Muscle VIII

Question 1

What is cardiac length tension relationship explained by

Answer 1

Degree of overlap between thick and thin filaments (increased chamber filling when slightly stretched)

Question 2

What does a slightly stretched sarcomere influence

Answer 2

1. Increases Ca sensitivity of myofilaments
   - stretched sarcomere has decreased diameter which reduces distance Ca needs to diffuse increasing cross bridge cycling (easier to get to troponin)
2. Puts additional tension on stress-activation Ca channels, increasing Ca entry from extracellular space and increasing Ca induced Ca release
3. Go to optimal length

Question 3

How is heart rate under tonic control

Answer 3

Autorhythmic cells modulated by sympathetic and parasympathetic neurons
- always input from both branches

Question 4

What is APs/min in autorythmic cells when there’s not autonomic stimulation

Answer 4

~90
- at resting parasympathetic is dominant 70-72bpm

Question 5

How does the parasympathetic neurons modulate pacemaker activity

Answer 5

Contain ACh and innervated SA node influencing autorhythmic myocardial cells, decreasing APs (decreasing HR)
- ACh acts on muscarinic (M2) cholinergic receptors opening K+ channels (hyperpolarize) and closing T-type Ca channels and HCN channels

Question 6

How does closing of HCN channels decrease APs

Answer 6

Pacemaker potential decreased because longer to depolarize so longer to threshold
Due to less Na coming in HCN

Question 7

How does opening of K+ channels decrease APs

Answer 7

Increases K+ leakage
-starting at more negative membrane potential so longer to reach threshold

Question 8

How does the closing of T-type Ca channels decrease APs

Answer 8

Takes longer to reach positive threshold

Question 9

How does the sympathetic NS modulate pacemaker activity

Answer 9

Beta 1 adrenergic receptors activated by NE from postganglionic sympathetic neurons or E from adrenal medulla
- activates adenylate cyclase pathway
- increased NA conductance through HCN (by cAMP) and Ca through T-type channels (protein kinase)

Question 10

How does the increase Na conductance through HCN channels and Ca through T-type channels in sympathetic modulation affect pacemaker activity

Answer 10

• cells reach threshold more rapidly
• decrease level of repolarization (decreased distance to travel)