Electrical properties of the heart Flashcards
What generates the action potentials that cause contraction of the heart muscle
Pacemaker cells
Describe how action potentials reach muscle cells
They travel extracellularly in transverse tubules
Describe how an action potential causes contraction of a cardiac muscle cell
AP causes voltage gated L type Ca2+ channel to open allowing extracellular Calcium ions to flow into the cell
Ca2+ ions induce RyR channel on Sarcoplasmic reticulum to open causing massive release of more Ca2+ ions from SR
Calcium spark from many different SR’s summate to generate Ca2+ signal
Ca2+ signal causes contraction
Describe the components of the contractile fibre thing in cardiac muscle cells
Thick myosin filament
- with cross bridges
Thin actin filament
Troponin binding site for Ca2+
Z lines
What junctions exist between cardiac muscle cells?
What is their purpose?
Desmosomes - hold them together
Gap junctions - allow electrical connection between cells
What is the name given to the bit where cells join together, where the junctions are found?
The intercalated disc
Easily visible histologically
Why does the AP potential graph of cardiac muscle have a plateau bit on it, whereas skeletal muscle doesnt?
Myocardial cells have a longer action potential than skeletal/neurones due to Ca2+ entry
Plateau stage:
- Increase in Permeability to Ca2+
- Decrease in Permeability to K+
In skeletal/neurones, the rapid depol is due to Na+ entry and the repol is due to K+ leaving the cell
How long is the AP in cardiac muscle, compared to skeletal muscle?
Cardiac muscle - 250 ms
Skeletal muscle - 2 ms
Describe how the prolonged action potential means that cardiac muscle can not exhibit tetanus
Long refractory period means AP’s cant re-fire before the muscle has relaxed
Describe why cardiac muscle gives graded contractions
Ca2+ release does not fully saturate troponin binding sites
Thus more/less can be released to vary the strength of contraction = graded
What property of pacemaker cell’s resting membrane potential makes them different from the contractile cells?
It is unstable
In a non-pacemaker AP:
What causes the initial depolarisation
Increase in permeability to Na+
In the non-pacemaker AP:
What causes the plateau
(i know its already been asked but shut up)
Increase in Permeability to Ca2+ (VGCCs)
Decrease in permeability to K+
In a non-pacemaker AP:
What causes the repolarisation to RMP?
Decrease in perm to Ca2+
Increase in perm to K+
High resting permeability to K+ generates the RMP
In the pacemaker AP:
What causes the pacemaker potential (pre potential)?
gradual decrease in PK+
early increase in PNa+ (= PF)
late increase in PCa2+ (T-type)
In the pacemaker AP:
What causes the action potential?
increase in PCa2+ (L-type)
What are the modulators of electrical activity of the heart?
Sympathetic & parasympathetic systems
Drugs
Temperature
Hyperkalemia (high plasma K+)
Hypokalemia (low plasma K+)
Hypercalcemia (high plasma Ca2+)
Hypocalcemia (low plasma Ca2+)
How do drugs affect the heart?
Ca2+-channel blockers – decrease force of contraction
Cardiac glycocides – increase force of contraction
How does temperature affect the heart?
increases ~10 beats/min/ºC
How does Hyperkalemia affect the heart?
Fibrillation & heart block
How does hypokalemia affect the heart?
fibrillation & heart block
How does hypercalcemia affect the heart?
Increased HR & increased force of contraction
How does hypocalcemia affect the heart?
Decreased heart rate & decreased force of contraction
What are the components of the conducting system of the heart?
Sinoatrial node
Atrioventricular node
Bundle of His + Purkinje fibres
Where is the SA node located and what does it do?
Superior part of the right atrium
It is the ‘main’ pacemaker
0.5 m/sec
Where is the AV node located and what does it do?
Inferior part of the right atrium
It is the delay box
0.05 m/sec (slow conducting)
What is attached to the AV node?
Bundle of His
How are the Purkinje fibres attached to the Bundle of His?
What is their function?
Left & right bundle branches run down through the interventricular septum and then up the walls of the ventricles
Purkinje fibres attached to the branches when they are running up the walls of the ventricles
Rapid conduction system (5 m/sec)
What is the annulus fibrosis of the heart?
Non-conducting, Fibrous ring surrounding the atrioventricular orifaces
What are the 3 main stages in the cardiac cycle?
Atrial excitation
Ventricular excitation (+ AR)
Ventricular relaxation
On an ECG, there are the P QRS and T waves
What causes each wave?
P - atrial depolarisation
QRS - ventricular depolarisation
T - ventricular repolarisation
