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
