Cardiac Physiology Flashcards
What are the main features of the conduction system of the heart?
SAN AV node AV bundle Right t& left bundle fibres Purkinje fibres
What does the SAN do?
Generate spontaneous A.Ps
Pass them to atrial muscle cells + AV node
What is the structure of SAN?
Specialised cardiac cells
What does the AV node do?
A.Ps conducted more slowly
= ventricles receive signal to contract after atria contract
What does AV bundle do?
Pass through hole in cardiac skeleton to reach interventricular septum
What does the right & left bundle branches do?
Extend beneath endocardium to apices of right + left ventricles
What do Purkinje fibres do?
Conduct A.Ps to ventricular muscle cells (myocardium)
What is the structure of Purkinje fibres?
Large diameter cardiac muscle cells with few myofibrils
Many gap junctions
Describe conduction system of heart
Depolarisation from SAN across atria
Conduct to AVN
Conduction slow in AVN
Atrioventricular bundle of His connects to branches
Purkinje fibres = terminal bundle branches
Myocardial fibres
Why is conduction slow in AVN?
To allow atria to empty into ventricles before ventricular systole
Describe structure of cardiac muscles
Faintly striated
Branched, mono-nucleated cells
Connected by intercalated discs ( gap junctions + desosomes)
Why is there slow propagation in cardiac muscles?
Gap junctions + small diameter
Why is propagation faster in skeletal muscles?
Larger diameter fibres
Where does A.P occur in cardiac muscles?
Cell to cell
Where does A.P occur in skeletal muscles?
Along length of single fibre
Describe sliding filament theory
Ca2+ binds to TN-C on thin filament
Exposes site on actin = can bind to myosin head
ATP hydrolysis supplies energy for actin-myosin conformational change
“Ratcheting” of actin-myosin + shortening of sacromere occurs
Ca2+ dissociates from TN-C + myosin unbinds with energy from ATP
Cycle ends when ATP binds to myosin + sacromere returns to original length
Describe cardiomyocyte contraction
Ca2+ enters through L-type channel Ca2+-induced release occurs Stimulates Ca2+ release from SR Intracellular [Ca2+] increases Ca2+ interacts with troponin-C Myosin binding site free on actin Actin moves over myosin = myocyte contraction
What happens after cardiomyocyte contraction has taken place?
Ca2+ reabsorbed into SR via sarco-endoplasmic reticulum Ca-ATPase pump
Removed from cell by Na+/Ca2+ exchanger + Ca2+ pump
ATP required to unbind myosin from action + resent sarcomere
What is the charge of the inside of cell?
Negative
What is charge after A.P?
Positive
What are the three factors that control charge?
Conc of ions in + out cell
Ion conductance
Activity of electrogenic pumps
What is value of resting potential?
-90mV
What is value of depolarisation threshold value?
-65mV
How many phases is there?
5
What happens in phase 0?
Opens Na+ gates
Membrane potential increases
+40mV
Na+ gated “inactivated” = Na+ permeability falls
What happens in phase 1?
Ca2+ gates open = inward flow of Ca2+
What happens in phase 2?
K+ balances Ca2+ flow = plateau
Generates cardiomyocyte contraction
Ca2+ channels inactivated
What happens in phase 3?
REPOLARISATION
K+ channels reopen
Membrane potential falls
What happens in phase 4?
Resting potential
Return to -90mV
What is pacemaker potential?
After A.P, membrane potential drifts up until threshold met
Na+ channels open = AP triggered
Where is the SAN?
Right atrium
close to vena cava
Describe A.P on SAN pacemaker cells
Phase 4: spontaneous depolarisation Inward movement of Na+, outward K+ K+ movement decays with time Pacemaker potential depolarises to -55mV Ca2+ inward current accelerates threshold to -40mV Phase 0: depolarisation Ca2+ increases = L-type Ca2+ channels Phase 3: repolarisation K+ channels open + help recognise cell as Ca2+ channel inactive
Describe parasympathetic regulation of cardiac cycle
Cardiomyocyte centre (vagus nerve)
Nerve branches to SA + AV nodes
Secrete acetylcholine; hyperpolarises heart
Decreases HR
Describe sympathetic regulation of cardiac cycle
Cardioaccelerator centre activates sympathetic neurons (cardiac nerves)
Secreted norepinephrine
Increases force of contractions = increases HR
What is a P-wave?
Depolarisation of atrial myocardium
What does P-wave show on electrocardiogram?
Signals onset of atrial contraction
On ECG = positive, not large or peaked
What is QRS complex?
Ventricular depolarisation
What does QRS complex show on electrocardiogram?
Signals onset of ventricular contraction
On ECG = negative deflection
What is a T-wave?
Repolarisation of ventricles
What is the PR interval?
Start of atrial depolarisation to start of ventricular depolarisation = contract + reflex
What does PR interval indicate?
Damage to conducting pathway or AVN
What is the QT interval?
Time required for ventricles to undergo single cycle of depolarisation + repolarisation
What is the QT interval lengthened by?
Electrolyte disturbances, conduction problems, ischemia + damage
