3.2 Cellular and molecular events in the heart Flashcards
What do the pacemaker cells of the heart have the intrinsic ability to do ?
spontaneously depolarise and trigger action potentials
Action of pacemaker cells of the heart don’t require what ?
extrinsic input
what do Non-contractile nodal cells intrinsically initiate ?
ion-dependent electrical events at regular intervals (60-80 per minute)
..1… electrical impulses propagate from the ..2.., via the ..3… septum to the cardiac apex to initiate contraction of the ..4..
This spreads through the ..5.. to produce a co-ordinated heartbeat
- Ion-dependent
- SA node to the AV node
- intraventricular
- contractile cardiomyocytes
- myocardium
Cardiomyocyte contraction involves a force generated by what ?
a contractile apparatus where actin-myosin generates tension where intracellular calcium concentration rises.
this tension is proportional to intracellular calcium concentration
What is the troponin complex ?
a component of the thin filaments in striated muscle complexed to actin
what are the 3 types of troponins ?
- troponin T (tropomyosin binding)
- troponin I (inhibitory protein)
- troponin C (calcium binding )
How do internal calcium concentration determine cardiomyocyte contraction / relaxation ?
intracellular calcium concentration must:
* rise to allow cardiomyocyte contraction during systole
* fall (to allow cardiomyocyte relaxation) during diastole
How is intracellular calcium concentration elevated and then returned to basal levels ? [changes]
influx and efflux mechanisms of cardiac myocytes
What do voltage-gated Ca2+ channels (VGCCs) of heart allow ?
An influx of Ca2+ into cells across the plasma membrane by opening in response to an increase in membrane potential (i.e. depolarisation)
Where is Ca2+ released from in the heart ? [change!!]
rapidly-releasable intracellular stores
what does the SERCA pump stand for?
sarco/endoplasmic reticulum Ca2+ ATPase
what does CICR stand for ?
Ca2+ - induced Ca2+ release
List the steps involved in the cardiac excitation-contraction coupling [change!!]
- action potential enters from adjacent cell
- voltage-gated Ca2+ channels open. Ca2+ enters cell
- Ca2+ induces Ca2+ release through ryanodine receptor-channels (RyR)
- Local release causes Ca2+ spark
- summed Ca2+ sparks create a Ca2+ signal
- Ca2+ ions bind to troponin to initiate contraction
- relaxation occurs when Ca2+ unbinds from troponin
- Ca2+ is pumped back into the sarcoplasmic reticulum for storage
What does the elevated intracellular calcium concentration occur due to ?
an action potential in the cell membrane
what is an action potential of the membrane potential of the cell ?
a characteristic disturbance
- What does it mean by cell membranes being ‘capacitors’?
- what are membranes charged by ?
- what are electrical current in biological systems carried by ?
- they store electrical charge
- movement of ions across them
- ions not electrons
When is an equilibrium established in the action potential of cardiomyocytes ?
at a membrane potential determined by the ratio of K+ concentrations inside and outside the cell
where the resting membrane potential of myocardial fibres is approximately -90mV
What does it mean by each ion having an equilibrium potential ?
the hypothetical membrane potential that would develop if that ion were the only ion that could cross the membrane
What are the equilibrium potentials of the following ?
1. K+
2. Na+
3. Cl-
4. Ca2+
- -90mV
- +50mV
- -90 mV
- +125mV
What will happen to the membrane potential if the permeability of the membrane to different ions changes ?
it will change accordingly
What channels do myocardial cell membranes have ?
- voltage-gated K channels
- voltage-gated Na channels
- voltage-gated Ca channels
- In diastole which ion is the cell membrane of myocardial cells mostly peremable to ?
- so which ion equilibrium is the membrane potential close to ?
- K+
- K+ equilibrium potential
- What are ventricular cell stiumlated by ?
- what does this do to the cell ?
- spread of electrical activity from pacemaker cells
- depolarises it = making less negative inside the cell
What does the initial depolarisation of the cardiac action potential do ?
The depolarisation to a threshold opens ‘ fast’ voltage-gated sodium channels
So the membrane moves towards the sodium equilibrium potential (+50mV) and depolarises further
why doesn’t the membrane potential repolarise quickly in cardiac myocytes when voltage gated sodium channels close ?
voltage-gated calcium channels open
Why is there a plateau phase in the cardiac action potential ?
There is a lot more Ca2+ outside cells than inside cells so the Ca Equi Pot is +ve inside and the opening of calcium channels keeps the membrane depolarised
these channels stay open for 250ms so cell contraction is sustained for a long time but eventually close allowing membrane to repolarise
What stimulates the calcium induced calcium release ?
open calcium channels allow calcium ions into the cell, which stiumlates release of calcium from the sarcoplasmic reticulum
resulting in Ca2+ allows actin-myosin interaction and cardiomyocyte contraction
What happens at the same time that the Ca2+ channels close ?
extra potassium channels open making the cell repolarise faster
so calcium is sequestered within the cell, so intracellular calcium concentration falls, and the cell relaxes - diastole.
Another name for pacemaker cells ?
non-contractile nodal cells
Where are pacemaker cells found (1) and what can they generate (2) ?
- in SA and AV node
- action potentials spontaneously
