Electrical Properties of the Heart Flashcards
What makes up the intercalated discs in cardiac muscle?
Desmosomes and Gap junctions
What are gap junctions?
Electrical connection
Narrow channels
What are desmosomes?
Physical connection
Stitch cardiac muscle cells together
What does cardiac muscle form?
Functional syncytium - independent cells
Explain cardiac muscle and action potentials
Has long action potential and long refractory period so cannot exhibit tetanic contraction
How does Ca2+ regulate contraction?
Ca2+ from outside cell
Ca2+ release does not saturate troponin so regulation of Ca2+ release can vary the strength of contraction
How does Ca2+ regulate contraction?
Ca2+ from outside cell
Ca2+ release does not saturate troponin so regulation of Ca2+ release can vary the strength of contraction
What is a pacemaker cell?
Cells that have an unstable resting membrane potential so are spontaneously contracting
What is the normal resting membrane potential of a non-pacemaker?
-90mV
Describe the basic electrophysiology of non-pacemaker cells?
High resting PK+
Initial depolarisation happens due to increase in PNa+
Plateau is caused by increase of PCa2+ (L-type) and decrease of PK+
Repolarisation is caused by decrease in PCa2+ and increase in PK+
What does L-type mean?
Long and large
Stay open longer and large channels causing large depolarisation
Describe pacemaker action potential
Action potential - increase in PCa2+ (L-type)
Pacemaker potential - gradual decrease in PK+, early increase in PNa+ (If) and late increase in PCa2+ (T-type)
What does Ca2+ T-type mean?
Tiny and transient
Don’t let a lot of Ca2+ into cell and don’t sty open long
Describe the sinoatrial node
Fastest pacemakers are here so depolarise to threshold first - 0.5m/s
Wave of depolarisation in atrium which leads to atria contracting and squeezing blood into ventricle
Describe the annulus fibrosis
Ring of non-conducting tissue which prevents depolarisation immediately form atrium to ventricle
Describe the atrioventricular node
Conducts slowly - 0.05m/s
Acts as a delay box so atria have time to depolarise and contract before ventricle
Describe the bundle of His and Purkinje fibres
Rapid conducting system - 5m/s
Ensure coordinated contraction of the heart
Describe an AP in a single myocyte
It evokes a very small extracellular electrical potential
Describe what happens when lots of small extracellular electrical potentials are evoked?
Due to many cells depolarising and repolarising at same time
Can summate to create large extracellular electrical waves
This can be recorded in periphery by electrocardiogram
What does the P wave represent on ECG?
Atrial depolarisation
What does the QRS complex represent on ECG?
Ventricular depolarisation
What does T wave represent on ECG?
Ventricular repolarisation
What is the U wave on an ECG?
Why cant atrial repolarisation be seen on ECG?
Atrial repolarisation gets lost by QRS complex as happen at same time and QRS complex is bigger
What is the RR interval?
Between the R intervals
Can measure heart rate manually
Large square is 0.2
Describe first degree heart block on an ECG?
PR interval is increased
Disruption of conduction of depolarisation to AV node onto ventricles
Describe 2nd degree heart block
QRS drops as the P interval increases
Some failure of conduction
Describe 3rd degree heart block
QRS is seen because pacemaker take over and cause contraction
No AV conduction - depolarisation not getting to ventricles
Describe atrial flutter
Atria conduct faster than they should
Lots of p waves before QRS
Describe atrial fibrillation
Failure of pacemaker spread of wave of depolarisation to atria
Cell are contracting and depolarising at different times so uncoordinated
Describe ventricular fibrillation
Uncoordinated contraction of ventricles and means heart not pumping blood effectively
Not getting to brain so can cause unconsciousness
