Heart muscle function - PP Flashcards
Does cardiac muscle does require
innervation to be activated?
No. Unlike skeletal muscle, cardiac muscle does not require innervation to be activated.
What makes heart transplant possible?
Cardiac muscle does not require action potentials from nerves to activate its own electrical activity (a fact that makes heart transplantation operations possible).
What innervates the heart?
Both branches of the autonomic nervous system (ANS) innervate the heart, the ANS modulates cardiac function.
Nexi:
All myocardial cells are coupled electrically through gap junctions at points called nexi.
When are all cardiac cells activated?
During systole, all cardiac cells are activated
Automaticity:
Automaticity - unique ability to generate own action potentials
Rhythmicity
Rhythmicity - ability to generate these potentials in a regular, repetitive manner
Where does automaticity occur?
automaticity can occur in atrial and ventricular myocytes as well as in the Purkinje fibers.
What initiates cardiac electrical activity?
Normally, prior to each contraction of the heart, cardiac electrical activity is initiated by a modified set of muscle cells of the right atrium called the sinoatrial (SA) node
Pulse generators:
- SA : approx. 80 /min
- AV : approx. 50/min
- Purkinje fib.: approx. 20/min
Where are fast response action potentials observed?
Fast response action potentials – observed in atrial and ventricular muscle and Purkinje fibers
Where are slow response action potentials observed?
Slow response action potentials – observed in sinoatrial (SA) node and atrioventricular(AV) node
What are the phases of fast response action potential?
- Phase 0
- Phase 1
- Phase 2
- Phase 3
- Phase 4
What happens in Phase 0 (in fast response action potential)?
Phase 0: membrane depolarisation from -85mV up to at least -55mV opens voltage-sensitive Na+ channels, allowing a rapid influx of Na+ to a cell – self reinforcing and self limiting depolarisation - few milliseconds later 99% of Na channels are inactivated.
- They can’t be open again unless the cell membrane becomes repolarized below -50mV
What happens in Phase 1 (in fast response action potential)?
Phase 1: opening transient outward K+ channels allows small temporary repolaryzation
What happens in Phase 2 (in fast response action potential)?
Phase 2: Opening of a calcium L channels » influx of a Ca 2+ to a cardiac cell. In the same time there is opening of a K+ channel - efflux of a K+. Both currents approximately matches to each other » potential remains relatively constant at a positive value - plateu
What happens in Phase 3 (in fast response action potential)?
Phase 3: rapid repolarization by efflux of a K+
What happens in Phase 4 (in fast response action potential)?
Phase 4.: resting membrane potential
Excitability:
- Is the ability of cardiac cells to initiate action potentials in response to depolarizing current.
- Reflects the recovery of channels that carry the inward currents for the upstroke of the action potential.
- Changes over the course of the action potential are described by refractory periods
Absolute refractory period (ARP):
- Begins with the upstroke of the action potential and ends after the plateau.
- Reflects the time during which no action potential can be initiated, regardless of how much inward current is supplied.
Relative refractory period (RRP):
- Is the period immediately after the ARP when repolarization is almost complete
- Is the period during which an action potential can be elicited, but more than the usual inward current is required
Supranormal excitability (SNP):
A period during which the treshold for producing a second action potential is less than it is in the steady state
What are the phases in slow response action potential?
- Phase 0
- Phase 3
- Phase 4
What happens in Phase 0 (in slow response action potential)?
The action potential is slower (than in myocardium and Purkinje fibers), carried entirely by slow L-type voltage-gated Ca2+ channels (inward current)
What happens in Phase 3 (in slow response action potential)?
Outward K+ current
What happens in Phase 4 (in slow response action potential)?
Unique for SA and AV spontaneous, progresive, depolarization caused by progresive reduction in K+ outward current, and Na + inward current (If)
Chronotropic positive:
activation of If current
↓
spontaneus depolarisation is faster
↓
Heart Rate (HR)↑Dromotropic positive:
activation of CaL channel
↓
amplitude of action potential ↑
↓
Conduction velocity ↑Chronotropic &
dromotropic negative:
activation K+ outward channel
↓
slowing down spontaneus depolarisation & decreasing amplitude of action potential
How are myocardial contraction is modified?
Myocardial contraction is modified by altering the gating of the L /DHP Ca2+ channels in the cell membrane that are activated during phase 2 of the action potential.
Inotropic positive – increasing intracellular Ca:
- β receptor agonists (norepinefrine, epinefrine);
- Digitalis (inhibition of a Na/K ATP-ase ⇛ intracellular Ca↑)
- Inhibitors of cAMP phosphodiesterase (↑ cAMP): caffeine, theophyline
Inotropic negative – decreasing intracellular Ca:
- Β - blockers
- Ca channel blockers,
- Acetylocholine (weak inotropic negative in atria),
- Ishemia (lack of ATP)
- Acidosis ( ↑H+)
End diastolic volume (EDV):
End diastolic volume (EDV) - volume of blood in ventricle at the end of diastole (maximal volume during cardiac cycle)
End systolic volume (ESV):
End systolic volume (ESV) - volume of blood in ventricle at the end of systole (minimal volume during cardiac cycle)
Stroke volume (SV):
Stroke volume (SV) - volume of blood ejected from a vetricle during 1 cycle. SV = EDV – ESV
Cardiac output (CO):
Cardiac output (CO) - volume of blood ejected from the heart per unit time, usual resting values for adults are 5 to 6 L/min, or approximately 8% of body weight per min. CO = SV × HR
Cardiac index:
CO divided by body surface area - cardiac index.
Ejection fraction (EF):
Ejection fraction (EF) = (SV/EDV) x 100%
STUDY PAGE 21 AND UP ON YOUR OWN
……….
