2/ electrical activity of the heart Flashcards
why is cardiac muscle referred to as a functional syncytium ?
- electrical connection via gap junctions
- physically connected by desmosomes
- these form the intercalated discs
what is the length of cardiac muscle action potential and how does this compare to skeletal muscle?
250 msec compared to about 2 msec in skeletal muscle
what is the point of a long action potential?
- long refractory period, so cannot exhibit titanic contraction
- Ca2+ entry from outside cell can regulate concentration (Ca2+ release does not saturate the troponin, so regulation of Ca2+ release can be used to vary the strength of contraction)
why would some cardiac cells have unstable resting membrane potential?
they act as pacemakers, because they can only reach threshold once in a while and require no stimulus
what protein accounts for non-pacemaker cells resting membrane potential?
high resting Pk+
how does a non-pacemaker cell depolarise initially?
increase in Pna+
what accounts for the plateau in non-pacemaker cells’ AP?
increase in Pca2+ and decrease in Pk+
what accounts for the depolarisation phase in non-pacemaker cells’ AP?
decrease in Pca2+ (L-type) and increase in Pk+
what protein concentrate variation accounts for the triggering of an AP in pacemaker cells?
increase in Pca2+ (L-type)
what happens after the AP (pre-potential)?
- gradual decrease in Pk+
- early increase in Pna+ (=Pf)
- late increase in Pca2+ (T-type)
what does the cells’ pacemaker property allow them to do?
autorhythmicity and pacemaker ability is a basis for understanding modulation of the activity of the heart
what modulates this electrical activity?
- sympathetic and parasympathetic systems
- drugs
- temperature
- hyperkalemia (high plasma K+)
- hypokalemia (low plasma K+)
- hypercalcemia (high plasma Ca2+)
- hypocalcemia (low plasma Ca2+)
how does temperature affect electrical activity?
10 beats/min/°C increase
what drugs modulate electrical activity?
Ca2+ channel blockers decrease force of contraction, cardiac glycosides increase force of contraction
how does hyperkalemia affect electrical activity?
fibrillation and heart block
how does hypokalemia affect electrical activity?
fibrillation and heart block (anomalous)
how does hypercalcemia affect electrical activity?
increased HR and force of contraction
how does hypocalcemia affect electrical activity?
decreased HR and force of contraction
what are the conducting parts of the ‘special’ conducting system?
- sinoatrial node
- atrioventricular node
- bundle of His
- left and right bundle branch
- Purkinje fibres
what is the role of the sinoatrial node? how fast does it conduct?
- pacemaker
- 0,5m/sec
is the annulus fibrosus a conducting or non-conducting tissue?
annulus fibrosus
what is the role of the atrioventricular? how fast does it conduct?
- delay box
- 0,05m/sec
what is the role of the Purkinje fibres? how fast do they conduct?
- rapid conduction system
- 5m/sec
what place does the atrial excitation begin at?
SA node
what place does the atrial excitation end at?
AV node
what place does the ventricular excitation begin at?
atrial relaxation
what place does the ventricular excitation end at?
signal sent to both ventricles
what place does the ventricular relaxation happen at?
left / right bundle branch
how are large extracellular electrical waves created if AP in a single myocyte evokes a very small extracellular electrical potential?
lots of small extracellular electrical potentials evoked by many cells depolarising and repolarising at the same time can summate to create large extracellular waves
where are these extracellular electrical waves recorded? as what?
at the periphery as the ECG
ECG- what does the first wave (P) correspond to?
atrial depolarisation (atrial excitation complete)
ECG- what does the QRS complex (QRS) correspond to?
ventricular depolarisation
ECG- what does the last wave (T) correspond to?
ventricular depolarisation
