Muscle Flashcards
What are the three major muscle types?
Skeletal
Cardiac
Smooth
What do muscles develop from?
Mesoderm - somites - myotome
How do myoblasts differentiate, what do they differentiate to?
Myoblasts withdraw from the cell cycle and express transcription factors MyoD and Myf5 to activate genes for myocyte formation.
The myoblasts fuse to form multinucleate myotubes which develop into mature myofibres.
What are contractile elements of all muscle?
Actin and myosin
Do all muscle cells have permanently oriented contractile machinery?
Yes
What are the role of muscles?
Change body position
Act as mechanical pumps
Move viscera
Regulate blood pressure.
What is the role of skeletal muscle?
Voluntary movement
How does skeletal muscle attach to bones?
Via tendons
Describe the nuclei number and position of skeletal muscle cells
Multinucleate with the nuclei peripherally located
Why are skeletal muscle nuclei peripherally located?
Fibrils fill the central part of the cell
What cells surround the surface of skeletal muscle cells?
Satellite cells
What is the role of satellite cells?
Stem cells with regenerative capacity
What are single skeletal muscle cells called?
Muscle fibres
What do muscle fibres contain (contraction)?
Myofibrils
What makes up a myofibril?
Myofilaments (actin and myosin)
What are T-tubules?
Invaginations of the sarcolemma that go deep into the cell to surround each myofibril. T-tubules are associated with SR cisternae.
What gives the striated banding pattern to skeletal muscle?
Thin actin filaments and thick myosin repeat along the length of the myofibril. Repeating sarcomeres
What is actin structure?
Actin is a globular protein which polymerises into a fibres, it has a myosin binding site
What is myosin structure?
6 twisted polypeptides which forms a fibrous helix with globular ends (heads) that will bind to the binding site on actin.
What is the structure of tropomyosin?
Fibre like proteins which wraps around the actin filament. Blocks the attachment side for myosin cross-bridge
What is the structure of troponin C?
Fibre like proteins which wraps around the actin filament. Blocks the attachment side for myosin cross-bridge
What is a sarcomere?
Repetitive subunits of contractile apparatus including the filaments along with the associated proteins
Describe a sarcomere?
Z-line at each end attached to actin filaments, I band is actin only, light striation
Actin overlaps with myosin filaments
Myosin alone at A-band dark striation
M line is centre
What happens to regions of the sarcomere when myofibril contracts?
Z discs move closer together
I and A band widths are reduced
How many actin surrounds each myosin?
6
What is the role of titin?
Connects the Z line to the M-line, acts as a molecular spring for passive elasticity of muscle. It is the largest known protein.
What is the largest known protein?
Titin
What is nebulin?
Actin binding protein localised to actin regulating its length.
What makes up the Z line?
Alpha actinin
What is the endomysium?
Connective tissue layer that surrounds each muscle fibre. Reticular fibres and basal lamina.
What is the perimysium?
Connective tissue layer that surrounds the cell bundles (fascicles) of muscle fibres, derived from the epimysium.
What is the epimysium?
Connective tissue layer that surrounds the entire muscle and is dense and irregular connective tissue.
What are bundles of muscle fibres called?
Fasicles
How is the cytoskeleton of muscles attached to the ECM?
By a protein complex called dystrophin-associated glycoprotein complex (DGC).
Dystrophin maintains mechanical integrity of cell during contraction by anchoring cytoskeletal elements.
What leads to muscular dystrophy?
Genetic defects in DGC proteins leads to muscular dystrophy.
As force is no longer transmitted from myocytes to bones and so cannot move at joint.
Is cardiac muscle striated?
Yes
What gives rise to the cardiac epicardium and myocardium?
Splanchnic mesenchyme and myoepicardium mantel cells
What gives rise to the endocardium?
Vascular endothelial progenitors
What are cardiomyocytes?
Specialised branched striated muscle cells
Describe the number and position of nuclei in cardiomyocytes
Single centrally located nuclei, one or two per cell
How are cardiomyocytes arranged?
Layers
What is a distinguishing feature of cardiomyocytes?
Intercalated discs
What are intercalated discs?
Darkly stained transverse lines that are regions of complex cell junction networks
What is the role of intercalated discs?
Support synchronised contraction
What are the three elements of intercalated discs?
Fascia adherens - Anchoring sites between cells of actin filaments.
Desmosomes - Cell to cell adherens junctions - using desmin filaments
Gap junctions - Provide chemical continuity between cells allowing ions to pass through. Bind cells together with connexins that form connexons.
What do fascia adherens do?
Anchoring sites between cells of actin filaments.
What are purkinje fibres?
Specialised cardiac muscle to enable synchronisation of vesicle contraction.
How do purkinje fibres look different to cardiomyocytes?
Stain much paler as they contain less contractile elements, no t-tubule system and more mitochondria and glycogen.
What is the innervation of cardiac muscle?
Autonomic
What is the innervation of skeletal muscle?
Somatic
What is the innervation of smooth muscle?
Autonomic
Describe the shape of smooth muscle cells?
Elongated, spindle shaped (fusiform) shape
Are smooth muscle cells striated?
No
Describe the nuclei of smooth muscle cells
Centally located, mononucleate
What are the two ways smooth muscle can act?
Unitary syncytium with gap junctions for mass control
Multiunit (isolated) for finer control
Why is smooth muscle non-striated?
Not aligned in parallel/regular arrays
Instead lattice like network
What are distinguishing features of smooth muscle?
Focal densities
Dense bodies
Caveolae
Gap junctions
What are focal densities?
Attachment junctions, actin to the sarcolemma.
What are dense bodies?
Attachments between intracellular actin filaments (like the Z-line) maintain the alignment of thin filaments.
What are caveolae?
Depression of the plasma membrane in the system, act a bit like t-tubules involved in transport of the depolarisation signal.
What muscle cell type has the greatest regenerative capacity?
Smooth muscle
Specialised structures in skeletal muscle cells found roughly at the boundary of the A and I bands
Triads
During contraction, which of the following remains constant in length?
A band
