Regulation of Cardiac Function 2 Flashcards
Describe a ventricular action potential
Rapid depolarisation
Long plateau phase
What is the resting membrane potential in the ventricles?
-90mV
What is the resting membrane potential in the atria?
-90mV
What is the resting membrane potential in the Purkinje fibres?
-90mV
Describe the ionic currents during a ventricular action potential (3)
Opening of Na+ channels = rapid depolarisation (voltage-gated Na+ channels and positive feedback) due to Na+ influx
Ca2+ channels open at peak depolarisation and Na+ channels close = plateau due to Ca2+ influx
K+ channels open after plateau and Ca2+ channels close = repolarisation due to K+ efflux
What occurs in skeletal muscle when there is insufficient time to remove Ca2+?
Tetany
Why is tetany not desirable in cardiac muscle?
Discrete contractions
Maximise volume of blood pumped
How are discrete cardiac muscle contractions achieved?
Length of action potential and length of contraction and relaxation is synonymous
How does lowering the Ca2+ concentration affect the ventricular action potential/contraction?
Shorter plateau
Less tension generated
What is the decaying pacemaker potential?
Describes the SAN membrane potential
Gradual spontaneous depolarisation to allow spontaneous action potential to be generated
What cells have a decaying pacemaker potential?
SAN
AV node and Purkinje fibres have some in case SAN is damaged
Is the upstroke/depolarisation faster in the SAN or the ventricles?
Ventricles
Describe the currents underlying the SAN action potential (4)
Long-acting Ca2+ channels generate slow-rising action potential by Ca2+ influx
At peak, K+ channels open and Ca2+ channels close = repolarisation by K+ efflux
Closure of K+ channels and opening of ‘funny current’ Na+ channels = start of pacemaker potential by Na+ influx
Towards end of pacemaker potential, transient and long-acting Ca2+ channels open = depolarisation
What kind of ion channel is mostly absent in nodal cells?
Voltage-gated Na+ channels
Why is it called the ‘funny current’ Na+ channel?
Opens on hyperpolarisation rather than depolarisation
What is the difference between transient and long-acting Ca2+ channels?
Transient = open for less time but let in more Ca2+ at one time
Long-acting = open for longer but let in less Ca2+ at one time
What drugs inhibit long-acting Ca2+ channels in the SAN?
Verapamil
Nifedipine
What inhibits K+ channels in the SAN?
Barium
What drug inhibits the ‘funny current’ Na+ channels in the SAN?
Ivabradine
What type of agents modulate the pacemaker potential decay rate?
Chronotropic agents
What do chronotropic agents modulate and how?
Decay of pacemaker potential
Affect ion conductance of channels
What are two examples of chronotropic agents and what do they do?
Noradrenaline (sympathetic) = faster decay = faster heart rate
Acetylcholine (parasympathetic) = slower decay = slower heart rate
What are the units that make up a gap junction?
Connexons
What are the subunits of a connexon?
6 connexins
Where are the gap junctions in the heart?
Intercalated discs
Why are there gap junctions in the heart?
Allow ions to move between cells/electrically coupled
So act as an electrical syncitium
Smooth contraction = maximum blood ejection
What two factors affect the rate of action potential conduction?
Resistance of gap junctions
Membrane capacitance
How can you change a gap junction to give faster action potential conduction?
More connexons (less resistance)
What is the conduction pathway in the heart?
SAN
AVN
Bundle of His
Left and right bundles
Purkinje fibres
How fast is action potential conduction in the atria?
1m/s
How fast is action potential conduction via the AVN?
0.05m/s
How fast is action potential conduction through the Bundle of His, left and right bundles and Purkinje fibres to the ventricular mass?
4m/s
How fast is action potential conduction from the endocardium to the epicardium?
0.3m/s
Why is conduction speed so slow in the AVN?
Allow last of atrial blood to enter ventricles
What type of cells make up the conduction pathway in the heart?
Muscle
What part of the nervous system modulates heart rate?
Autonomic NS
What is Ca2+-induced Ca2+ release?
Action potential causes long-acting Ca2+ channels to open in T-tubules = Ca2+ influx
Ca2+ binds to ryanodine receptors on sarcoplasmic reticulum to allow Ca2+ release from internal store
What receptor do Ca2+ ions bind to on the sarcoplasmic reticulum?
Ryanodine
What protein does Ca2+ bind to in muscle contraction?
Troponin
Why must Ca2+ be removed after heart contraction?
Allow relaxation before the next contraction
How is Ca2+ removed after heart contraction?
Active Ca2+ uptake into sarcoplasmic reticulum via SERCA pump
Ca2+ removal by Na-Ca exchanger
What pump is used in the uptake of Ca2+ into the sarcoplasmic reticulum?
SERCA
How many ions are transported by the Na-Ca exchanger at one time?
1 Ca2+ out
3Na+ in
How does noradrenaline act as an positive inotropic agent?
Faster decay of pacemaker potential = faster heart rate (positive chronotropic agent)
Less time to remove Ca2+ so builds up
Ca2+ required for contraction so increased contractile force (staircase/Treppe effect)
What is the Treppe effect?
Increased heart rate increases contractile force
What is the difference between chronotropic agents and inotropic agents?
Chronotropic agents affect rate
Inotropic agents affect contractility
Give an example of a positive chronotropic agent
Noradrenaline
Give an example of a negative chronotropic agent
ACh
Propanolol (b-blocker)
Atenolol (b1-blocker)
Adenosine
Digoxin (glycosides)
How does adenosine decrease heart rate?
Activates nodal K+ channels = hyperpolarisation
How do glycosides decrease heart rate?
Increase vagal tone
Give an example of a positive inotropic agent
Noradrenaline
Isoprenaline, adrenaline (b-agonists)
Digoxin (glycosides)
How does digoxin increase contractility?
Inhibits Na-K ATPase
Na+ not removed = Ca2+ not removed
What can act as a negative inotropic agent?
Ischaemia
Hypoxia
Acidity
How does an electrocardiogram work?
Measures alterations in currents at skin’s surface to detect de/repolarisation of heart muscle
What is Einthoven’s triangle?
Placement of electrodes in a 3-lead ECG
Which type of 3-lead ECG is used most often and why?
Lead II
Depolarisations measured are parallel to the interventricular septum (best view of PQRST waves)
What is the positioning of electrodes in the Lead II 3-lead ECG?
Cathode on right clavicle
Anode on last left rib
Earth on left clavicle
What is the positioning of electrodes in the Lead III 3-lead ECG?
Cathode on left clavicle
Anode on last left rib
Earth on right clavicle
What is the positioning of electrodes in the Lead I 3-lead ECG?
Cathode on right clavicle
Anode on left clavicle
Earth on last left rib
Why do we take 6 or 12 lead ECGs?
Allows us to see different planes of heart to determine where a blockage in conduction is
In a Lead II 3-lead ECG, why is there positive and negative deflection?
Positive = depolarisation in same direction of anode
Negative = depolarisation in opposite direction to anode
