Mechainics For Myocardial Cotraction Flashcards
What does the myocardium consist of?
Contractile tissue
Connective tissue - keeps contractile tissue together
Fibrous frame - sits in between filling and pumping chambers, surrounds and holds valves together.
Specialised conducting system
What is a cardio myocyte?
It is a specialised cardiac muscle cell.
How are the contractile proteins activated in a cardiac myocyte?
By a signalling process known as excitation-contraction coupling
What is excitation-contraction coupling?
It begins when the action potential depolarises the cell and ends when Ca2+ appears in the cytosol and binds to Ca2+ receptors of the contraction apparatus.
The movement of Ca2+ into the cytosol is a passive process mediated by Ca2+ channels.
The heart relaxes when ion exchanges and pumps transport Ca2+ out of the cytosol via active transport.
Facts about cardiac muscle cells:
Striated muscle
Myogenic
Intercalated discs with gap junctions and desmosomes
Central nuclei
Narmoxaemic myocardial metabolism:
Myocardial metabolism under normal oxygen levels.
The most important heart substrate is fatty acids.
When there is not enough oxygen to breakdown fatty acids it will start to an anaerobically respire glucose producing lactate - during ischaemia
What is myosin?
What is actin?
What is the A band?
What is the z line?
What is the I band?
Thick fibrous protein involved in muscle contraction.
Protein that binds together with myosin to cause contraction.
The A band = where the myosin and actin overlaps - appears dark on an electron microscope image.
Z line = the lateral boundaries of a sarcomere that anchors the myosin via titin.
I band = the part of the sarcomere that contains thin filaments only.
H zone = part of the sarcomere that contains myosin only.
Sarcomere =
Sarcoplasmic reticulum =
Sarcolemma =
Sarcoplasm =
Sarcomere = the functional unit of the contractile apparatus between:
Z line — I band — A band (with ho zone in middle) — I band — Z line
Sarcoplasmic reticulum = membrane network that surrounds the contractile proteins. Responsible for the cytoplasmic concentration of Ca2+.
Sarcolemma = membrane of the muscle cell
Sarcoplasm = cytoplasm of a muscle cell
What is the transverse tubular system?
The function of T-TUBULES is to conduct impulses from the surface of the cell (SARCOLEMMA) down into the cell and, specifically, to another structure in the cell called the SARCOPLASMIC RETICULUM.
Cardiac transverse tubules (t-tubules) are highly branched invaginations of cardiomyocyte sarcolemma that are rich in ion channels important for excitation-contraction coupling. They synchronise the release of Ca2+ and hence synchronise contraction.
Myosin ultra structure:
Made up of 2 heavy chains, 4 light chains.
At rest the myosin heads are perpendicular on the thick filament.
What is Tropomyosin and what is its function?
It is an elongated molecule made up of 2 helical peptide chains.
It occupies each of the longitudinal grooves between the 2 actin strands.
It stops myosin and actin cross bridges from forming and changes shape due to the release of calcium.
What are the three types of troponin and what is their function?
I - along with Tropomyosin it inhibits actin and myosin interaction
T - binds with troponin complex to Tropomyosin
C - has a high affinity for Ca2+, this bond drives troponin I away from actin allowing interaction with myosin. It also causes re-alignment of Tropomyosin in the groove which exposes actin to the myosin heads therefore contraction can take place.
Troponin is released into the blood during a heart attack
Overview of the sliding filament mechanism:
Actionable potential travels to the muscle via the neuromuscular junction
Opens Ca2+ channel which binds to troponin C, driving troponin I away and also realigning Tropomyosin in the groove which exposes actin to myosin.
ATP binds to the myosin head and is hydrolysed to ADP causing it to move and bind with the actin, when the ADP is realised it pulls on the actin causing muscle contraction (power stroke). The process is then repeated.
In other word the sarcomere shortens - 2 z lines get closer together.
What are the contractile proteins so the heart?
Myosin - thick filament - hydrolyses ATP via ATPase - and interacts with actin
Actin - thin filament - interacts with myosin
Tropomyosin - thin filament - modulates actin, myosin interactions
Troponin C - thin filament -binds Ca2+
Troponin I - thin filament - inhibits actin- myosin interaction
Troponin T - thin filament - binds troponin complex to thin filament
Summary of contraction o the myocardium:
It involves 6 proteins - myosin, actin, Tropomyosin and the 3 troponin molecules.
These interactions are controlled by the movement of Ca2+ into the cytosol (excitation contraction coupling) and then Ca2+ transport out of the cytosol.
