Action potential

Question 1

What is the “time constant” with respect to the cell membrane?

Answer 1

1. The amount of time it takes for the voltage to change by a certain percentage (63%) of the eventual new steady state value 2. Function of the axon’s electrical resistance and capacitance (tao = RC)

Question 2

What is the “space constant” with respect to the cell membrane?

Answer 2

1. Distance between the injection site of current and the point where the steady state transmembrane voltage change has decayed by 63% from its peak value2. Describes how signal decays over distance

Question 3

What are the properties of the action potential?

Answer 3

1. Spread in nondecremental fashion - stay same size as spread2. Threshold for generation of the action potential - if a membrane depolarization reaches threshold, it induces an action potential through a self-regenerative process3. All or none - size of action potential does NOT depend on the size of the triggering stimulus

Question 4

What creates depolarization phase?

Answer 4

1. Opening of sodium channels (activation gate)2. Sodium rushes in making cell membrane less negative

Question 5

What creates the repolarization phase?

Answer 5

1. Sodium channels close (inactivation gate)2. Potassium channels open so K rushes out of cell

Question 6

What creates the hyperpolarization phase?

Answer 6

If K channels remain open long enough, cell gets close to K Nernst potential

Question 7

When can the voltage gated Na channel switch from inactivated to resting?

Answer 7

Only when the membrane is repolarized toward its negative resting potential

Question 8

What are the properties of the voltage gated K channel?

Answer 8

1. Activated after a brief delay by strong depolarizations and inactivates very slowly2. Responsible for repolarization phase of action potential3. Probability that K channels will open is less than that for Na for same amount of membrane depolarization

Question 9

What channel is responsible for repolarization phase of action potential?

Answer 9

K channel

Question 10

What are two concepts of threshold?

Answer 10

1. Sufficient depolarization to trigger an action potential2. Value of membrane potential at which inward flow of Na exceeds the passive outward flow of K

Question 11

What factors influence the AP threshold?

Answer 11

1. Na channel2. K channel3. Calcium concentration

Question 12

How does hypocalcemia affect the Na channel?

Answer 12

When the extracellular concentration of calcium is lowered the probability that a sodium channel will be opened at any voltage is increased

Question 13

How is the threshold affected by lowered extracellular calcium concentrations?

Answer 13

1. A smaller than normal amount of depolarization will bring an excitable membrane to its firing threshold2. Hypocalcemia causes neuromuscular excitability

Question 14

What are the symptoms of hypocalcemia?

Answer 14

1. Neuropsychiatric2. Neuromuscular 3. Cardiovascular4. Autonomic

Question 15

What is Chvostek’s sign?

Answer 15

1. A possible sign of latent tetany associated with hypocalcemia2. Contraction of the muscles of the eye, mouth, or nose elicited by tapping over the facial nerve in front of the ear3. Neither sensitive nor specific - absent in 1/3 of patients and present in 10% of normal people

Question 16

What is Trousseau’s sign?

Answer 16

1. Muscle spasms in forearm and hand as a result of BP cuff inflation greater than systolic 2. More sensitive and specific than Chvostek’s sign

Question 17

What happens to the AP threshold as external calcium concentration increases (hypercalcemia)?

Answer 17

1. Moves further away from resting potential 2. A larger amount of depolarization is needed to bring membrane to firing level

Question 18

How does external calcium influence the AP threshold?

Answer 18

By changing sodium channel opening probability (if low, membrane is more likely to depolarize)

Question 19

What does a reduction in external sodium concentration do to the rise and shape of an AP?

Answer 19

1. Reduces rate of rise2. Reduces peak amplitude

Question 20

What are the properties of the absolute refractory period?

Answer 20

1. Roughly the same time as AP spike2. Dependent on phenomenon of Na channel inactivation - a stimulus cannot open enough Na channels to cause another AP3. Known as effective refractory period in cardiac tissue

Question 21

What are the properties of the relative refractory period?

Answer 21

1. Period of time after the absolute refractory period during which you need stronger than normal stimulus to elicit a new AP2. Main reason is delayed K channel opening and closing3. There is a maintained K conductance due to slow closing of K channels 4. K ions leaving the cell oppose the depolarizing effect of opening Na channels

Question 22

What are the steps of AP propagation in an unmyelinated nerve?

Answer 22

1. One patch of membrane is depolarized2. Current flows between depolarized patch and adjoining resting membrane3. Na channels open as previously resting patch is depolarized 4. Ap is constantly regenerated in new patches as it progagates

Question 23

What sets the upper limit of firing frequency?

Answer 23

Refractory period

Question 24

What is the main factor controlling conduction velocity?

Answer 24

Axon diameter

Question 25

Why does increasing axon diameter increase conduction velocity?

Answer 25

1. The larger the axoplasmic resistance, the slower the conduction velocity2. The larger the capacitance, the more charge has to be moved to change the voltage across the membrane3. With increasing diameter, electrical resistance of the axoplasm decreases in proportion to square of the radius 4. When the diameter increases, the surface area increases and thus capacitance increases in direct proportion to radius of the axon 5. Since electrical resistance decreases faster than the capacitance increases, increasing diameter increases speed

Question 26

What factors contribute to AP conduction velocity?

Answer 26

1. Axon diameter2. Axon length3. Time constant

Question 27

How does myelination cause insulation?

Answer 27

Decreases axon membrane capacitance and increases membrane resistance

Question 28

Where are APs generated in myelinated nerves?

Answer 28

Only at Nodes of Ranvier

Question 29

Why is AP conduction faster in myelinated nerves?

Answer 29

1. No waste of time in space between Nodes of Ranvier2. Capacitance of myelinated nerves is much lower than that of unmyelinated

Question 30

How is a myelinated nerve more efficient?

Answer 30

1. Less membrane generates APs2. Less Na flows into cell - less work for Na/K pump

Question 31

What is the cause of Guillain-Barre syndrome?

Answer 31

1. Damage to peripheral myelin2. Muscle weakness, paralysis3. Can recover since remyelination in peripheral nervous system is possible

Question 32

What cell is responsible for myelination in the CNS?

Answer 32

Oligodendrocyte

Question 33

What cell is responsible for myelination in the PNS?

Answer 33

Schwann cell