Exam #1: Action Potential

Question 1

What is the depolarization phase of an action potential? What ion is responsible for depolarization?

Answer 1

Portion of the Na+ channels are activated, resulting in the rising phase of the action potential (increase in membrane potential)

Question 2

What is the overshoot of an action potential? What ion is responsible for over shoot?

Answer 2

Activation of Na+ channels leads to depolarization above 0mV

Question 3

What is the repolarization phase of an action potential? What ion is responsible for repolarization?

Answer 3

Potassium channel activation leads to the efflux of K+ & decreased in membrane potential

Question 4

What is the undershoot of an action potential? What ion is responsible for undershoot?

Answer 4

Prolonged opening of K+ channels leads to a dip in membrane potential below resting membrane potential

Question 5

What is the hyperpolarization phase of an action potential? What ion is responsible for hyperpolarization?

Answer 5

Prolonged opening of K+ channels leads to a dip in membrane potential below resting membrane potential

Question 6

Draw an action potential. Label ion movement for each phase of the action potential.

Answer 6

N/A

Question 7

What happens to the conductance of Na+ & K+ in the action potential?

Answer 7

• Pk > PNa at rest
• PNa > Pk during depolarization
• PNa > Pk at over shoot
• PNa waning during repolarization
• Pk > PNa during hyperpolarization/ undershoot
• Pk > PNa at rest

Question 8

What are the three possible states for the rapidly inactivating Na+ channel?

Answer 8

Sodium channels are voltage-gated and have 2x gates, an activation gate & an inactivation gate. The two gates give rise to three states:

1) Resting= activation gate closed & inactivation gate open
2) Activated= both open
3) Inactivated= activation gate open & inactivation gate closed

Question 9

What is the absolute refractory period?

Answer 9

A second response is not possible regardless of strength or duration of the stimulus
- Na+ channel inactivation

Question 10

What is the relative refractory period?

Answer 10

A second response can be elicited but at a greater cost i.e. strength or duration
- K+ channel prolonged opening

Question 11

What factors control the speed of action potential conduction?

Answer 11

Size

Question 12

How does axon diameter relate to speed of action potential conduction?

Answer 12

Larger= faster

Directly proportional to length constant
Inversely proportional to time constant

Question 13

Where are voltage-gated Na+ channels located in myelinated & unmyelinated nerves?

Answer 13

Nodes of Ranvier

Question 14

Which conducts more rapidly, myelinated or unmyelinated nerves? Why?

Answer 14

Myelinated

• Myelination greatly decreases axon membrane capacitance and increases membrane resistance

Question 15

Which conducts more efficiently, myelinated or unmyelinated nerves?

Answer 15

Myelinated

Question 16

What is saltatory conduction?

Answer 16

Jumping of current from node to node

Question 17

Which causes spontaneous muscle twitches, hypo or hypercalcemia?

Answer 17

Hypocalcemia

Question 18

What causes Trousseau’s sign?

Answer 18

Much more sensitive and specific sign of hypocalcemia than Chvostek’s sign
- BP cuff inflation leading to muscle spasm of arm & hand

Question 19

What causes Chvostek’s sign?

Answer 19

A possible sign of latent tetnay associated with hypocalcemia
- Contraction of muscles of the eye, mouth, nose elicited by tapping over facial nerve in front of ear

Question 20

What change can cause muscle weakness, fatigue, mental confusion, and coma?

Answer 20

Hypercalcemia

Question 21

What cell type is responsible for myelination in the CNS?

Answer 21

Oligodendrocytes

Question 22

What cell type is responsible for myelination in the PNS?

Answer 22

Schwann Cells

Question 23

What is Guillian-Barre Syndrome? What part of the nervous system is affected?

Answer 23

Demyelination of the PNS (Schwann cells)

• Autoimmune disease
• Can undergo remyelination

Question 24

What is Multiple Sclerosis? What part of the nervous system is affected by MS?

Answer 24

Demyelination of the CNS (oligodendrocytes & axons)

• Autoimmune disease
• Cannot undergo remyelination

Question 25

What is the time constant?

Answer 25

Amount of time it takes for the voltage to change by a certain percentage

Question 26

What is the time constant a function of?

Answer 26

Product of the axon’s electrical resistance (R) & capacitance (C)

Question 27

What is the length constant?

Answer 27

Distance between current injection site & point where that voltage has decayed 63%

Question 28

What two tissues types are considered excitable?

Answer 28

Nerve & muscle

Question 29

What is TTX?

Answer 29

Tetrodotoxin from the puffer fish, a Na+ channel blocker

Question 30

What is the mechanism of action of Lidocaine? What is Lidocaine used for?

Answer 30

• Na+ channel antagonist

- Local anesthesia

Question 31

What is the definition of AP threshold?

Answer 31

Lowest voltage or minimal depolarization needed to drive Na+ channels into the fast, positive, feedback loop

Question 32

What factors influence AP threshold?

Answer 32

Na+ Channel
K+ Channel
Ca++ ECF

Question 33

What are the CATS of hypocalcemia?

Answer 33

Convulsions
Arrhythmia
Tetany
Stridor

Question 34

How does Ca++ change threshold?

Answer 34

External Ca++ influences AP threshold by changing Na+ open probability (% of channels that will open)

Question 35

Experimentally, if an axon is stimulated somewhere in the middle, which direction will current flow? Does this happen in life?

Answer 35

• Both
• No, the axon hillock has the lowest AP threshold
• K+ channels in subsequent Nodes of Ranvier prevent backward direction of AP propagation