Action events

Remember, there is no substitute for judgment and experience. Consider these quantitative exercises as toois or methods to guide your team s determination of the right priorities, not as absoiute formuias. in addition, keep in mind that your goai is to improve the overaii status of process safety as part of a continuing effort—not as a singie action, event, or siiver builet. ... 

Operator action event trees are treelike diagrams that represent the sequence of various decisions and actions that the operating team is expected to perform when confronted with a particular process event. Any omissions of such... 

External T/F External items including weather and external third party actions/events were not creating out-of-design conditions. 

Write down short accounts of three occasions when you did not get the outcome you wanted. So that you can reflect dispassionately upon these episodes, record only actions, events and feelings. Omit all explanations, reasons, and rationalizations. These wiil obscure any recurring patterns. 

To address the second type of complaint, reports of any adverse re-action/event should be entered in a separate register in accordance with national and international requirements. An investigation should be conducted to find out whether the adverse reaction/event is due to a quality problem and whether such reactions/events have already been reported in the literature or whether it is a new observation. In either case, complaint records... 

The possible actions/events causing the leakage to occur are listed in the following ... 

Action events involve a multiphase biochemical-bioelectric process. A localized stimulus to an excitable cell can launch a series of cascading molecular events affecting the membrane s ionic permeability. The accompanying changes in TMP feed back on the membrane by way of voltagegated channels and magnify the effect of the stimulus. If the stimulus amplitude reaches a threshold value, this causes further and more dramatic changes in the membrane s ionic permeability. 

The threshold behavior of excitable cells can be observable by placing a tiny (usually a 1 -jiim tip diameter) microelectri e inside a cell. Sufficient charge injection from an external voltage source can drive the transmembrane potential toward zero. Now if the transmembrane potential is moved stepwise more positive an action event will occur above some threshold. At this point, the membrane potential will suddenly jump of its own accord from a negative TMP to a positive viilue. Figure 17.8 shows a setup where a battery is used to stepwise drive the cell TK more positive. 

FIGURE 17.9 Illustration of the cycle of ionic conductances associated with action events. 

The depolarization and repolarization phases of action events can occur quickly over intervals of tens of microseconds, although the actual durations depend very much on die cell type. During the time when the cell is depolarized, it cannot be restimulated to another action event. This interval is known as the cell s absolute refractory period. The cell s relative refractory period is the interval... 

In an action potential event, the total eunount of charge Q transferred across the membrane by movements of sodium and potassium is relatively little, just sufficient to charge the membrane capacitance C . The amplitude of potential change and time course of an action event depend on the type of excitable cell being studied. 

Membranes also have a finite and distributed electric resistance both across their thickness and along their length. These resistances tend to short circuit the membrane capacitor and cause its stored charge to decay to zero if the membrane ion pumps cease. There are several different models of the cell membrane that describe its static and transient bioelectric behavior. Models of the cell membrane are useful because they help explain the propagation of action events, such as that along a nerve. 

With local membrane depolarization caused by some stimulus, the longitudinal membrane conduction causes the local rise in TMP to spread along the adjacent regiims of the memlnane. Adjacent voltage-gated membrane channels are driven above threshold, propagating the depolarization event. This effect on adjacent membrane is illustrated in Fig. 17.11 for a nerve axcm. By diis mechanism, bioelectric action events spread outward from the point of stimulus, move along the cell membrane, and cease when the entire membrane is depolarized. 

The velocity of action event propagation depends on the nerve diameter d as well as on passive electric properties of the membrane. Assuming that these electric properties are constant along the membrane, it can be shown that the depolarization wave velocity of a nerve is proportional to... 

However, there are still electrode-detectable bioelectric fields from myelinated nerves during action events, since volume currents flow, as shown in Fig. 17.13. These bioelectric currents are dipolar in nature, meaning that ionic currents flow spatially from the source at the leading edge of depolarization to a sink at the repolarization edge. Separation distances between these nodes in mammals are on the order of a millimeter. 

Single motor units are actuated by a nerve fiber that terminates at die synaptic cleft of the myoneural junction. The neurotransmitter acetylcholine tqiens up m brane ion channels, allowing the influx of sodium and causing an action event followed by a short, twitchlike mechanical contraction. 

The action potential event of the heart is different from that of skeletal muscle. It has a prolonged depolarization phase that varies from tens of milliseconds for cells of the atria to about 250 to 400 ms for cells of the ventricle. The general waveform shape for an action event of the ventricle is shown in Fig. 17.15. 

The prolongation of the heart action event results from the activity of slow calcium channels in the cell membranes. These act in conceit with the fast sodium channels to maintain a longer depolarization phase of the heart. This prolongation is important in the physiological function of the heart since it defines the timing and duration of the mechanical events required for a contraction. 

The mechanical contraction of the heart follows the time course of its longer bioelectric current flows. Bioelectric currents initiate and define the motor activity of the heart cells. If die heart cells simply twitched during an action event rather than producing a sustained force over a longer duration, there would not be time for the myocardium to move against the inertia of blood in the ventricles. 

The frequency-dependent behavior of electrode impedance is of more concern in intracellular microelectrode recording aqyplications, where electrode resistances are very high. In conjunction with the electrode capacitance and that of the recording circuit, a low-pass filter is formed at the microelectrode interface that tends to distort high-frequency portions of action events. 

In muscular disease, needle EMG waveforms vary in strength, waveshape, and frequency content. For example, in muscular dystrophy, the EMG action events are low in amplitude, short in duration (1 to 2 ms), and high in frequency (up to 40 per second). When the bioelectric signals are introduced into a loudspeaker so that they can be heard, there is a high-pitched characteristic sound. These diseased muscles fatigue easily, and this is reflected in their action events that are greatly reduced with sustained contraction. 

When the innervation of muscles is severed by accident or disease, spontaneous action events known as fibrillation waves appear several weeks after the trauma. These waves result from weak random contraction and relaxation of individual motor units and produce no muscle tension. Fibrillation potentials persist as long as denervation persists and as long as the muscles are present. The... 

The propagated bioelectric response is detected with skin bioelectrodes some distance away from the stimulus. Nerve action events are transient and not synchronous with other fibers within a nerve bundle, so these action currents do not sum together in the same way as they do, for example, in the heart. Detection of the skin surface potential created by an action event from a single nerve fibril within a larger bundle is very difficult, pertiaps impossible. 

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