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R Bertram

Publications and source records attributed to R Bertram.

30 records · Page 2Linked to original sources
PubMed1996

Single-domain/bound calcium hypothesis of transmitter release and facilitation.

1. We describe a model of transmitter release that is based on the finding that release can be gated during the opening of individual Ca2+ channels, suggesting that the release site can be activated by the Ca2+ domain under a single channel. In this model each release site contains four independent Ca2+ binding sites or gates with unbinding kinetics graded from slow to fast and affinities ranging from high to low. All four gates must be bound for release to occur. Thus synaptic dynamics are governed by the kinetics of Ca2+ binding and unbinding from release sites, not Ca2+ diffusion. 2. Fast facilitation occurs when an action potential invades a terminal with one or more ions remaining bound to the release sites. Residual free Ca2+ is not necessary for facilitation with this mechanism, but if present it would enhance facilitation by binding to high-affinity gates between pulses. 3. This model can account for key features of release. These include fourth-power cooperativity with regard to external Ca2+; a release time course that is virtually independent of an increase in quantal content; an inverse relation between external Ca2+ and the degree of facilitation; and a steplike increase in facilitation with increasing stimulus frequency, with each step corresponding to a unitary decline in the Ca2+ cooperativity. 4. Facilitation of single-channel-based secretion is shown to be robust even if channel opening is stochastic. Spontaneous release of transmitter, assumed to be due in part to spontaneous Ca2+ channel openings, is shown to be elevated during and after a train of impulses. 5. An extension of the model to include multiple Ca2+ channels per release site demonstrates that one role of overlapping Ca2+ domains may be to accentuate depolarization-evoked release relative to spontaneous release.

Action Potentials↗
PubMed1995

Topological and phenomenological classification of bursting oscillations.

We describe a classification scheme for bursting oscillations which encompasses many of those found in the literature on bursting in excitable media. This is an extension of the scheme of Rinzel (in Mathematical Topics in Population Biology, Springer, Berlin, 1987), put in the context of a sequence of horizontal cuts through a two-parameter bifurcation diagram. We use this to describe the phenomenological character of different types of bursting, addressing the issue of how well the bursting can be characterized given the limited amount of information often available in experimental settings.

Animals↗
PubMed1995

A role for calcium release-activated current (CRAC) in cholinergic modulation of electrical activity in pancreatic beta-cells.

S. Bordin and colleagues have proposed that the depolarizing effects of acetylcholine and other muscarinic agonists on pancreatic beta-cells are mediated by a calcium release-activated current (CRAC). We support this hypothesis with additional data, and present a theoretical model which accounts for most known data on muscarinic effects. Additional phenomena, such as the biphasic responses of beta-cells to changes in glucose concentration and the depolarizing effects of the sarco-endoplasmic reticulum calcium ATPase pump poison thapsigargin, are also accounted for by our model. The ability of this single hypothesis, that CRAC is present in beta-cells, to explain so many phenomena motivates a more complete characterization of this current.

Acetylcholine↗
PubMed1994

Reduced-system analysis of the effects of serotonin on a molluscan burster neuron.

The mathematical model described in Bertram (1993) is used to carry out a detailed examination of the manner in which the neurotransmitter serotonin modifies the voltage waveform generated endogenously by burster neuron R15 of Aplysia. This analysis makes use of a reduced system of equations, taking advantage of the slow rate of change of a pair of system variables relative to the others. Such analysis also yields information concerning the sensitivity of the neuron to brief+ synaptic perturbations.

Action Potentials↗
PubMedDate not supplied

Implications of G-protein-mediated Ca2+ channel inhibition for neurotransmitter release and facilitation.

G-protein-mediated inhibition of Ca2+ current is ubiquitous in neurons, and in synaptic terminals it can lead to a reduction in transmitter release (presynaptic inhibition). This type of Ca2+ current inhibition can often be relieved by prepulse depolarization, so the disinhibition of Ca2+ current can combine with Ca2+ -dependent mechanisms for activity-induced synaptic facilitation to amplify this form of short-term plasticity. We combine a mathematical model of a G-protein-regulated Ca2+ channel with a model of transmitter secretion to study the potential effects of G-protein-mediated Ca2+ channel inhibition and disinhibition on transmitter release and facilitation. We investigate several scenarios, with the goal of observing a range of behaviors that may occur in different synapses. We find that the effects of Ca2+ channel disinhibition depend greatly on the location and distribution of inhibited channels. Facilitation can be greatly enhanced if all channels are subject to inhibition or if the subpopulation of channels subject to inhibition are located closer to release sites than those insensitive to inhibition, an arrangement that has been suggested by recent experiments (Stanley and Mirotznik, 1997). We also find that the effect of disinhibition on facilitation is greater for longer action potentials. Finally, in the case of homosynaptic inhibition, where Ca2+ channel inhibition occurs through the binding of transmitter molecules to presynaptic autoreceptors, there will be little reduction in transmitter release during the first of two successive bursts of impulses. The reduction of release during the second burst will be significantly greater, and if the unbinding rate of autoreceptors is relatively low, then the effects of G-protein-mediated channel inhibition become more pronounced as the duration of the interburst interval is increased up to a critical point, beyond which the inhibitory effects become less pronounced. This is in contrast to presynaptic depression due to the depletion of the releasable vesicle pool, where longer interburst intervals allow for a more complete replenishment of the pool. Thus, G-protein-mediated Ca2+ current inhibition leads to a reduction in transmitter release, while having a highly variable amplifying effect on synaptic facilitation. The dynamic properties of this form of presynaptic inhibition are very different from those of vesicle depletion.

Animals↗
PubMedDate not supplied

[Dorsal lumbar interbody implantation of cages for stabilizing segmental spinal instabilities].

PURPOSE: The technique of posterior lumbar interbody fusion (PLIF) has been critically discussed due to a high degree of complications, including the development of pseudarthrosis. With the recent establishment of intercorporeal implants new aspects have to be contemplated in surgical techniques, especially concerning the posterior approach. In this study we present our first results after intercorporeal stabilisation of segmental spinal instabilities utilising carbon and titanium cages. METHOD: 45 spinal instabilities were surgically stabilised in 42 patients who were evaluated on average for 2.8 years post-operatively. 12 patients had isthmic and 19 patients degenerative instabilities while 11 patients suffered from instabilities resulting from prior spinal surgery. RESULTS: Assessed according to the Hambly-score, 69% of the patients had an excellent or good result; 2 (4.8%) patients were subjectively worse off than before surgical treatment. After implantation of cages precise radiological evaluation of bony ingrowth is frequently impaired by artefacts. We found that three months after implantation of a titanium cage, which had to be removed after incorrect placement, no bony consolidation was visible. Persisting or recurrent instabilities in fused segments were not recorded. CONCLUSION: By means of PLIF and implantation of cages the interbody space is reconstructed and jeopardized neural structures are decompressed. In addition to this, the frequently osteochondrotically degenerated segment is immobilized. The posterior approach allows decompression of neural structures and, with comparable results concerning stability, the considerable risks of the ventral approach are avoided.

Adult↗
PubMedDate not supplied

[Comparison of dorso-lateral and dorso-ventral stabilization procedures in the treatment of vertebral fractures].

AIM: The radiological and clinical outcome of surgical treated thoracolumbal fractures were followed up after 6.7 years. METHOD: The study encompassed 97 patients. The degrees of the bony deformation and the local kyphosis were measured on the lateral view X-ray at 4 different time points: post trauma, after the operation, before implant removal and at the follow up examination. The clinical outcome was evaluated by the Oswestry score. RESULTS: 74 fractures were treated with a fixateur intern and a dorsal fusion by apposition of autologous bone postero-laterally. 23 fractures were fused in a combined dorso-ventral manner by intervertebral fusion with tricortical autologous bone. The lateral X-rays showed a loss of correction up to 48 % in the dorsal fused group and 10 % loss of correction in the combined group. The local kyphosis increased up to 84 % in the dorsal operated group due to degeneration of the injured disk. The clinical Oswestry score showed no significant difference in both groups. CONCLUSION: The additional ventral surgery should be evaluated carefully.

Adult↗
PubMedDate not supplied

[Intermediate-term results with the Mecron screw cup acetabulum--a follow-up study].

ISSUE: The short term result of the cementless implanted titan screw cups (type Mec-Ring) has been very encouraging. The study was aimed to show the survival rate in the middle term. METHOD: The survival rate of 111 implanted acetabular titan screw cups (type Mec-Ring) was determined in a retrospective study. 85 patients (= 76.5%) could be followed up after 6.7 years (5-8.5 years). RESULTS: 38 Implants had to be removed during the follow up period (34.2%). Taking into account the radiologic loosening in addition 22 cups were defined as failure (19.8%). Overall 36 (32.4%) cups showed excellent or good clinical results with 85 points according to the Harris-Hip-Score. 15 of the 22 radiologic loose implants showed only little or no pain. Considering the removed implants the probability of survival is 69% after 7 years. Defining the radiologic loose implants as failure the survival rate is 54%. CONCLUSION: X-rays have to be taken frequently to determine bone loosening early with subsequent revision operation. The survival rate is to low compared to other types of fixation. Therefore acetabular titan screw cups (type Mec-Ring) should not be used any more.

Acetabulum↗
PubMedDate not supplied

[Species specific motility patterns of hyperactivated mammalian spermatozoa and quantitative analysis of the hyperactivation of bull spermatozoa].

Movement tracks of spermatozoa of human, lion, tiger, cow, pig and sheep are recorded by dark field photography (fluid layer thickness 16.7 microns, exposure time 1 s). Comparison before and after capacitation by an incubation of 2 h in modified tissue culture medium TCM 199 with 10% fetal calf serum resulted in two quite different patterns of hyperactivated spermatozoa: 1) Tracks are broadened due to enlarged lateral head displacement or radius of rotating head movements respectively (tiger, lion) and show beside that a markedly increase in erratic motility (human). 2) In the studied species of artiodactyla, cow, pig and sheep, a qualitative new, panicle-like pattern arised as a result of superposition of spermatozoa head pendular movements around the axis of forward motility and the other one around the head axis. This new type of tracks allows a simple quantitative analysis of hyperactivation of bull spermatozoa, first described in this report. The method is applied to investigations on efficacy of capacitation media and provides evidence for high individual differences of semen donors in capacitation success.

Animals↗