Sodium ions, nerve cells

Fig. 6.24 A hypothetic scheme of the time behaviour of the spike linked to the opening and closing of sodium and potassium channels. After longer time intervals a temporary hyperpolarization of the membrane is induced by reversed transport of potassium ions inside the nerve cell. Nernst potentials for Na+ and K+ are also indicated in the figure. 

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. 

Cell membranes contain selective ion channels that are highly discriminatory for potassium ions, sodium ions, calcium ions, and the proton. For instance, the highly selective potassium channels of nerves show selectivity for ions as Li < Na K > Rb > Cs, and calcium channels show selectivity as Mg Ca > Sr > Ba. Ion selection operates on the basis of size and repulsion, not... 

Sodium ions A marked and rapid increase in the Na ion transport across the plasma membrane into a nerve or muscle cell, via the Na ion channel, causes a depolarisation of the membrane that initiates a transient flux of electrical activity along the nerve or muscle (that is, an action potential) (Chapters 13 and 14). 

Figure 2.2 Diagram of a voltage-activated sodium channel protein. The channel is composed of a long chain of amino acids intercormected by peptide bonds. The amino acids perform specific functions within the ion channel. The cylinders represent amino acid assemblies located within the membrane of the nerve cell and responsible for the foundation of the ion pore.

Neurotransmitters can either excite or inhibit the activity of a cell with which they are in contact. When an excitatory transmitter such as acetylcholine, or an inhibitory transmitter such as GABA, is released from a nerve terminal it diffuses across the synaptic cleft to the postsynaptic membrane, where it activates the receptor site. Some receptors, such as the nicotinic receptor, are directly linked to sodium ion channels, so that when acetylcholine stimulates the nicotinic receptor, the ion channel opens to allow an exchange of sodium and potassium ions across the nerve membrane. Such receptors are called ionotropic receptors. 

The NMDA receptor complex has been extensively characterized and its anatomical distribution in the brain determined. The NMDA receptor is analogous to the GABA-A receptor in that it contains several binding sites, in addition to the glutamate site, whereby the movement of sodium and calcium ions into the nerve cell can be modulated. 

In simple terms, messages travel along neurons (nerve cells) in the form of an electrical current that moves from one end of the neuron to its opposite end. The electric current is produced by a flow of sodium ions (Na ") and potassium ions (K ) across the nerve membrane, as shown in the diagram on page 11. When the electrical current reaches the end of the neuron, it causes the release of a chemical known as a neurotransmitter. Some examples of neurotransmitters are acetylcholine, serotonin, dopamine, GABA (gamma-aminobutyric acid), and norepinephrine. 

In a resting condition, there is a specific rest potential between the axoplasm and the inner parts of the cell. This rest potential is maintained by relative concentration of sodium and potassium ions along the membrane of the nerve. During nerve stimulation, the membrane is depolarized and sodium channels in that area are opened, allowing sodium ions to rush into the cell. At the peak of depolarization potassium channels are opened. The last ones leave the cell and the cell is repolarized. 

It is suspected that these drugs selectively bind with the intracellular surface of sodium channels and block the entrance of sodinm ions into the cell. This leads to stoppage of the depolarization process, which is necessary for the diffusion of action potentials, elevation of the threshold of electric nerve stimulation, and thus the elimination of pain. Since the binding process of anesthetics to ion channels is reversible, the drug diffuses into the vascular system where it is metabolized, and nerve cell function is completely restored. 

The next event in the generation of a nerve signal involves the opening of sodium channels within the membrane. What will happen considering the imbalance of sodium ions Of course, they will rush into the cell The sodium channel opening is following after a brief interval by the opening of... 

Sodium and potassium are not the only ions which can participate in pumps and channels. Calcium is also pumped, channeled, exhanged,and stored. See Figure 23. Calcium concentration within the cell cytoplasm is very low. This allows the calcium to play a pivotal role in cellular activity. The cytoplasmic protein calmodulin binds and stores calcium ion. Various intracellular structures and organelles such as the mitochondria and sarcoplasmic reticulum also store calcium. Calcium is vital to such functions as the release of neurotransmitters from nerve cells. There are at least seven known modes of biochemical action for this ion, one of the most important of which involves stimulation of cardiac muscle protein (actin-myosin). Certain types of angina (heart pain) are believed to be caused by abnormal stimulation of cardiac arteries and muscle (coronary spasm) A relatively new class of drugs, known as the calcium channel blockers, has brought relief from pain and arrhythmias (irregular heart beats). 

In most instances the arrival of a nerve signal at the presynaptic end of a neuron causes the release of a transmitter substance (neurohormone). Tire transmitter passes across the 10-50 nm (typically 20 nm) synaptic cleft between the two cells and induces a change in the electrical potential of the postsynaptic membrane of the next neuron (Fig. 30-10).149 401 Excitatory transmitters usually cause depolarization of the membrane. By this we mean that the membrane potential, which in a resting neuron is -50 to -70 mv (Chapter 8), falls to nearly zero often as a consequence of an increased permeability to Na+ and a resultant inflow of sodium ions. The resulting postsynaptic... 

We may now assemble the foregoing information into a molecular description of a few biological processes in which the interaction between water and metal ions plays an important role. First some problems related to signal transfer in nerve cells are discussed. This is followed by some comments on the mechanism operating at nerve synapses in which, in addition to the sodium and potassium ions, a specific transmitter substance and calcium ions take part. 

Sodium and potassium ions are vital to the normal functioning of the nerve cell. The ions are separated by the cell membrane with sodium on the outside and potassium on the inside of the resting cell. A model of the basic membrane, which in principle was built as a bilayer according to the well known Davson-Danielli-Robertson scheme but which... 

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