Mechanisms Of Cardiac Myocytes Flashcards
What is the myocardium composed of
Contractile tissue
Connective tissue
Fibrous frame
Specialised conduction system
What is a myocyte
This is the cell of the muscle of the heart
What does the pumping action of the heart depend on
The pumping action depends on the interactions made between the contractile proteins in the muscular walls of the heart
Where do the interactions occur in the muscular heart wall
Between the muscle fibrils
What do the interactions between the fibrils in the muscle wall do
The interactions between the fibrils in the muscle wall transforms the chemical energy from the ATP into mechanical work which can then move blood under a high pressure
What excites the myocardium of the heart
The electrical impulse which is sent from the sinoatrial node of the heart is which activates the contractile proteins in the muscle wall by signalling
What is excitation-contraction coupling
The electrical impulse is what excites the myocardium
The mechanical impulse is the contraction
Explain the process of excitation-contraction coupling
This process begins with the action potential depolarising the cell membrane, a passage of calcium ions from outside the cell and from internal stores into the cytosol to bind with the Ca2+ receptors within the myofibrils which creates a connection between the the fibrils which allows then to glide between each other and cause a contraction. The cell then uses enters by breaking a high energy phosphorus bond to remove the calcium from the myofibrils and put it back into the internal stores in the cytoplasm
Characteristics of a myofibril in the ventricle
Striated muscle
Has intercalated discs
Nucleus is present
What are features of a myocardial cell
Myofibrils
Mitochondria
Tubules of the cellular membrane
Reticulum which carries the free calcium
What fibrils are found in the myofibrils of the myocardial cell
Actin (thin fibrils)
Myosin (thick fibrils)
Where do the heads of the myosin sit on the sacromere
They sit at every level every 40 degrees
What do the heads of the myosin do
They heads of the myosin interact with a section of the actin
When can the heads of the myosin interact with the actin
When calcium arrives which links to troponin-C which lifts off the collection of troponin- T,I,C off the actin, the heads of the myosin can interact
What does the interaction between the myosin and the actin produce
This will produce shortening of the sarcomere which will produce a contraction of the muscle fibre
What is titin
This s protein which keeps the whole section of the sarcomere tight and together and stops it from stretching too much
What happens when titin is defective
The muscle of the heart will stretch and not be able to recoil causing dilated cardial myopathy and heart failure
What is a genetic disorder which can lead to heart failure
Defective titin protein causing dilated cardial myopathy
On a length of actin which is attached to the tropomyosin
Troponin T
Troponin I
Troponin C
What happens to the troponin complex when calcium binds to it
When calcium binds to troponin C the troponin complex including Troponin I, T and C will lift of the tropomyosin
When the troponin complex lifts what does this allow
This allows interaction of the heads of the myosin
What is troponin I’s function
Troponin I works with tropomyosin to inhibit actin and myosin interacting
What is the function of Troponin T
Troponin T binds the troponin complex to tropomyosin
What is the function of troponin C
Troponin C has a high affinity to calcium, which then can start the signal for contraction
What is a sacromere made up of
The sarcomere is made up of 2 light bands of actin, 1 dark band which is made up of both actin and myosin during contraction but mostly myosin during relaxation
This means a sarcomere is made up of 2 1/2 I bands and 1 A band (this is what causes the striated muscle)
What is myosin and what are its properties
Myosin is a protein found in the thick filaments of the heart which hydrolyses ATP and interacted with actin
What is actin and what are its properties
Actin is a protein located in the thin filaments of the heart which activates myosin ATP and interacted with the thick filament myosin
What is tropomyosin and what does it do
This is a protein found in the thin filament of the heart which modulates the actin and myosin interactions when calcium is present
What are the 6 proteins which the contraction of the myocyte involves
Myosin of the thick filament interacting with actin of the thin filament via tropomyosin due to the lifting of troponin C, I and T due to the binding of calcium to protein Troponin C
How is the interactions of actin and myosin controlled
By the downhill movement of Ca2+ into the cytosol which begins excitation-contraction coupling and active transport of Ca2+ out of the cytosol
When does the heart relax
The heart relaxes when the ion exchangers and pumps transport calcium uphill and out of the cytosol
What does myocardial metabolism rely on
It relies on free fatty acids during aerobic metabolism to have efficient energy production
What happens to myocardial metabolism during hypoxia
There would be no free fatty acids available so anaerobic metabolism would begin relying on metabolising glucose anaerobically to produce sufficient energy so the muscle can carry on contracting
How is myosin arranged
Myosin has 2 heavy chains and 4 light chains
What type of protein is actin and what shape is it
Actin is a globular protein which forms a double stranded macro molecular helix to form F actin
What is tropomyosin made up of and where is it found
Tropomyosin is a elongated molecule which is made up of 2 helical peptide chains which reside in the groove between the 2 actin strands
How does a skeletal muscle contract
A skeletal muscle contraction starts at the neuromuscular junction where an action potential arrives and stimulates the release of acetylcholine. The acetylcholine binds to the receptors on the post-muscle membrane. This stimulates sodium to enter the muscle cell which creates an end plate potential. The end plate potential will then reach a threshold which will produce a new action potential which will propagate along the muscle membrane and down t-tubules which stimulates the calcium Chanels to open along with the ryanodine receptor to allow stored calcium to exit the scaroplamic reticulum. The calcium then binds to troponin C which stimulates the Troponin complex to move off the tropomyosin and allows myosin heads to interact with the actin. The myosin heads then bend at the hinge region and causing the thin filaments to be dragged along. The sliding of the thing filaments over the thick filaments causing a shortening of the sacromere which results in the muscle contraction
How does a skeletal muscle relax
When the muscle contraction has occurred the ATP receptor on the sarcoplasmic reticulum pumps calcium back in to the intracellular stores reducing the calcium levels and causes the muscle to relax
How does the cardiac muscle contract
The sinoatrial node sends the action potential to the atrioventricular and ventricular fibres. This stimulates calcium channels on the membrane of the t-tubules to open so calcium can enter the scarcoplamic reticulum, this increases intracellular calcium levels but this is not enough to envoke a contraction within the muscle. The rise of intracellular calcium levels induces the calcium from the internal stores to be released, this is known as calcium induced calcium release. 2 sources of calcium then binds to troponin C to cause the troponin complex to lift so the actin and myosin heads can bind and shorten the sacromere and cause the muscle to contact
How does the cardiac muscle relax
At the end of the plateau of calcium entering the sacromere the calcium ions are pumped out and back into the sarcoplamic reticulum and the extracellular fluid, this causes sodium ions to enter the cell and then get pumped out via the sodium potassium pump which causes the muscle to relax
What increase contractility in cardiac muscle
Increased Ca2+ ions would result in bigger muscle contractions due to more calcium binding to troponin C
What is increased contractility an example of
Positive inotropic effects
