Lecture 3: Striated Muscle

Question 1

What is muscle?

Answer 1

complex tissue composed of myocytes (muscle cells) and many other types of cells

• myocytes are contractile – contractile apparatus uses actin and myosin

Question 2

How are muscle types classified?

Answer 2

mostly based on microscopic appearance, but can also be categorized using other characteristics

Question 3

What are the two main types of muscle?

Answer 3

• striated muscle: skeletal and cardiac
• smooth muscle

Question 4

Which muscle types are voluntary? Which muscle types are involuntary?

Answer 4

• voluntary muscle: skeletal
• involuntary muscle: cardiac and smooth

Question 5

What are thick filaments?

Answer 5

myosin

• polymers of approximately 300 myosin II hexamers (2 myosin II motor and 4 light chains)
• typical vertebrate myosin thick filament is 1.6 μm with an axial helical repeat of 429 Å

Question 6

How do thick filaments assemble?

Answer 6

myosin molecules assemble in a manner that creates a bipolar filament with a central bare zone

Question 7

What are thin filaments?

Answer 7

actin

• thin filaments are polymers of alpha-actin (while microfilaments are polymers of beta-actin)
• capped by special proteins at the ends to stabilize structure
• associated with several structural proteins (ie. troponin and tropomyosin)

Question 8

Which filaments are troponin and tropomyosin associated with? What do they do?

Answer 8

• associated with thin filaments
• regulate interaction between actin and myosin in striated muscle

Question 9

What is tropomyosin?

Answer 9

long, thin, double-stranded protein that extends over about 7 actin monomers

Question 10

What is troponin?

Answer 10

trimer of TnC, TnI, and TnT that binds to every 7th actin on the thin filament

• TnC: binds Ca2+ (calcium sensor)
• TnI: inhibitory, binds troponin to actin
• TnT: binds troponin to tropomyosin

Question 11

What are sarcomeres?

Answer 11

functional unit of a muscle fibre

• thick filament surrounded by an array of thin filaments, in association with structural proteins
• in vertebrate striated muscle, thick filament is surrounded by 6 thin filaments on either end

Question 12

What is the Z-disk?

Answer 12

protein plates at the end of the sarcomere where the (+) end of actin thin filaments are attached

• composed of actin, titin, and other proteins

Question 13

What is the M-line?

Answer 13

centre of the sarcomere between (-) ends of actin

• region where thick filaments do not overlap with thin filaments

Question 14

What is the A-band?

Answer 14

region where thick filaments occur

Question 15

What is the I-band?

Answer 15

portion of thin filaments that does not overlap with thick filaments

• spans the Z-disk

Question 16

What is the H-zone?

Answer 16

portion of thick filaments that does not overlap with thin filaments

Question 17

What is titin?

Answer 17

protein that anchors thick filament to Z-disk and provides elasticity

Question 18

What is nebulin?

Answer 18

protein that twists around thin filament to establish the appropriate length for the filament

Question 19

What is tropomodulin?

Answer 19

protein that caps the (-) end of thin filament

Question 20

What is capZ?

Answer 20

protein that anchors (+) end of thin filament to Z-disk

Question 21

What are some similarities and differences in flight muscle sarcomeres of highly divergent groups?

Answer 21

similarities:

• sarcomeres from insect (invertebrate) and vertebrate flight have the same basic structure

differences:

• some unique proteins
• insect sarcomeres are longer
• myosin arrangement is different

Question 22

Describe sarcomere contraction.

Answer 22

• during its power stroke, each myosin head pulls a thin filament towards M-line
• combined effort of all these power strokes pulls Z-disks closer together, which decreases sarcomere length

Question 23

Length-Force Relationship

When can cross-bridges form?

Answer 23

can only form when myosin heads of a thick filament can interact with actin units of a thin filament

Question 24

Length-Force Relationship

What increases/decrease the amount of force that a sarcomere can produce during contraction?

Answer 24

• force increases as the number of myosin heads that can contact a thin filament increases
• force decreases as the overlap between thin filaments of adjacent Z-disks increases – at the shortest sarcomere length, thick filaments will collide with Z-disks and no further contraction is possible

Question 25

What is a myofibril?

Answer 25

composed of many sarcomeres arranged in series

• runs the entire length of a muscle cell

Question 26

What is a myofibre?

Answer 26

striated muscle cell that contains many myofibrils arranged in parallel

• parallel arrangement is important for force transmission (generates more force)

Question 27

How does striated muscle grow in length?

Answer 27

adds more sarcomeres to the ends of each myofibril

Question 28

How does striated muscle grow in diameter?

Answer 28

increases the number of myofibrils