7-muscle Flashcards
(18 cards)
Describe EC coupling in skeletal muscle
“1. The action potential propagates along the myofibril membrane (sarcolemma) and the T-tubules.
2. Depolarisation activates dihydropyridine receptors causing a conformational change in the channel protein.
3. This conformational change is transmitted to ryanodine receptors (RyR) on the SR, resulting in their opening and Ca2+ release from intracellular stores.
4. Thus depolarisation leads to an increase in intracellular Ca2+
”
How is muscle contraction initiated?
By an increase in cystolic calcium concentration
“
Skeletal muscle cells maintain a low cytosolic Ca2+ (below 100 nM) due to the actions of a Ca2+-ATPase that continuously pumps Ca2+ from the cytosol into the sarcoplasmic reticulum (SR).
Describe the sliding filament theory
“In the presence of Ca2+, there is a slight movement of the troponin molecule from the tropomyosin chain
This movement exposes a myosin binding site on the surface of the actin chain
The ‘charged’ myosin heads bind to the tropomyosin on the actin filament
The binding and subsequent discharge of ADP causes the myosin head to pivot (the ‘power stroke’), pulling actin filament towards the centre of the sarcomere
Binding of ATP then releases the myosin head from the actin filament
Hydrolysis of the ATP molecule provides the energy to ‘recharge’ the myosin head
Describe ec coupling in cardiac muscle
““The action potential propagates along the cardiomyocyte membrane and the T-tubules
Depolarisation opens voltage-gated Ca2+ channels (VGCCs) allowing Ca2+ influx
Within the cardiomyocyte the Ca2+ has three main effects:
i. Causes Ca2+ induced Ca2+ release (CICR) by binding to ryanodine receptors (RyR) on the SR
ii. Initiates contraction by binding to troponin
iii. Causes further depolarisation
Thus depolarisation leads to increased intracellular Ca2+ and muscle contraction.”
Describe ec coupling in smooth muscle
“1. Depolarisation activates VGCCs which allows the influx of Ca2+ into the cells
2. The Ca2+ binds to the intracellular protein calmodulin (CaM) forming a complex
3. This Ca2+-CaM complex activates myosin light chain kinase (MLCK)
4. MLCK phosphorylates the myosin light chains allowing them to form cross-bridges with the actin filaments resulting in contraction
”
“Cardiac muscle: explain the structure, physiology and function of cardiac muscle, and compare with skeletal muscle.
“Cardiomyocytes are connected to each other at specialised regions known as intercalated disks, containing numerous gap junctions.
Depolarisation activates voltage-gated Ca2+ channels (VGCCs) which allows the influx of Ca2+. The Ca2+ binds to ryanodine receptors (RyR) on the SR, causing CICR.
”
What is an antagonistic pair made up of?
Flexor, extensor
What are the 2 types of isotonic contraction?
Concentric, eccentric
Describe the ultra structure of a muscle fibre
Myofibre bundle, myofibres,myofibrils.
What is the SR?
Sarcoplasmic reticulum, a network of calcium stores around the myofibril
What are t tubules?
Membrane invaginations in myofibril, in contact with extra cellular fluid
What are the sacromere components.
Z line Myosin Actin Nebulin Titin Tropomysin Cap z Tropomodulin
How does tension relate to load in isotonic and isometric contraction?
Isotonic: tension>load
Isometric: tension=load
What is the difference in ec coupling between skeletal and cardiac muscle
DIFFERENCE: THERE IS NO CONTACT BETWEEN VGCC and RyR
In cardiac The Ca2+ then binds to the RyR and causes CALCIUM INDUCED CALCIUM RELEASE (CICR)
How is contraction different in smooth muscle
Depolarisation activates and open the VGCC (slightly different from the VGCC in cardiomyocytes) causing Ca2+ influx
• The calcium moves into the cell and binds to Calmodulin (CaM) forming a Ca2+- CaM complex
• Ca2+-CaM complex activates Myosin Light Chain Kinase (MLCK)
• MLCK phosphorylates myosin light chains (MLC20)
• This changes the appearance of smooth muscle cells from elongated to
contracted
• This leads to vasoconstriction
What is the sarcolemma
The surface membrane of the muscle fibre
Describe actin
“A polymeric thin filament protein that is composed of two twisted α-helices. Along with the actin another rope-like molecule called tropomyosin forms a chain along each actin filament and associated with the tropomyosin is another protein, troponin.
”
Summarise skeletal muscle contraction
nerve impulse causes AP to travel along the surface membrane of the muscle cell
2) this causes membrane depolarisation
3) depolarisation leads to Ca2+ release from the intracellular Ca2+ stores
4) Ca2+ binds to troponin-C (TN-C, a protein in thin filaments) and a conformational change in troponin (TN) causes a shift in the position of tropomyosin.
5) tropomyosin (in the presence of Ca2+), moves into groove in the thin filament and allows myosin to interact with myosin-binding site on actin. NB in relaxed muscle, tropomyosin prevents interaction of myosin and actin.
6) relaxation: the sarcoplasmic reticulum (SR) Ca-ATPase pumps Ca2+ back into the SR continuously. Once t-tubule membrane has been repolarised, Ca2+ channels in SR close. Once Ca2+ release from the SR stops, net Ca2+ uptake into the SR occurs and Ca2+ comes off TN-C.
