Threshold potential

Figure 1.4 Ionic basis for excitatory postsynaptic potentials (EPSPs) and inhibitory postsynaptic potentials (IPSPs). Resting membrane potential ( — 70 mV) is maintained by Na+ influx and K+ efflux. Varying degrees of depolarisation, shown by different sized EPSPs (a and b), are caused by increasing influx of Na. When the membrane potential moves towards threshold potential (60-65 mV) an action potential is initiated (c). The IPSPs (a b ) are produced by an influx of Cl. Coincidence of an EPSP (b) and IPSP (a ) reduces the size of the EPSP (d)...

The ventricular action potential is depicted in Fig. 6-2.2 Myocyte resting membrane potential is usually -70 to -90 mV, due to the action of the sodium-potassium adenosine triphosphatase (ATPase) pump, which maintains relatively high extracellular sodium concentrations and relatively low extracellular potassium concentrations. During each action potential cycle, the potential of the membrane increases to a threshold potential, usually -60 to -80 mV. When the membrane potential reaches this threshold, the fast sodium channels open, allowing sodium ions to rapidly enter the cell. This rapid influx of positive ions... 

Drugs may have antiarrhythmic activity by directly altering conduction in several ways. Drugs may depress the automatic properties of abnormal pacemaker cells by decreasing the slope of phase 4 depolarization and/or by elevating threshold potential. Drugs may alter the conduction characteristics of the pathways of a reentrant loop. 

It should be emphasized that the enzyme activity of the molecular interfaced FDH was reversibly controlled by electrode potential in the potential range from 0.1 to 0.6V. It is also noted that the enzyme can be activated and inactivated at a threshold potential of 0.07V which corresponds to the redox potential of the prosthetic group PQQ. 

FIGURE 2.46. Variation of the photo-injected charge with the dc electrode potential (a) and extraction of the apparent number of electron (b, c). A is the electrode dc potential and E,ilr is the threshold potential. Adapted from Figure 2 of references 51, with permission from Elsevier. 

Phase 0 Spontaneous baseline drift results in the threshold potential being achieved at 40 mV. SlowL-type Ca2+ channels are responsible for further depolarization so you should ensure that you demonstrate a relatively slurred upstroke owing to slow Ca2+ influx. 

Phase 0 Rapid depolarization occurs after threshold potential is reached owing to fast Na+ influx. The gradient of this line should be almost vertical as shown. 

This is the time from the absolute refractory period until the cell s membrane potential is less than the threshold potential. It corresponds to the period of increased K+ conductance. 

Phase 1 The curve should cross the y axis at approximately —70 mV and should be shown to rapidly rise towards the threshold potential of —55 mV. 

Thus, a 10 1 transmembrane gradient of a single monovalent ion, say potassium, will generate a membrane potential of 58 mV. See Resting Potential Action Potential Depolarization Threshold Potential Nernst Equation Goldman Equation Patch-Clamp Technique... 

PERMEABILITY PERMEABILITY CONSTANT MEMBRANE POTENTIAL ACTION POTENTIAL DEPOLARIZATION GOLDMAN EQUATION NERNST EQUATION RESTING POTENTIAL THRESHOLD POTENTIAL PATCH-CLAMP TECHNIQUE Membrane protein dynamics,... 

NUCLEAR MAGNETIC RESONANCE THREONINE DEHYDRATASE THRESHOLD POTENTIAL THROMBIN... 

Transmembrane action potential of a sinoatrial node cell. In contrast to other cardiac cells, there is no phase 2 or plateau. The threshold potential (TP) is -40 mV. The maximum diastolic potential (MDP) is achieved as a result of a gradual decline in the potassium conductance (gK+). Spontaneous phase 4 or diastolic depolarization permits the cell to achieve the TR thereby initiating an action potential (g = transmembrane ion conductance). Stimulation of pacemaker cells within the sinoatrial node decreases the time required to achieve the TR whereas vagal stimulation and the release of acetylcholine decrease the slope of diastolic depolarization. Thus, the positive and negative chronotropic actions of sympathetic and parasympathetic nerve stimulation can be attributed to the effects of the respective neurotransmitters on ion conductance in pacemaker cells of the sinuatrial node. gNa+ = Na+ conductance. 

Effects of norepinephrine and acetylcholine on spontaneous diastolic depolarization automaticity) in a pacemaker cell for the sinoatrial node. The pacemaker cell discharges spontaneously when the threshold potential (TP) is attained. The rate of spontaneous discharge is determined by the initial slope of the membrane potential and the time required to reach the threshold potential. 

There is a decrease in the slope of diastolic depolarization as well as hyperpolarization of the cell. The time to reach the threshold potential is prolonged, with the net effect being a decrease in the rate of spontaneous depolarization. 

In the second experimental approach, the Cgo molecules are deposited on top of an insulating self-assembled monolayer, thus creating a double barrier tunnel junction coimected in series and sharing an electrode [66, 67]. Under these conditions current steps in the I-V graph are observed, because when a potential is applied the capacitances of each junction has to be charged to a threshold potential before an electron can tunnel through the junction and when it is favorable for an electron to sit in the middle electrode the amount of current that flows through the junctions increases [68],... 

When the RMP becomes less negative the cell undergoes depolarisation and when the RMP becomes more negative the cell undergoes hyperpolarisation. When the RMP increases from -80 mV to a critical value of -55 mV (the threshold potential) an action potential ensues. If an individual spike potential fails to reach the threshold value then an action potential is not generated. This is an all-or-none phenomenon. 

Local anaesthetics slow the rate of rise of the action potential and reduce its height. They also slow impulse conduction and lengthen the refractory period (Figure 5.7). They may elevate the threshold potential but do not affect the RMP. As more and more Na+ channels are blocked by local anaesthetic the value of each successive spike potential gradually decreases to the point where it fails to achieve the value of the threshold potential (Figure 5.7). At this point nerve conduction ceases. 

Figure 5.7 Effect of local anaesthetics on the propagation of an action potential. A full action potential occurs when a spike potential reaches the level of the tfireshold potential. Local anaesthetics decrease the rate of rise and the frequency of spike potentials. When the value of the spike potential falls below the threshold potential neural transmission ceases.

Naturally, one must supply a minimum energy to remove an electron from the electrode thus there is a red-limit wavelength above which photoejection is very improbable [33,34,55]. Since the Fermi level depends on electrode potential, this limit shifts to shorter wavelengths as the potential becomes more positive. Thus there is a threshold potential for photoemission and the emission becomes more probable at more negative potentials [59,62]. 

Fig. 8.3 Effects of dietary selenium on p-adrenergic responses in rat heart, (a) L-type Ca2+ currents (I(a i recorded from ventricular myocytes with depolarization from —70mV to OmV, for 200 ms. Mean ( SEM) values of peak amplitudes of IcaL in both experimental and control groups. The cell capacitances of these three groups of cardiomyocytes were similar, (b) Average current-voltage relationships for peak IcaL (measured as the difference between the peak Ca2+ current and the end of 200-ms depolarization). The maximums of IcaL of both experimental groups were shifted to the right with respect to the control, (c) The threshold potentials were significantly more negative and activation potentials were more positive in both experimental groups with respect to the control. (Adapted from Sayar et al. 2000.)...

Walmsley, H.L., Threshold potentials and discharge charge transfers for the evaluation of electrostatic hazards in road tanker loading, J. Electrostatics, 26, No 2, 1991. 

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