Excitable Cells Part 2 (6/19/15)

Question 1

Describe the 6 steps of establishing a resting membrane potential….

Answer 1

1. There is a concentration gradient across the plasma membrane for K+.
2. Permeability for K+ is greater so K can leave the cell (more K inside the cell to begin with)
3. K+ ions move down concretion gradient leading to the development is a diffusion potential.
4. Equilibrium is reached when the force in the concentration gradient for K+ ions is balanced out by the opposing force in electrical gradient which favors keeping the K+ ions IN the cell.
5. At this point, membrane potential is very close to the equilibrium potential for K+. Resting membrane potential does not = Equilibrium potential for K+ because Na+ still affects it some. (Na is diffusing into the cell down both its concentration and electrical gradient, keeping the resting membrane potential slightly lower than EK.)
6. Therefore, K+ ions diffuse out of the cell and Na+ ions diffuse into the cell at rest!

Question 2

True for False, Resting membrane potential is not steady.

Answer 2

False, it is! It is not a transient diffusion potential.

Question 3

What does the Na/K pump do?

Answer 3

Uses ATP energy to pump K+ ions back into the cell and Na+ out of the cell, when the cells are at rest.

Question 4

What happens if the Na/K pump gets poisoned?

Answer 4

A slow decrease in the resting membrane potential (moves toward 0 mV)

Question 5

_______ moves toward 0 mV.

Answer 5

Depolarization (Rising phase)

Question 6

_______ moves toward resting potential.

Answer 6

Repolarization (Falling phase)

Question 7

_______ increase in membrane potential.

Answer 7

Hyperpolarization

Question 8

______ is a membrane portion found within almost all cells.

Answer 8

Na/K pump

Question 9

Na/K pump is an example of _____.

Answer 9

Homeostasis

Question 10

What is an Action Potential?

Answer 10

A large transient change in membrane potential (Neurons and muscle cells)

Question 11

Where is the Action potential initiated?

Answer 11

in the Initial segment of the neuron (never in cell body or dendrite)

Question 12

What are the steps to CREATING an action potential?

Answer 12

Stimulus –> depolarizes membrane (dendites of neuron) –> Depolarizes cell body –> AP first develops at initial segment –> action potential propagates down axon.

Question 13

What are the steps DURING the development and Time course of an Action potential?

Answer 13

1. Increase in membrane permeability to Na+ ions.
2. Na+ ions enter through Na channels driven by the concentration and electrical gradient, carrying + charges with them. *These Na channels are voltage regulated!
3. The membrane potential move closer to 0 mV (rising phase/depolarization)
4. If depolarization is sufficient, threshold point will be reached and many more Na channels will open and more Na+ enters the cell!
5. Now the membrane potential move VERY RAPIDLY toward 0 mV and overshoots, reaching a peak @ around +40 mV.
6. K channels open (increasing K+ permeability) because they too are voltage aged and K+ ions cross over to outside of cell.
7. Membrane potential abruptly reverses direction and rapidly returns to resting value as K+ ions diffuse out of the cell. *When 0 mV is hit during the falling phase the electrical gradient will be lost for K+ which will inhibit K+ moving out of the cell. In other words, K+ will move out of cell via electrical gradient as long as potential is +).
8. Overshoots Hyperpolarization

Question 14

T or F, a relatively small number of ions can result in a large change in membrane potential.

Answer 14

True!

Question 15

What is the latent period?

Answer 15

A very brief period between applying a stimulus and beginning of depolarization during the rising phase of an AP.

Question 16

The latent period precedes the ________.

Answer 16

Foot of the AP which is the portion of the rising phase before the threshold is reached.

Question 17

Why does the peak of the AP stop at approx. 40 mV?

Answer 17

B/C peak of AP is approaching Eq of NA (58 mV) which it cannot reach unless K+ channels do not open.

Question 18

What is the relationship between the concentration gradient and electrical gradient of Na+ at peak of AP?

Answer 18

The electrical and Concentration gradients are nearly equal at Peak AP.

Question 19

The End of the repolarization phase (The resting membrane potential) is close to _______.

Answer 19

equilibrium K (eK)
*B/C permeability of K is very high during the falling phase of AP until the concentration and electrical gradient are nearly balanced at  end of AP.

Question 20

________ is insufficient to take membrane to threshold.

Answer 20

Sub-threshold stimuli

Question 21

________ is just sufficient to take membrane to threshold.

Answer 21

Threshold stimulus

Question 22

________ is Greater than a threshold stimulus.

Answer 22

Supra-threshold stimulus

Question 23

Events resulting from a sub threshold stimuli is confined to where?

Answer 23

The immediate region of the membrane

Question 24

________ are small depolarizations or hyper-polarizations that do not move along the membrane.

Answer 24

Local responses

Question 25

_____ responses are proportional to stimulus strength.

Answer 25

Graded responses

Question 26

What happens if threshold is reached?

Answer 26

An AP WILL occur, it is automatic at that point.

Question 27

What is an Absolute refractory period?

Answer 27

Brief period when a second threshold stimulus cannot elicit a second AP.

Question 28

What is a Relative Refractory period?

Answer 28

For a longer period after and AP a supra-threshold can elicit another AP, but a threshold stimulus cannot.
*Basically it imposes an upper limit on the frequency at which cells can fire AP’s. This is important in the heart.

Question 29

_______ binds to Na channels and blocks sodium influx.

Answer 29

Tetrodotoxin (TTX) from pufferfish

Question 30

What is the stimulus strength-duration curve?

Answer 30

Curve that relates strength of an instantaneous stimulus to the duration of that same stimulus that is required to elicit a response.

Question 31

A Drug that enhances stimulus sensitivity would shift the stiumuls strength-duration curve to the _______.

Answer 31

Left!

Question 32

A Drug that enhances stimulus sensitivity would shift the stimulus strength-duration curve to the ______.

Answer 32

Right!

Question 33

What is Rheobase?

Answer 33

The magnitude of least intense stimulus that can elicit a response.

Question 34

What is Utilization time?

Answer 34

duration required to elicit response by a stimulus with a Rheobase magnitude.

Question 35

What is Chronaxie?

Answer 35

duration required to elicit a response by a stimulus TWICE the strength the magnitude of the Rheobase stimulus *used to compare the excitability of different cells.

Question 36

_____ are passive currents (do not propagate) that occur at immediate site of stimulation.

Answer 36

Local currents

Question 37

T or F, AP’s only go in one direction.

Answer 37

True, due to Na+ channel inactivation.

Question 38

_____ wrap extensive layers of plasma membrane around axon except at periodic nodes (nodes of ranvier)

Answer 38

Schwann cells

Question 39

Action potential in a nerve bundle is known as _______.

Answer 39

Compound action potential

Question 40

Axons with larger diameters have ______ velocities.

Answer 40

Higher velocities