Potential in nerve cells

Voltage-Gated Ion Channels and the Propagation of Action Potentials in Nerve Cells... 

Fundamentally, the eel is simply a living battery. The tips of its head and tail represent the poles of the eel s battery . As much as 80 per cent of its body is an electric organ, made up of many thousands of small platelets, which are alternately super-abundant in potassium or sodium ions, in a similar manner to the potentials formed across axon membranes in nerve cells (see p. 339). In effect, the voltage comprises thousands of concentration cells, each cell contributing a potential of about 160 mV. It is probable that the overall eel potential is augmented with junction potentials between the mini-cells. 

This results in the extrusion of three positive charges for every two that enter the cell, resulting in a transmembrane potential of 50-70 mV, and has enormous physiological significance. More than one-third of the ATP utilized by resting mammalian cells is used to maintain the intracellular Na+-K+ gradient (in nerve cells this can rise up to 70%), which controls cell volume, allows neurons and muscle cells to be electrically excitable, and also drives the active transport of sugars and amino acids (see later). 

You will be expected to have an understanding of action potentials in nerves, cardiac pacemaker cells and cardiac conduction pathways. 

Some of the main types of cellular regulation associated with rhythmic behavior are listed in Table III. Regulation of ion channels gives rise to the periodic variation of the membrane potential in nerve and cardiac cells [27, 28 for a recent review of neural rhythms see, for example, Ref. 29]. Regulation of enzyme activity is associated with metabolic oscillations, such as those that occur in glycolysis in yeast and muscle cells. Calcium oscillations originate... 

As indicated above, theoretical models for biological rhythms were first used in ecology to study the oscillations resulting from interactions between populations of predators and preys [6]. Neural rhythms represent another field where such models were used at an early stage The formalism developed by Hodgkin and Huxley [7] stiU forms the core of most models for oscillations of the membrane potential in nerve and cardiac cells [33-35]. Models were subsequently proposed for oscillations that arise at the cellular level from regulation of enzyme, receptor, or gene activity (see Ref. 31 for a detailed fist of references). 

A further intercellular communication mechanism relies on electrical processes. The conduction of electrical impulses by nerve cells is based on changes in the membrane potential. The nerve cell uses these changes to communicate with other cells at specialized nerve endings, the synapses (see chapter 16). It is central to this type of intercellular commimication that electrical signals can be transformed into chemical signals (and vice versa, see chapter 16). 

Ion channels - regulated by membrane potential or ligands - provide and support complex signalling processes in nerve cells, surrounded by membranes that act as insulators all the way down the signal flow to the target or as communicators at the synapses. We have learned to interfere with these processes with modern therapeu-... 

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. 

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. 

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 ... 

In living cells, ion channels and energy-dependent pumping mechanisms (with more or less specificity) facilitate or catalyze the permeation of certain ions in one direction or the other across the lipid bilayers that constitute the cell membranes. This mechanism makes the permeation of certain ions several orders of magnitude faster than the permeation of others which thereby accentuates the transmembrane potentials discussed above. These processes are particularly important in the creation and the transmission of the action potential in nerves. 

In the exocrine pancreatic cells. In many animal cells, the combined force of the Na" concentration gradient and membrane electric potential drives the uptake of amino acids and other molecules against their concentration gradient by lon-llnked symport and antiport proteins (see Section 7.4). And the conduction of action potentials by nerve cells depends on the opening and closing of Ion channels In response to changes In the membrane potential (see Section 7.7). 

In this example, the Na concentration gradient and the membrane electric potential contribute almost equally to the total AG for transport of Na Ions. Since AG Is <0, the inward movement of Na Ions Is thermodynamically favored. As discussed in the next section, certain cotransport proteins use the inward movement of Na to power the uphill movement of other ions and several types of small molecules into or out of animal cells. The rapid, energetically favorable movement of Na ions through gated Na channels also is critical in generating action potentials in nerve and muscle cells. 

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