L1 Electrical Excitability and AP

Question 1

Describe the shape/configuration of neuronal APs

Answer 1

Short (2ms), RMP about -70mV, designed to trigger NT release and recover quickly

Question 2

Describe the shape/configuration of skeletal APs

Answer 2

Short (5ms), RMP about -90mV

Question 3

Describe the shape/configuration of cardiac APs

Answer 3

Long (200ms), slower for best heart fxn (pump blood at 1 Hz freq)

Question 4

Membrane resistance: neuronal conduction

Answer 4

Increased membrane resistance (increased myelin)= increased neuronal conduction and lambda

Question 5

Internal resistance: neuronal conduction

Answer 5

Decreased internal resistance (bigger diameter) = increased neuronal conduction and lambda

Question 6

Define the space/length constant

Answer 6

Lambda= distance over which a subthreshold depolarization (local response) will travel and influece the next membrane segment (point where it has lost 2/3 of it’s strength, not effective beyond 1/3 strength)

Question 7

What is lambda’s role in neuronal conduction?

Answer 7

Longer lambda = more rapid conduction

Question 8

Which ionic current mechanisms are responsible for neuronal APs?

Answer 8

Na and K currents

Question 9

Describe the gating properties of Na channels

Answer 9

Na channels have 2 gates to open: m gate is activation gate, h gate is inactivation gate

• At rest (-90mV), channel is closed b/c m gate is closed
• Em >0, channel activated and both gates open
• Right after activation= inactivation, h gate closes

Question 10

When is the m gate open and closed?

Answer 10

m gate is closed at rest, open at activation (depolarization), open at the beginning of inactivation, then closes once cell is polarized again
- open as long as cell is depolarized

Question 11

When is the h gate open and closed?

Answer 11

h gate is open at rest and activation, causes inactivation by closing shortly after m gate opens b/c time dependent to protect cell from hyperexcitability
- determines duration of AP/Na influx

Question 12

Relationship between RMP and Na channel availability

Answer 12

Inactivated channel isn’t ready to trigger AP= UNAVAILABLE
The more depolarized you become, the less channels available to trigger AP
- Channels open at positive potentials

Question 13

Define absolute refractory period

Answer 13

Time during which a stimulus can’t elicit any response (no AP can be triggered)
-At more positive voltages, Na channels inactivate, become unavailable

Question 14

Define relative refractory period

Answer 14

Time during which a stimulus can elicit a response, but the signal has to be sufficiently strong enough

• As Em repolarizes, Na channels recover and can be activated
• During hyperpolarization, have to overcome the more negative potential

Question 15

How does myelination affect neuronal conduction

Answer 15

Increased myelination= increased conduction velocity and increased lambda

Question 16

Hypercalcemia

Answer 16

Elevated plasma Ca, Raises threshold for Na channel activation and DECREASES membrane excitability

Question 17

Hypoventilation

Answer 17

Increase in CO2= acidosis = increase in free plasma Ca= DECREASE in membrane excitability

Question 18

Hypocalcemia

Answer 18

Low plasma Ca, Decreases threshold for Na channel activation and INCREASES membrane excitability

Question 19

Hyperventilation

Answer 19

Decrease in CO2= alkalosis = decrease in free plasma Ca= INCREASE in membrane excitability

Question 20

How does Ca affect neuronal cell excitability?

Answer 20

Ca alters membrane surface charge by binding to the negative charges around the Na channel, DOESN’T change the RMP

Question 21

How do EPSPs originate?

Answer 21

Binding of excitatory NT (ACh, glutamate)= influx of positive ions (Na in, K out)= depolarization= MP becomes more positive and AP probability increases

Question 22

How do IPSPs originate?

Answer 22

Binding of inhibitory NT (GABA, glycine)= Influx of negative ions (Cl in)= Em towards -65mV and clamped here= AP probability decreases

Question 23

K channel properties

Answer 23

Voltage dependent, only have activation gate
As long as cell is depolarized, K channels will be open
K outflow from cell is responsible for repolarization

Question 24

Hyperkalemia

Answer 24

Elevated plasma K= more positive RMP= Na channels less available/inactive= Na current decrease and conduction slows= Slow mentation, muscle weakness, coordination issues

Question 25

Depolarization

Answer 25

Membrane potential becoming more positive, Na channels becoming less available

Question 26

Hyperpolarization

Answer 26

Membrane potential becoming more negative, Na channels becoming more available

Question 27

Regenerative depolarization

Answer 27

Na moves rapidly into cell down gradiants to depolarize Em= increase in Na permeability/ more channels open= more depolarization (positive feedback)