L25 - Cardiac Muscle Flashcards
What is cardiac muscle composed of?
Branched interconnecting cardiomyocytes
What are cardiac muscle cells?
Branched cells w/ single polyploid nucleus (human)
- 1% are non-contractile
What is a syncytium?
Cardiac cells are separated from each other but are electrically coupled
How are cardiac muscle cells connected?
Intercalated discs
- work as a single functional syncytium
- located at Z-line on the sarcomere
What are the different types of intercalated discs? (3)
- intedigitating folds
- mechanical junctins
- electrical junctions
What are interdigitating folds?
Infolds at the end of the cells that increase the SA of cell-cell connection
What are examples of Mechanical and electrical junctions? (2)
- Fascia adherens and desmosomes
- gap junctions
What do fascia adherens do?
Couple the membrane to the contractile apparatus of the cardiomyocytes
- transmission of contractile force between cardiomyocytes
What do desmosomes do?
Provide strong structural support between cardiomyocytes
- ensure that they can withstand contractile forces of the heart
What do gap junctions do?
Electrically couple the cytoplasm of neighbouring cells
- fascilitate passage of ions between cells
= cardiac action potentials to spread, producing depolarisation of the heart muscle
What is the structure of gap junctions?
- hexamer composed of connexins provided by each cell
= form gap junction with 12 subunits
What does the gap junction serve as?
Low-resistance pathway between cells
- cell-cell conductance - elctrical coupling
- 5-500 gap junctions represent gap junction plaque
What is the blood supply like in cardiac muscle? (3)
- requires continuous supply of oxygen
- rich capillary supply
- short diffusion distances for oxygen and waste products
What is excitation-contraction coupling? (4)
- actin-myosin based contraction
- fundamental unit is sarcomere
- calcium initiates contraction
- regulated by troponin - tropomyosin complex
Why are cardiac pacemaker cells needed?
Cardiac muscle contractions are not initiated by nerve stimulation
What are pacemaker cells?
Specialised cardiomyocytes
- sponstaneuosly generate cardiac action potentials
What are the primary pacemaker cells?
Sinoatrial node (SA node)
- near the superior vena cava entrance
What are the phases in the sinoatrial node action potential?
- phase 4
- phase 0
- phase 3
What happens in phase 4 in the sinoatrial node action potential?
Hyper-polarised membrane potential
- activation of funny channels
- inward flow of Na++ causes gradual increase in potential
- T-type Ca2+ channels open
What happens in phase 0 in the sinoatrial node action potential?
Rapid depolarisation
- opening of L-type Ca2+ channels
What happens in phase 3 in the sinoatrial node action potential?
Repolarisation
- L-type channels close
- K+ channels open
What are the phases in cardiac (ventricular) action potential?
- phase 4
- phase 0
- phase 1
- phase 2
- phase 3
What happens in phase 4 in cardiac (ventricular) action potential?
Diastole
- resting potential -90mV
What happens in phase 0 in cardiac (ventricular) action potential?
Rapid depolarisation
- VGNC activated by AP from neighbouring cell
What happens in phase 1 in cardiac (ventricular) action potential?
- rapid inactivation of Na+ channels
- opening and closing of K+ channels
= brief efflux of K+ ions - ‘notch’ on AP waveform
What happens in phase 2 in cardiac (ventricular) action potential?
Plateau phase - MP remains almost constant
- membrane slowly repolarises
- L-type Ca2+ channels allow movement of Ca2+ into cell
What happens in phase 3 in cardiac (ventricular) action potential?
Rapid repolarisation phase
- L-type Ca2+ channels close
- VGKC open
How does Ca2+ induce Ca2+ release?
- 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
Whar does the sarco-endoplasmic reticulum calcium ATPase (SERCA)?
Calcium is returned back into the SR
How is calcium removed out of the cell?
Na+/Ca2+ exchanger
How is the membrane repolarised?
Potassium channels at the end of the AP
What happens when the heart muscle is damaged?
Cardiac troponins are fragmented and released into the blood
- used to detect myocardial infarction
What is dilated cardiomyopathy (DCM) caused by?
Mutations in a number of genes including those encoding actin, myosin, a-actinin, titin or troponin
What is DCM characterised by?
- dilation of the left ventricle
- progressive contractile failure
- cardiomyocyte hypertrophy
- loss of myofibrils
- interstitial fibrosis
What may individuals with DCM be like?
- initially asymptomatic
- develop shortness of breath
- fatigue as the left ventricle fails
What is mortality significant through?
Progressive heart failure or sudden arrhythmia
