Cardiovascular Lecture 3 Flashcards
What structure of cardiac muscle causes the striated appearance?
The regular arrangement of actin and myosin that generate force by the sliding filament mechanism. This arrangement is the sacromere.
What are the main structural features of a sacromere?
Z line, M line, I band, A band, Myofibril, Titin
What is the Z line?
Site of actin attachment
What is the M line?
Site of myosin attachment
What is the I band?
Actin filaments
What is the A band?
Myosin filaments
What is myofibril?
Filament bundles
What is titin?
Protein that tethers myosin to the Z line
How is tropomyosin removed, and what does this do?
Tropomyosin is removed by Ca2+ binding to troponin C, allowing myosin binding and cross-bridge formation
Describe the location of sacroplasmic reticulum with respect to other structures
The SR surrounds the myofibrils, and is closely associared with T-tubules.
What is in the sacroplasmic reticulum?
An abundance of mitochondria and internal Ca2+ stores
What are T-tubules?
Sacrolemmal invaginations positioned at the Z bands
What molecular ion is necessary for cardiac contraction?
Extracellular free Ca2+
What happens during the action potential plateau on the molecular level?
Ca2+ enters the cell through voltage gated L-type Ca2+ channels, localized in T-tubules. The amount entering is small, but causes a large release of Ca2+ from the adjacent junctional SR (Ca2+-induced Ca2+ release). This CICR occurs through ryanodine receptors (RyR), a Ca2+ gated Ca2+ channel in the SR membrane.
What happens when intracellular Ca2+ concentration is elevated?
Actin-myosin interaction can occur, causing cardiac contraction.
How does relaxation after contraction occur?
Removal of Ca2+ from the cytosol by:
- Re-accumulation in the SR by the SR Ca2+-ATPase (SERCA)
- Extrusion from the cell by the sacrolemmal Na+-Ca2+ antiporter (3:1 Na+:Ca2+) and ATP-driven Ca2+ pump
Why can’t more muscle cells be recruited to increase contractile force?
All cardiac muscle cells contract during the heart beat.
How is the contractile force increased?
Sympathetic stimulation and stretch
Describe sympathetic stimulation
a. increases size of intracellular Ca2+ transient, resulting in a more forceful contraction (positive intropy)
b. increases rate of relaxation, resulting in shorter contraction (positive lusitropy)
c. Increases frequency of contractions (positive chronotopy)
Describe strech
a. Occurs by the Frank-Starling mechanism, with increased venous return of blood to the heart, allowing the heart to pump whatever volume it recieves
b. Involves increases in myosin-actin interactions
c. Myosin-actin interactions are not increased by greater overlap of thick and thin filaments, but rather increased sensitivity to Ca2+ and reduced interfilament spacing, mediated by titin
What represents the muscle force-length relationship?
Pressure-volume (PV)
Describe PV during relaxation/filling (diastole)
Ventricular PV curve is relatively flat, indicating increased volume is accommodated with small increases in pressure -compliant (inverse of stiff)
Describe PV with greater filling
Ventricle becomes much less distensible, reflecting the abundance of connective tissue
What mechanism does developed pressure follow?
The Frank-Starling mechanism
What is preload?
Force that stretches relaxed muscle (blood filling)
What is afterload?
Force against which muscle must contract (pressure in artery against which ventricle ejects)
What cardiac and vascular factors are affected by preload and afterload?
- venous tone
- peripheral resistance
- rate
- stroke volume
What is contractility?
It defines performance at a given preload and afterload, which will determine the peak isometric force (isovolumic pressure) at a given initial fibre length (end-diastolic volume)
Is cardiac muscle voluntary?
No, it is involuntary.
What makes cardiac muscle different from skeletal muscle?
An influx of extracellular Ca2+ is required for contraction.
Force of contraction is increased by stretch (Frank-starling mechanism) and sympathetic stimulation, unlike skeletal muscle, which recruits muscle fires and undergoes tetanny
