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Introduction to physiology. Your colleague , Firas alkhalayleh A 3’rd year medical student. Nerve and Muscle Physiology. Plasma Membranes of Excitable tissues Ref: Guyton, 12 th ed: pp: 57-69. 11th ed: p57-71 ,. Resting membrane potential. {NA+} inside = 14 mEq/L
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Introduction to physiology Your colleague , Firas alkhalayleh A 3’rd year medical student
Nerve and Muscle Physiology • Plasma Membranes of Excitable tissues • Ref: Guyton, 12th ed: pp: 57-69. 11th ed: p57-71,
{NA+} inside = 14 mEq/L • {Na+} outside = 142 mEq/L • {K+} inside = 140 mEq/L • {K+} outside = 4 mEq/L
The diffusion potential for K+ only is found to be -94 millivolte • The diffusion potential for Na+ only is found to be +61 millivolte
Nernest equation • E (mV) = - 61.log (Ci/Co) • E = Equilibrium potential for a univalent ion . • Ci = conc. inside the cell. • Co = conc. outside the cell.
The greater the ratio {I}/{o} , the greater tendency to diffuse in one direction . • At normal body tempreture EMF=+/- 61 * log {I}/{O} EMF= electromotive force .
Nernst equation means ……. !!!! • If EMF is + the moving ions are – • If EMF is - the moving ions are +
Multiple ions involvement • Depends on : • Polarity of the electrical charges . • Permeability of the membrane . • The {I} + {O}
Goldman-Hodgkin-Katz equation EMF (mV) = - 61. log [( CiNa+. PNa+ + CiK+. PK+ + CoCl- .PCl- ) (CoNa+ PNa+ + CoK+ PK+ + CiCl- PCl-)] Ci = Conc. inside Co = Conc. outside P = permeability of the membrane to that ion.
During transmission of nerve impulses , permeability of the Na+ and K+ undergoes rapid change , whereas that of Cl- doesn't change greatly .
K+ leak channels are 100 times more permeable to K+ than Na+ .
Action potential • A rapid change in the membrane potential. • Resting stage . • Depolarization stage . • Repolarization stage .
When the membrane potential becomes less negative than during the resting state , rising from -90 toward 0 , it finally reaches a voltage – usually between -70 and -50 – that cause a sudden conformational change in the activation gate , flipping it all the way to the open position.This is called the activated state , during this state , Na+ ions pour inward through the channel increasing the Na+ permeability of the membrane as much as 500-to 5000-fold .
The inactivation state • The same increas in the voltage that open the activation gate also closes the inactivation gate . The inactivation gate , however, closes a few 10,000ths of a second after the activation gate opens . That is, the conformational change that flips the inactivation to the closed state is a slower process than the conformational change that opens the activation gate . Therefore after the Na+ channel has remained open for a few 10,000ths of a second, the inactivation gate closes, and the Na+ no longer can pour to the inside of the membrane . At this point, the membrane potential begins to recover back toward the resting state, which is repolartization state .
During the resting state, the gate of the K+ channel is closed and K+ are prevented from passing to the exterior. When the membrane potential rises from -90 toward 0 , this voltage causes a conformational opening of the gate . However, because of the slight delay in the opening of the K+ channel, for the most part, they open just at same time that the Na+ channels are beginning to close because of inactivation. Thus, the decrease in Na+ entry to the cell and the simultaneous increase in K+ exit from the cell combine to speed the repolarization process, leading to full recovery of the RMP .
