Muscle Excitation and Signaling and Contraction

Question 1

At what voltage do Na+ activation gates open?

Answer 1

-70 to -20mV

Question 2

At what voltage do Na+ inactivation gates open?

Answer 2

-80 to -40mV (at -40, most will be closed)

Question 3

What happens with Na+ channels if the cell’s resting potential is depolarized?

Answer 3

More inactivation Na+ channels will be closed, and less will be available to allow Na+ flow up to threshold

-this occurs in ischemia, infarction, inc K+, TTX

Question 4

What is an M gate

Answer 4

Activation Na channel

Question 5

What is an H gate?

Answer 5

Inactivation channel

Question 6

M/H positions at resting potential

Answer 6

M: Closed
H: Open

Question 7

M/H positions at upstroke

Answer 7

Both open

Question 8

M/H positions at Plateau

Answer 8

M: Open
H: Closed

Question 9

At what voltage does phase 4 depol occur in SA node?

Answer 9

-70 to -50mV

Question 10

What does the ST segment represent?

Answer 10

Ventricular plateau

Question 11

At what voltage does purkinje fiber phase 4 depol occur?

Answer 11

-90 to -70mV

Question 12

What determines the amplitude of electrical vectors?

Answer 12

1. Mass of muscle
2. Conduction velocity
3. Degree of cancellation of electrical forces

NOT the rate of phase 4 depol

Question 13

What is similar about contraction and relaxation of myosin?

Answer 13

Both are energy dependent!

• ->ATP req to disassociate myosin from thin filament
• ->ATP req to pump Ca2+ back into SR

Question 14

How is Ca2+ removed from the myoplasm?

Answer 14

1. Ca2+ pump: active transporter on SR membrane
2. Na+/Ca2+ exchanger pumps in 3 Na+ for 1 Ca2+ out of the cell, operates on Na+ electrochemical gradient
3. 3Na+ out/2K+in exchanged via ATPase

Question 15

What’s the difference between Ca2+ activation in skeletal vs cardiac muscle?

Answer 15

In cardiac: ca induced ca release (via L type Ca channels). Muscle is activated via Ca from SR and from EC space. Ca is also pumped back out into EC space. Muscle can only stay contracted for as long as Ca is there.

In skeletal: Voltage dependent and only pumps out of SR and then back into SR. doesnt leave cell space, calcium doesn’t come from outside space. Muscle can stay contracted for longer than Ca is there

Question 16

What is twitch duration controlled by in cardiac vs skel m

Answer 16

Cardiac: AP duration
Skel: not AP duration

Question 17

What 3 mechanisms dec HR?

Answer 17

1. Reduce rate of phase 4 depol
2. Less neg threshold (takes longer to get there)
3. More neg max diastolic pot (ie lower resting potential)

Question 18

How does vagus regulate HR?

Answer 18

Ach binds K+ channel and allows more K+ out of the cell

Question 19

How does SNS regulate HR

Answer 19

Norepi and epi bind GPCRs leading to increase in Ca2+ current and increased rate of diastolic depol

Question 20

What regulates strength of contraction?

Answer 20

1. Amt of Ca2+ in the cell and response to Ca2+
   - ->faster heart beats, stronger beats are because Ca2+ doesn’t have time to leave the area
2. Concentration of Na+
   - ->cardiac cells contract more strongly with reduced Na+ because it dec efficacy of Na/Ca2+ exchanger
3. Concentration of K+
   - ->If you increase concentration of K+, you shorten the AP because you increase the resting potential
4. Catecholamine regulation
   - ->Inc cAMP leads to phosphorylation of phospholamban which increases the rate of Ca2+ uptake into the SR
   - ->Increase strength of contraction by inc magnitude of activator Ca2 current into cell
   - ->Decrease duration of contraction by inc SERCA pumping and phosphorylating troponin I and dec its affinity for Ca2+ (modulates other effects of catecholamines)

Question 21

Explain Frank-Starling law

Answer 21

Greater T = greater F of contraction

-stretch=closer thick/thin filaments making easier for myosin to generate greater F

Question 22

Role of phospholamban

Answer 22

Normally inhibits SERCA. Inactivated when phosphorylated. Normally inactivated via catecholamine regulation

Question 23

What is the rate staircase?

Answer 23

The rate of relaxation needs to be increased with higher HR so that the chamber can fill completely in diastole (faster the stim interval, the longer duration of rest is req)

–>rate staircase is when HR is inc, strength of contraction inc due to inc in Ca2+ reserve, not dependent on catecholamine effect