lecture 6 - CVS 2: electrical activity & muscle contraction Flashcards
what does auto-rhythmic mean?
can excite itself independently
heart muscle is auto-rhythmic
what are the 2 groups of myocytes in the heart?
work cells - generate force
conducting cells - fast spread of electrical events throughout the heart
where does the initiation of the heartbeat start?
in the SAN in the right atrium
what do we need to get heart muscle to contract?
calcium
to get it to relax we lower the calcium levels to stop the calcium-myosin interaction
what 2 ways does the spread of excitation occur
cell to cell via gap junctions - direct spread
via the specialised conducting fibres in the atria and ventricles
how are work cells organised?
they are stuck together by the intercalated disks to form a functional syncytium
what is a syncytium?
a single cell or cytoplasmic mass containing several nuclei, formed by fusion of cells or by division of a nuclei
process of the electrical pathway in the heart
1) SAN sends out AP to both atria to stimulate them
2) slight delay gives atria time t contract before ventricles contract - allows blood to empty
3) signal converged to AVN
4) bundle of His
5) purkinje fibres
6) spreads upwards to rest of ventricular muscle
7) repolarisation leads to muscle relaxation
why is the AVN essential?
there is a fibrous, non-conducting layer between the atria and ventricles which electrically insulates the chambers from each other
electrical activity can only go through the AVN
what happens if the SAN is dysfunctional?
SAN determines HR
if dysfunctional, other conducting cells can take over the role but these have a slower firing rate
ionic basis of AP in SAN
at resting membrane potential:
• K+ channels close and slow Na+ channels open
• sodium enters
• leads to depolarisation
when threshold reached: • activate vg Ca++ channels • can only stay open for a short time • influx of Ca++ • depolarises cell more
when maximum reached:
• Ca++ channels close
• K+ channels open
• repolarisation
how does an AP in heart muscle trigger muscle contraction?
1) AP enters from adjacent cell
2) vg Ca++ channels open
3) Ca++ induces Ca++ release through ryanodine receptor channels (RyR) - calcium induced calcium release
4) local release causes Ca++ spark which creates a Ca++ signal
5) Ca++ binds to troponin to innate contraction
6) relaxation occurs when the Ca++ unbinds from the troponin
7) Ca++ pumped back into SR for storage
8) Ca++ os changed with Na+ by the NCX antiporter
9) Na+ gradient is maintained by the Na+/K+ ATPase
what is the refractory period in the heart?
the contraction period
muscle is fully relaxed before another AP can be fired - different to skeletal muscle
prevents summation
prevents tetanus
what is tetanus?
the prolonged contraction of the muscle caused by rapidly repeating stimuli
what is an electrocardiogram (ECG)?
a recording of the electrical activity of the heart made by placing electrodes on the skin
