Electrical Signals in Neurons

Ch. 12

ObjectivesDescribe the factors that maintain a resting

membrane potential.List the sequence of events that generates an

action potential.

Path of a Nerve Impulse

Path of a Nerve Impulse

Membrane PotentialsResting membrane potential – potential for

current to passCreated by a difference in charge between the

inside and outside of the cellMost cells are -70mVThe greater the difference, the greater the

potential Graded potential – a change in resting potentialAction potential – the passing of electrical

current as a result of the graded potentialThis is how neurons send signals

Resting Membrane Potential

Resting Membrane Potential – based on diffusionInside – more negative Outside – more

positiveHigh in K+

High in proteins (-)

- ions can’t leaveThis keeps inside more -

alsoK+ tends to diffuse out of

the cellThis results in an

increase – charge here

High in Na+

High in Cl-

K+ diffuses outThis results in an

increase + charge here

However, K+ will also go back inside because of attraction to - charge

Resting Membrane Potential – based on pump actionNa+ action

Will diffuse inwardThis could destroy RMP

sodium-potassium pumpPumps 3 Na+ out and 2 K+ inMaintains RMP

Action PotentialsAlso called impulseOccurs 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 PotentialChannels

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-NoneWhen depolarization reaches the threshold (-

55mV) the action potential occursNo matter how great the difference is, it will

occurLike dominoes

It doesn’t matter how hard or soft your push the first domino, once you do all the rest will fall

Depolarizing PhaseInward movement of Na+ is favoredNa+ channels have 2 gates

Activation and inactivationIn 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 PhaseAfter activation gates open, inactivation gates

closeInactivated channelNa+ – K+ pumps return to Na+ to the ECF

Repolarizing PhaseAt threshold – depolarization opens K

channelsThis allows K+ to flow out of the cell

the neuron is returned to resting state

Action Potential

ReviewDescribe a neuron at rest. Where are the

ions? What are the charges?List the steps in an action potential.

StepsRMP (-70mV)Action potential is triggered by

neurotransmitterDepolarization - Na+ gates open and Na+

enters the cell (+30mV)Repolarization – K+ gates open and K+ leaves

the cell Pumps return everything to RMP (-70mV)