Lecture 9: Ionic Basis of Membrane Potentials Part 1

What is bioelectricity?

electrical signals mediated by cell membranes

Electrical currents are caried by charged ions such as what?

• Na+

• K+

• Ca2+

• Cl-

• HCO3-

What do electrical current across membranes flow via?

• ion channels

• electrogenic ion transporters

• electrogenic ion pumps

True or False: Passive ion diffusion across cell membranes require channels.

true

Without an ion channel, Na+ would have to dissolve in the lipid bilayer. Why is this unfavorable?

• the energy required to transfer Na+ ion from water to interior of phospholipid membrane is about 36 kcal/mole

• 30 kcal/mole is about 60-fold higher than molecular thermal energy at room temperature

• physically speaking, the probability of ion dissolving in the bilayer is about 0

What do channels provide?

a favorable polar environment

What does the variable epsiolon mean?

dielectric constant (polarizability)

What is the polarizability for water and the lipid bilayer?

• water: high polarizability

• lipid bilayer: low polarizability

What is membrane potential (Vm)?

the difference between the electrical potential in cytoplasm and the extracellular space

What is the electrical potential difference generated by? What does this create?

by the separation of charge across the membrane, which creates an electrical gradient

True or False: In a biological context, the membrane potential is typically expressed in millivolts (mV).

true

What is the resting membrane potential?

steady-state value of Vm of electrically excitable cells when not active (no action potential)

What is the resting membrane potential in skeletal and cardiac muscle?

-60 to -90 mV

What is the resting membrane potential in smooth muscle?

-55 mV

What is an electric field?

the force that a charged particle senses in vicinity of a charge source

What is the relationship between Vm and E (electric field)?

E = Vm / a where a is the distance across the membrane (thickness)

True or False: Despite small transmembrane voltage, cell membranes sustain large and strong electric field.

true

True or False: Electric fields in the brain produce signals strong enough to be recorded on the scalp (EEG).

true

Briefly describe the microelectrode method.

• intracellular recording

• “sharp electrodes”

• measures electrical difference

• voltage recorded at the cell body

Briefly describe the fluorescent dye method.

• contains: light source, dichroic mirror, and photodetector

• fluorescent dye in the cell membrane

• voltage recording at the processes

True or False: The microelectrode and fluorescent dye method have good correspondence.

true

What do GEVIs stand for?

genetically encoded voltage indicators

True or False: GEVIs and electrode recordings have good correspondence.

true

Review the following diagram.

True or False: Cell membrane potentials depend on ionic concentration gradients.

true

The Vrest of large cells is maintained for at least some time after ATP-dependent metabolism is blocked. What does this show?

the contribution of the Na/K pump to maintaining the Vrest is small

Large cells have large surface-to-volume ratios. How does this affect the ion gradient?

the ion gradients run down slowly

What happens to the membrane potential as extracellular potassium increases?

Vm depolarizes

Describe planar lipid bilayers.

• artificial model of cell membrane

• bilayer made of synthetic lipids

• solubilized, purified channel proteins are reconstituted into membrane

What can planar lipid bilayers determine?

determines minimal components for generating a membrane potential and manipulate solutions on both sides of the membrane

What do planar lipid bilayers examine?

examines channel function without the influence of cell metabolism

True or False: Planar lipid bilayers can investigate the direct role of ion gradients in determing Vm.

true

Describe the planar lipid bilayer with K+-selective channels.

• 4mM KCl on external

• 155 mM KCl on internal

• mannitol added on external side to balance osmolarity across membrane

• assume K+ channels are in the open state and membrane is only permeable to K+

• electrodes are in contact with solution on either side