SKELETAL MUSCLE: Overview

Question 1

Provide an overview on skeletal muscle: what makes up the whole muscle and define them. (4)

Answer 1

Muscle → fasciculus → fiber → myofibril

• Whole muscle: acts on the skeleton to produce movement. Made up of fascicle bundles and surrounded by epimysium (connective tissue)
• Fasciculus: made up of bundles of muscle fiber and surrounded by perimysium
• Muscle fiber: represents a single individual muscle cell and made up of bundles of myofibrils. Surrounded by endomysium. multinuclei
• Myofibrils: contracticle units of the muscle

Question 2

Define and describe myogenesis

Answer 2

• Myogenesis: myocyte development
• Myoblasts fuse to form myotubes then undergo terminal differentiation into myocytes (muscle fibres). This is why muscle fibers/myocytes are multi-nuclei
• Myofibrils are assembled in the cytoplasm

Question 3

Define endomysium, perimysium, epimysium

Answer 3

• Endomysium: connective tissue that separates single muscle fibres from one another
• Perimysium: the sheath of connective tissue surrounding a bundle of muscle fibers.
• Epimysium: connective tissue that surrounds the entire muscle circumference

Question 4

Describe the length of muscle fibers as well as the differences between pennate and parallel muscle fiber arrangement

Answer 4

• ~2 cm long
• Parallel: lower force, good endurance, e.g. sartorius
• Pennate: higher force, lower range of movement e.g. rectus femoris

Question 5

What makes up the thin and thick filaments respectively?

Answer 5

Made up of actin and myosin

Question 6

Define A bands, I bands, M Lines, Z Lines, and H Zones

Answer 6

A band: the entire length of thick filament. Thick filament is made up largely by myosin
M line: exists in the centre of the A band
I band: distance between the end of one thick filament to the beginning of the adjacent thick filament. Made up of thin filament. DO NOT assume that the I band is the entire length of the thin filament.
Z line: anchor point for actin. Where thin filaments insert
H zone: distance between end of one thin filament and the start of the next thin filament. Made up of thick filament
M Lines: line at the center of a sarcomere to which myosin bind

Question 7

Define sarcolemma

Answer 7

Sarcolemma: membrane that surrounds each fiber and defines the limits of the muscle cell

Question 8

Define T-tubule

Answer 8

T- tubule: extensions of the cell membrane that penetrate into the center of skeletal and cardiac muscle cells

Question 9

Define sarcoplasmic reticulum

Answer 9

Sarcoplasmic reticulum: Specialized endoplasmic reticulum - dumps Ca+ into myofibrils and takes it up again

Question 10

Define actin and describe how it forms into thin filaments

Answer 10

Actin: protein. Often used as thin filament for strength. Generally globular but forms thin filaments to construct the cytoskeleton of the muscle fibre. Helical coils of g-actin polymerize to form f-actin

F-actin is unstable, other proteins needed to stabilize (3):
Nebulin, tropomyosin, troponin

Question 11

Define myosin and describe how it forms

Answer 11

• Myosin: force generating ATPase that binds to actin
• Myosin subunits polymerize in a tail to-tail formation
• Each myosin has a tail region and a crossbridge region (arm and globular heads)
• Globular heads contain light chains important for myosin ATPase activity

Question 12

Describe the other important myofibril components that make up thin filaments

Answer 12

capZ, a-actinin, tropomodulin

Troponin is a trimer
TnC, TnT, TnI
TnC has Ca2+ binding

Question 13

Describe the other myofibril components that make up thick filaments

Answer 13

titin, myomesin, C-protein

Myosin heads contain:
- Heavy chain (MHC),
- Essential light chain (MLC-1)
- Regulatory light chain (MLC-2)

Question 14

What proteins helps the muscle cell stabilize their membrane and withstand forces?

Answer 14

Dystroglycans
Sarcoglycans

Loss of proteins such as dystroglycans, sarcoglycans, can cause different forms of muscular dystrophy

Major component is the dystrophin-glycoprotein complex, made up of (transmembrane) sarcoglycans and (membrane associated) dystrophin
- Strengthen individual sarcoplemma

Question 15

Why do muscle cells need dystroglycans, sarcoglycans, and dystrophin-glycoprotein complex?

Answer 15

Need to link muscle cytoskeleton to ECM through ECM and membrane proteins to maintain structural integrity and prevent sarcolemma rupture through shearing
Requires multiple membrane proteins at/within the sarcolemmal membrane - Help the muscle cell stabilize their membrane and withstand forces.

Question 16

What do defects in the dystrophin gene cause?

Answer 16

Defects in the dystrophin gene cause Duchenne Muscular Dystrophy, a degenerative muscle disease

Question 17

Describe how the number of sarcomeres in series or in parallel help determine the properties of muscle

Answer 17

many in series = high velocity and range ofmotion

manyin parallel = high force orientation