Cardiomyocytes Flashcards
List the differences between skeletal and cardiac muscle
fibres are long, cylindrical shaped and run along the whole length of the muscle.
Multinucleated, nuclei at the surface (except in disease). Voluntary control.
The heart is a striated muscle made up of cardiomyocytes
Size of cells~ 30x100 μm, striations ~2 µm
Mononucleated.
Jagged cell edges allow branching.
Joined end to end via the intercalated discs.
Name the components of the intercalated disc. explain function
Three types of cell junction are present at the intercalated disc:
- gap junctions,
- fascia adherens junctions
- desmosomes.
They serve two main purposes:
- Mechanical linkage for the transmission of force (fascia adherens which connect actins, and desmosomes which connect intermediate filaments)
- Electrical and chemical connection for the flow of action potentials and for equilibration, regulation, growth and development (gap junctions)
Gap junctions are composed of connexons (one from each cell), which are in turn composed of connexins (6 per connexon).
What is the volume ratio of mitochondria and cardiomyocyte?
30-40% of the volume of a cardiomyocyte is taken up by mitochondria.
How are thick filaments formed?
The thick filaments are made of a protein myosin, which has a long straight tail with two globular heads each of which contains an ATP-binding site and an actin-binding site.
Length 150 nm.
Myosin filament has 3-fold rotational symmetry.
Crown separation is 14.3 nm, repeats after 43nm.
Myosin molecules assemble in anti-parallel fashion to form the central bare-region and assemble in parallel to form the crossbridge region on either side. 3 myosin molecules are added every 14.3 nm along a 3-strand helix (approx). The periodicity is 42.9 nm.
2 heavy chains and 2 pairs of light chains.
C-terminus: α-helical coiled-coil tail. N-terminus: folding of each heavy chain to form globular pear-shaped head.
Enzyme action defines the different parts of myosin: the myosin head S1, the rod/neck S2 and LMM, rod/tail.
What are the components of thin filaments?
The thin filaments consists of several proteins including actin—two-helical strands of polymerized subunits (g-actin) with sites that interact with the heads of myosin molecules to form cross-bridges with the thick filaments;
tropomyosin—a regulatory fibrous-type protein lying in the groove of the actin -helix, which prevents actin from interacting with myosin when the muscle is at rest;
troponin—a regulatory protein consisting of three subunits:
- troponin C – binds Ca2+ during activation and initiates the configurational changes in the regulatory proteins that expose the actin site for cross-bridge formation;
- troponin T – anchors the troponin complex to tropomyosin;
- troponin I – participates in the inhibition of actin–myosin interaction at rest.
Explain the sliding filament hypothesis
Sliding filament hypothesis
what is titin? size, location, structure
Titin is a giant 3MDa protein
- Length - 1 μm, spans half sarcomere, linking the Z-disc to the M-band.
- Titin binds to the Z-disc, M-band and thick filament, but ♦ runs free in the I-band.
Composed of a linear arrangement of ~300 100-amino acid domains which are immunoglobulin and fibronectin type 3 domains
• In the I-band it has a region composed of repeats of PEVK residues, the number varies with muscle type → provides elasticity to sarcomere
What is the role of titin?
Titin extends from the Z disk to the M line and contributes significantly to the passive stiffness of cardiac muscle over its normal working range
What is the main component of the M-band?
Myomesin is the main component of the M-band
What is the main component of the Z-disc?
the main component of the z-disc is α-actinin.
What is the difference in the Z-disc of slow or cardiac and fast muscle?
the width:
Minimum 2 layers in fast muscle (30-50nm);
Up to 6 layers (100nm) in slow and cardiac muscle.
Names the types of cells found in the heart.
vContractile cells - cardiomyocytes
vConduction cells eg Purkinje fibres
vEndothelial cells – capillaries
vFibroblasts (ECM)
Although cardiomyocytes occupy 2/3 volume of the heart, the myocytes population is 1/3 of the total number of cells. Fibroblasts are the most numerous cell-type in heart, 2/3 of the total. Every myocyte is bordered by at least 1 fibroblast.
Capillaries: Every myocyte is flanked by at least 2 capillaries
What constitutes the cardiac interstitium (ECM)?
ECM comprises non-myocyte cells (fibroblasts, endothelial cells), fibrillar collagen and blood vessels
Collagen surrounds every cell and forms inter-cell tethers
What type of cell has highest number in the heart?
Fibroblasts, although they take up less volume than cardiomyocytes
Name the main components of the conduction system and how excitation spreads
Sinoatrial node
Atrioventricular node
His fibres (left and right bundle branches)
Purkinje fibres
Cardiac excitation initiated in the sinoatrial (SA) node, cardiac pacemaker – in right atrium, and spreads to both atria via gap junctions in the intercalated discs of atrial fibres. The AP enters the atrioventricular (AV) node (bundle of His), the only electrical connection between the atria and ventricles. The AP travels down the left and right bundle branches to the apex of the heart and then along Purkinje fibres up the ventricle walls.
