Resting Membrane Potential

Fig. 12.09a,b

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.

Recording of Resting and action potentials It is recorded by cathode ray oscilloscope it is negative in polarized (resting, the membrane can be excited) state with the potential difference inside the cell membrane is negative relative to the outside. – -70 mV + + 0 mV Voltmeter + – Dentistry 07

Terminology Associated with Changes in Membrane Potential F8-7, F8-8 Depolarization- a decrease in the potential difference between the inside and outside of the cell. Hyperpolarization- an increase in the potential difference between the inside and outside of the cell. Repolarization- returning to the RMP from either direction. Overshoot- when the inside of the cell becomes +ve due to the reversal of the membrane potential polarity.

Types of Ion Channels

Membrane Potential: Goldman Equation P = permeability at rest: PK: PNa: PCl = 1.0 : 0.04 : 0.45 Net potential movement for all ions known Vm:Can predict direction of movement of any ion ~

The action potential (AP) An action potential is: A regenerating depolarization of membrane potential that propagates along an excitable membrane. [propagates = conducted without decrement (an ‘active’ membrane event)] [excitable = capable of generating action potentials] 1 ms +70 (mV) -80 ENa EK Action potentials: are all-or-none events need to reach threshold have constant amplitude do not summate are initiated by depolarization involve changes in permeability rely on voltage-gated ion channels downstroke

Recording membrane potential + 60 - + 30 - 0 - - 30 - - 60 - - 90 - mV Electrotonic potential Localized non propagated Action potential

Threshold and Action Potentials Threshold – membrane is depolarized by 15 to 20 mV Established by the total amount of current flowing through the membrane Weak (subthreshold) stimuli are not relayed into action potentials Strong (threshold) stimuli are relayed into action potentials All-or-none phenomenon – action potentials either happen completely, or not at all

The Action Potential Equilibrium potential of sodium (+60 mV) - 75 mV Resting potential (-75 mV) Equilibrium potential of potassium (-95 mV) Equilibrium potential of sodium (+60 mV) K Na - 75 mV Passive increase in positive charge Electrotonic potential

The Action Potential threshold Resting potential (-75 mV) Equilibrium potential of potassium (-95 mV) Equilibrium potential of sodium (+60 mV) - 55 mV Na Na K K K Opening of voltage-gated sodium channel threshold Electrotonic potential

The Action Potential Equilibrium potential of sodium (+60 mV) - 40 mV Resting potential (-75 mV) Equilibrium potential of potassium (-95 mV) Equilibrium potential of sodium (+60 mV) Depolarisation due to sodium influx - 40 mV Na Na K K K Opening of voltage-gated sodium channel Electrotonic potential

voltage-gated sodium channels turn to the inactivation phase The Action Potential voltage-gated sodium channels turn to the inactivation phase Resting potential (-75 mV) Equilibrium potential of potassium (-95 mV) Equilibrium potential of sodium (+60 mV) Depolarisation due to sodium influx + 50 mV Na Na K K K Inactivation of voltage-gated sodium channel Electrotonic potential

The Action Potential Equilibrium potential of sodium (+60 mV) + 50 mV Resting potential (-75 mV) Equilibrium potential of potassium (-95 mV) Equilibrium potential of sodium (+60 mV) Depolarisation due to sodium influx opening of voltage-gated potassium channel K + 50 mV Na Electrotonic potential

The Action Potential Equilibrium potential of sodium (+60 mV) - 85 mV Resting potential (-75 mV) Equilibrium potential of potassium (-95 mV) Equilibrium potential of sodium (+60 mV) Depolarisation due to sodium influx Repolarization due to potassium influx opening of voltage-gated potassium channel K - 85 mV Na Electrotonic potential

Resting potential (-75 mV) Equilibrium potential of sodium (+60 mV) The Action Potential Membrane potential approaches the ENa and voltage-gated sodium channels turn to the inactivation phase Resting potential (-75 mV) Equilibrium potential of sodium (+60 mV) Depolarisation due to sodium influx K - 75 mV Na repolarization due to potassium influx closing of voltage-gated potassium channel Electrotonic potential Repolarisation due to potassium influx Hyperpolarising afterpotential

The Action Potential threshold Inactivation of voltage-controlled sodium channel Equilibrium potential of sodium (+60 mV) Opening of voltage-controlled sodium channel Opening of voltage-controlled potassium channel threshold Electrotonic potential Resting potential (-75 mV) Hyperpolarization due to more outflux of potassium ions Dentistry 07

The Action Potential (excitability changes) Absolute refractory period Relative refractory period ENa (+60 mV) Depolarisation (due to sodium influx) Resting potential (-75 mV) Polarized state (resting) Hyperpolarising afterpotential EK (-95 mV)

The Action Potential threshold

neuron from the inferior olive