Action potential curves

Electrical stimulation (or depolarization) of the cell will result in changes in membrane potential over time or a characteristic action potential curve (Fig. 17-1). The action potential curve results from the transmembrane movement of specific ions and is divided into... 

FIGURE 25.3 Simulated sarcolemmal and tubular action potentials of frog sartorius muscle fiber, (a) Temporal membrane action potentials calculated in a transverse plane 5 mm from the end-plate zone of a fiber with radius a = 50 /xm. Curve 1 Sarcolemmal action potential curves 2 to 4 action potentials in tubular membrane patches at r = a (2), r = a/2 (3), and r = a/20 (4). (b) Sarcolemmal action potentials for fibers with radius 70 (1), 50 (2), and 30 (3) /xm. The time axes have been expanded and truncated. 

An action potential curve shows the changes in a cell s electrical charge during the five phases of the depolarization-repolariza-tion cycle. These graphs show electrical changes for pacemaker and nonpacemaker cells. 

As the graph below shows, the action potential curve for pacemaker cells, such as those in the sinoatrial node, differs from that of other myocardial cells. Pacemaker cells have a resting membrane potential of -60 mV (instead of -90 mV), and they begin to depolarize spontaneously. Called diastolic depolarization, this effect results primarily from calcium and sodium leakage into the cell. 

Six helpful appendices are included Quick guide to cardiac arrhythmias summarizes the details of 20 arrhythmias Cardiac drug overview covers commonly used cardiac drugs Best monitoring leads shows the most beneficial leads to monitor for the most challenging arrhythmias Depolarization-repolarization cycle explains the five phases of this cardiac cycle Action potential curves reviews the cellular changes that occur during the depolarization-repolarization cycle and Cardiac conduction system reviews how electrical impulses affect heart function. 

Fig. 4.6. Photocurrent action spectra (curves 1, 2) and absorption spectrum (curve 3) of aggregated Dye III embedded in the nanostructurated TiOz film electrolyte 0.25 M Na2S04 light intensity 10 4 W cm 2 electrode potential +0.3 V (curve 1), -0.2 V (curve 2). Absorption spectrum of dyed Ti02 colloid (50 mmol/1 of titania + 1 mmol/1 of Dye III) used for fabrication of volume-sensitized nanostructurated Ti02 electrode. Dye III exhibits J-aggregation upon adsorption at Ti02 particles.

The existence of a curved conformation associated with the action potential is supported by the fact that the ion influx at the spike will induce an increased average wedge-shape of the molecules, due to electrostatic screening of the lateral repulsion of phosphatidylserine molecules. Furthermore a conformation associated with the spike would directly relate action potential propagation to the mass-cooperative vesicular fusion, involved in the chemical signal transfer by transmitter molecules at the pre-synaptic membrane. Experimental support for this concept has been recently reported [39]. This well-controlled fusion process of numerous "vesicles" with the presynaptic membrane must take place as a phase transition. The... 

The larger the value of the exponent, the more curved (steeper, concave downwards) is the line. A very high exponent can be viewed as indicating an all-or-none effect (e.g. the development of an action potential in a nerve). Within a narrow concentration range, the observed effect goes from all to nothing or vice versa. An exponent less than unity (<1) sometimes indicates active metabolites and/or multiple receptor sites. 

Fig. 2. Action potential generation. Opening and closing kinetics of sodium and potassium channels during an action potential. The curved arrows indicate the predominant conformation of the channels during the different phases of an action potential.

Braitman (1989) found that CN (10-200 pM) rapidly depressed synaptic transmission between Schaffer collateral-commissural fibres and pyramidal cells. Analysis of input/output curves revealed both a decrease in excitatory postsynaptic potential generation and an increase in action potential threshold these suggested that the rapidity of action was due to a direct effect on CNS neurones. However, most studies emphasize a role for biochemical mechanisms as being mainly responsible for neurotoxicity. 

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