25. Electric activity of the heart, electromechanical coupling Flashcards
1
Q
Excitable tissues
A
- working muscle fibers
- pacemaker cells
- conduction fibers
2
Q
Excitable tissues
- working muscle fibers
A
Working fibers: generate unique, elongated AP(plateau), which prevents the heart from early secondary contraction.
3
Q
Excitable tissues:
- pacemaker cells
A
Pacemakers: has no permanent resting membrane potential, but turns into constant depolarization (lifetime pacemaker)
4
Q
Excitable tissues:
- conduction fibers
A
Conductive system provides rapid spreading of stimuli, hence providing synchronized contraction between atria and ventricles.
5
Q
Excitable tissues:
- Additional elements
A
Additional elements: anulus fibrosus (non conducting), and Aschoff-Tawara node(delayes the atrial signal), so the synchronous atrial contraction precedes the synchronous ventricular one.
6
Q
Working fibers
A
- Constitute a syncitium, i.e. that the muscle cells are connected via gap junctions that ensure an instant conduction of electrical activities from one cell to another.
- The properties of the action potential generated by the muscle fibers vary in different parts of the heart.
- An action potential of cells closer to the base and to the endocardium exhibit a longer plateau phase, than that of those situated in the vicinity of the apex and the epicardium. This influences the final picture of the total electrical activity of the heart.
7
Q
Action potential of working fibers
A
- Average resting membrane potential is -90 mV.
- The fiber is stimulated by an electrical impulse so that the RMP shifts towards and reaches the threshold potential.
- Reaching the threshold potential: the voltage sensitive fast sodium channels open and a sudden influx of the sodium from the EC occur. With this change one enters the so-called 0. phase of the AP.
- 0. phase - depolarization: Influx of sodium continues, membrane potential reaches approximately +25 mV. At this point the channels are inactivated and the flow of sodium stops.
- 1. phase – overshoot: Depolarization is stopped. Repolarization begins. Short chloride influx and potassium efflux.
- 2. phase – plateau: Calcium channels open, calcium influx. At the same time potassium channels open, potassium efflux. The balance of these processes causes the elongation of this phase.
- 3. phase – full repolarization: The late potassium channels open and potassium rapidly flows out of the cell according to its electrochemical gradient, while the calcium channels close.
- The role of the plateau phase is to prevent a premature generation of a new AP.
- A stimulus given after the beginning of phase 0 and before the end of phase 2, can not elicit a new action potential in the working fiber of the heart independently of the strength of the stimulus. This period is called the “Absolute Refractory Period” (ARP)
- A stimulus given after the end of the plateau phase but before reaching the threshold potential elicits an answer in function of the strength of the stimulus. Strong stimuli can elicit the formation of a new action potential. This phase is therefore named as “Relative Refractory Period”, RRP.
- The most important period is the one between the threshold and the RMP. Already a slight stimulus can elicit a new AP, an thus produce a premature newcontraction. This period is called SNP (Supernormal Phase or Period).
8
Q
Electromechanical Cupling
A
- Structural unit is the DIAD (at sceletal muscle:Triad) T-tubuls and Sarcoplasmic Retic.(SR) are in contact here. AP – huge Ca-transient (tubular L-type Ca channel(voltage gated) open, + rianoid-Ca ch open + Cadependent Ca-channel (from SR) + membrane Ca-dependent Ca channel from EC space are open: huge amount of IntraCytoplasmic Ca is around the sarcomers – results in contraction.
- After (during) contraction: ATP-dependent Ca pump drives back the Ca to the SR, plus Na/Ca antiport pumps back the Ca to EC space – IC Ca conc. Drops - resulting in relaxation.
