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M Demirci

Publications and source records attributed to M Demirci.

28 records · Page 2Linked to original sources
PubMed1998

Differences between the time constant of sensory and motor peripheral nerve fibers: further studies and considerations.

Using a method of latent addition, we previously demonstrated that sensory fibers had time constants that were about three times longer than those of motor fibers. The aim of the present work was to confirm this difference by determining the time constants for single sensory axons by using microneurography and for single motor axons by recording single motor units with bipolar concentric needle electrodes. To determine the influence of the conditioning pulse on the neural time constant, we used both depolarizing and hyperpolarizing conditioning pulses. When hyperpolarizing conditioning pulses at comparable intensity were applied, the tendency was to find shorter time constants than when depolarizing pulses were applied, although still with the motor time constant being slightly shorter. Although the absolute values varied with the different methods, the sensory time constant was generally three times the motor time constant for depolarizing conditioning stimuli, whereas for hyperpolarizing conditioning stimuli the difference dropped to about one and a half. These characteristics improve understanding of the behavior of sensory and motor axons, and, in particular, explain the differential excitability. Determination of neural time constants might prove valuable for clinical use.

Adult↗
PubMed1998

Pulp reaction to a tri-cure resin-modified glass ionomer.

OBJECTIVE: This study evaluated the pulp response to a tri-cure resin-modified glass ionomer cement and compared this response to those elicited by a zinc oxide-eugenol cement and a silicate cement. Materials were placed in nonexposed class V cavity preparations on human teeth. STUDY DESIGN: Sixty premolars were selected in orthodontic patients in a voluntary group of teenagers. All cavities were prepared according to International Dental Federation specifications. Resin-modified glass ionomer and silicate applications were carried out according to the manufacturers' instructions. RESULT: All teeth were asymptomatic. Pulp responses to the resin-modified glass ionomer cement were slightly greater than to the zinc-oxide-eugenol cement. Bacterial staining attempts were inconclusive with regard to time intervals and pulp responses. No necrotic pulps were seen in any teeth. CONCLUSION: The tri-cure resin-modified glass ionomer cement elicited a slightly greater adverse pulp response than did the zinc-oxide-eugenol cement with respect to the healing process of the pulp.

Composite Resins↗
PubMed1997

Monitoring cellular edema at single-neuron level by electrical resistance measurements.

Electrical resistance measurements have been used for investigating extracellular volume fraction (EVF) of brain tissue. Conventional techniques using multiple metal electrodes are limited in their spatial resolution, and thus not suitable for detecting local EVF changes at cellular level. We used a multibarrelled glass microelectrode to monitor cellular swelling locally at single-neuron level. The microelectrode was placed in CA1 region of the rat hippocampus, in situ. A constant current pulse was applied between one of the barrels and a reference electrode placed in the neck. The resultant voltage drop, which was directly proportional to the resistance of the immediate environment surrounding the tip of the microelectrode, was recorded through another barrel. A third barrel was used for iontophoretic injection of N-methyl-D-aspartate (NMDA) for inducing local cellular edema. The effect of diffuse edema induced by bilateral carotid artery ligation on EVF was also investigated. NMDA application increased the local tissue resistance by 2.0-, and ischemia, by 3.4-folds. We conclude that the method described can detect changes in EVF of minute volumes of brain tissue, and is suitable for monitoring very local effects of drugs or changes in the metabolism on cell volume.

Animals↗
PubMed1997

A mismatch between kinesthetic and visual perception in Parkinson's disease.

Kinesthesia may be defective in patients with Parkinson's disease (PD), and this defect conceivably has a role in parkinsonian hypokinetic symptoms. In the present study, PD patients used kinesthetic perception to estimate the amplitude of passive angular displacements of the index finger about the metacarpophalangeal joint and to scale them as a percentage of a reference stimulus. The reference stimulus was either a standard kinesthetic stimulus preceding each test stimulus (task K) or a visual representation of the standard kinesthetic stimulus (task V). In task V, the PD patients' underestimation of the amplitudes of finger perturbations was significantly greater than that of normal subjects, but not for task K. PD patients' underestimation was also greater in task V than in task K; the difference between the underestimations was significantly greater than for normal subjects. These results suggest that, when kinesthesia is used to match a visual target, distances are perceived to be shorter by the PD patients. Assuming that visual perception is normal, kinesthesia is "reduced" in PD patients. This reduced kinesthesia, when combined with the well-known reduced motor output and probably reduced corollary discharges, implies that the sensorimotor apparatus is "set" smaller in PD patients than in normal subjects.

Aged↗
PubMed1996

Menthol and nonmenthol cigarettes and smoke exposure in black and white women.

Purposes of this investigation were to compare smoke constituent exposure (CO and nicotine boosts) and smoking topography parameters between black and white women, and between women regularly using menthol or nonmenthol cigarettes. A two-factor factorial design with a sample of 37 women stratified by race and menthol or nonmenthol cigarette use was implemented. There were significant main and interaction effects of race and menthol/nonmenthol use on CO boost. Black women had a mean CO boost of 10.1 ppm vs. 7.2 ppm for white women, while women using nonmenthol cigarettes had a higher CO boost (mean = 10.6 ppm) compared to those regularly using menthol cigarettes mean = 6.5 ppm). White menthol smokers had the lowest CO boost of all subgroups. There was a trend for black women to have higher nicotine boost than white women (21.4 ng/ml vs. 15.9 ng/ml). Black women had nonsignificantly higher puff volumes compared to white women (mean = 48.4 vs. 43.5 ml), while nonmenthol smokers had nonsignificantly higher puff volumes than menthol smokers (mean = 48.5 vs. 42.7 ml). Lower CO boost with mentholated cigarettes suggests factors beyond mentholation may affect elevated smoke constituent exposure among black women.

Adult↗
PubMed1996

The effect of rapid maxillary expansion on conductive hearing loss.

The effects of rapid maxillary expansion (RME) on conductive hearing loss were investigated in 14 subjects (11 females and 3 males). The subjects ranged in age from 10 years 4 months to 16 years 9 months (mean age 12 years 11 months +/- 1 year 9 months) and had narrow maxillary arches and conductive hearing loss. Hearing levels were determined by means of pure-tone audiometric records. Three records were taken for each subject. The first was taken before RME, the second after sufficient midpalatal suture opening was achieved (mean = 15 days), and the third after the retention period (mean = 4.5 months). All the audiometric records were assessed by an otolaryngologist. Changes in both hearing level and air-bone gap were investigated by means of analysis of variance. It has been determined that hearing improved at a statistically significant level (P < 0.05) after the active treatment period, but that the improvement reversed at the end of the retention period. Five patients experienced significant and stable hearing improvement over the duration of this study.

Adolescent↗
PubMed1996

Effects of cerebral ischemia on N-methyl-D-aspartate and dihydropyridine-sensitive calcium currents. An electrophysiological study in the rat hippocampus in situ.

BACKGROUND AND PURPOSE: During cerebral ischemia, both promoting and limiting factors are present for activation of the N-methyl-D-aspartate (NMDA) receptor ion channel and the dihydropyridine (DHP)-sensitive Ca2+ channels. We investigated the activity of these channels during ischemia and reperfusion in the rat hippocampus in situ. METHODS: Reversible ischemia was induced by bilateral carotid artery ligation. NMDA and BAY K8644 were applied by iontophoresis or pneumatic ejection, and extracellular field potential and resistance changes were recorded from the CA1 region of the rat hippocampus. Resting membrane potentials of the CA1 neurons were also recorded. RESULTS: DC potential shifts produced by NMDA and BAY K8644 were reduced when ischemia depressed the evoked activity more than 50%. They disappeared on total failure of synaptic transmission and recovered during reperfusion. When the evoked activity was depressed less than 50%, DC shifts were greater than their preischemic values; however, BAY K8644-induced potentiation did not reach statistical significance. CA1 neurons were depolarized during ischemia. CONCLUSIONS: These data suggest that ischemia severe enough to cause transmission failure inactivates NMDA and DHP-sensitive Ca2+ currents. During less intense ischemia and reperfusion, NMDA and DHP-sensitive Ca2+ channels are functional, and their overactivation may lead to neurotoxicity.

3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethy↗
PubMed1992

Spastic paraparesis associated with portal-systemic venous shunting due to congenital hepatic fibrosis.

We report a case of progressive spastic paraparesis in a 45-year-old man with total portal-systemic shunting, which developed spontaneously due to congenital hepatic fibrosis. Cellular functions of the liver, except for an elevated blood ammonia level, were within normal limits, as is usual in congenital hepatic fibrosis. This case shows that spastic paraparesis following portal-systemic shunting may occur without liver failure.

Ammonia↗