Lecture 21 Flashcards
What does cardiac muscle look like histologically?
cylindric, branched cells that have 1-2 nuclei centrally located
What makes up the transverse and lateral components of the intercalated disks?
transverse = fascia adherens and desmosomes
lateral = gap junctions (do not allow spatial summation
all cells are interconnected)
Duration of the AP in cardiac muscle?
much longer compared to those found in skeletal muscle and neurons
contraction in cardiac muscle?
- During an AP, the DHP channels (within the T-tubule) open and allows for extracellular calcium to enter the cell.
- The extracellular Ca then activates the RyR-2 which releases Ca from the SR
- calcium then binds to troponin C
- tropomyosin moves away from myosin binding site on actin
- Contraction
Relaxation in cardiac muscle?
- Ca will unbind from troponin due to decreased levels of Ca
- tropomyosin again blocks the binding site
- Ca is pumped back into the SR (80%) and some is moved to the ECM (20%) by Ca/Na channel
differences between skeletal and cardiac tissue?
skeletal cells are multinucleated and the nuclei are peripheral, while cardiac muscle has 1-2 centrally located nuclei
skeletal cells are non-branched while cardiac cells are branched
skeletal = short AP, while cardiac = long AP
Ca just comes from the SR in SM, while Ca comes from the SR and EC in cardiac cells
skeletal cells are isolated, while cardiac cells are all interconnected by gap junctions
Inotropism?
the ability of myocardial cells to change force/strength of contraction at cellular level
how?
- Increasing Ca influx from DHP channels
- increasing release of Ca from RYR-2
- sensitizing troponin C to Ca
- anrep effect - increase in afterload
- bowditch effect - increase heart rate
Gs, Gi, Gq protein receptors on cardiac cells
Gs = beta-1 receptor = stimulate muscle contractibility
Gi = M2 receptors = decrease HR and conduction velocity
Gq = increase contraction
Digoxin
Beta agonist: Gs protien linked
cardiac glycoside
directly inhibits the Na/K ATPase, which also inhibits the Na/Ca exchanger due to low Na
used for systolic heart failure (increased contraction)
Striated vs smooth muscle regarding muscle contraction
striated muscle has Troponin C, while smooth muscle does not.
striated muscle has a sarcomere, while smooth muscle does not.
smooth muscle has caldesmon and calponin as thin filaments
striated muscle = bipolar arrangement and a higher amount of myosin II
smooth muscle = side-polar arrangement and a lower amount of myosin II
Striated= 1:2 thick to thin filament ratio smooth = 1:15 thick to thin filament ratio
Multi-unit and single-unit smooth muscle
Multi:
each fiber is innervated by a single nerve ending
each fiber can contract independently
single-unit:
electrically coupled (syncytium)
fibers contract together by gap junctions
Smooth muscle contraction and relaxation
- increased calcium intracellularly
- formation of the Ca/calmodulin complex
- activation of myosin light chain kinase
- phosphorylate myosin light chain
- contraction
- myosin light chain phosphatase can un-phosphorylate the myosin light chain, thus leading to relaxation
actomyosin ATPase activity is much slower in smooth muscle compared to skeletal muscle
Calcium entry into smooth muscle cells?
- voltage-gated calcium channels (electromechanical coupling)
- ligand-gated calcium channels (pharmacomechanical coupling: contraction with AP)
- stretch activated calcium channels
- leak calcium channels (unregulated)
Calcium channel blockers
inhibit voltage-dependent L-type calcium channels
thus decreased muscle contractibility
Calcium entry using secondary messengers regarding smooth muscle
- IP3 pathway- increase of calcium
- cAMP pathway - either the inhibition of adenylyl cyclase or the activation
decreased cAMP = muscle relaxation - NO-cGMP pathway- muscle relaxation
