Muscle Tissue

Question 1

What is a myoblast?

Answer 1

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.

Question 2

How are skeletal muscle cells formed?

Answer 2

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.

Question 3

What are myosatellite cells? What is their function?

Answer 3

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.

Question 4

What forms cardiac muscle cells?

Answer 4

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.

Question 5

What makes up smooth muscle?

Answer 5

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

Question 6

What surrounds all types of muscle cells?

Answer 6

External (basal) lamina

Question 7

How many nuclei does each muscle type have?

Answer 7

Skeletal - multinucleated
Cardiac - mono-nucleated or binucleated
Smooth - mono-nucleated

Question 8

What is a myocyte?

Answer 8

A skeletal muscle cell, also called a muscle fiber. Each muscle fiber is composed of myofibrils, which are long, tubular, striated elements.

Question 9

What makes up a myofibril?

Answer 9

Repeating subunits of sarcomeres. Each sarcomere is composed of parallel arrays of actin and myosin filaments.

Question 10

What are the three connective tissue layers surrounding muscle cells?

Answer 10

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

Question 11

What goes to every myocyte?

Answer 11

Each myocyte is surrounded by an external lamina and is innervated by a single axon.

Question 12

What makes up the gross structure of a skeletal muscle cell?

Answer 12

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)

Question 13

What are the three bands clearly visible in striated muscle by H&E?

Answer 13

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.

Question 14

what defines every sarcomere and what do they make up?

Answer 14

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

Question 15

What proteins keep adjacent myofibrils aligned?

Answer 15

Desmin and vimentin are intermediate filaments which link adjacent myofibrils toegher at the Z-line, and bind them to the plasma membrane.

Question 16

What links actin filaments of sarcomere to the Z-disc?

Answer 16

alpha-actinin - It is cross-linking actin. Same protein used in stereocilia for the terminal web.

Question 17

What is the M-line?

Answer 17

Myosin filament bundles making up the thick filament A-band are anchored in the center of the sarcomere via the protein myomesin.

Question 18

What is the structure of a myosin molecule?

Answer 18

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.

Question 19

What is titin?

Answer 19

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

Question 20

How is a myosin filament made?

Answer 20

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.

Question 21

What is the function of tropomyosin?

Answer 21

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

Question 22

What is the function of troponin?

Answer 22

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

Question 23

How does a powerstroke work?

Answer 23

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.

Question 24

What causes rigor mortis?

Answer 24

Lack of ATP disallows muscles to relax, since ATP is required to break myosin - actin complex.

Question 25

What important organelles and inclusions lie between myofibrils?

Answer 25

Mitochondria - for ATP production Glycogen - for glucose storage Smooth endoplasmic reticulum - sarcoplasmic reticulum, for sequestering calcium ions to regulate contraction

Question 26

What calcium binding protein is used by the sarcoplasmic reticulum?

Answer 26

calsequestrin

Question 27

What is the function and orientation of T-tubules?

Answer 27

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)

Question 28

What channels increase the amount of calcium affecting muscle cells?

Answer 28

Voltage-gated dihydropyridine receptor in T-tubules opens calcium channels (Ryanodine) in the SR, releasing calcium ions into the cytoplasm

Question 29

What provides a backup supply of ATP if mitochondria cannot produce enough?

Answer 29

Phosphocreatine

Question 30

How is a muscle cell returned to rest following contraction?

Answer 30

Ca+2-ATPase membrane proteins on the SR pump Ca+2 back in

Question 31

What is the function of dystrophin?

Answer 31

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

Question 32

How do reticular fibers link a myofibril to a bone?

Answer 32

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

Question 33

What is muscular dystrophy?

Answer 33

Defects or absence of dystrophin results in a progressive weakening of skeletal muscles

Question 34

What type of muscle cell is most likely to contain lipofuscin granules?

Answer 34

Cardiac muscle, because they are so long-lived

Question 35

What organelle is very large in cardiac muscle compared to skeletal muscle?

Answer 35

Mitochondria - they can extend the full length of the sarcomere

Question 36

Where are nuclei located in cardiac muscles?

Answer 36

Centrally located, organelles near the poles of elongated nuclei

Question 37

Why are there dyads rather than triads typically in cardiac muscle?

Answer 37

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

Question 38

What is an intercalated disc?

Answer 38

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

Question 39

What are the three parts of the intercalated disc?

Answer 39

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

Question 40

What are the Purkinje fibers?

Answer 40

Specialized cardiac muscle cells, conducting signals to regulate contraction. Uses gap junctions of intercalated discs heavily to spread this charge

Question 41

What are caveolae?

Answer 41

Spherical / omega shaped vesicles found along plasma membrane of smooth muscle thought to correspond to SR and T-tubules of striated muscle

Question 42

What is the organization of organelles / nuclei in smooth muscle?

Answer 42

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.

Question 43

What is the function of the electron dense patches / cytoplasmic densities in smooth muscle?

Answer 43

They anchor bundles of actin and intermediate filaments via alpha-actinin.

Question 44

How does smooth muscle contraction work?

Answer 44

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.

Question 45

How are smooth muscle actin and myosin different than skeletal muscle?

Answer 45

besides being disorganized, smooth muscle actin lacks troponin, and myosin filaments are organized in smaller bundles

Question 46

What is the function of calmodulin?

Answer 46

It binds calcium in smooth muscle, and activates myosin light chain kinase

Question 47

How does the efficiency of smooth muscle compare to skeletal muscle?

Answer 47

It is much slower than skeletal (may take a second or more), but is very efficient, using only 1/5 as much ATP.

Question 48

What are the junctions between smooth muscle cells?

Answer 48

Gap junctions

Question 49

Can smooth muscle cells be regenerated?

Answer 49

Yes, throughout life

Question 50

What is the function of integrin in smooth muscle tissue?

Answer 50

A transmembrane receptor, binds smooth muscle cytoskeleton to ECM, They can sense forces on ECM from surrounding smooth muscles and cause contraction as well.

Question 51

What tissue surrounds cardiac muscle?

Answer 51

Epicardium and endocardium

Question 52

What tissue surrounds smooth muscle?

Answer 52

Connective tissue