L25 - Cardiac Muscle

Question 1

What is cardiac muscle composed of?

Answer 1

Branched interconnecting cardiomyocytes

Question 2

What are cardiac muscle cells?

Answer 2

Branched cells w/ single polyploid nucleus (human)
- 1% are non-contractile

Question 3

What is a syncytium?

Answer 3

Cardiac cells are separated from each other but are electrically coupled

Question 4

How are cardiac muscle cells connected?

Answer 4

Intercalated discs
- work as a single functional syncytium
- located at Z-line on the sarcomere

Question 5

What are the different types of intercalated discs? (3)

Answer 5

• intedigitating folds
• mechanical junctins
• electrical junctions

Question 6

What are interdigitating folds?

Answer 6

Infolds at the end of the cells that increase the SA of cell-cell connection

Question 7

What are examples of Mechanical and electrical junctions? (2)

Answer 7

• Fascia adherens and desmosomes
• gap junctions

Question 8

What do fascia adherens do?

Answer 8

Couple the membrane to the contractile apparatus of the cardiomyocytes
- transmission of contractile force between cardiomyocytes

Question 9

What do desmosomes do?

Answer 9

Provide strong structural support between cardiomyocytes
- ensure that they can withstand contractile forces of the heart

Question 10

What do gap junctions do?

Answer 10

Electrically couple the cytoplasm of neighbouring cells
- fascilitate passage of ions between cells
= cardiac action potentials to spread, producing depolarisation of the heart muscle

Question 11

What is the structure of gap junctions?

Answer 11

• hexamer composed of connexins provided by each cell
  = form gap junction with 12 subunits

Question 12

What does the gap junction serve as?

Answer 12

Low-resistance pathway between cells
- cell-cell conductance - elctrical coupling
- 5-500 gap junctions represent gap junction plaque

Question 13

What is the blood supply like in cardiac muscle? (3)

Answer 13

• requires continuous supply of oxygen
• rich capillary supply
• short diffusion distances for oxygen and waste products

Question 14

What is excitation-contraction coupling? (4)

Answer 14

• actin-myosin based contraction
• fundamental unit is sarcomere
• calcium initiates contraction
• regulated by troponin - tropomyosin complex

Question 15

Why are cardiac pacemaker cells needed?

Answer 15

Cardiac muscle contractions are not initiated by nerve stimulation

Question 16

What are pacemaker cells?

Answer 16

Specialised cardiomyocytes
- sponstaneuosly generate cardiac action potentials

Question 17

What are the primary pacemaker cells?

Answer 17

Sinoatrial node (SA node)
- near the superior vena cava entrance

Question 18

What are the phases in the sinoatrial node action potential?

Answer 18

• phase 4
• phase 0
• phase 3

Question 19

What happens in phase 4 in the sinoatrial node action potential?

Answer 19

Hyper-polarised membrane potential
- activation of funny channels
- inward flow of Na++ causes gradual increase in potential
- T-type Ca2+ channels open

Question 20

What happens in phase 0 in the sinoatrial node action potential?

Answer 20

Rapid depolarisation
- opening of L-type Ca2+ channels

Question 21

What happens in phase 3 in the sinoatrial node action potential?

Answer 21

Repolarisation
- L-type channels close
- K+ channels open

Question 22

What are the phases in cardiac (ventricular) action potential?

Answer 22

• phase 4
• phase 0
• phase 1
• phase 2
• phase 3

Question 23

What happens in phase 4 in cardiac (ventricular) action potential?

Answer 23

Diastole
- resting potential -90mV

Question 24

What happens in phase 0 in cardiac (ventricular) action potential?

Answer 24

Rapid depolarisation
- VGNC activated by AP from neighbouring cell

Question 25

What happens in phase 1 in cardiac (ventricular) action potential?

Answer 25

• rapid inactivation of Na+ channels
• opening and closing of K+ channels
  = brief efflux of K+ ions
• ‘notch’ on AP waveform

Question 26

What happens in phase 2 in cardiac (ventricular) action potential?

Answer 26

Plateau phase - MP remains almost constant
- membrane slowly repolarises
- L-type Ca2+ channels allow movement of Ca2+ into cell

Question 27

What happens in phase 3 in cardiac (ventricular) action potential?

Answer 27

Rapid repolarisation phase
- L-type Ca2+ channels close
- VGKC open

Question 28

How does Ca2+ induce Ca2+ release?

Answer 28

• L-type Ca channel open during cardiac AP
• trigger opennig of Ryanodine receptor Ca channel in the SR membrane
• Ca is released from SR and binds to troponin to trigger cross bridge cycling

Question 29

Whar does the sarco-endoplasmic reticulum calcium ATPase (SERCA)?

Answer 29

Calcium is returned back into the SR

Question 30

How is calcium removed out of the cell?

Answer 30

Na+/Ca2+ exchanger

Question 31

How is the membrane repolarised?

Answer 31

Potassium channels at the end of the AP

Question 32

What happens when the heart muscle is damaged?

Answer 32

Cardiac troponins are fragmented and released into the blood
- used to detect myocardial infarction

Question 33

What is dilated cardiomyopathy (DCM) caused by?

Answer 33

Mutations in a number of genes including those encoding actin, myosin, a-actinin, titin or troponin

Question 34

What is DCM characterised by?

Answer 34

• dilation of the left ventricle
• progressive contractile failure
• cardiomyocyte hypertrophy
• loss of myofibrils
• interstitial fibrosis

Question 35

What may individuals with DCM be like?

Answer 35

• initially asymptomatic
• develop shortness of breath
• fatigue as the left ventricle fails

Question 36

What is mortality significant through?

Answer 36

Progressive heart failure or sudden arrhythmia