Electric impulse

Fig. 6. A pacemaker provides electrical impulses to the heart in an effort to correct potentially fatal arrhythmias.

The Cardiac Cycle. The heart (Eig. lb) performs its function as a pump as a result of a rhythmical spread of a wave of excitation (depolarization) that excites the atrial and ventricular muscle masses to contract sequentially. Maximum pump efficiency occurs when the atrial or ventricular muscle masses contract synchronously (see Eig. 1). The wave of excitation begins with the generation of electrical impulses within the SA node and spreads through the atria. The SA node is referred to as the pacemaker of the heart and exhibits automaticity, ie, it depolarizes and repolarizes spontaneously. The wave then excites sequentially the AV node the bundle of His, ie, the penetrating portion of the AV node the bundle branches, ie, the branching portions of the AV node the terminal Purkinje fibers and finally the ventricular myocardium. After the wave of excitation depolarizes these various stmetures of the heart, repolarization occurs so that each of the stmetures is ready for the next wave of excitation. Until repolarization occurs the stmetures are said to be refractory to excitation. During repolarization of the atria and ventricles, the muscles relax, allowing the chambers of the heart to fill with blood that is to be expelled with the next wave of excitation and resultant contraction. This process repeats itself 60—100 times or beats per minute... 

Neurons have three parts the cell body and dendrites, the axon, and axon terminals. The cell body contains the nucleus and the organelles needed for metabolism, growth, and repair. The dendrites are branched extensions of the cell body membrane. The axon is a long, thin structure which transfers electrical impulses down to the terminals. The axon divides into numerous axon terminals and it is in this specialized region that neurotransmitters are released to transmit information from one neuron to its neighbors. The synapse has been defined as the space between two subsequent interrelated neurons. ... 

American inventor and educator Alexander Graham Bell develops the telephone by converting electrical impulses into sound. 

The sinus rhythm is the heart rhythm in which the sinus node generates an electrical impulse that travels through specialized cells (that form a conduction system) and leads to a ventricular contraction. 

These bridge wire devices are relatively safe from initiation by direct application of heat and external mechanical influences or from most electrical inputs. They are designed for highly specialized electrical impulses (Ref 13)... 

The cardiotonics affect the transmission of electrical impulses along the pathway of the conduction system of tiie heart. The conduction system of die heart is a group of specialized nerve fibers consisting of die SA node, die AV node, the bundle of His, and die branches of Purkinje (Fig. 39-2). Each heartbeat (or contraction of tiie ventricles) is tiie result of an electrical impulse tiiat normally starts in tiie SA node, is tiien received by die AV node, and travels down die bundle of His and through tiie Purkinje fibers (see Fig. 39-2). The heartbeat can be felt as a pulse at the wrist and otiier areas of die body where an artery is close to the surface or lies near a bone When the electrical impulse reaches the... 

The cardiac muscle (myocardium) has attributes of both nerve and muscle and therefore has the properties of both. Some cardiac arrhythmias are caused by the generation of an abnormal number of electrical impulses... 

Muscle activity is accompanied by cellular pumping of sodium ions. The energy requirements of the sodium pump have been studied on an individual cardiac muscle mounted inside a tiny differential calorimeter and stimulated by electrical impulses. The heat evolved was different in the presence and absence of a known inhibitor of the sodium pump. 

A number of plants can eat insects. These plants are called insectivorous plants. Examples include the Drosera, Dionaeva, Venus-flytrap, pitcher plants, sundew, and blad-derworts. Insectivorous plants are most often found in moist and nutrient-poor habitats. The insects, which the plants trap, provide mineral nutrients. The amazing speeds of electrical impulses are illustrated by the capture of insects by insect-eating plants and during the tropism. Action potentials in the plants not processing motor activity are similar in their characteristics to the action potentials of sensitive plants of the... 

The generation and propagation of action potentials and electrical impulses between the tissues in higher plants can be measured by reversible nonpolarizable electrodes [1]. Since both Ag/AgCl electrodes are identical, we decided to call them reference and working electrodes as shown in Fig. 4. The reference electrode (—) was usually inserted in the stem or in a root of a soybean plant, and the upper (working) electrode (-I-) inserted in the stem or a leaf of the plant. 

Following initiation of the electrical impulse from the SA node, the impulse travels through the internodal pathways of the specialized atrial conduction system and Bachmann s bundle (Fig. 6-1 j.1 The atrial conducting fibers do not traverse the entire breadth of the left and right atria, as impulse conduction occurs across the internodal pathways, and when the impulse reaches the end of Bachmann s bundle, atrial depolarization spreads as a wave similar to that which occurs upon throwing a... 

After an electrical impulse is initiated and conducted, there is a period of time during which cells and fibers cannot be depolarized again. This period of time is referred to as the absolute refractory period (Fig. 6-2),2 and corresponds to phases 1,2, and approximately half of phase 3 repolarization on the action potential. The absolute refractory period also corresponds to the period from the Q wave to approximately the first half of the T wave on the ECG (Fig. 6-2). During this period, if there is a premature stimulus for an electrical impulse, this impulse cannot be conducted, because the tissue is absolutely refractory. 

Abnormal initiation of electrical impulses occurs as a result of abnormal automaticity. If the automaticity of the SA node increases, this results in an increased rate of generation of impulses and a rapid heart rate (sinus tachycardia). If other cardiac fibers become abnormally automatic, such that the rate of initiation of spontaneous impulses exceeds that of the SA node, other types of tachyarrhythmias may occur. Many cardiac fibers possess the capability for automaticity, including the atrial tissue, the AV node, the Purkinje fibers, and the ventricular tissue. In addition, fibers with the capability of initiating and conducting electrical impulses are present in the pulmonary veins. Abnormal atrial automaticity may result in premature atrial contractions or may precipitate atrial tachycardia or atrial fibrillation (AF) abnormal AV nodal automaticity may result in junctional tachycardia (the AV node is also sometimes referred to as the AV junction). Abnormal automaticity in the ventricles may result in ventricular premature depolarizations (VPDs) or may precipitate ventricular tachycardia (VT) or ventricular fibrillation (VF). In addition, abnormal automaticity originating from the pulmonary veins is a precipitant of AF. 

The mechanism of abnormal impulse conduction is traditionally referred to as reentry. Reentry is often initiated as a result of an abnormal premature electrical impulse (abnormal automaticity) therefore, in these situations, the mechanism of the arrhythmia is both abnormal impulse formation (automaticity) and abnormal impulse conduction (reentry). In order for reentry to occur, three conditions must be present. There must be (1) at least two pathways down which an electrical impulse may... 

O Atrial fibrillation may be caused by both abnormal impulse formation and abnormal impulse conduction. Traditionally, AF was believed to be initiated by premature impulses initiated in the atria. However, it is now understood that in many patients AF is triggered by electrical impulses generated within the pulmonary veins.20 These impulses initiate the process of reentry within the atria, and AF is believed to be sustained by multiple reentrant wavelets operating simultaneously within the atria.21 Some believe that, at least in some patients, the increased automaticity in the pulmonary veins may be the sole mechanism of AF and that the multiple reentrant wavelet hypothesis may be incorrect.21 However, the concept of multiple simultaneous reentrant wavelets remains the predominant hypothesis regarding the mechanism of AF. 

Ventricular Rate Control is achieved by inhibiting the proportion of electrical impulses conducted from the atria to the ventricles through the AV node. Therefore, drugs that are effective for ventricular rate control are those that inhibit AV nodal impulse conduction P-blockers, diltiazem, verapamil, and digoxin (Tables 6-5 and 6-6). Amiodarone also inhibits AV nodal conduction, but is not a preferred drug for ventricular rate control in AF due to its unfavorable adverse-effect profile (Table 6-6). 

O Paroxysmal supraventricular tachycardia is caused by reentry that includes the AV node as a part of the reentrant circuit. Typically, electrical impulses travel forward (antegrade) down the AV node and then travel back up the AV node (retrograde) in a repetitive circuit. In some patients, the retrograde conduction pathway of the reentrant circuit may exist in extra-AV nodal tissue adjacent to the AV node. One of these pathways usually conducts impulses rapidly, while the other usually conducts impulses slowly. Most commonly, during PSVT the impulse conducts antegrade through the slow... 

Ventricular premature depolarizations (VPDs) are ectopic electrical impulses originating in ventricular tissue, resulting... 

Electroconvulsive therapy (ECT) is the application of prescribed electrical impulses to the brain for the treatment of severe depression, mixed states, psychotic depression, and treatment-refractory mania in patients who are at high risk of suicide. It also may be used in pregnant women who cannot take carbamazepine, lithium, or divalproex. 

Reentry Circular movement of electrical impulses a mechanism of many arrhythmias. 

Counter, Gas-flow Proportional (GPC)—P-particles are detected by ionization of the counter gas which results in an electrical impulse at an anode wire. If a sufficient amount of radiostrontium is present and the ionization efficiency is calibrated, the quantity of radiostrontium can be determined. 

The discovery of galvanic electricity (i.e. electrical phenomena connected with the passage of electric current) by L. Galvani in 1786 occurred simultaneously with his study of a bioelectrochemical phenomenon which was the response of excitable tissue to an electric impulse. E. du Bois-Reymond found in 1849 that such electrical phenomena occur at the surface of the tissue, but it was not until almost half a century later that W. Ostwald demonstrated that the site of these processes are electrochemical semipermeable membranes. In the next decade, research on semipermeable membranes progressed in two directions—in the search for models of biological membranes and in the study of actual biological membranes. 

The transport of information from sensors to the central nervous system and of instructions from the central nervous system to the various organs occurs through electric impulses transported by nerve cells (see Fig. 6.17). These cells consist of a body with star-like projections and a long fibrous tail called an axon. While in some molluscs the whole membrane is in contact with the intercellular liquid, in other animals it is covered with a multiple myeline layer which is interrupted in definite segments (nodes of Ranvier). The Na+,K+-ATPase located in the membrane maintains marked ionic concentration differences in the nerve cell and in the intercellular liquid. For example, the squid axon contains 0.05 MNa+, 0.4 mK+, 0.04-0.1 m Cl-, 0.27 m isethionate anion and 0.075 m aspartic acid anion, while the intercellular liquid contains 0.46 m Na+, 0.01 m K+ and 0.054 m Cl-. 

According to Fig. 6.17 the nerve cell is linked to other excitable, both nerve and muscle, cells by structures called, in the case of other nerve cells, as partners, synapses, and in the case of striated muscle cells, motor end-plates neuromuscular junctions). The impulse, which is originally electric, is transformed into a chemical stimulus and again into an electrical impulse. The opening and closing of ion-selective channels present in these junctions depend on either electric or chemical actions. The substances that are active in the latter case are called neurotransmitters. A very important member of this family is acetylcholine which is transferred to the cell that receives the signal across the postsynaptic membrane or motor endplate through a... 

To understand the chemical-imbalance theory, it will be helpful to first review some basic aspects of how the brain functions. The human brain contains about ioo billion nerve cells called neurons. Each neuron is like an electrical wire with many branches. When a neuron fires, electrical impulses travel along its length from one end to the other. When an impulse reaches the end of a branch, it may stimulate the next neuron, influencing whether or not it fires. 

Figure 4.4 Saltatory conduction. Transmission of electrical impulses in a myelinated axon occurs by way of saltatory conduction. Composed primarily of lipid, the myelin sheath insulates the axon and prevents generation of membrane potentials. Membrane potentials occur only at gaps in the myelin sheath, referred to as the nodes of Ranvier. Therefore, transmission of the impulse, or generation of action potentials, occurs only at the nodes.

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