Cardiac Myocyte and Cardiac electrophysiology

Question 1

Where is the heart located?

Answer 1

Thoracic cavity medial to the lungs,posterior to breastbone

Question 2

What are the three layers of the heart?

Answer 2

Epicardium
Myocardium
Endocardium

Question 3

What is the epicardium?

Answer 3

Thin layer of serous membrane that helps protect and lubricate the outside of the heart

Question 4

What is the myocardium?

Answer 4

Muscular middle layer of heart
contains cardiac muscle tissue
Makes up thickness of the heart and is responsible for pumping blood

Question 5

What is the endocardium?

Answer 5

Simple Squamous endothelium layer that lines inside of the heart.
Stops blood from sticking inside the heart (which could cause blood clots)

Question 6

What are the 4 chambers of the heart?

Answer 6

Right Atrium
Right Ventricle
Left Atrium
Left Ventricle

Question 7

What is the role of the atria?

Answer 7

Receiving chambers of the heart
Connected to veins that carry blood to the heart

Question 8

What is the role of the Ventricles?

Answer 8

Larger, stronger pumping chambers that send blood out the heart through the arteries

Question 9

Why is the right side ventricle myocardium thinner than the left?

Answer 9

Sends blood to the nearby lungs compared to whole body

Question 10

What is the role of the heart (atrioventricular) valves?

Answer 10

Prevent blood flowing back (regurgitating)
Located in the middle of atrium and ventricles

Question 11

Which valve is in between the right atrium and ventricle?

Answer 11

Tricuspid valves (three flaps)

Question 12

Which valve is in between the left atrium and ventricle?

Answer 12

Mitral valve/bicuspid valve

Question 13

What controls the valves (connected to them?)

Answer 13

Chordae Tendineae

Question 14

What are the semilunar valves?

Answer 14

Named after their crescent moon shaped cusps
Located between ventricles and arteries that carry blood away from heart

Question 15

Right side semilunar valve?

Answer 15

Pulmonary Valve
Prevents backflow from pulmonary trunk into ventricles

Question 16

Left side semilunar valve?

Answer 16

Aortic valve
prevents aorta sending blood back

Question 17

How do they close?

Answer 17

They are smaller- no chordae tendineae
Rely on blood pressure to snap shut
Are cup shaped

Question 18

Blood flow through heart

Answer 18

(superior+inferior) Vena Cava—> right atrium—-> tricuspid valve—-> right ventricle—-> pulmonary semilunar valve—-> pulmonary trunk—-> lungs—–> Pulmonary vein—-> left atrium—-> bicuspid valve—–> left ventricle—-> aortic semi lunar valve—-> aorta—–> whole body

Question 19

What is the sinoatrial node?

Answer 19

Pacemaker cells located in the upper wall of the right atrium, at junction where superior vena cava enters

Question 20

How are they special?

Answer 20

They have natural automaticity- generate their own impulses

Question 21

What is the Atrioventricular node?

Answer 21

Where the electrical impulses that have spread across the atria converge
Located near the AV septum (near openings of coronary sinus)

Question 22

How is the AV node special?

Answer 22

Delays impulses by 120 ms to ensure atria can completely eject blood into ventricles before ventricular systole

Question 23

What is the AV bundle?

Answer 23

(Bundle of His) continuation of the specialised tissue of the AV node
transmits electrical impulse from AV node to Purkinje fibre in ventricles

Question 24

How does it (AV Bundle) split?

Answer 24

Goes down membranous part of intraventricular septum
divides into right bundle branch and left bundle branch

Question 25

What are the purkinje fibres?

Answer 25

Sub endocardial plexus of conduction cells Lots of glycogen lots of gap junctions Transmit cardiac action potentials from AV bundle to Myocardium of ventricles

Question 26

What does this allow for?

Answer 26

Coordinated Ventricular contraction (systole) Moves blood from ventricles to leaving blood vessels

Question 27

What overrides the intrinsic AV node and purkinje fibre pacemaker activity?

Answer 27

SA node

Question 28

Sequence of electrical events?

Answer 28

SA----> Atrial contraction--->AV node signal delay----> Bundle of his conduction----> His and purkinje fibre signal spread---> ventricular systole

Question 29

Pacemaker potential

Answer 29

Occurs at end of one action potential and just before start of next Slow depolarisation of pacemaker cells (SA) towards depolarisation called a funny current (or IF)

Question 30

How is pacemaker potential achieved?

Answer 30

Activation of HCN channels (Hyper Polarisation activated cyclic nucleotide gated channels) Allow Na+ entry into cells-ensures slow depolarisation Channels activated at membrane potentials more negative than -50mV

Question 31

Action potential in SA node

Answer 31

Once HCN has brought membrane potential to -40mV, Ca2+ voltage gated channels open Produces faster rate of depolarisation for positive membrane depolarisation HCN channels inactivate At peak of actionpotential, Ca2+ channels inactivate-K+ channels open

Question 32

What effect does opening k+ channels have?

Answer 32

Movement of K+ causes repolarisation

Question 33

What is different in the SA node actional potential?

Answer 33

Opening of CA 2+ channels are not sustained- no plateau so action potential is triangular in shape

Question 34

What are gap junctions?

Answer 34

Regulated pores that exist between cardiac myocytes Made of connexins which form a unit called connexon

Question 35

Where are connexons embedded?

Answer 35

Plasma membrane of adjacent cells located at either end of cell at region of intercalated disks Connecting connexons allows for cytosol of two cells to mix

Question 36

Allowing cytosol to mix freely is helpful because?

Answer 36

Ions easily pass from cell to cell-electrically coupled

Question 37

What does electric coupling ensure?

Answer 37

Synchronised heart activity large electrochemical gradient lots of passive diffusion Unidirectional spread of action potential

Question 38

What makes cardiac action potential different from other action potential?

Answer 38

Duration is longer (200-400 ms) Calcium ions play a large role in depolarization Calcium influx prolongs duration of action potential

Question 39

Phase 4 in non pacemaker cells

Answer 39

Phase 4- true resting membrane potential (-90mV) due to potassium channels being open (positive K+ leaves cell making membrane potential) (Fast sodium channel and slow l type calcium channel closed)

Question 40

Phase 0 in non pacemaker

Answer 40

Rapidly depolarised to around -70 Due to increase in fast sodium channel Potassium channels close Conductance changes move membrane potential away from EK (negative) to equilibrium potential for sodium

Question 41

Phase 1-Notch

Answer 41

Transient opening og K+ channels rapidly repolarise cell. Set membrane potential of plateau phase More K+: more repolarisation-plateau at lower voltages Less K+: less repolarisation- plateau at higher voltages

Question 42

Phase 2 of non pacemaker cells

Answer 42

L-Type Ca 2+ channels in t tubules penetrating cell Channels close to Sarcoplasmic reticulum Ca 2+ entering them bind to ryanodine receptor Calcium induced calcium release Essential for excitation contraction coupling of cell Ca binds to troponin C causing muscle contraction

Question 43

Phase 3 non pacemaker

Answer 43

As Ca2+ channels close, K+ channels repolarise the cell-driving membrane potetial towards equillibrium K Membrane potential more negative

Question 44

Cardiac Myocyte

Answer 44

specialized muscle cell myocyte-> myofibril-> myofilaments Have sarcomeres within it

Question 45

cardiac excitation-contraction coupling

Answer 45

1)Action potentials traveling along the sarcolemma and down into the transverse tubule (T-tubule) system depolarize the cell membrane. 2) Voltage-sensitive dihydropyridine (DHP) receptors (L-type calcium channels) open to permit calcium entry into the cell during phase 2 of the action potential. 3)Calcium influx triggers a subsequent release of calcium that is stored in the sarcoplasmic reticulum (SR) through calcium-release channels ("ryanodine receptors"), and increases intracellular calcium concentration from about 10-7 to 10-5 M 4) Free calcium binds to troponin-C (TN-C) that is part of the regulatory complex attached to the thin filaments. When calcium binds to the TN-C, this induces a conformational change in the regulatory complex such that troponin-I (TN-I) exposes a site on the actin molecule that is able to bind to the myosin ATPase located on the myosin head. This binding results in ATP hydrolysis that supplies energy for a conformational change to occur in the actin-myosin complex. The result of these changes is a movement ("ratcheting") between the myosin heads and the actin, such that the actin and myosin filaments slide past each other thereby shortening the sarcomere length. Ratcheting cycles occur as long as the cytosolic calcium remains elevated 5) At the end of phase 2, calcium entry into the cell slows and calcium is sequestered by the SR by an ATP-dependent calcium pump (SERCA, sarco-endoplasmic reticulum calcium-ATPase), thus lowering the cytosolic calcium concentration and removing calcium from the TN-C. To a quantitatively smaller extent, cytosolic calcium is transported out of the cell by the sodium-calcium-exchange pump. Unbinding of calcium from TN-C induces a conformational change in the troponin complex leading, once again, to TN-I inhibition of the actin binding site. At the end of the cycle, a new ATP binds to the myosin head, displacing the ADP, and the initial sarcomere length is restored.

Question 46

Cardiac Muscle

Answer 46

Long branching cells Less abundant SR Gap junctions and intercalated discs Myogenic Plateaud action potential 25-40% of cell volume

Question 47

What is chronotrophy?

Answer 47

Rate the heart beats

Question 48

Through which receptor does the parasympathetic system act to influence the heart

Answer 48

M2 ACHR

Question 49

Which of these is not an action of the sympathetic nervous system (SNS) (fight or flight) on the heart?

Answer 49

Reduce AV delay