Patient - Excitable Cells Flashcards
Describe how the heart works with regards to external control and external modulation
Does not require external control, contraction cycle is spontaneous, without external trigger. Cycle involves fine coordination of different parts of the heart.
Heart function is responsive to external modulation, however
Give examples of external modulation of the heart
Autonomic nervous system, other hormones, physical factors
Why is the cardiac function able to be spontaneous?
Cells store energy as a concentration difference of ions (inside/outside)
Membrane proteins control passage of ions, creating electrical activity and excitability
electrical signal can travel rapidly to neighbouring cells
electrical activity is coupled to mechanical action (contraction)
Where are the specialised conducting cells in the heart
sinoartial node atrioventricular node Bundle of his Bundle branches Purkinje fibres
Describe the appearance of cardiac muscle on a cellular level
Short (100-200um)
Strong, branching cells held together at intercalated discs
Electrical (gap) junctions allow cell-to-cell spread of action potentials (in 3D) without chemical synpases
What is the first stage of the cardiac cycle?
Sinoatrial node activity and atrial activation begins
What happens after the atria activate with regards to excitability of cells?
Stimulus spreads across the atrial surfaces and reaches the AV (atrioventricular) node
What happens after AV node activation?
There is a 100millisecond delay at the AV node. Atrial contraction then begins,
Where does the impulse go after the atria have contracted?
The impulse travels along the interventricular septum within the AV bundle and the bundle branches to the Purkinje fibres and, via a moderator band, to the papillary muscles of the right ventricle
What happens after the impulse has reached the papillary muscles of the right ventricle?
The impulse is distributed by the Purkinje fibres and relayed throughout the ventricular myocardium. Atrial contraction is completed, and ventricular contraction begins
Summarise the route of cardiac action potentials generated and spread by the pacemaker/conducting system
1) Produced by SA node cells
2) Depolarise and contract atrial muscle cells
3) Atrial APs depolarise AV node
4) Spread along right and left bundle branches
5) Into Purkinje cell network to contact ventricles
6) Repeated to produce cardiac cycle
Which cardiac conductive tissue is responsible for keeping atrial and ventricular systole separate?
The AV node as it delays the signal before it goes to the bundle of his
What percentage of cardiac cells are specialised for contraction? i.e. regular muscle cells
~99%
What percentage of cardiac cells are pacemaker and “specialised conducting” cells?
~1%
typical Na+ in mM:
Extracellular and intracellular
extracellular: 145mM
intracellular: 14mM
typical amount of K+ in mM
Extracellular and intracellular
Extracellular: 4mM
Intracellular: 140mM
Typical amount of Ca2+ in mM
extracellular and intracellular
extracellular: 2mM
intracellular: <0.001mM
Describe the plasma membrane in 3 statements
Bilayer of phospholipids.
Proteins inserted into the bilayer.
Proteins associated with bilayer surface.
Is the lipid bilayer an electrical insulator or an electrical conductor? What does this mean?
Electrical insulator - stops stuff going past i.e. segregate charged particles
Barrier to passage of most molecules
Does the lipid bilayer allow water flow?
Yes
What is the lipid bilayer permeable to?
Small lipophilic (nonpolar) molecules
2 kinds of ion channel
Ion selective
Gated
4 kinds of gated ion channel
- Ligand gated (fast) receptor and gate in the same unit
- Second-messenger, enzyme linked, such as GPCR
- Voltage-gated, sensitive to transmembrane electrical potential
- Mechanical (stretch) gated
What creates the transmembrane electrical potential?
Selective permeability to charged solutes
Define Nernst (equilibrium) potential
The electrical potential that balances the chemical potential (so no net movement of ions)
give the nerst (equilibrium) potential equation
Vm= (60mV/z) x log ([X]out/{X}in)
Where Vm= voltage inside relative to outside, units of millivolt
[X} is the concentration of X
z is the charge of ion X (e.g. -1, +2 etc)
How to calculate the Vm (voltage inside relative to outside) when there are multiple penetrating solutes? which equation?
Goldman-Hodgkin-Katz (GHK) equation
Contribution of membrane potential is weighted according to permeability
Van’t Hoff equation states what?
Chemical energy is proportional to solute concentration
What is the implication of the 3 equations below combined?
Van’t Hoff equation
Nernst equation
Goldman/GHK equation
Gated ion channels can rapidly change the membrane voltage, without any change in bulk intracelluar concentrations, on a time scale as fast as ~1ms (nerve action potential)
What is the cause of differences in action potential shapes?
Different gated cation channels
Describe the pacemaker potential in a nodal cell
Cation permeabilities vary over time, affected by autonomic nervous system inpit
What is the action potential upstroke in a nodal cell graph due to?
Due to voltage-gated Ca2+ channels
What is the action potential downstroke due to?
in nodal cells
Increase in concentration of K+
What are the currents in nodal cells activated by?
sympathetic stimulation (AdRs, e.g. noradrenaline) which open Na+ and Ca2+ channels and increase their concentration
In pacemaker cells, describe how spontaneous slow depolarisation occurs
Na+ (and K+)
Transient Ca2+ channels
In pacemakers cells, describe how rapid depolarisation occurs
due to slow Ca2+ channels
How does repolarisation in pacemaker cells occur
K+ channels
Describe atrial and ventricular contractile muscle contractile cells action potential
1) Resting membrane potential
2) Depolarisation
3) Slight repolarisation
4) Plateau to maintain contraction
5) Repolarisation (further)
6) Resting membrane potential
Describe which channels open and close and relative concentrations at the following moments with a star on
1) Resting membrane potential
2) Depolarisation*
3) Slight repolarisation*
4) Plateau to maintain contraction*
5) repolarisation (further)*
2) FAST Na+ channels open therefore influx of Na+
3) Na+ close therefore efflux of Na+
4) Open Slow Ca2+ channels and close Ka+ therefore influx in Ca2+ and decrease of intracellular K+
5) slow Ca2+ channels close therefore efflux of Ca2+ and opening of K+ channels
Resting membrane potential of a contractile cardiac cell
-90mV
What is the depolarised membrane potential of a contractile cardiac cell when it is at plateau stage and the membrane depolarisation is extended?
How long does this last?
~0mV for >100ms
