Lecture 8: Passive and Activr Membrane Properties of Muscle/neurons

Question 0

Active properties

Answer 0

Passive electric signal: a membrane potential (transient voltage change) that dissipates as it propagates in space and time.

Active electric signal: a membrane potential that is augmented or transmitted without decrement as it propagates in space and time. Action potential signals do not degrade with distance.

Question 1

Membrane potential

Answer 1

Ions with greater permeability have greater influence on Vm. The larger the influence of an ion X, the closer Vm will be to Ex.

At rest mostly permeable to K+ so Vm close to Ek

Question 2

Action potential is a cyclic process

Answer 2

1. Resting membrane potential
2. Depolarizing stimulus
3. Membrane depolarizes to threshold. Voltage gated Na+ channels open quickly and Na+ enters cell. Voltage gated K+ channels begin to slowly open.
4. Rapid Na+ entry depolarizes cell
5. Na+ channels close and slower K+ channels open.
6. K+ moves from extra cellular fluid.
7. K+ channels remain open and additional K+ leaves cell, hyper polarizing it
8. Voltage gated K+ channels close, less K+ leaks out of the cell
9. Cell returns to resting ion permeability and resting membrane potential.

Question 3

Electrical signals: voltage gated Na+ channels

Answer 3

Na+ channels have two gated: activation and inactivation gates

At resting membrane potential the activation gate closes the channel

Depolarizing stimulus arrives at the channel. Activation gate opens.

With activation gate open, Na+ enters the cell

Inactivation gate closes and Na+ enters stops

During Repolarization caused by K+ leaving the cell, the two gates reset to their original positions

Question 4

Electrical signals: refractory period

Answer 4

Absolute refractory period: Na+ channels close and k+ channels open
(No signal can cause depolarization at this time)

Relative refractory period: Na+ channels reset to original position while k+ channels remain open.
(Strong enough signal can cause depolarization)

Question 5

Hyperkalemic periodic paralysis

Answer 5

A common form of defective inactivation exhibited by mutant NaVChs associated with hyperkalemic periodic paralysis. The defect is caused by incomplete closure of the inactivation gate resulting in an increased level of persistent sodium influx

Question 6

Activation potentials travel forward

Answer 6

After one segment of axon membrane is very depolarized, nearby membrane patches are also depolarized.

The neighboring patch of membrane then goes through the action potential steps.

Sections that just finished the action potential are refractory they are resistant to depolarization because the Na+ channels need time to recover.

Thus the action potential moves forward.

Question 7

Cardiac action potentials

Answer 7

1. Rapid depolarization due to opening of voltage gated fast Na+ channels
2. Plateau (maintained depolarization) due to opening of voltage gated slow Ca2+ channels and closing some K+ channels
3. Repolarization due to opening of voltage gated K+ channels and closing of Ca2+ channels