Cardiac muscle

What’re the general features of cardiac muscle cells?

• Contractile

• Uni-nucleated

• Cells align end to end, can also align parallel

What’re cardiac muscle cells interconnected by and whats within them?

• They’re interconnected by intercalated discs and from functional syncytia

• Within intercalated discs theres 2 kinds of membrane junctions-

1. Desmosomes = There for structure to tightly join cardiac muscles so they don’t pull apart when contracted

2. Gap junctions = Connected by connexins, works like an electrical synapse

What’re the types of cardiac muscle cells?

1. Myocardial contractile cells (working cells)

2. Cardiac conducting cells

Explain myocardial contractile cells

• 99% of cardiac muscle cells

• Contractile, MUSCLE part of the heart, do mechanical work of pumping

• Joined electrically by gap junctions

Explain cardiac conducting cells

• Myocardial Autorhythmic cells = Initiate and maintain electrical activity in the heart, DO NOT CONTRACT

• Conducting cells = Conduct electrical signals throughout the heart (Purkinje fibres), DO NOT CONTRACT

What’re the steps of electrical conduction in heart?

1. Cardiac impulse originates at SA node (AP originates here)

2. AP spreads throughout right and left atria

3. Impulse passes from atria into ventricles through AV node (only point of electrical contact atrial and ventricular chambers)

4. AP briefly delayed at AV node, this ensures atrial contraction precedes ventricular contraction to allow complete ventricular filling

5. Impulse travels rapidly down interventricular septum by means of bundle of His

6. Impulse rapidly disperses throughout myocardium by means of purkinje fibres

7. Rest of ventricular cells activated by cell-to-cell spread of impulse through gap junctions

In terms of AP in pace makers explain the intrinsic conduction system

• Has autorhythmic cells which initiate action potentials

• No real “resting membrane potential”

• Pacemaker potential membrane slowly depolarizes “drifts” to threshold, initiates AP, membrane depolarizes to -60 mV

What’re the different kinds of autorhythmic cells and what do they do?

• IF = causes pacemaker potential/ where pacemaker current is found, a Na+ current

• ICaT = Fast calcium current, turns on and off quickly

• ICaL = Slow Ca++ current, responsible for main depolarization

Understand this diagram

What’re the features of AP of contractile cells?

• Rapid depolarization

• Rapid, partial early repolarization

• Prolonged period of slow repolarization called the “plateau phase”

• Rapid final repolarization phase

What’re the different phases of AP of contractile cells?

0. Na+ channels open

1. Na+ channels inactivate

2. Ca++ channels open, K+ channels open and balance Ca++

3. Ca++ channels close; slow K+ channels open

4. Resting potential

Why is AP of contractile cells so long?

• Because plateau is primarily due to prolonged activation of “slow Ca++ channels” (L-type Ca++ channels)

• Ensures adequate ejection of blood out of heart

Whats the difference in refractory period of skeletal vs cardiac muscle cell?

• Skeletal muscle fast-twitch fiber = The refractory period is very short compared with the amount of of time required for the development of tension, skeletal muscles that are stimulated repeatedly will exhibit summation and tetanus

• Cardiac muscle fibers = The refractory period lasts almost as long as the entire muscle twitch, long refractory period in a cardiac muscle prevents tetanus

How does the long AP of contractile cells avoid tetanus?

• Long AP causes long refractory period and long contraction

Explain excitation contraction coupling in cardiac contractile cells

• Ca++ entry through Ca++ channels in T tubules triggers massive release of Ca++ from SR

  • Ca++ induced Ca++ release leads to cross bridge cycling and contraction