Nerve cell potential difference

In a neuron (a nerve cell), the concentration of K ions inside the cell is about 20 to 30 times as great as that outside. What potential difference between the inside and the outside of the cell would you expect to measure if the difference is due only to the imbalance of potassium ions ... 

The electricity-producing system of electric fishes is built as follows. A large number of flat cells (about 0.1 mm thick) are stacked like the flat unit cells connected in series in a battery. Each cell has two membranes facing each other. The membrane potentials of the two membranes compensate for each other. In a state of rest, no electrostatic potential difference can be noticed between the two sides of any cell or, consequently, between the ends of the stack. The ends of nerve cells come up to one of the membranes of each cell. When a nervous impulse is applied from outside, this membrane is excited, its membrane potential changes, and its permeability for ions also changes. Thus, the electrical symmetry of the cell is perturbed and a potential difference of about 0.1 V develops between the two sides. Since nervous impulses are applied simultaneously to one of the membranes in each cell, these small potential differences add up, and an appreciable voltage arises between the ends of the stack. 

The initiation of an epileptic attack involves "pacemaker" cells these differ from other nerve cells in their unstable resting membrane potential, i.e a depolarizing membrane current persists after the action potential terminates. 

Molecular mechanisms in biology, too, depend to a great extent on electrified interfaces. Thus, the mechanism by which nerves carry messages from brain to muscles is based on the potential difference across the membrane that separates a nerve cell from the environment. What are the laws that apply to this electrified interface If this question is answered, then the mechanism by which nerves transmit messages may be determined at the molecular level and the process concerned may become controllable. 

Action potentials are waves of depolarization and repolarization of the plasma membrane. In a resting nerve cell, the electric potential gradient (At//) across the plasma membrane is about —70 mV, inside negative. This potential difference is generated mainly by the unequal rates of diffusion of K+ and Na+ ions down concentration gradients maintained by the Na+-K+ ATPase. 

Using the information given in Table 2.1, calculate the potential difference (in mV) across a squid nerve cell. 

The normal potential difference between the inner and outer parts of nerve cells is about -70 mv as estimated above. Transmission of a nerve impulse is initiated by a lowering of this potential difference to about -20 mv. This has the effect of temporarily opening the Na+ channel the influx of these ions causes the membrane potential of the adjacent portion of the nerve to collapse, leading to an effect that is transmitted along the length of the nerve. As this pulse passes, K+ and Na+ pumps restore the nerve to its resting condition. 

As mentioned previously, there are many potential targets available in nerve cells for interaction with a drug. Some targets are general to all nerve cells, and drugs that affect them will produce widely dispersed effects. Other targets are found only in a subset of nerve cells, and in these cases drug effects will be restricted to them. The most common reason specific interactions occur is that neurons differ in the transmitter system they possess. 

The ions can move in solutions and conduct small electrical currents within cells and between cells of the body. These often control the release of hormones and maintain the effectiveness of neurotransmitters in the brain. A neuron (nerve cell) message is passed by a flow of Na+ ions into a neuron and a flow of K+ ions out, giving about 100 mV potential difference at the cell membrane only. This passes along a neuron at about 120m/s. 

Many cells and subcellular organelles maintain an electrostatic potential difference across their membranes. This potential typically is important to the operation of the cell or organelle. For example, in nerve cells and other cells with excitable membranes such as muscle cells, the electrostatic potential is an important signal that governs cellular behavior. In these cells, some form of electrochemical signal that is sent to the cell can elicit an action potential - a transient change in the membrane potential that can trigger intracellular events, such as contraction of a muscle cell. 

Figure 7.8 Simulated action potential from the Flodgkin-Huxley model. The upper panel plots action potential for three different values of applied current. The lower panel plots the predicted conductances of the sodium and potassium channels for the case of Iapp = 6.2 qA-cm-2, for which sustained period firing of the nerve cell is predicted.

There are two major differences between action potentials that occur in nerve cells or skeletal muscle cells on the one hand, and in heart muscle cells on the other ... 

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