Lecture 7 Flashcards
Know anatomy of the heart including chambers, valves and pathway of blood
See figure.
Describe histology of cardiac muscle tissue and compare the skeletal muscle tissue
Sarcomeric Arrangement (striated)
Mononucleated
Central nuclei
Syncytium
Intercalated discs
Cells may branch
Compare the skeletal muscle fibers with cardiac muscle fibers with regard to the T-tubules and sarcoplasmic reticulum
SKELETAL Muscle Fibers:
T-tubules are at the ends of Thick Filaments
Two Cisternae per T-Tubule
T-Tubules form triads with the SR.
SR is MORE extensive in skeletal muscle fibers.
Motor unit arrangement (one 1 nerve fiber synapses with one or more skeletal muscle fiber)
CARDIAC Muscle Fibers:
T-Tubules are found along the Z-line
One Cisterna per T-tubule
T-Tubules form Diads with SR.
SR is LESS extensive
Cardiac Muscle cells form a Synctium
Compare the physiology of fast and slow cardiac muscle action potentials
FAST:Due to changes in conductance of Potassium, Sodium, and Calcium ions.
Conductance pattern is due to Voltage dependent gates.
Faster conductance: Greater AP amplitude, More rapid rate of rise of phase 0, Larger Cell Diameter.
SLOW:NO fast Sodium ion gates
Upstroke (negative to positive) of AP is due to CALCIUM (so it goes slow)
Resting phase potential 4 is -60mV (rather than -90mV)
Change in potential (Amplitude) is less than that for fast APs
SA & AV nodal tissue will spontaneously depolarize slowly to reach threshold during phase 4 (resting phase)
Describe the activities that occur during each of the five phases of a cardiac muscle potential
Phase 0: Depolarization
Fast Sodium Channels open and allow Sodium Ions to rapidly flow into the cell. Membrane Potential reaches +20mV.
(L-type Calcium Channels open up)
Phase 1: Initial Repolarization
Fast Sodium Channels close and cell begins to repolarize.
Potassium channels open and Potassium leaves cell.
(L-Type Calcium channels open up)
Phase 2: Plateau
Calcium Channels open and [fast] Potassium channels close.
Brief initial repolarization occurs and AP plateaus because of Increase Calcium Ion permeability & Decreased Potassium Ion Permeability.
(Calcium Ion channels open slowly during phases 1 & 0)
Phase 3: Rapid Repolarization
Calcium Channels Close and Slow Potassium Channels Open
Closure of Calcium Channels & Potassium Ions exiting the cell ends the plateau
Cell membrane return to resting level
Phase 4:
Resting Membrane Potential
-90mV.
Describe fast cardiac action potentials
Fast: • Found in atria, ventricles and conduction system • Very rapidly conducting but non-contractile in Purkinje fibers • Rapidly conducting and contractile in atrial and ventricular fibers • High amplitude (100 mV) • Fast action potentials are due to changes in conductance of potassium, sodium, and calcium ions. • Conductance pattern is mostly due to voltage dependent gates• Greater AP amplitude, • More rapid rate of rise of phase 0 • Larger cell diameter
Describe slow cardiac action potentials
Slow: • Found in SA and AV nodal tissues • Conducts slowly • Automatically depolarizes during resting phase: • More rapidly in SA node than in AV node • Low amplitude (60 mV)• No fast sodium ion gates • Upstroke (negative to positive) of action potential is due to calcium (therefore it proceeds slowly). • Resting phase potential 4 is close to -60 mV rather than -90 mV characteristic of fast action potentials. • Change in potential (amplitude) is less than that for fast action potentials. • SA and AV nodal tissue will spontaneously depolarize slowly to reach threshold during phase 4 (resting phase).
3 Characteristics of fast type contractile myocytes
• Large diameter • High amplitude • Rapid onset of action potential
What are 2 Characteristics of fast type non-contractile myocytes
• Very large diameter • Very rapid upstroke
3 Characteristics of slow type non-contractile myocytes
• Small diameter • Low amplitude • Slow rate of depolarization
Explain the role of Ca2+, Na+ and K+ in the creation of cardiac muscle action potential plateau
• In skeletal muscle, the sodium channels close rapidly. • In cardiac muscle the sodium channels also close rapidly, but the calcium channels open slowly and stay open for a longer period of time. • In cardiac muscle there is also a delay in the opening of the potassium channels. • The large concentration of both calcium ions and potassium ions is responsible for the plateau.
Describe role of SA node as the heart’s pacemaker
Resting membrane potential of SA node fiber: • -55 to -60 mV (Threshold ≈ -40 mv) • Fast sodium channels are already inactivated (blocked). • Inactivation gates close when membrane potential is less negative than -55 mV. • Therefore, only slow sodium-calcium channels can open. • Therefore, atrial nodal action potential is slower to develop. • Therefore, repolarization is also slower. • There is a slow leak of sodium ions back into the cells. • Membrane potential becomes more positive. • At -40 mV, sodium-calcium channels become activated. • Sodium-calcium channels are inactivated within 100-150 msec after opening
Compare a sinus rhythm with an ectopic focus
• Action potentials that originate anywhere else are said to be from an ectopic focus or pacemaker.
Describe the mechanism of calcium release during the contraction of a cardiomyocyte with regard to DHP and ryanodine channels and compare with calcium release in skeletal muscle fibers
Calcium floods in the SR and completes the electromechanical coupling process.
Far FEWER Calcium-Induced Calcium Release Channels in Cardiac Muscle compared to skeletal muscle.
This allows Fine control over Sarcoplasmic Calcium concentration and contractility.
In Skeletal Muscle, the excitation always triggers the MAXIMUM release of Calcium from the SR.
After the AP travels along the sarcolemma of cardiac myocytes, it enters the T-Tubules
Calcium then enters from the Extracellular Fluid through the Dihydropyridine Receptor Channels of the T-tubules.
Elevated cytoplasmic Calcium triggers more Calcium to enter from the Cisternae of Sarcoplasmic Tubules through Ryanodine Receptors.
Elevated Cytoplasmic Calcium binds to Troponin and Myofilament Contraction occurs.
Describe the role of two Ca2+ transporters involved in cardiac muscle relaxation
SERCA: Sarcoplasmic Reticulum Calcium ATPase
Stimulated by the Phosphorylation via an integral SR protein called “Phospholambian”, reduces its ability to inhibit SERCA pump when phosphorylated.
Returns Calcium to SR during DIASTOLE
This will allow for an even greater Calcium release on the next beat.
Also allows for Fast Clearance of Calcium form the Sarcoplasm.
Sodium-Calcium Exchanger in Sarcolemma:
Transports Calcium out of the Cell.
What is this Valve?
Mitral Valve
What is this valve?
Aortic Valve
What happens during each of these 4 phases?
Phase I: Period of Filling
Phase II: Isovolumic Contraction
Phase III: Period of Ejection
Phase IV: Isovolumic relaxation
