Membrane potential ch 5 neurons

Depolarization/Resting Membrane Potential Ch 5

Electricity Review1. Law of conservation of electrical charges2. Opposite charges attract; like charges repel each

other3. Separating positive charges from negative

charges requires energy4. Conductor versus insulator

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Figure 5.23a Separation of electrical charge: the membrane potential difference (1 of 4)

Cell and solution areelectrically and chemicallyat equilibrium.

The cell membrane acts asan insulator to prevent freemovement of ions betweenthe intracellular and extra-cellular compartments.

Figure 5.23b Separation of electrical charge: the membrane potential difference (2 of 4)

Energy is used to pump one cation out of the cell,leaving a net charge of 1 in the cell and 1 outsidethe cell. Cell and solution are now in chemical andelectrical disequilibrium.

Intracellular fluid Extracellular fluid

Figure 5.23c Separation of electrical charge: the membrane potential difference (3 of 4)

Intracellular fluid Extracellular fluid

2 1

1

2

22

1

1

0

0

Intracellular fluid Extracellular fluid

Absolute chargescale

Relative charge scale – extracellular fluid set to 0 (ground).

On an absolute charge scale, theintracellular fluid (ICF) would be at 1and the extracellular fluid (ECF) at 1.

Physiological measurements, however, arealways on a relative scale, on which theextracellular fluid is assigned a value of 0.This shifts the scale to the left and givesthe inside of the cell a relative chargeof 2.

Figure 5.23d Separation of electrical charge: the membrane potential difference (4 of 4)

The voltmeter measures the differencein electrical charge between the insideof a cell and the surrounding solution.This value is the membrane potentialdifference, or Vm.

The membrane potentialcan change over time.

Output

Input

The ground ( ) orreference electrodeis placed in the bath and givena value of0 millivolts (mV).

Saline bath

Cell

In the laboratory, a cell’s membranepotential is measured by placing oneelectrode inside the cell and a secondin the extracellular bath.

A recordingelectrodeis placed

inside the cell.

Figure 5.24a Equilibrium potentials (1 of 4)

Artificial cellAn artificial cell whose membraneis impermeable to ions is filled withK+ and large protein anions. It isplaced in a solution of Na+ and Cl-.Both cell and solution areelectrically neutral.

K+

K+

K+

K+

Na+ Na+

Na+ Na+

Cl- Cl-

Cl-Cl-

Pr-

Pr-

Pr-Pr-

Figure 5.24b Equilibrium potentials (2 of 4)

K+ leak channel

A K+ leak channel is inserted into the membrane.

Na+ Na+

Na+Na+

Cl-

Cl-

Cl-Cl-

K+

K+K+

K+

Pr-

Pr-

Pr-

Pr-

K+ leaks out of the cellbecause there is a K+ concentration gradient.

In the white boxes write the netelectrical charge of theintracellular and extracellularcompartments as shown.

FIGURE QUESTION

Figure 5.24c Equilibrium potentials (3 of 4)

Why don’t Na+, Cl-, and theproteins (Pr-) cross themembrane?

FIGURE QUESTION

K+

K+

K+

K+

Pr-

Pr-

Pr-

Pr-

Na+

Na+

Na+ Na+

Cl-

Cl-

Cl- Cl-

1

1

The negative membrane potentialattracts K+ back into the cell. Whenthe electrical gradient exactlyopposes the K+ concentrationgradient, the resting membranepotential is the equilibriumpotential for K+.

Concentrationgradient

Electricalgradient

A K+ leak channel is inserted into the membrane.

Figure 5.24d Equilibrium potentials (4 of 4)

Na+

Na+

Na+

Na+

Na+

Cl-

Cl-

Cl-Cl-

K+

K+ K+

Pr- Pr-

Pr-

150 mM0 mV

15 mM60 mV

The Na+ concentration gradient inthis artificial cell is exactly opposedby a membrane potential of 60 mV.

Now a Na+ leak channel replaces the K+ channel.

Na+ equilibrium potential (ENa) 60 mV.

Figure 5.25 Resting membrane potential in an actual cellResting cells are permeableto both K+ and Na +.

Intracellular fluid70 mV

Extracellular fluid0 mV

K+ K+

Na+

Na+

ATP

FIGURE QUESTIONS• What force(s) promote(s) Na+ leak into the cell?• What force(s) promote(s) K+ leak out of the cell?

Figure 5.26 Membrane potential terminology

Membrane potential difference (Vm)

Time (msec)

Vm decreases Vm

increases

Depolarization Repolarization Hyperpolarization

Mem

bran

e po

tenti

al (m

V)

120

20

100

80

60

40

20

0

40

Figure 5.27a Insulin secretion and membrane transport processes (1 of 2)

Beta cell at rest. The KATP channel is open, andthe cell is at its resting membrane potential.

Low glucoselevels in blood.

Metabolismslows.

ATPdecreases.

KATP channels open.

Cell at restingmembrane potential.No insulin is released.

K+ leaksout

of cell

K+

Voltage-gatedCa2+ channelclosed

ATPMetabolismGlucose

GLUTtransporter

No insulinsecretion

Insulin insecretory vesicles

Figure 5.27b Insulin secretion and membrane transport processes (2 of 2)

Beta cell secretes insulin. Closure of KATP channeldepolarizes cell, triggering exocytosis of insulin.

High glucoselevels in blood.

Metabolismincreases.

ATPincreases.

KATP channelsclose.

Cell depolarizes andcalcium channelsopen.

Ca2+ entryacts as anintracellularsignal.

Ca2+

Ca2+

Ca2+ signaltriggersexocytosisand insulinis secreted.

ATP Glycolysisand citricacid cycle

Glucose

GLUTtransporter

Slide 7

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