Membrane potentials / EC Gradient
Electrochemical Gradients
- ions move through ion channels in response to chemical and electrical gradients
Chemical Gradients
Electrolyte Concentrations in Plasma (mEq/L) |
|
Na+ |
136-145 |
K+ |
3.5-5.0 |
Ca2+ |
1-2 |
Cl- |
95-105 |
- with respect to neural communication, the critical ions are Na+, K+, Ca2+ and Cl-
- the other primary electrolytes are Mg2+, hydrogen phosphate (HPO42-) and hydrogen carbonate (HCO3-)
- extracellular (plasma) electrolyte concentrations are determined by the balance between oral intake and renal excretion, with electrolyte homeostasis being under hormonal control
- disturbances in electrolyte concentrations (as occurs during dehydration or overhydration, for example) can cause severe cardiac and neurologic complications, which can be life-threatening
- membrane transporters determine the intracellular concentrations of the various ions, and they vary with cell type
- in neurons, the concentration gradients are generally so large that, under normal conditions, no amount of flow through ion channels will change the intra- or extra-cellular concentrations
Intracellular Ion Concentrations (mM) |
||
Myocyte |
Neuron |
|
Na+ |
10 |
5-15 |
K+ |
155 |
140 mM |
Ca2+ |
0.0001 mM |
0.0001 mM |
Cl- |
10-20 |
4-30 mM |
- in considering the neuronal membrane potential, the critical concentration gradients are those of Na+, K+ and Cl-
- in neurons, the Na+ and K+ gradients are established by Na+/K+ ATPase
- Na+/K+ ATPase is an active transporter that exchanges 3 intracellular Na+ for 2 K+ extracellular ions, expending 1 ATP molecule in the process of moving the ions from one side of the membrane to the other
- the slow speed of the transport, and the fact that there is only a movement of +1 for each pump cycle means that Na+/K+ ATPase is NOT responsible for generating the large negative resting membrane potential of neurons
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The primary function of neuronal Na+/K+ ATPase is the establishment of the concentration gradients for Na+ and K+ that are used to establish the resting, graded and action potentials of neurons and myocytes. The actions of Na+/K+ ATPase are only mildly electrogenic: the net result of the actions of Na+/K+ ATPase is a Vm of ~ -5 to -12 mV. |
- decreasing the function of Na+/K+ ATPase (e.g., by anoxia, which decreases availability of ATP or by excessive administration of antiarrhythmic drugs that block this transporter) can be fatal
- most critical is the resulting depolarization of cardiac myocytes --> arrhythmias (including asystole) --> death
- most critical is the resulting depolarization of cardiac myocytes --> arrhythmias (including asystole) --> death
Electrical Gradients
- electrical gradients are relatively simple to understand: cations are attracted by negative potentials and repelled by positive (the opposite is true for anions)
- i.e., a negative Vm will tend to cause cations (esp. Na+ and K+) to move into the cell