Ion channels opening and closing

During excitation, ion channels open and close and a few ions flow 98 Gating mechanisms for Na+ and K+ channels in the axolemma are voltage-dependent 98... 

To produce membrane depolarization, a current stimulus of sufficient intensity to exceed the outward K+ current must be appUed to the cell. If the depolarizing stimulus raises the membrane potential above a threshold value, sodium channels within the sarcolemmal membrane change their conformation and open their ion-selective pore, allowing Na to enter the cell driven by the electrochemical gradient. The open sodium channels raise the membrane potential toward the equilibrium potential of sodium (-f65 mV) and set into motion the intricate and precisely coordinated series of ion channel openings and closings leading to the characteristic action potential. 

Now what s the molecular basis of different permeabilities for different ions This is where the channels come in. Without a specific channel, no ion can effectively cross the membrane, so its permeability will be very small only ions for which specific channels exist will therefore have a say in determining the membrane potential. Furthermore, as ion channels open and close, the changing permeabilities can shift the weight from one ion to the other. The most important example is the transient opening of sodium channels, which according to the Goldman equation will cause the... 

This pump does not operate equally in both directions, and two to three sodium ions are transported out of the cell for each potassium ion that enters the cell. Also, cell membranes are almost 100 times more permeable to potassium ions than to sodium ions, so that it is reasonable to assume the potassium ion concentration difference across a cell is an equilibrium state. On average, for a resting nerve fiber, the sodium ion concentrations are 10 millimolar (mM) within cells and 142 rruVI outside the cell, while the potassium ion concentrations are 5 mM outside the cell (in the extracellular fluid, ECF) and 140 mM inside the cell. (Note that the actual situation is further complicated by the fact that the ion channels open and close depending on the physiological situation. For example, during a nerve impulse the permeability of the membrane to sodium ions can increase by a factor of a thousand, almost eliminating the sodium ion concentration difference.)... 

Calcium channels are members of the large family of proteins, including Na and channels, which become incorporated into plasma membranes, and which form intermittent aqueous pathways through which ions can move. The channels open and close. As is the case generally for membrane spanning proteins, a Ca channel is formed by a set of helical units, in this case seven, which associate to form the channel. 

The permeability selectivity ratio for P < /Pci estimated to be 5 for the channel of 5 pS. The selectivity again favors cation and the ratio resembles the value obtained by supramolecular oligoether channels. Open and closed transitions were relatively slow and the time distributions were 300 ms and 400 ms, respectively. Therefore, the ion pair, trans-2l, can be concluded to show typical characteristics of the single-ion channel in all respects. 

Potassium ions arc also used in conduction of impulses along nerves and muscles. The brief inflow of Na ions results In neutralization of the electrical charge across the membrane. The charge difference is restored by potassium ions. Immediately after the Na channels have opened and dosed, K channels open and close. This latter event results in momentary flow of potassium out of the cell, which restores the relative negative charge within the cell. The nerve or muscle cell is then able to transmit another impulse. 

Ligand— Any molecule that interacts with an ion channel and regulates channel opening and closing by producing conformational changes. 

Fig. 1. Comparison of the current noise due to a carrier with that due to a pore. (A) Current fluctuations in the presence of valinomycin with lithium (which is poorly transported) and the rubidium (which is well transported) as the cation. Noise increases at high frequencies because the empty carrier must return after carrying one ion across the membrane. (From [8].) (B) Current fluctuations in the presence of alamethicin. Noise decreases at high frequencies because the channels open and close at a limited rate. (From [7].)...

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