The Propagation of Nerve Impulses

The idea that signals are transmitted along the nerve channels as an electric current had arisen as early as the middle of the nineteenth century. Yet even the first measurements performed by H. Helmholtz showed that the transmission speed is about lOm/s (i.e., much slower than electric current flow in conductors). It is known today that the propagation of nerve impulses along the axons of nerve cells (which in humans are as long as 1.5m) is associated with an excitation of the axon s outer membrane. 

Recent drug development studies have centered on the capacity of known antiepileptic drugs (AEDs) to interact with ion channels, and it is now established that several agents appear to be exerting their effects primarily by inhibiting ion channels. Modulation of neuronal sodium channels decreases cellular excitability and the propagation of nerve impulses. Inhibition of sodium channels appears to be a major component of the mechanism of action of several anticonvulsant drugs. 

How do ion chaimels, vital to a wide array of biological functions, operate at a molecular level We will examine three chaimels important in the propagation of nerve impulses the ligand-gated channel the acetylcholine receptor channel, which communicates the nerve impulse between certain neurons and the voltage-gated Na+ and K+ channels, which conduct the nerve impulse down the axon of a neuron. 

A biological cell can be compared to a concentration cell for the purpose of calculating its membrane potential. Membrane potential is the electrical potential that exists across the membrane of various kinds of cells, including muscle cells and nerve cells. It is responsible for the propagation of nerve impulses and heart beat. A membrane potential is established whenever there are unequal concentrations of the same type of ion in the interior and exterior of a cell. For example, the concentrations of ions in the interior and exterior of a nerve cell are 400 mM and 15 mM, respectively. Treating the situation as a concentration cell and applying the Nemst equation, we can write... 

The feasibility of phospholipid phase transition phenomena in the propagation of nerve impulse as well as in information transfer in biological systems in general has been noted earlier. The phase transition of model and biomembranes exhibits considerable hysteresis.The possible role of fast and slow hysteresis phenomena in biomembranes for information storage was put forward by Trauble. " ... 

Voltage-regulated sodium channels are the major participants in propagation of nerve impulses. Tire large 260-kDa a subunit of the sodium channel of nerve membranes contains four homologous repeat sequences, each of which may form transmembrane helices and also contain a loop that may participate in forming a pore similar to the K+ pore of Fig. 8-21.509 510a However, the structure is uncertain.511 Tire channel complex also contains 36- or 33-kDa Pj and P2 subunits that appear to be members of the Ig superfamily. 

Chloroprocaine, like other local anesthetics, blocks the generation and the conduction of nerve impulses, presumably by increasing the threshold for electrical excitation in the nerve by slowing the propagation of the nerve impulse and by reducing the rate of the rise of action potential. In general, the progression of anesthesia is related to the diameter, myelination, and conduction velocity of affected nerve fibers. Clinically, the order of loss of nerve function is as follows pain, temperature, touch, proprioception, and skeletal muscle tone. 

As monotherapy bupivacaine blocks the generation and the conduction of nerve impulses by increasing the threshold for electrical excitation in the nerve, slowing the propagation of the nerve impulse, and reducing the rate of elevation of the action potential. 

Helmholtz, H. L. F. (1850). Measurements on the time of twitching of animal muscles and the velocity of propagation of nerve impulses. Mullers Archiv fiir Anatomie und Wissenschaftliche Medizin, 276-364. 

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