the molecular basis for myocardial contraction Flashcards
what are the main components of the myocardium
- contractile tissue
- connective tissue
- fibrous frame
- specialised conduction system
what does the cardiac myocyte do
1) pumping action of heart depends on interactions between contractile proteins in its muscular walls
2) the interactions transform chemical energy from ATP into mechanical work that movies blood under pressure from great veins into pulmonary artery and from the pulmonary veins into the aorta
3) the contractile proteins are activated by a signalling process called excitation - contraction coupling
4) this begins when action potential depolarises the cell and it ends when ionised calcium Ca2+ that appears within the cytosol binds to the Ca2+ receptor of the contractile apparatus
5) movement of Ca2+ into the cytosol is a passive process mediated by Ca2+ channels
6) the heart relaxes when ion exchangers and pumps transport Ca2+ uphill, out of the cytosol
structure of working myocardial cell
regular arrangement of cross striated myofibrils
these are actin and myosin
they are contractile proteins - they mediate sliding of contractile fibres (contraction) of a cell’s cytoskeleton, and of cardiac and skeletal muscle
function of the working myocardial cell do
cells that expand and contract in response to electrical impulses from the nervous system. These cardiac cells work together to produce the rhythmic, wave-like contractions that is the heartbeat
what does the plasma membrane of myocardial cell do
- regulates excitation-contraction coupling and relaxation
- separates the cytosol from extra cellular space and sarcoplasmic reticulum
where are myocardial cells found
cardiac muscle/myocardium
structure of myosin
- forms majority of THICK filament
- 2 large polypeptide heavy chains
- 4 light smaller light chains
- the polypeptides combine to form a molecule that consists of two globular heads (containing heavy and light chains) and a long tail formed by the two intertwined heavy chains.
what does myosin tail do during contraction
-the tail of each myosin molecule lies along the axis of the thick filament
and the two globular heads extend out to the sides forming cross-
bridges, which make contact with the thin filament and exert force
during muscle contraction.
structure of myosin globular head
- each globular head contains two binding sites, one for attaching to the thin filament and one for ATP.
-attached to the myosin head is an inorganic phosphate molecule (Pi) and ADP. - the ATP binding site also serves as an enzyme - an ATPase that hydrolyses the
bound ATP, harnessing its energy for contraction
structure of actin
- forms majority of THIN filament
- globular protein
- composed of single polypeptide that
polymerises with other actin monomers to form a polymer made up of two
INTERTWINED, helical chains. - these chains make up the core of the thin filament.
- each actin molecule contains a binding site for myosin
what is thin filament composed of
mainly actin
some troponin & tropomyosin
structure of tropomyosin
- elongated molecule
- 2 helical peptide chains
- occupies each of the longitudinal grooves between the two actin strands,
- overlies MYOSIN binding sites on actin
what does troponin do
- protein that changes shape when Ca2+ binds to it, when it does it
changes shape in doing so pushes the tropomyosin - EXPOSING myosin binding
sites on actin enabling contraction to occur
what is main source of energy for cardiac muscle
fatty acids
myocardial metabolism relies on free fatty acids during aerobic metabolism for efficient energy production
describe metabolism in hypoxia
hypoxia is low levels of oxygen in body tissues
during hypoxia there is no FFA (free fatty acids) metabolism,
so anaerobic metabolism occurs
what is A band
the region of the sarcomere occupied by thick and a few overlapping thin
filaments -
overall there are twice as many thin as thick filaments in the region of
filament overlap
what is I band
occupied only by thin filaments that
extend to the centre of the sarcomere from the Z lines
also contains tropomyosin and troponin
what are Z lines
two successive Z lines
defines the limits of one sarcomere.
they bisect each I band
what is the H zone
contains only thick myosin filaments
what is the M line
- in the centre of the H-zone,
comprised entirely of thick filament myosin. - Corresponds to proteins that link together the central region of adjacent thick filaments
what is the sarcomere
the basic contractile, functional unit of a myofibril
is the region between a pair of Z lines
what does the sarcomere contain (5)
2 half I-bands
1 A-band
1 H-zone
1 M-line
2 Z-lines
what is the sarcoplasmic reticulum
a membrane that surrounds the contractile proteins
consists of the sarcotubular network at the centre of the sarcomere and the subsarcolemmal cisternae
what does sarcoplasmic reticulum do
releases Ca2+ when Ca2+ binds to it ryanodine receptor
what is transverse tubular system (T tubule)
a network of interconnecting rings, each of which surrounds a myofibril.
is lined by a membrane that is continuous with the sarcolemma, so that the lumen of the T tubules carries the extracellular space toward the centre of the myocardial cell
what controls the contractile cycle (3)
- calcium ions
- troponin phosphorylation
- myosin ATPase
myosin quick summary
thick filament
hydrolyses ATP
interacts with actin
actin quick summary
thin filament
activates myosin ATP
interacts with mysoin
tropomyosin quick summary
thin filament
modulates actin - myosin interaction
troponin C quick summary
thin filament
binds Ca2+
troponin I quick summary
thin filament
inhibits actin - myosin interaction
troponon T quick summary
thin filament
binds troponin complex to thin filament
summary of myocardial contraction
- contraction of myocardium working cells involves interactions between 6 proteins:
- myosin
- actin
- tropomyosin
- troponin C
- troponin I
- troponin T
- the interactions are controlled by downhill movement of Ca2+ into cytosol (excitation-contraction coupling) and active Ca2+ transport out of the cytosol
- all of these Ca2+ fluxes are highly regulated, which provides for the changes in cardiac muscle chemistry that give rise to changes in myocardial contracility
