Action potential impulse

Musha, T., Kosugi, Y., Matsumoto, G. (1981) Modulation of the time relation of action potential impulses propagating along an axon. IEEE Trans. Biomed. Eng. 28, 616... 

Conductivity. Conductivity is an electrical property of excitable tissue which ensures that if one area of a membrane is excited to full activity, that area excites adjacent areas. Conduction of an impulse varies direcdy with the rate of development of phase 0 and the ampHtude of the action potential. Phase 0 is faster, and ampHtude of the action potential is greater, the more negative the transmembrane potential at the time of initiation of the impulse. Conduction velocity is faster when phase 0 is fast. 

Each nervous impulse is a group of consecutive action potential bursts. It had been pointed out above that these bursts are identical in shape. The information to be transmitted by the impulse is encoded in terms of the number of individual bursts in a group and of the intervals between bursts. 

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. 

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. 

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.

A second factor that influences the velocity of action potential conduction is the diameter of the axon. The greater the diameter is then, the lower the resistance to current flow along the axon. Therefore, the impulse is... 

The mechanism of action of these anesthetics involves the blockade of sodium channels in the membrane of the second-order sensory neuron. The binding site for these anesthetics is on a subunit of the sodium channel located near the internal surface of the cell membrane. Therefore, the agent must enter the neuron in order to block the sodium channel effectively. Without the influx of sodium, neurons cannot depolarize and generate an action potential, so the second-order sensory neuron cannot be stimulated by impulses elicited by pain receptors associated with the first-order sensory neuron. In other words, the pain signal is effectively interrupted at the level of the spinal cord and does not travel any higher in the CNS. In this way, the brain does not perceive pain. 

As mentioned previously, a single action potential lasting only 2 msec causes a muscle twitch that lasts approximately 100 msec. If the muscle fiber has adequate time to completely relax before it is stimulated by another action potential, the subsequent muscle twitch will be of the same magnitude as the first. However, if the muscle fiber is restimulated before it has completely relaxed, then the tension generated during the second muscle twitch is added to that of the first (see Figure 11.3). In fact, the frequency of nerve impulses to a muscle fiber may be so rapid that there is no time for relaxation in between stimuli. In this case, the muscle fiber attains a state of smooth, sustained maximal contraction referred to as tetanus. 

Verapamil (Class IV antiarrhythmic drug) is an effective agent for atrial or supraventricular tachycardia. A Ca++ channel blocker, it is most potent in tissues where the action potentials depend on calcium currents, including slow-response tissues such as the SA node and the AV node. The effects of verapamil include a decrease in heart rate and in conduction velocity of the electrical impulse through the AV node. The resulting increase in duration of the AV nodal delay, which is illustrated by a lengthening of the PR segment in the ECG, reduces the number of impulses permitted to penetrate to the ventricles to cause contraction. 

Action potential, or nerve impulse The wave of electrical activity that passes from the dendrites of the neuronal cell body, down the axon to the synaptic bouton. 

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