Wavelength dependence of the optically recorded action potentials in guinea pig atrial muscles.

The optically recorded action potentials of the frog atrial muscles which lack transverse tubules showed different features from those reported by Heiny and Vergara (Heiny JA, Vergara J. Optical signals from surface and T system membranes in skeletal muscle fibers. J Gen Physiol 1982;80:203-230) in skeletal muscles (Fujishiro N, Kawata H. The wavelength dependence of optically recorded action potentials in the atrial muscles of the bullfrog (Rana catesbeiana). Comp Biochem Physiol 1996;114A:153-157). We examined whether or not the differences were consistent in other atrial muscles which lack transverse tubules with guinea pig atrial muscles. Two dyes (merocyanine rhodanine and merocyanine oxazolone) were used, and the dependence of the maximum rising phase of the optical signals on the wavelength of the incident beam was analyzed. No dependence was observed between them, and this finding was consistent with the structure of the membrane system of the guinea pig atrial muscles. The optical signals recorded at 718 nm of the incident beam from the guinea pig atrial muscles which stained with merocyanine oxazolone showed a more prominent second rising phase after the initial rapid rising phase of the optical signal than that recorded in the frog atrial muscles. This phase was not observed in the optical signals recorded at other wavelengths. The features of the optically recorded action potentials in guinea pig atrial muscles were consistent with those recorded in frog atrial muscles. Nifedipine did not affect the second rising phase.     

An eikonal-curvature equation for action potential propagation in myocardium

We derive an eikonal-curvature equation to describe the propagation of action potential wavefronts in myocardium. This equation is used to study the effects of fiber orientation on propagation in the myocardial wall. There are significant computational advantages to the use of an eikonal-curvature equation over a full ionic model of action potential spread. With this model, it is shown that the experimentally observed misalignment of spreading action potential ellipses from fiber orientation in level myocardial surfaces is adequately explained by the rotation of fiber orientation through the myocardial wall. Additionally, it is shown that apparently high propagation velocities on the epicardial and endocardial surfaces are the result of propagation into the midwall region and acceleration along midwall fibers before reemergence at an outer surface at a time preceding what could be accomplished with propagation along the surface alone.Research was supported in part by NSF Grant DMS-8801446     

Sodium and potassium currents recorded during an action potential

A simple method was used to measure directly sodium and potassium currents underlying the action potential in single nerve fibres of Xenopus laevis. A short rectangular stimulus under current-clamp conditions elicited an action potential which was digitally stored and later used as command when voltageclamping the same fibre. The currents thus obtained nearly reproduced the original rectangular stimulus. Adding first 100 nM TTX and subsequently 100 nM TTX plus 10 mM TEA to the extracellular Ringer solution revealed the sodium and the potassium currents during an action potential. They were converted to permeabilities by use of the constant-field equation and are in good agreement with the curves which had been calculated from conventional voltage-clamp data. Thus experimentally determined currents and permeabilities are shown as they are changing during an action potential.     

Effect of activation sequence on transmural patterns of repolarization and action potential duration in rabbit ventricular myocardium

Although transmural heterogeneity of action potential duration (APD) is established in single cells isolated from different tissue layers, the extent to which it produces transmural gradients of repolarization in electrotonically coupled ventricular myocardium remains controversial. The purpose of this study was to examine the relative contribution of intrinsic cellular gradients of APD and electrotonic influences to transmural repolarization in rabbit ventricular myocardium. Transmural optical mapping was performed in left ventricular wedge preparations from eight rabbits. Transmural patterns of activation, repolarization, and APD were recorded during endocardial and epicardial stimulation. Experimental results were compared with modeled data during variations in electrotonic coupling. A transmural gradient of APD was evident during endocardial stimulation, which reflected differences previously seen in isolated cells, with the longest APD at the endocardium and the shortest at the epicardium (endo: 165 ± 5 vs. epi: 147 ± 4 ms; P < 0.05). During epicardial stimulation, this gradient reversed (epi: 162 ± 4 vs. endo: 148 ± 6 ms; P < 0.05). In both activation sequences, transmural repolarization followed activation and APD shortened along the activation path such that significant transmural gradients of repolarization did not occur. This correlation between transmural activation time and APD was recapitulated in simulations and varied with changes in intercellular coupling, confirming that it is mediated by electrotonic current flow between cells. These data suggest that electrotonic influences are important in determining the transmural repolarization sequence in rabbit ventricular myocardium and that they are sufficient to overcome intrinsic differences in the electrophysiological properties of the cells across the ventricular wall.     

Effects of electroporation on optically recorded transmembrane potential responses to high-intensity electrical shocks

The outcome of defibrillation shocks is determined by the nonlinear transmembrane potential (DeltaVm) response induced by a strong external electrical field in cardiac cells. We investigated the contribution of electroporation to DeltaVm transients during high-intensity shocks using optical mapping. Rectangular and ramp stimuli (10-20 ms) of different polarities and intensities were applied to the rabbit heart epicardium during the plateau phase of the action potential (AP). DeltaVm were optically recorded under a custom 6-mm-diameter electrode using a voltage-sensitive dye. A gradual increase of cathodal and well as anodal stimulus strength was associated with 1) saturation and subsequent reduction of DeltaVm; 2) postshock diastolic resting potential (RP) elevation; and 3) postshock AP amplitude (APA) reduction. Weak stimuli induced a monotonic DeltaVm response and did not affect the RP level. Strong shocks produced a nonmonotonic DeltaVm response and caused RP elevation and a reduction of postshock APA. The maximum positive and maximum negative DeltaVm were recorded at 170 +/- 20 mA/cm2 for cathodal stimuli and at 240 +/- 30 mA/cm2 for anodal stimuli, respectively (means +/- SE, n = 8, P = 0.003). RP elevation reached 10% of APA at a stimulus strength of 320 +/- 40 mA/cm2 for both polarities. Strong ramp stimuli (20 ms, 600 mA/cm2) induced a nonmonotonic DeltaVm response, reaching the same largest positive and negative values as for rectangular shocks. The transition from monotonic to nonmonotonic morphology correlates with RP elevation and APA reduction, which is consistent with cell membrane electroporation. Strong shocks resulted in propidium iodide uptake, suggesting sarcolemma electroporation. In conclusion, electroporation is a likely explanation of the saturation and nonmonotonic nature of cellular responses reported for strong electric stimuli.     

The phenomenon of reversal of reticulum action potentials recorded from the body surface of sheep

The experiments were carried out on sheep with reticulum fistula. The biopotentials and transformed mechanogram of the reticulum were recorded simultaneously on the body surface with an electrocardiograph. Lactic acid solution 1.6 mol/l was administered through the fistula into the reticulum in an amount of 15 mmol/kg of body weight. It was found that administration of this acid caused reversal of the biopotentials of the reticulum. This phenomenon lasted from 10 to several scores of hours and occurred with pH of reticulum contents below the normal range as well as after return of normal pH.     

Active calcium responses recorded optically from nerve terminals of the frog neurohypophysis

Voltage-sensitive dyes were used to record by optical means membrane potential changes from nerve terminals in the isolated frog neurohypophysis. Following the block of voltage-sensitive Na+ channels by tetrodotoxin (TTX) and K+ channels by tetraethylammonium (TEA), direct electric field stimulation of the nerve terminals still evoked large active responses. These responses were reversibly blocked by the addition of 0.5 mM CdCl2. At both normal and low [Na+]o, the regenerative response appeared to increase with increasing [Ca++]o (0.1-10 mM). There was a marked decrease in the size of the response, as well as in its rate of rise, at low [Ca++]o (0.2 mM) when [Na+]o was reduced from 120 to 8 mM (replaced by sucrose), but little if any effect of this reduction of [Na+]o at normal [Ca++]o. In normal [Ca++]o, these local responses most probably arise from an inward Ca++ current associated with hormone release from these nerve terminals. At low [Ca++]o, Na+ appears to contribute to the TTX-insensitive inward current.     

Extracting message inter-departure time distributions from the human electroencephalogram

The complex connectivity of the cerebral cortex is a topic of much study, yet the link between structure and function is still unclear. The processing capacity and throughput of information at individual brain regions remains an open question and one that could potentially bridge these two aspects of neural organization. The rate at which information is emitted from different nodes in the network and how this output process changes under different external conditions are general questions that are not unique to neuroscience, but are of interest in multiple classes of telecommunication networks. In the present study we show how some of these questions may be addressed using tools from telecommunications research. An important system statistic for modeling and performance evaluation of distributed communication systems is the time between successive departures of units of information at each node in the network. We describe a method to extract and fully characterize the distribution of such inter-departure times from the resting-state electroencephalogram (EEG). We show that inter-departure times are well fitted by the two-parameter Gamma distribution. Moreover, they are not spatially or neurophysiologically trivial and instead are regionally specific and sensitive to the presence of sensory input. In both the eyes-closed and eyes-open conditions, inter-departure time distributions were more dispersed over posterior parietal channels, close to regions which are known to have the most dense structural connectivity. The biggest differences between the two conditions were observed at occipital sites, where inter-departure times were significantly more variable in the eyes-open condition. Together, these results suggest that message departure times are indicative of network traffic and capture a novel facet of neural activity.     

Membrane potential, action potential and activation potential of eggs of the sea urchin, Lytechinus variegatus

Unfertilized Lytechinus variegatus eggs in sea water in their normal physiological state have membrane potentials that approximate ?70 to ?80 mV. This conclusion is based on microelectrode measurements and on computation from the Na⁺ and K⁺ fluxes. The ?8 to ?15 mV values for the membrane potential previously reported and which are generally measured are the consequence of depolarization by impalement. The activation potential in inseminated eggs with an initial membrane potential more negative than ?60 mV is a compound event involving sperm-induced as well as voltage dependent conductance changes. The sperm-induced mechanism is a two-phase conductance increase which involves both Na⁺ and Ca²⁺ during the first phase, and Na⁺ alone during the second phase. In addition, the sperm-induced depolarization at the beginning of the first phase activates a voltage dependent Ca²⁺-conductance mechanism resulting in generation of an action potential.     

Electrocardiograms recorded from the body surface of the carp, Cyprinus carpio

The electrocardiograms (ECGs) in five kinds of bipolar leads were recorded from the body surface of the carp under water. Each component constituting the ECG wave could be detected by the present method. The potentials of the ECGs recorded by the present method were relatively small, the values being 2-64 microV. The electrical axis in most carp was directed toward the right lateral side.     

Some properties of motor unit action potential trains recorded during constant force isometric contractions in man

A specially designed needle electrode was used to record motor unit action potentials for the complete time duration of constant force isometric contractions varying in discrete steps from minimum to maximum force levels. A total of 70 motor unit action potential trains were recorded and analyzed.Several properties of the motor unit action potentials were observed. The inter-pulse intervals between adjacent motor unit action potentials of a particular motor unit action potential train were measured and subsequently analyzed as a real continuous random variable. The distribution of the values of the inter-pulse intervals was described by the Weibull probability distribution function with time and force dependent parameters. Furthermore the Survivor function and the Hazard function of the Weibull probability distribution function described certain characteristics of the motor unit firing intervals. Most important of all, it became possible to derive an equation that would generate a real continuous random variable whose properties would be identical to those of the inter-pulse intervals.     

Extracting dwell time sequences from processive molecular motor data

Processive molecular motors, such as kinesin, myosin, or dynein, convert chemical energy into mechanical energy by hydrolyzing ATP. The mechanical energy is used for moving in discrete steps along the cytoskeleton and carrying a molecular load. Single-molecule recordings of motor position along a substrate polymer appear as a stochastic staircase. Recordings of other single molecules, such as F1-ATPase, RNA polymerase, or topoisomerase, have the same appearance. We present a maximum likelihood algorithm that extracts the dwell time sequence from noisy data, and estimates state transition probabilities and the distribution of the motor step size. The algorithm can handle models with uniform or alternating step sizes, and reversible or irreversible kinetics. A periodic Markov model describes the repetitive chemistry of the motor, and a Kalman filter allows one to include models with variable step size and to correct for baseline drift. The data are optimized recursively and globally over single or multiple data sets, making the results objective over the full scale of the data. Local binary algorithms, such as the t-test, do not represent the behavior of the whole data set. Our method is model-based, and allows rapid testing of different models by comparing the likelihood scores. From data obtained with current technology, steps as small as 8 nm can be resolved and analyzed with our method. The kinetic consequences of the extracted dwell sequence can be further analyzed in detail. We show results from analyzing simulated and experimental kinesin and myosin motor data. The algorithm is implemented in the free QuB software.     

Structural three-dimensional constitutive law for the passive myocardium

A three-dimensional constitutive law is proposed for the myocardium. Its formulation is based on a structural approach in which the total strain energy of the tissue is the sum of the strain energies of its constituents: the muscle fibers, the collagen fibers and the fluid matrix which embeds them. The ensuing material law expresses the specific structural and mechanical properties of the tissue, namely, the spatial orientation of the comprising fibers, their waviness in the unstressed state and their stress-strain behavior when stretched. Having assumed specific functional forms for the distribution of the fibers spatial orientation and waviness, the results of biaxial mechanical tests serve for the estimation of the material constants appearing in the constitutive equations. A very good fit is obtained between the measured and the calculated stresses, indicating the suitability of the proposed model for describing the mechanical behavior of the passive myocardium. Moreover, the results provide general conclusions concerning the structural basis for the tissue overall mechanical properties, the main of which is that the collagen matrix, though comprising a relatively small fraction of the whole tissue volume, is the dominant component accounting for its stiffness.     

Convergent properties of 3 types of visual cortex neurons recorded with various action potential amplitudes

Responses of visual cortex neurones recorded with large (N1), mean (N2) and small (N3) amplitudes of action potentials to photic, acoustic and electro-cutaneous stimuli were studied. It was shown that N1 neurones as differing from N3 were predominantly polymodal, often reacted with inhibitory responses and their background activity was characterized by random distribution of interstimuli intervals. Inputs for heteromodal stimuli were found to differ in N1 and N3 neurones. The results suggest existence of functional heterogeneity of neurones, recorded with different action potential amplitudes.