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Physiology of the Nerve Impulse

All of the cells in the body have an electric potential difference across their plasma membrane. This means that the inside of each cell is negatively charged with respect to the outside and this difference in voltage, just like a chemical concentration gradient, can be used to do cellular work. The potential difference across a cell membrane (usually about –70 mV) is referred to as the resting potential, and exists due to the maintenance of unequal concentrations of positively and negatively charged ions inside and outside of the cell. 
In addition, a transmembrane protein, the sodium-potassium pump, constantly uses active transport to move sodium (Na+) ions out of the cell and potassium (K+) ions into the cell, resulting in an electrochemical gradient of sodium and potassium ions across the cell membrane. There is always much more sodium outside of the cell than inside, and potassium is in greater concentration internally. While all cells exist in this state of disequilibrium, it is especially important for the functioning of nervous and muscular tissue cells. The resting potential is maintained due to the impermeability of the cell membrane to any charged atoms or molecules because of its hydrophobic nature (see Chapter 11). Thus, the only way for ions to traverse the membrane is through protein channels which tend to be very specific for a particular type of ion. Since these channels can be open or closed, they are referred to as gated, and we will be most interested in the gated sodium channels and gated potassium channels.
In a resting neuron (refer to Figure 13.1 for structure), almost all of the gated channels are closed. When a proper stimulus is present, the neuron reacts by opening the gated sodium channels in the region being stimulated (usually a dendrite). The result is a sudden influx of positively charged sodium ions, which follow their electrochemical concentration gradient and cause the membrane to become depolarized. This is a temporary state during which the polarity of the potential difference is reversed, and the inside of the cell becomes positively charged with respect to the outside. This movement of charges and subsequent depolarization is a form of electrical energy, and causes the gated potassium channels to open. As you would expect, the result is a movement of potassium ions from the inside to the outside of the cell, restoring the resting potential in a process known as repolarization. The gated channels of both types then close, and the Na+/K+ pump restores the concentrations of these ions to their original states. All of these changes taken together are called the action potential, and requires only about a thousandth of a second to occur. An important feature of the action potential is its ability to cause similar changes in adjacent regions of the cell; the net result is the unidirectional propagation of a nerve impulse down the neuron. After the passage of a nerve impulse through any portion of a neuron, there is a brief refractory period, during which the cell is “resetting” itself to be able to receive another stimulus and is unexcitable for a short period of time (about 1/2,500 of a second).

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