Muscles

Question 1

Where is the z-disk in a sarcomere? What proteins are in the disk?

Answer 1

Z-disks are located at the ends of sarcomeres and contain an actin capping protein (Cap-Z) that binds the plus or barbed end of actin filaments. In addition, Z disks contain actin crosslinking proteins that maintain regular spacing between filaments.

Question 2

What protein prevents myosin from binding actin?

Answer 2

Actin filaments are wrapped by tropomyosin that prevents myosin from binding to actin in the absence of calcium.

Question 3

What protein binds Ca++ and allows tropomyosin to release from actin?

Answer 3

Troponin

Question 4

How are myosin proteins oriented in the sarcomere?

Answer 4

Myosin filaments are bipolar with motor domains on one side of the M-line oriented toward the Z-disk and the motor domains on the other side of the M-line oriented toward the opposite Z-disk.

Question 5

Where is titan located in the sarcomere and what is its function?

Answer 5

It extends from the Z disk to the M-line and contacts the myosin filament at numerous points. Titan likely functions as a molecular spring to prevent sarcomeres from overextending

Question 6

Where are actin filaments anchored in smooth muscle?

Answer 6

Some filaments have their plus ends anchored at points in the plasma membrane termed dense plaques, whereas other filaments have plus ends anchored in dense bodies in the cytosol

Question 7

What neurotransmitter is used at skeletal NMJs to promote contraction? What kid of receptor does it bind to?

Answer 7

ACh, nicotinic AChR

Question 8

What ions cause depolarization in skeletal muscle?

Answer 8

Na+

Question 9

How does SR release of Ca++ occur?

Answer 9

Opening of sodium channels in T-tubules causes voltage-gated calcium channels in sarcoplasmic reticulum to open, increasing cytosolic calcium and initiating muscle contraction.

Question 10

Myosin filaments are also activated by increases in cytosolic calcium but by a different mechanism than in skeletal/cardiac muscle. How does this process occur in smooth muscle?

Answer 10

Myosin light chain kinase is activated by calcium and phosphorylates a small protein on myosin. Once phosphorylated, the myosin is now active and begins contraction

Question 11

How many neurons innervate each muscle cell? How many muscle cells does each neuron innervate?

Answer 11

Each muscle cells is innervated by only one motor neuron but a motor neuron can innervate many muscle cells

Question 12

Contraction places great strain on muscle cell membranes. What structure exists to stabilize them?

Answer 12

Linkages to an extracellular basal lamina via dystrophin (connected to the cytoskeleton), dystroglycan, and laminin (in the basal lamina)

Question 13

What is Duchenne muscular dystrophy?

Answer 13

Mutations in dystrophin which disrupt the connection between muscle cell cytoskeleton and basal lamina. The plasma membrane is weakened and calcium enters muscle cells leading to necrosis of muscle cells.

Question 14

Which has faster contractions: skeletal or smooth muscle? Why?

Answer 14

Skeletal; T tubules spread action potential quickly throughout the large interconnected cell.

Question 15

What is endomysium?

Answer 15

The reticular fibers that surround each muscle fiber.

Question 16

What is perimysium?

Answer 16

Collagenous tissues that group muscle fibers into fasciculi

Question 17

What is epimysium?

Answer 17

A heavy connective tissue layer that encloses numerous fasiculi into muscle

Question 18

What landmarks of the sarcomere correspond to the lengths of the actin and myosin, respectively?

Answer 18

The I-band (isotropic), and the A-band (anisotropic)

Question 19

Characterize slow twitch muscle fibers

Answer 19

Slow-twitch (type I) muscle fibers mainly rely on aerobic metabolism. They contain large amounts of mitochondria and myoglobin, an oxygen storage molecule. The reddish color of myoglobin is why these fibers may be referred to as red fibers. These muscles can maintain continuous contraction and are useful in activities such as the maintenance of posture.

Question 20

Characterize fast twitch muscle fibers

Answer 20

Fast-twitch (type II) muscle fibers employ anaerobic metabolism. These fibers contain few mitochondria and little myoglobin, but are rich in glycogen and glycolytic enzymes. The lack of myoglobin results in a paler color than the slow twitch muscles, and fast-twitch fibers may therefore be referred to as white fibers. These muscles are important for intense but sporadic contractions; for
example, those that take place in the biceps.

Question 21

What is a T-tubule?

Answer 21

An invagination of the sarcolemma along each sarcomere, allowing the depolarizatoin to come in close contact with all parts of the sarcoplasmic reticulum, allowing for concerted muscle contraction

Question 22

What is Atrial natriuretic factor?

Answer 22

A hormone released upon excess filling of the atria and opposes the action of angiotensin II in production of aldosterone