phys 5: basic electrophysiology

Vm is the sum of the net electrical potential enegy differnece for each permeable ion. The Vm results from separation of positive charge form negative charge within the membrane during diffusion of each ion down its gradient

1. its membrane permeability which is determined by the number of open channels per unit area of membrane 2. the magnitude and direction of its electrichemical gradient

A steady state potential requires a continual supply of metabolic energy to maintain the concentration gradient for each permeable ion not at its equilibrium potential. In contrast, no energy is required to maintain the equibrium potential for for each ion, no metablic energy is required to maintain its transmembrane concentration gradient.

3 Na out, 2 K in (each ion is pumped against its [ ] gradient)

1. the membrane is hoogenous 2. the electrical potential gradient is linear 3. The ions diffuse independently of one another 4. The permeability coefficient for each ion is constant

1. the transmembrane potential Vm 2. The internal and external concentration 3. the ion permeability

The force generated by the transmembrane concentration difference is equal and opposite to the force generated by the electrical potential energy difference

0, the outward and inward flow are equal and opposite because the two forces (chemical gradient and electrical potential) are equal, note also that since the net flux is 0, the current is 0

[inside]/[outeside] (proper direction is crucial!)

the log of the ratio of its intracellular to extracellular concentration

0 because Vm is not changing and there is no net flux of ions, this does not mean that every current is zero, only that their sum is zero. Note that this allwos you to derive the resting Vm by substituting the individual ion currents and setting their I total to zero

Cl isin equilibrium across the membrane and there are no active trnasport pumps. Under resting conditions, the transmembrane Icl is zero so it does not contribute to I total so Em=Ecl, note that this does not mean that the transmembrane Icl is at zero at all times

During trasient depolarization and action potential of the muscle cell, an inwardly directed electrical force moves Cl into the cell generating a transient inwardly-directed Icl that helps to repolarize the cell and terminte the action potential

If Cl can't enter the skeletal muscle cell, the ability of the cell to repolarize is compromised. The result is a prolonged partial depolariation of the cell leading to an abnormally prolonged tiem required for relaxation of the contracted muscle to its resting state.

Na and K (not Cl because it's in equilibrium)

Increased extracellualr K would derease the absolutel magnitude of Vm (the cell would become depolarized) this occurs because there is a reduced electrochemical gradient force (the outside is now more like the inside) which causes a reduced Ik and a reduction in Vm

Increasing the extracellular K brings the outside of the cell closer to the inside of the cell. THis decreases the electrochemical gradient thus decreasing the Ik which in turn depolarizes the cell. The cell can depolarize to at or near the threshold potenial resulting in hyperexcitability and spontaneous firing of action potentials

The absolute value of Vm is inversely proportional to alpha. As alpha increases (sodium moves in easier or potassium moves out easier) the membrane becomes less polarized. This is the basis upon which action potentials are generated, opening Na channels increases Pna allowing Na to rush in and depolarize the cell

a rapid, transient, and reversible depolarization of an electrically excitable cell membrane generated by a sequence of rapid, transient and reversible increases in ion conductances (pereabilities)

1. local depolarization causes a rise in both pNa and pK due to an opening of their channels.2. The channels automatically close and pNa and pK return to resting levels. Note that the kinetics of the change in permeability are different. The peak pNa occurs earlier than the peak pK, this determines the amplitude and shape of the action potential.