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Electrophysiology of Neurons

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BIOL 1117
Christopher Richardson

L23-Nervous Tissue 11/4/2013 • Electrical Potentials and Currents o An important focus of neurophysiology is the study: 1) the mechanism for generating an electrical signal in a neuron and 2) the mechanism for transmitting useful information from one cell to another cell o electrophysiology: cellular mechanisms for producing electrical potentials and currents  basis for neural communication and muscle contraction o electrical potential: a difference in the concentration of charged particles between two points which has the potential to do work o Recall that electrical potential or voltage is the energy required to move a unit electrical charge in a static electric field o electrical current: a flow of charged particles from one point to another  electrical potential between charges tends to make charged particles flow  If charges are opposite, current will bring them together; but if charges are the same, then the current will separate them  voltage (or electrical potential)/resistance = current (V/R = I)  For given R, if increase V increase I  For given V, if increase R decrease I; materials like myelin with high electrical R which then reduce current are known as electrical insulators while those with low R will increase current and are known as electrical conductors o in the body, currents are movement of ions, such as Na or K through gated channels in the plasma membrane; open or closed by stimuli which enables the cell to turn electrical current on or off o When an electrical potential exists between two points, such as the poles of a battery, they are said to be in a polarized state o living cells are polarized o resting membrane potential (RMP): charge difference across the plasma membrane  -70 mV in a resting, unstimulated neuron  negative value means there are more negatively charged particles on the inside of the membrane than on the outside • Resting Membrane Potential o RMP exists because of unequal electrolyte distribution between extracellular fluid (ECF) and intracellular fluid (ICF). o Thus, tiny excess of negative ions are inside the cell with o electrical attraction across membrane of cations and anions to each other o More negative inside means cell attracts ECF cations to its outer surface while repelling the ECF anions o RMP results from the combined effect of two factors:  specific ions that diffuse down their concentration gradient through the membrane  plasma membrane is selectively permeable and allows some ions to pass more easily than others • Creation of Resting Membrane Potential + o potassium ions (K ) have the greatest influence on RMP  plasma membrane is more permeable to K than any other ion  Membrane is most leaky to K+ because a few K+ channels stay open even at rest  cytoplasmic anions can not escape due to size or charge (phosphates, small organic acids, proteins,ATP, and RNA)  K leaks out due to diffusion down concentration gradient  negative electrical charge of ICF starts to attract K back in and + equilibrium is reached and net diffusion of K outward stops  Movement of K+ ions out of cell due to concentration gradient is now equal but opposite the movement back in of K+ due to the electrical gradient which attracts positive ions back towards negative charge of cell  The membrane potential is now negative enough to produce opposite but equal flux to concentration gradient.  The magnitude of the equilibrium potential (in mV) for any ions type depends on the concentration gradient for that ion across the membrane  The larger the concentration gradient, the larger the equilibrium potential required because a larger, electrically driven movement of ions will be required to balance the movement of ions due to the concentration gradient (concentration difference)  At equilibrium, K+ is about 30-40x as concentrated in the ICF as in the ECF  *Potassium has single greatest influence on resting membrane potential of neuron  As potassium leaves, you see an increasing net negative charge in the ICF o resting membrane much less permeable to high concentration of sodium (Na ) found outside the cell  But some leaks in; diffuses into the cell down its concentration gradient  Sodium leaking back in makes the RMP of cell less negative than if due to only K+ leaking out  Nonetheless, the RMP generated across the membrane is due more to K+ flux down its concentration gradient out of the cell than sodium flux down its concentration gradient into the cell  At equilibrium, Na is about 10-12 times more concentrated in the ECF than the ICF o Na /K pumps works continuously to compensate for Na+ and K+ leakage  creates the concentration gradients which indirectly contribute to the membrane potential  3 Na out for every 2 K it brings in using oneATP  The unequal transport of positive ions also directly contributes to membrane potential by making the inside of the cell more negative than it would be by simple ion diffusion alone  The Na+–K+ pump accounts for about 70% of theATP requirement of the nervous system
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