Electrical Activity of the Heart Flashcards
Name the two fibres of the sacomere.
Actin and myosin
Where are the actin anchored together?
At the Z lines
What is the membrane of a muscle cell called?
Sarcolemma
What are the names given to the deep invaginations of the sarcolemma>
T tubules
What is stored in the sarcoplasmic reticulum?
Intracellular calcium
What happens when a motor neuron fires an action potential?
Releases neurotransmitter acetylcholine and binds to cholinergic nicotinic receptors on the end plate.
What triggers an end plate potential?
The binding of acetylcholine to cholinergic nicotinic receptors.
What happens to allow actin and myosin to interact with each other?
Calcium released from stores in the sarcoplasmic reticulum which binds to troponin, allowing them to interact
What is formed when multiple muscle cells fuse together?
True syncytium
What does the true syncytium contain?
Many nuclei
What is formed in cardiac muscle instead of a true syncytium?
Functional syncytium
There is no fusion of cardiac muscle cells but they work together.
How are they joined together?
Physically and electrically
How are muscle cells in cardiac muscle joined electrically?
By gap juntcions
How are msucle cells physically connected?
By desmosomes
What are the two components of intercalated discs?
Gap junctions
Desmosomes
Compare the length of the action potential in skeletal and cardiac muscle.
Skeletal- short, 1-2 ms
Cardiac- long, 200-250 ms
What are cardiac muscle cells mediated by?
Voltage gated sodium ion channels
Voltage gated calcium ion channels
When do voltage gated calcium channels open?
When the cell depolarises
What does the calcium do?
Depolarises the cell
Binds to sites on troponin, increasing the number of crossbridges between actin and myosin
What can change the strength of contraction of cardiac muscle?
Able to modulate how much calcium is released, affecting the number of cross bridges hence changing the strength of the contraction.
What is meant by refractory periods?
Period immediately following stimulation during which a nerve or muscle is unresponsive to further stimulation.
Compare the refractory periods in skeletal and cardiac muscle.
Skeletal- short as short action potential
Cardiac- longer as much longer action potential
What helps the cardiac muscle to contract and relax contineously?
Long action potentials
Why can cardiac muscle not exhibit tetanic contraction?
Very long refractory periods
What can regulate contraction?
Ca2+ entry from outside cell
What do cells which have unstable resting membrane potential act as?
Pacemakers
Compare the resting membrane potential of non-pacemaker cells and pacemaker cells.
Non-pacemaker action cells have a standard resting membrane potential of -90mv
Pacemaker cells do not have a standard resting membrane potential and can depolarise spontaneously.
What are leaky potassium channels?
Channels which are open at rest and continually allow some potassium ions to leak out of the cell down its concentration gradient.
Is there a high conc. of potassium inside or outside the cell?
Inside
Why is their more potassium in the cell?
Sodium potassium pumps continuously pumps potassium into the cell and sodium out.
Are voltage gated sodium ion channels usually open or closed at rest?
Closed
When do voltage gated sodium ion channels open?
When the cell reaches threshold
When do the voltage gated calcium channels open?
When the cell reaches threshold
Discuss the action potential of non-pace maker cells.
Resting membrane potential
High resting PK+
Initial depolarisation
Increase in PNa+
Plateau
Increase in PCa2+ (L-type) and decrease in PK+
Repolarisation
Decrease in PCa2+ and increase in PK+
Resting membrane potential
What causes the rapid depolarisation of non-pacemaker cells?
Rapid entry of sodium into the cell
When do calcium ion channels tart to open?
At the same time as the voltage gated sodium ion channels but don’t open until the plateau phase as take so long to fully open- by which the sodium channels have already shut
What is meant by L-type calcium channels?
Very large- let in lots of Ca- and stay open for a long period of time
Describe the action potential of pacemaker cells.
Action potential
Increase in PCa2+ (L-type)
Pacemaker potential (= pre-potential)
Gradual decrease in PK+
Early increase in PNa+ (= If)
Late increase in PCa2+ (T-type)
Why do pacemaker cells spontaneously depolarise?
-> Gradual decrease in the permeability of the cell to potassium.
-> Early increase in the permeability of the cell to sodium ions.
Where in the heart are the fastest pacemaker cells found?
In the Sinoatrial node
What is the speed of the depolarisation of pacemaker cells in the Sinoatrial node?
0.5 m/s
What is the Annulus fibrosus?
Non-conducting ring of tissue
What is the function of the Annulus fibrosus?
Prevent the depolarisation immediately spreading from the atria to the ventricles.
In a healthy heart, what is the only way depolarisation spreads?
Via the Atrioventricular node
What is the speed of conduction of the Atrioventricular node?
0.05 m/s
What makes up the rapid conduction system of the heart?
-Bundles of His fibres (both right and left)
- Purkinje fibres
What do the his fibres separate into?
Purkinje fibres
Discuss how the special conducting system works.
- Pacemaker cells in the sinoatrial node spontaneously depolarising to threshold
- This threshold gradually spreads over both atria
- Atria contracts and pushes blood into the ventricles
- Depolarisation slowly spreads to the atrioventricular node, allowing the atria time to depolarise and contract.
- Depolarisation spreads very rapidly through the bundle of His and the Purkinje fibres
- Ventricular contraction takes place
- Ventricles relax
Describe the electrical potential evoked by a myocyte (muscle cell).
Very small extracellular electrical potential
What’s the results of lots of myocytes depolarising and repolarising at the same time?
The small extracellular electrical potentials summate to create large extracellular electrical waves
What are the three waves of an ECG (electrocardiogram)?
P wave
QRS complex
T wave
(alphabet :) PQRST)
Which electrical event does P wave refer to?
Atrial depolarisation.
Which electrical event does the QRS complex refer to?
Ventricular depolarisation
Which electrical event does the T wave refer to?
Ventricular repolarisation
What are some of the advantages of ECG?
Cheap
Quick
Non-invasive
What does an ECG tell us about?
- Special conducting system of the heart
- Heart’s rhythm
What’s the normal heart rate when measuring ECG?
60-100 bpm
How long is the usual PR distance of an ECG?
0.2 seconds
What might you see in 1st degree heart block?
PR interval increases so conduction from atria to ventricles is taking longer
What might you see in 2nd degree heart block?
PR interval increases over time
Conduction fails so only every second depolarisation is conducted from the atria to the ventricles.
This is shown as a QRS complex gets dropped every know and then.
What might you see in 3rd degree block?
No atrioventricular conduction
What is 3rd degree block also known as?
Complete block
How can 3rd degree block be treated?
Installing a pacemaker device into the heart
What happens in atrial flutter?
Atria depolarise and contract quicker than they should
What does arterial flutter look like on an ECG?
Several P waves before QRS complex
What is atrial fibrillation?
Failure of pacemaker
Cells depolarise and contract at different times
What does arterial fibrillation look like on an ECG?
No clear P waves
What is ventricular fibrillation?
‘He’s in V-fib, pushing one of epi :) ‘
Uncoordinated contraction in the ventricles.
What might happen after ventricular fibrillation?
Patient may lose consciousness due to lack of oxygen
How can you resolve ventricular fibrillation?
Using a defibrillator
What do defibrillators do?
Depolarise all cells at the same time so they enter their refractory periods at the same time.
