Lecture 30: Skeletal Muscle Part I

Question 1

What are the features of skeletal muscle?

Answer 1

• Under voluntary control
• Striated
• Single long cylindrical cells
• Multiple peripheral nuclei

Question 2

What are the features of cardiac muscle?

Answer 2

• Striated
• Branched cells with 1-3 central nuclei
• Connected via intercalated discs
• Invountary control

Question 3

What are the features of smooth muscle?

Answer 3

• Involuntary
• Found in the walls of internal organs
• Spindle shaped, uninucleated cells
• Not striated

Question 4

What are the contractile units that make up muscle fibres called?

Answer 4

Myofibrils

Question 5

How are skeletal muscles attached within the body?

Answer 5

They are attached to bones via tendons

Question 6

Where are thick filaments found on a sarcomere?

Answer 6

Running the length of the A band

Question 7

Where are thin filaments found on a sarcomere?

Answer 7

Running the length of I band and partway into the A band

Question 8

What is the Z disc?

Answer 8

Sheet of proteins that anchor thin filaments and connects myofibrils to one another

Question 9

What is the H zone?

Answer 9

The part of the A band in which only thick filaments exist

Question 10

What is the M line?

Answer 10

Line of protein myomesin that holds thick filaments together - located within the H zone

Question 11

What are T-tubules?

Answer 11

Deep infoldings of sarcolemma that circle each sarcomere at the junctions of A and I bands.

Question 12

What is the function of T-tubules?

Answer 12

Carry action potentials deep into muscle cell

Question 13

What is the SR?

Answer 13

Sarcoplasmic reticulum is the storage site for calcium - they surround T-tubules forming a triad

Question 14

What are thick filaments?

Answer 14

Parts of myofibrils made up of myosin -> each myosin has a globular head and a tail

Two myosin molecules come together as the two tails intertwine to form a helix

The head allows myosin molecules to bind to actin -> enzyme that is able to breakdown ATP

Question 15

How are thick filaments arranged within myofibrils?

Answer 15

Myosin heads project away from M line

Question 16

What are thin filaments?

Answer 16

Parts of myofibrils that are made up of globular actin proteins

Filaments are composed of double stranded helical actin chain

Thin filaments have regulatory proteins called troponin and tropomyosin

Question 17

What is troponin?

Answer 17

Regulatory protein found on thin filaments -> when bound by calcium ions they cause tropomyosin to move away from the actin binding site allowing myosin heads to form cross bridges with the actin chain

Question 18

What is tropomysoin?

Answer 18

Regulatory protein found on thin filaments -> at rest, they cover the actin binding site preventing muscle contraction from occurring

Question 19

How does the sarcomere change as muscle contraction occurs?

Answer 19

Z discs pulled towards M line

I band and H zone become narrower

Question 20

What are the 4 steps of the cross bridge cycle?

Answer 20

1. Cross-bridge formation
2. Power stroke
3. Detachment
4. Energisation of myosin head

Question 21

What occurs in the cross-bridge formation stage of the cross bridge cycle?

Answer 21

Calcium ions bind to troponin causing tropomyosin to move positions exposing actin binding site

ATP binds to myosin head and it broken down to ADP and Pi allowing myosin head to be in the cocked position

Release of inorganic phosphate ion allows myosin heads to bind

Myosin head binds to actin binding site on thin filament

Question 22

What happens in the power stroke stage of the cross bridge cycle?

Answer 22

ADP is released providing energy for the myosin head to rotate to its lower energy state pulling the actin filament along

-> sarcomere is therefore shortened

Question 23

What occurs in the detachment stage?

Answer 23

A new ATP molecule binds to the myosin head -> this weakens the actin-myosin bond causing myosin to detach from the thin filament

Question 24

What happens in the energisation of the myosin head stage?

Answer 24

Myosin head breaks down the ATP to ADP and Pi

Hence, energy is released allowing myosin to return to its higher energy state (position) making it ready for muscle contraction again

Question 25

What is the role of calcium in muscle contraction?

Answer 25

It is the on switch for muscle contraction to begin

-> the cross-bridge cycle will continue if calcium levels are above a critical threshold (0.001-0.01mM)

Question 26

How is the release of calcium to allow muscle contraction regulated?

Answer 26

Calcium ions are released from the sarcoplasmic reticulum by the opening of calcium channels

Active transport pumps (Ca2+ ATPase) constantly move Ca2+ ions from the muscle fibres back to the sarcoplasmic reticulum

Question 27

What is isotonic contraction?

Answer 27

Muscle contraction in which there is a shortening of the muscle.

• > tension is constant
• > velocity variable

Question 28

What is isometric contraction?

Answer 28

Muscle contraction in which there is no shortening of the muscle

• > length of muscle constant
• > tension is variable