Muscle Morphology Flashcards
Three types of muscle
Skeletal, cardiac and smooth
7 muscle proteins (in spec)
Actin, myosin, tropomyosin, troponin, dystrophin, alpha-actinin, titin
Myosin structure
Large complex with two heavy chains and two pairs of light chains. The two thick chains are twisted together as myosin tails. Globular projections are present on each of the heavy chains forming the myosin heads.
Actin structure
Thin helical filaments formed of two chains of actin wrapped around each other, with a G actin monomer containing a binding site for myosin.
Tropomyosin structure
Long coil of two polypeptide chains located in the groove between the two twisted actin strands
Troponin structure
A complex of three subunits, TnT which attaches to tropomyosin, TnC which attaches to calcium ions and TnI which regulates the myosin- actin interactions
Explain the striated appearance of skeletal muscle fibres
The dark parts of myofibrils are called A bands and contain myosin. The I bands are lighter and contain only actin.
Connective tissue of the muscle (3) + location
Each muscle fibre is invested by the endomysium.
A bundle of 10-100 muscle fibres and their associated endomysium form fascicles, which are each covered in perimysium.
The individual muscles are then surrounded by the epimysium.
Structures of the three connective tissues
Endomysium- thin, reticular fibres and scattered fibroblasts
Perimysium- nerves, blood supply and lymphatic vessels
Epimysium- dense, irregular connective tissue. Septa extend inwards to allow large nerves and blood vessels to enter the muscle.
Function of connective tissues
Blood and nerve supply and continuity of connective tissues ensure that the force generated at the ends of the fibres are transmitted to the tendon at the end of the whole muscle- continuous with myotendinous junctions.
Stages of muscle name
Progenitor cells give rise to myoblasts which fuse to form multinucleate cells called myotubes.
Myotubes develop into a mature multinucleate syncytium
For muscle cells- myocytes, muscle fibres, myofibres
What are found within myofibrils?
Myofibrils composed of my-filaments, such as actin and myosin.
Sarcomere definition
Repetitive functional subunit of the contractile apparatus, extending from Z disc to Z disc.
Sarcomere structure
2.5 micrometers long, between the Z discs
Central A and Peripheral I zones due to actin and myosin respectively. A band also includes some actin.
H zone in the centre, only contains myosin.
M line in the centre of the sarcomere.
A-actinin function
Actin binding protein that attaches actin to the Z discs
Titin function
Links myosin to Z line and maintains sarcomere architecture, allowing molecular spring for passive elasticity of muscle, whilst promoting passive stiffness. (largest known protein).
Dystrophin associated glycoprotein complex function
Connects cytoskeleton of muscle fibre to surrounding extracellular matrix through the cell membrane. Helps scaffold various signalling and channel proteins.
Dystrophin structure and location
Rod shaped cytoplasmic protein. Located between the sarcolemma and outermost layer of the myofilaments in the muscle cell.
Structure of skeletal muscle fibres
Sarcoplasmic reticulum, sarcolemma with T tubules
Sarcoplasmic reticulum function
Contains pumps and other proteins for calcium sequestration. Calcium release from cistern triggered by membrane depolarisation.
Transverse tubule structure and function
Infoldings of the sarcolemma that penetrate deep into the cell that allows an action potential to travel through into the cell, enabling simultaneous contraction.
What is the triad?
Two terminal cistern adjacent to a T tubule.
Motor unit definition
All the muscle cells supplied by one motor neurone
Satellite cell function
Precursors to skeletal muscle cells that can give rise to differentiated skeletal muscle cells and or myonuclei. Key function is muscle repair.
Satellite cell location
Located between the basement membrane and the sarcolemma of muscle fibres.
Fast fibres adaptations to functions
specialised for rapid, short contractions, have few mitochondria and rely largely on anaerobic respiration. Rapid contractions lead to rapid fatigue as lactic acid builds up from glycolysis
Slow fibres adaptations to function
specialised for slow, long contractions without fatigue. Large number of mitochondria and many surrounding capillaries
Skeletal muscle repair forms
Satellite muscle cells become activated and re-enter the cell cycle, known as transit amplifying pool cells and form new myotubes. Fibroblasts may also deposit scar tissue which can impair muscle function.
What characterises cardiomyocytes?
Branching mesh of centrally located- mononuclear striated cells, joined and electrically coupled by intercalated discs
What are intercalated discs?
Presence of transverse lines that cross fibres at irregular intervals where myocardial cells join.
What junctions are present and why?
Fascia adherens and desmosomes to provide strong intercellular adhesion
Gap junctions- serve as electrical synapses producing a electrical syncytium
What connective tissue is present?
Endomysium around muscle cells, thicker layer of perimysium surrounds bundles and layers of the muscle fibres. Outer epicardium.
Cardiac sarcomere structure
Have Z lines, A and I bands
Myofilaments form a continuous mass within the cell interrupted by extensions of sarcoplasm containing more mitochondria and SR.
Dyad structure
One cisternae and one T-tubule form the dyad, where electrical excitation coupling takes place.
Larger T tubule lumen in ventral side compared to arterial.
Purkinje fibers location and function
Located in ventricular walls of the heart, just beneath the endocardium. Specialised conducting fibres formed of electrically excitable cells with fewer myofibrils and may mitochondria compared to the cardiomyocytes. Allow the heart’s conduction system to occur in a synchronised manner,
Regeneration of cardiac muscle explained
Cardiac muscle cells cannot proliferate, instead scar tissue is deposited if there is damage. They hypertrophy however, through the synthesis of extra myofibrils.
Smooth muscle characteristics
Fusiform cells that lack striations, no sarcomeres and have slow, involuntary contractions
Smooth muscle fibre structure
Long, elongated, spindle shaped each enclosed by a lamina and collagen fibres comprising the endomysium. Elongated nucleus in the broadest part.
Smooth muscle location
contractile walls of hollow organs- respiratory tract, gut, genitourinary system
Smooth muscle arrangement of cells
Single unit and multi unit
Single unit appearance and function
Behave as one, enabled by electric coupling via gap junction. Allow the muscle to behave as a syncytium
Multi-unit appearance
Each cell isolated and stimulated independently, allowing finer control of movement, such as the cilliary muscles.
Sarcoplasm structure
Contains actin and myosin filaments and a cytoskeleton. Acting binding dense plaques present, linked to intermediate cytoskeletal filaments.
Invaginations of the sarcolemma name and function
Caveoli- vesicular invaginations that increase the surface area. Possibly regulate calcium entry into the cell.
Structure of actin and myosin
bundles of thick and thin filaments criss cross the sarcoplasm. Myosin filaments have a less regular arrangement . Actin filaments not associated with tropomyosin or troponin. - arranged as a lattice
Dense bodies structure and function
anchoring cytoplasmic and plasmalemma associated dense Bodie, containing alpha actinin which anchors the actin my-filaments.
Submembranous dense bodies contain cadherins and desmosomes.
Serve as points for transmitting contractile force within cells and between cells
Where are junctions located?
Focal densities
Smooth muscle cell regeneration
Smooth muscle cells undergo mitosis and replace damaged tissue
