Muscles

Question 1

How many skeletal muscles do humans have?

Answer 1

~660 skeletal muscles

Question 2

What is the average muscle mass in adult females?

Answer 2

13-30 kg

Question 3

What is the average muscle mass in adult males?

Answer 3

18-40 kg

Question 4

What is the diameter range of muscle fibers?

Answer 4

10-100 μm

Question 5

What is the length range of muscle fibers?

Answer 5

1-500 mm

Question 6

What are the three layers of connective tissue surrounding muscle fibers?

Answer 6

• Epimysium: Encloses muscles. (The connective tissue)
• Perimysium: Surrounds the groups of fibres that form muscle fascicles.
• Endomysium: Encloses each muscle fibre, and it consists of Extracellular matrix (made of collagen).

Question 7

What is the basic contractile unit of muscle?

Answer 7

Sarcomere: Consists of myofilaments within myofibrils which are arranged in a series of repeating units.

Question 8

What type of proteins are troponin and tropomyosin classified as?

Answer 8

Regulatory proteins

Question 9

What are the two types of myofilaments found in myofibrils?

Answer 9

• Thick filaments
• Thin filaments

Question 10

What proteins make up thick and thin filaments?

Answer 10

Thick Filaments: Myosin and Myosin-binding proteins (C protein, H protein, M protein, myomesin).

Thin filaments: Actin, tropomyosin, and troponin complex (TN-T, TN-I, TN-C).

Question 11

What is the primary function of dystrophin in muscle fibers?

Answer 11

Force transmission

Question 12

What are the three types of muscle contractions?

Answer 12

• Eccentric contraction: muscle generates force when lengthening. (Where muscle force is at its greatest.
• Isometric contraction: Muscle generates force while length stays the same.
• Concentric contraction: Muscle shortens and generates force.

Question 13

In which type of contraction does the muscle generate force while lengthening?

Answer 13

Eccentric contraction

Question 14

What is myosin?

Answer 14

A myosin molecule is composed of six proteins:
- Two myosin heavy chains (MyHC)
- One essential myosin light chain (MLC) on each MyHC
- One regulatory myosin light chain (MLC) on each MyHC

MyHC contains ATP- and actin-binding sites (essential for muscle contraction)

Question 15

What is the role of titin in muscle fibers?

Answer 15

Acts as a molecular spring permitting return of stretched sarcomeres to ‘optimal’ length.

Question 16

What occurs during the power stroke in the cross-bridge cycle?

Answer 16

Dissociation of Pi results in 5-10 nm displacement

Question 17

True or False: Muscle force is greatest during concentric contractions.

Answer 17

False

Question 18

What is the relationship between force and velocity during muscle contraction?

Answer 18

Speed of contraction decreases with increasing resistance

Question 19

What does the sarcoplasmic reticulum do in muscle fibers?

Answer 19

Surrounds myofibrils and stores calcium

Question 20

What is the average resting myoplasmic calcium concentration?

Answer 20

~0.1 µM

Question 21

What is the maximum contraction myoplasmic calcium concentration?

Answer 21

~2 µM

Question 22

What is the delay between action potential and initiation of muscle contraction?

Answer 22

~20 ms

Question 23

What slows down the relaxation after fatiguing contractions?

Answer 23

Decrease in ATP concentration

Question 24

What provides the framework for actin and myosin filaments in the sarcomere?

Answer 24

Cytoskeletal proteins

Question 25

What links to the endomysium?

Answer 25

Structural Intracellular proteins of muscle fibre link to the endomysium via focal adhesions. (This is important for force transmission).

Question 26

What is the Contractile machinery of the muscle?

Answer 26

- Muscle fibres (10-100 μm) are covered by sarcolemma (≈7.5 nm thick). - T-tubules are invaginations of sarcolemma. - Sarcoplasmic reticulum is a network of tubes surrounding myofibrils. - Sarcoplasm is fluid enclosed within the fibre. - Myofibrils (~1 μm) are packed bundles of myofilaments running along the fibre.

Question 27

What do the different filaments in the sarcomere do?

Answer 27

- Thin filaments at one end project into the sarcomere while another connects the Z line. -Thin filaments contain troponin and tropomyosin proteins that participate in blocking and unblocking of thin filaments (regulatory proteins). - Thick filaments (myosin) are in the centre of sarcomere and overlap thin filaments from both sides

Question 28

Muscle Contraction:

Answer 28

Myosin heads drag thin filaments from both ends towards each other •The distance between Z lines shortens (sarcomere shortens) •Shortening of sarcomeres in series add up. •Example –10-cm fibre contains ~4,000,000 sarcomeres –Each sarcomere shortening by 1 µm shortens the fibre to 6 cm.

Question 29

How is force produced: the cross-bridge cycle

Answer 29

1. ATP hydrolysis increases affinity of myosin for actin. 2. Following Ca++ release from SR myosin binds actin. 3. ATP binds to myosin decreasing its affinity for actin. 4. Myosin dissociates from actin, ATP is hydrolysed.

Question 30

Why does eccentric contraction generate more force than concentric?

Answer 30

Eccentric - all associated cross-bridges contribute. Concentric - “power stroke” cross-bridges contribute.

Question 31

What slows down the relaxation after fatiguing contractions?

Answer 31

Possible cause of slowing relaxation is a decrease in concentration ATP leading to impaired dissociation of these metabolites from the myosin.

Question 32

Where is the muscle contractile force transmitted through?

Answer 32

Through the connective tissue: Endomysium, perimysium, epimysium, tendons).