Muscle Tissue Flashcards
What is a myoblast?
The muscle stem cell that forms from the mesodermal layers of the mesodermal layers of the embryo. They lack myofilaments, but contain many free ribosomes that will be used to synthesize them.
How are skeletal muscle cells formed?
They develop from myoblasts and become striated, and under control of somatic nervous system. Myoblasts will also become an undifferentiated cell type called satellite cells.
What are myosatellite cells? What is their function?
They are undifferentiated myoblasts which are located between the plasma membrane and external lamina of striated muscle. They allow for limited regeneration of skeletal muscle.
What forms cardiac muscle cells?
Mesenchymal cells from mesodermal embryonic connective tissue. They will become striated muscle but are branched. It is autorhythmic and will contract without nervous input, but is modulated by parasympathetic & sympathetic nervous systems. Capable of limited regen.
What makes up smooth muscle?
Derived from mesenchymal myoblasts, composed of nonstriated muscle, spindle-shaped (fusiform) cells. Usually found arranged in sheets around tubular organs, they are capable of moderate regeneration
What surrounds all types of muscle cells?
External (basal) lamina
How many nuclei does each muscle type have?
Skeletal - multinucleated
Cardiac - mono-nucleated or binucleated
Smooth - mono-nucleated
What is a myocyte?
A skeletal muscle cell, also called a muscle fiber. Each muscle fiber is composed of myofibrils, which are long, tubular, striated elements.
What makes up a myofibril?
Repeating subunits of sarcomeres. Each sarcomere is composed of parallel arrays of actin and myosin filaments.
What are the three connective tissue layers surrounding muscle cells?
Epimysium - thick, dense irregular connective tissue covering
Perimysium - Collagenous tissue which makes up the bundles (fascicles) of muscle cells walled off by septae
Endomysium - loose, highly vascularized connective tissue matrix containing type 1 and type 3 (reticular) collagen
What goes to every myocyte?
Each myocyte is surrounded by an external lamina and is innervated by a single axon.
What makes up the gross structure of a skeletal muscle cell?
They are unbranched, cylindrical, taper at their ends, and may be up to a few cm long. Nuclei are located just below plasma membrane near the noncontractile organelles (Golgi, rough ER)
What are the three bands clearly visible in striated muscle by H&E?
I-band = LIght band, lightly stained, formed by the empty regions filled with uncoiled titin
A-band = dArk band, dark stained, formed by myosin thick filaments running parallel
Z-line or Z-disc = line running through the middle of the I-band, which represents the actin backbone that forms the borders of the sarcomere.
what defines every sarcomere and what do they make up?
A sarcomere is a repeated pattern of Z-band to Z-band. Put back to back, they make up the myofibril, which fills the sarcoplasm (cytoplasm) of myocytes
Each sarcomere has an A-band and two half I-bands
What proteins keep adjacent myofibrils aligned?
Desmin and vimentin are intermediate filaments which link adjacent myofibrils toegher at the Z-line, and bind them to the plasma membrane.
What links actin filaments of sarcomere to the Z-disc?
alpha-actinin - It is cross-linking actin. Same protein used in stereocilia for the terminal web.
What is the M-line?
Myosin filament bundles making up the thick filament A-band are anchored in the center of the sarcomere via the protein myomesin.
What is the structure of a myosin molecule?
Two heavy chain proteins make up the helical backbone, with 2 globular heads each having two light chains. There is an ATP binding site, and actin binding site, and an ATPase.
What is titin?
A long spring-like protein which runs between the Z-line and the M-line. It helps center the myosin filaments in the sarcomere, and uses its spring to contribute to relaxation after contraction stops
How is a myosin filament made?
Much like actin, myosin filaments will come together spontaneously from myosin molecules. Half a filament is attached at the M line, and makes half an A-band.
What is the function of tropomyosin?
It is a long, rod-shaped protein which covers the actin filaments of sarcomeres. It is bound by troponin, and functions to prevent actin / myosin interaction
What is the function of troponin?
It is a protein that has multiple domains. It has a calcium binding domain as well as tropomyosin binding domain. When calcium is bound, the configuration of the actin-tropomyosin complex changes to reveal actin’s myosin binding site
How does a powerstroke work?
Myosin heads bind to the actin. ATP is required to break the interaction between actin and myosin. ATP charges myosin head to interact further down the actin, and uses phosphate to supply powerstroke that moves actin filaments towards to M-line. It is rotational movement.
What causes rigor mortis?
Lack of ATP disallows muscles to relax, since ATP is required to break myosin - actin complex.
What important organelles and inclusions lie between myofibrils?
Mitochondria - for ATP production
Glycogen - for glucose storage
Smooth endoplasmic reticulum - sarcoplasmic reticulum, for sequestering calcium ions to regulate contraction
What calcium binding protein is used by the sarcoplasmic reticulum?
calsequestrin
What is the function and orientation of T-tubules?
Transverse tubules - they number 2 per sarcomere and run perpendicular to the sarcomeres at the junction of A / I bands. They are invaginations of the plasma membrane to conduct electrical activity into the depths of the muscle cell. They closely contact with dilations of the SR (triad on TEM)
What channels increase the amount of calcium affecting muscle cells?
Voltage-gated dihydropyridine receptor in T-tubules opens calcium channels (Ryanodine) in the SR, releasing calcium ions into the cytoplasm
What provides a backup supply of ATP if mitochondria cannot produce enough?
Phosphocreatine
How is a muscle cell returned to rest following contraction?
Ca+2-ATPase membrane proteins on the SR pump Ca+2 back in
What is the function of dystrophin?
It is a peripheral protein that links actin of the myofibril to the plasma membrane. A dystroglycan complex will ultimately like dystrophin to the extracellular laminin
How do reticular fibers link a myofibril to a bone?
After dystrophin has been linked to laminin, they link the external lamina to the collagenous connective tissue of the tendon. That connective tissue is continuous with the periosteum of the bone and the Sharpey’s fibers of the bone it anchors into
What is muscular dystrophy?
Defects or absence of dystrophin results in a progressive weakening of skeletal muscles
What type of muscle cell is most likely to contain lipofuscin granules?
Cardiac muscle, because they are so long-lived
What organelle is very large in cardiac muscle compared to skeletal muscle?
Mitochondria - they can extend the full length of the sarcomere
Where are nuclei located in cardiac muscles?
Centrally located, organelles near the poles of elongated nuclei
Why are there dyads rather than triads typically in cardiac muscle?
Triads are in skeletal muscle, and formed by T-tubule between two terminal cisternae of the SR. There is much less SR in cardiac muscle, so typically T-tubule only aligns with one SR dilation
What is an intercalated disc?
A junctional complex found only in cardiac muscle. It is seen as a dark band between cardiac myocytes and consists of the fascia adherens, desmosome, and gap junction
What are the three parts of the intercalated disc?
Fascia adherens - dense plaque where actin filaments insert (anchors myofibril to end of cell)
Macula adherens - uses desmin and vimentin to bind adjacent cells together
Gap junctions - provide electrical continuity between adjacent cells
What are the Purkinje fibers?
Specialized cardiac muscle cells, conducting signals to regulate contraction. Uses gap junctions of intercalated discs heavily to spread this charge
What are caveolae?
Spherical / omega shaped vesicles found along plasma membrane of smooth muscle thought to correspond to SR and T-tubules of striated muscle
What is the organization of organelles / nuclei in smooth muscle?
They are single centrally located nuclei, with the Golgi and rough ER found near the poles of the nuclei. There are also gap junctions between adjacent cells.
What is the function of the electron dense patches / cytoplasmic densities in smooth muscle?
They anchor bundles of actin and intermediate filaments via alpha-actinin.
How does smooth muscle contraction work?
Intracellular Ca+2 rises (probably from caveolae in plasma membrane). calcium binds to calmodulin, which activates myosin light chain kinase. Once the light chains on the globular heads of the myosin molecules are phosphorylated, the actin binding site becomes exposed and contraction proceeds.
How are smooth muscle actin and myosin different than skeletal muscle?
besides being disorganized, smooth muscle actin lacks troponin, and myosin filaments are organized in smaller bundles
What is the function of calmodulin?
It binds calcium in smooth muscle, and activates myosin light chain kinase
How does the efficiency of smooth muscle compare to skeletal muscle?
It is much slower than skeletal (may take a second or more), but is very efficient, using only 1/5 as much ATP.
What are the junctions between smooth muscle cells?
Gap junctions
Can smooth muscle cells be regenerated?
Yes, throughout life
What is the function of integrin in smooth muscle tissue?
A transmembrane receptor, binds smooth muscle cytoskeleton to ECM, They can sense forces on ECM from surrounding smooth muscles and cause contraction as well.
What tissue surrounds cardiac muscle?
Epicardium and endocardium
What tissue surrounds smooth muscle?
Connective tissue
