Lecture 1-3 (Cardiac Muscle)

describe cardiac muscle characteristics

• Single nucleus bust post-mitotic

• Striated muscle

• Intercalated discs have desmosomes and gap junctions

• Generates synchronized and forceful contractions via gap junctions

ventricular muscle fiber action potential (steps and channels involved)

1. at resting potential, Kir channels open and contributes to very negative resting potential

2. V-gated Na+ channels open → rapid depolarization to threshold

3. At peak, V-gated Na+ channels inactivate

4. Just after peak, L-type Ca2+ channels (V-gated) open. Ca2+ balances K+ efflux = plateau

5. Near end of plateau: L-type Ca2+ channels inactivate. Delayed-rectifying V-gated K+ channels open (slow to open) → rapid repolarization

6. Kir channels maintains hyperpolarized Vm

   

how are DHP receptors in ventricular muscles similar and different to skeletal muscles

Ventricular muscle cells have DHP receptors in plasma membrane T-tubules, BUT they are NOT physically connected to RyR

what are DHP receptors in ventricular muscle membranes also referred as

L-type Ca2+ channels (V-gated channels): they open and allow Ca2+ influx from the ECF. Ca2+ then binds to and opens RyR on the SR membrane 

what is Ca2+-induced Ca2+ release

Opening of L-type Ca2+ channels (DHP receptors) causes Ca2+ influx. Ca2+ binds to RyR on the SR membrane. Most of the Ca2+ for contraction is released from the SR.

draw out DHP and RyR channels during CICR

what is on the sarcolemma

Ca2+-ATPases and Na+-Ca2+ exchangers

what is located on the SR membrane

SERCA and RyR

cardiac excitation-contraction coupling and relaxation steps

1. V-gated Na+ channels open (depolarizing phase of AP) 

2. L-type Ca2+ channels (DHP receptors) open (plateau phase of AP) 

3. Ca2+ influx → Ca2+ binds to RyR → RyR opens → 

4. Ca2+ release from SR → 

5. Ca2+ binds to troponin → 

6. Cross-bridge cycling starts → force production 

7. Ca2+ is removed by 3 active transporters to ECF or SR → relaxation 

8. Simultaneous with Ca2+ removal (step 7), delayed V-gated K+ channels open and L-type Ca2+ inactive then close → membranes repolarizes → V-gated Na+ channels close → another AP can be generated

why is it important for cardiac muscles to have a longer refractory period

cardiac muscle cannot undergo fused tetanic contractions (maintained contraction w/o relaxation)

desmosomes

hold the muscle cells together so they are not pulled apart during contraction