Ch. 12

Objectives Describe the factors that maintain a resting membrane potential. List the sequence of events that generates an action potential.

Path of a Nerve Impulse

Membrane Potentials Resting membrane potential – potential for current to pass Created by a difference in charge between the inside and outside of the cell Most cells are -70mV The greater the difference, the greater the potential Graded potential – a change in resting potential Action potential – the passing of electrical current as a result of the graded potential This is how neurons send signals

Resting Membrane Potential

Resting Membrane Potential – based on diffusion Inside – more negative Outside – more positive High in K + High in proteins (-) - ions can’t leave This keeps inside more - also K + tends to diffuse out of the cell This results in an increase – charge here High in Na + High in Cl - K + diffuses out This results in an increase + charge here However, K + will also go back inside because of attraction to - charge

Resting Membrane Potential – based on pump action Na + action Will diffuse inward This could destroy RMP sodium-potassium pump Pumps 3 Na + out and 2 K + in Maintains RMP

Action Potentials Also called impulse Occurs in 2 phases Depolarizing phase Negative membrane potential becomes less negative Reaches zero Then becomes positive Repolarizing phase Membrane potential is restored to -70mV (RMP)

Action Potential Channels Na + channels open Na + rushes into the cell → depolarizing phase K + channels open K + flows out → repolarizing phase depol. and repol. last about 1msec (.001 sec.)

All-or-None When depolarization reaches the threshold (-55mV) the action potential occurs No matter how great the difference is, it will occur Like dominoes It doesn’t matter how hard or soft your push the first domino, once you do all the rest will fall

Depolarizing Phase Inward movement of Na + is favored Na + channels have 2 gates Activation and inactivation In resting state Inactivation gate is open, activation gate is closed Na + cannot move into the cell through these gates At threshold (activated state) Both channels are open Na + rushes into the cell and depolarization occurs The more Na + rushing in, the more channels open

Depolarizing Phase After activation gates open, inactivation gates close Inactivated channel Na + – K + pumps return to Na + to the ECF

Repolarizing Phase At threshold – depolarization opens K channels This allows K + to flow out of the cell the neuron is returned to resting state

Action Potential

Review Describe a neuron at rest. Where are the ions? What are the charges? List the steps in an action potential.

Steps RMP (-70mV) Action potential is triggered by neurotransmitter Depolarization - Na + gates open and Na + enters the cell (+30mV) Repolarization – K + gates open and K + leaves the cell Pumps return everything to RMP (-70mV)