Electrical Activity of the Heart Flashcards
What can act as a large calcium store?
Sarcoplasmic reticulum
What does calcium bind to?
Troponin
Explain why cardiac muscle can act as a functional syncytium?
Cardiac muscle can function like one massive cell despite being composed of many different cells
State 3 ways in which cells are linked together.
- Desmosomes
- Intercalated discs
- Gap junctions
Desmosomes provide what type of connection?
Physical connection
Gap junctions provide what type of connection?
Electrical connection
_____ muscle can exhibit tetanus while there is no tetanus in _____ muscle
Skeletal
Cardiac
Compare the length of the action potential of cardiac muscle with that of skeletal muscle.
Cardiac - 250 ms (longer because more calcium is released)
Skeletal - 2 ms (shorter)
State why cardiac muscle cannot exhibit tetanic contraction.
Cardiac muscle has a long refractory period (ensures heart fully relaxes and prevents tetanic contraction)
Why do you not want tetanic muscle contraction in your heart?
It would mean your heart would beat once and there would not be another one causing ASYSTOLE (cardiac arrest).
What forms the intercalated discs?
Desmosomes and gap junctions
Calcium saturation troponin
LATER
What do cells which have unstable resting membrane potentials act as?
Pacemaker cells (these produce the electrical impulses that cause your heart to beat)
Graph of non - pacemaker vs pacemaker potential:
INSERT HERE
What is responsible for the resting membrane potential of non-pacemaker action potentials?
The permeability of potassium ions (PK+)
What is responsible for the initial depolarisation in non-pacemaker action potentials?
Increase in permeability to sodium ions (PNa+)
What is responsible for the plateau in non-pacemaker action potentials?
Increase in the permeability to calcium ions (PCa2+, L-type)
And a decrease in the permeability to potassium ions (PK+)
What is responsible for the repolarisation in non-pacemaker action potentials?
Decrease in the permeability to calcium ions (PCa2+)
And an increase in the permeability to potassium ions (PK+)
What is responsible for the action potential in pacemaker action potentials?
Increase in the permeability to calcium ions (PCa2+, L-type)
What is the pacemaker potential (pre-potential) due to?
- Gradual decrease in the permeability to potassium
- Early increase in the permeability to sodium
- Late increase in the permeability to calcium (T-type)
State 7 modulators of electrical activity.
- Sympathetic and parasympathetic systems
- Temperature
- Drugs
- Hyperkalemia
- Hypokalemia
- Hypercalcemia
- Hypocalcemia
Drugs: What do calcium channel blockers do to modulate electrical activity?
Decrease force of contraction
Drugs: What do cardiac glycosides do to modulate electrical activity?
Increase force of contraction
How does temperature modulate electrical activity?
Increases the heart rate by 10 beats/min/degree
How does hyperkalcemia modulate electrical activity?
Fibrillation and heart block
How does hypokalaemia modulate electrical activity?
Fibrillation and heart block (anomalous)
How does hypercalcemia modulate electrical activity?
Increased heart rate AND force of contraction
How does hypocalcemia modulate electrical activity?
Decreased heart rate AND force of contraction
List the sequence of events occurring during excitation-contraction coupling in cardiac muscle. (8)
- An action potential enters from adjacent cell
- Voltage-gated calcium channels open and calcium enter the cell
- Calcium induces calcium release form the sarcoplasmic reticulum through ryanodine receptor channels, which causes calcium to spark
- Summed calcium sparks create a calcium signal
- Calcium ions then bind to troponin, which cause tropomyosin to detach from the actin filament, exposing the actin active sites
- Contraction occurs, myosin cross bridges alternatively attach to actin and detach, pulling the actin filaments towards the centre of the sarcomere, release of ATP hydrolysis powers the cycling process
- Removal of calcium by active transport into the sarcoplasmic reticulum after the action potential ends
- Tropomyosin blockage restored on active sites, contraction ends and the muscle fibre relaxes
