Action Potential and Excitability PDF

Physiology of nerve  Resting Membrane Potential The chief ions on the outer surface are Na, Cl, HCO3, the chief ions on the inner surface are K, proteins  Potassium ions, concentrated inside 30 times greater than outside the cell &tend to move outward  Sodium ions, concentra...

Physiology of nerve  Resting Membrane Potential The chief ions on the outer surface are Na, Cl, HCO3, the chief ions on the inner surface are K, proteins  Potassium ions, concentrated inside 30 times greater than outside the cell &tend to move outward  Sodium ions, concentrated outside 15 times greater than inside  Under resting conditions, the membrane of the nerve is semipermeable:  1- the permeability to the hydrated Na is low  2- the permeability to the hydrated K is high (50-100 greater than Na)  Therefore, K diffuses from inside to outside→ loss of +ve ions from inside to outside→ ↑+ve charges outside  3- the membrane is impermeable to proteins → keeping –ve inside  4- the membrane is permeable to Cl- & HCO3- → diffuse to inside -90 mV Na-K pump Definition: It is the TRANSIENT change in membrane potential following stimulation of the nerve by an effective stimulus. Stages: 1- depolarization: opening of Na channels, slow till firing level -50mV, then rapid till +35.... Na inflow.... Magnitude=105 2- repolarization: inactivation of Na channels& opening of K channels, rapid till 70% then slows down (after repolarization)......K outflow till reaching the RMP, then hyperpolarized (hyperpolarization), then returns to its original RMP Shape and Phases of Action Potential +35 overshoot 0 depolarization repolarization mv After-repolarization -55 FL hyperpolarization -70 time Na Channel ions K Channel ions Mech. Channel  Na-K Pump ATP Potassium ions ATP Sodium ions Na channel ions Open closed In-active Action Potential Depolarization Na Na Na Na Na Na Na Repolarization K K K K K K K  K K K After hyperpolarization Shape and Phases of Action Potential +35 overshoot 0 depolarization repolarization mv After-repolarization -55 FL hyperpolarization -70 time Excitability changes during the action potential ARP RRP FL Supernormal Increased excitability excitability Subnormal excitability Excitability changes during the action potential 1- initial increase of excitability in the slow phase of depolarization 2- absolute refractory period: ARP excitability is lost, the nerve cannot respond to any stimulus, corresponds to the rapid depolarization & upper third of repolarization 3- relative refractory period:RRP Excitability of the nerve below normal ARP RRP Needs stronger stimulus to respond Recovered gradually FL Coincide with middle third of repolarization Supernormal 4- supernormal phase of excitability: Increased excitability excitability Coincides with after repolarization phase 5- subnormal excitability: coincide with the Subnormal after hyperpolarization excitability  ARP RRP FL Supernormal Increased excitability excitability Subnormal excitability Figure 7-16