utilize neuronally released glutamate for energy metabolism.

Animals↗
PubMed1991

Magnetic stimulation of muscle evokes cerebral potentials.

Somatosensory evoked potentials (SEPs) were recorded from the scalp in man to magnetic stimulation of various skeletal muscles. The potentials consisted of several components, the earliest of which decreased in latency as the stimulated site moved rostrally, ranging from 46 msec for stimulation of the gastrocnemius, to 14 msec for stimulation of the deltoid. Experiments were performed to distinguish the mechanisms by which magnetic stimulation of the muscle was effective in evoking these cerebral potentials. For the gastrocnemius, the intensity of the magnetic stimulus needed for evoking cerebral potentials was less than that required for activating mixed or sensory nerves in proximity to the muscle belly (eg, posterior tibial nerve in the popliteal fossa, sural nerve at the ankle). Vibration of the muscle or passive lengthening of the muscle, procedures which activate muscle spindles, were accompanied by a significant attenuation of the potentials evoked by magnetic stimulation of the muscle. Anesthesia of the skin underlying the stimulating coil had no effect on the latency or amplitude of the early components of the magnetically evoked potentials, whereas electrically evoked potentials from skin electrodes were abolished. Thus, the cerebral potentials accompanying magnetic stimulation of the muscle appear to be due to activation of muscle afferents. We suggest that magnetic stimulation of muscle can provide a relatively simple method for quantifying the function of muscle afferents originating from a wide variety of skeletal musculature.

Adult↗
PubMed1991

Physiology of short-term verbal memory.

These studies document a series of brain events accompanying short-term memory functions. For auditory verbal material the sequence involves at least two different sites within auditory cortex subserving sensory and cognitive processes of memorization. During the scanning of the short-term store structures within the medial temporal lobes, presumably the hippocampus, are active. There is an inconsistency between these results and the clinical observations of the need for an intact dominant parietal lobe for auditory short-term memory to function normally. Magnetic recordings showed no focal dipolar source of activity in the parietal lobe during any aspect of auditory short-term memory. The discrepancy could be accounted for by considering the parietal lobe lesion as "disconnecting" the lateral temporal cortex from the deep medial hippocampal structures thereby impeding auditory short-term functions (Geschwind, 1965). These studies show that the physiological analysis of brain events in the msec range can provide information about relatively complex cognitive processes underlying short-term memory. The magnetic and electrical recording methods provide a noninvasive way to study human brain functions involved in cognition that can then be correlated with behavioral measures of specific cognitive activities.

Brain↗
PubMed1991

Event-related potential correlates of the serial position effect in short-term memory.

Event-related potential (ERP) correlates of the serial position effect in short-term memory were investigated using a memory scanning task. Nine normal young adults (18-39 years) indicated whether a probe item was a member of a previously presented 5-item memory set by pressing 1 of 2 reaction-time buttons. Three types of stimuli were used: verbal digits presented both auditorily and visually, and musical notes presented auditorily. The ERPs to the probes were separately averaged according to the serial position of the probe (1, 2, 3, 4 or 5) in the memory set. The ERPs to the memory set items in positions 1, 3 and 5 also were separately averaged. Both baseline-to-peak and average amplitudes of a late positive parietal potential to the probes were larger to probe items presented in the last position in the memory set than to probes presented in the middle positions (2, 3 and 4), showing a significant recency effect, but only for auditory digits. Reaction time reflected significant recency effects for both auditory digits and notes, but not for visual digits. Response accuracy (percent correct) showed a significant recency effect only for notes. For each stimulus type, both the baseline-to-peak and average amplitudes of a late frontal component to the memory set items became more negative (in the case of the visual digits, less positive) in the third and last serial position of the memory set compared to the first. These findings provide electrophysiological evidence of serial position effects in short-term memory, which, during memory scanning, are dependent on stimulus modality (auditory, visual) and type (verbal, non-verbal).

Adolescent↗
PubMed1991

Effects of myelin or cell body brainstem lesions on 3-channel Lissajous' trajectories of feline auditory brainstem evoked potentials.

Auditory brainstem evoked potentials (ABEP) were recorded from 16 awake cats to obtain 3-Channel Lissajous' Trajectories (3CLTs) using three orthogonal differential electrode configurations (nasion-midline nuchal ridge, left-right mastoids, vertex-midline under the mandible). Potentials, evoked by monaural 80 dBnHL (re, human threshold) clicks, were studied before, and up to 7 weeks after inducing neuronal lesions localized to the cochlear nucleus (CN) or the superior olivary complex (SOC), or myelin lesions localized to the fibers of the trapezoid body connecting these two structures. Neuronal lesions were induced by injection of kainic acid (KA), while myelin lesions were induced by injection of L-alpha-lysophosphatidylcholine (LPC). With CN neuronal lesions the major changes in 3CLT were in the time domain of 'b', 'c' and 'd' (components P2, P3 and P4 of single-channel ABEP). With SOC neuronal lesions the major changes were in 'c' and 'd' of 3CLT (P3 and P4 of ABEP). With trapezoid body lesions the major change was in 'c' (P3 of ABEP). The results are compatible with the peripheral generation of the first ABEP components (P1a and P1b). The second component (P2) is generated by ipsilateral CN neurones and their outputs. The third component (P3) is generated primarily by ipsilateral SOC neurones and their outputs, with the ipsilateral CN providing input. The The fourth component (P4) is generated bilaterally by the SOC neurones and their outputs, receiving their inputs from ipsilateral CN. The fifth ABEP component (P5) is generated by structures central to the SOCs and their immediate outputs. Neither focal neuronal nor myelin lesions were sufficient to produce obliteration of any component, consistent with a set of generators for each of the ABEP components, consisting of both cell bodies and their output fibers, that is distributed spatially in the brainstem.

Animals↗
PubMed1991

Aortic valve replacement in patients over 80 years of age: a comparative standard for balloon valvuloplasty.

Elderly patients who develop symptomatic aortic valvular malfunction have a grave prognosis. Until recently they have not been seriously considered for active treatment, such as valvular surgery or balloon valvuloplasty. Between January 1972 and July 1989, 88 patients over the age of 80 years underwent aortic valve replacement and have been prospectively followed for a total of 185 patient-years. The majority were in New York Heart Association functional class III (48%) or IV (48%) preoperatively. Valvular pathology was pure aortic stenosis in 81%, regurgitation alone in 6%, and mixed lesions in 13% of the cases. Before 1982 the majority of patients received mechanical valves, whereas tissue valves have predominated since then (76% of total). Forty-three percent of the patients had concomitant coronary artery bypass grafting. The overall operative mortality was 16%. Emergency surgery, isolated aortic valve replacement, advanced preoperative functional class, and female gender carried a statistically higher operative mortality. The overall actuarial survival (standard error) at 5 years was 64(7)%. Survival was significantly higher at 5 years for concomitant coronary bypass grafting than for isolated aortic valve replacement, 70 (11)% versus 59(8%), and for males compared to females, 73(9)% versus 55(9)%. The 5-year event-free rates for valve-related death and valve re-replacement were 97(2)% and 93(5)%, respectively. These data provide a firm basis for aortic valve replacement as the standard form of treatment in patients over 80.

Actuarial Analysis↗
PubMed1990

Five-year results of coronary bypass grafting for patients older than 70 years: role of internal mammary artery.

Despite numerous references to the superiority of the internal mammary artery (IMA) over the saphenous vein for myocardial revascularization, its role in the elderly is still in question. From January 1984 through December 1988, 1,081 patients older than 70 years (mean age, 74.9 years) underwent bypass grafting, 354 (33%) receiving left IMA grafts based on the surgeon's preference and 727 (67%) receiving vein grafts only. Selection bias resulted in a higher incidence of known risk factors (such as cardiomegaly, arrhythmias, left ventricular failure, wall motion abnormalities, and preoperative combined New York Heart Association/Canadian Cardiovascular Association functional class IV) in patients in whom the IMA was not used. However, unstable angina, acute myocardial infarction, left ventricular dysfunction, and left main disease were not contraindications for using IMA grafts. The operative mortality rate was significantly lower in IMA patients (2.8% versus 7.6%). The actuarial 5-year survival rate (standard error) was higher in patients with IMA grafts, 89% (3%) versus 78% (2%), and postoperative functional class improved to a greater extent in IMA patients (87% of patients were in classes I and II). Arrhythmias and myocardial infarction were significant causes of late death only in patients with vein grafts. When patients are older than 70 years, patient selection factors clearly play an important role in the differential results between patients in whom the IMA is used and patients in whom vein grafts are used. As in younger patients, excellent results can be achieved in the elderly.

Actuarial Analysis↗
PubMed1979

Short latency mechanically evoked somatosensory potentials in humans.

Somatosensory potentials evoked by mechanical stimulation were recorded by surface electrodes over (1) the digital nerves in the index finger, (2) the median nerve at the wrist, (3) the median nerve near the axilla, (4) the brachial plexus, (5) the cervical cord at CII, (6) the scalp overlying the somatosensory cortex. Nerve conduction velocities varied inversely with age and ranged from 43 to 68 m/sec. Mechanically evoked potentials recorded from the electrodes overlying the digital nerves were an artifact of the finger movement. All other electrode configurations recorded potentials comparable to those evoked by electrical stimulation of nerves. These mechanically evoked potentials could prove useful in the assessment of clinical disorders of somatosensory function from receptor to cortex in man.

Adolescent↗
PubMed1979

Mechanically and electrically evoked somatosensory potentials in normal humans.

Somatosensory potentials evoked by mechanical stimulation of the fingernail and electrical stimulation of the nerve in the finger and at the wrist were recorded by surface electrodes over; (1) the digital nerve in the index finger, (2) the median nerve at the wrist, (3) the median nerve at the axilla, (4) the brachial plexus at Erb's point, (5) the cervical cord at C2, and (6) the scalp overlying the somatosensory cortex. Nerve conduction velocities were computed for two portions of the median nerve. Conduction times along the somatosensory pathway between spinal cord and cerebral cortex were also defined. The mechanically evoked potentials had less temporal dispersion, were of lower amplitude, and occasionally consisted of fewer components than the electrically evoked potentials. Electrical stimulation of the nerve trunk at the wrist evoked some additional components not detected by the other stimulation methods. Nerve conduction velocities and conduction times were comparable among the three methods of stimulation.

Arm↗