Skeletal muscle

Question 1

What are the basic characteristics of skeletal muscle?

Answer 1

Strong, quick, discontinuous, striated, voluntary

Question 2

Name 4 functions of skeletal muscle

Answer 2

• Force production
• Support of soft tissue
• Control of entrances/exits
• Heat production

Question 3

What are the 3 layers of connective tissue?

Answer 3

1. Endonysium - surrounds each muscle fiber
2. Perinysium - surrounds bundles of fibers
3. Epinysium - surrounds entire muscle

Question 4

What are the properties of muscle fibers?

Answer 4

• Smallest units which can give a normal physiological response
• Multinucleate but enclosed in a single plasma membrane
• Composed of 100s of myofibrils enclosed in intermediate filament netowrk
• repeating longitudional units (sarcomeres)

Question 5

In the sarcomeres what is the a) Z-line b) M-line c) I band d) A band e) H zone

Answer 5

a) separates sarcomeres, proteins which interconnect and anchor filaments
b) middle of sarcomere, proteins which stabilise thick filaments
c) thin filaments only
d) overlapping region
e) thick filaments only

Question 6

Which protein is responsible for the elastic properties of muscle?

Answer 6

titin restores sarcomere length, largest known protein

Question 7

What isoform of myosin is found in skeletal muscle?

Answer 7

Myosin II

Question 8

What are the structural properties of myosin?

Answer 8

• Head containing actin and ATP binding sites
• Neck associated with MLCs and MHCs forming a coiled-coil
• bipolar organisation

Question 9

Describe the properties of actin

Answer 9

• G actin can polymerise to become F-actin
• • end attached to Z line
• Each G-actin molecule has an active site that can bind a myosin head

Question 10

Describe the 5 stages of the sliding filament model

Answer 10

1) Myosin head bound to ATP in low energy state
2) Myosin hydrolyses ATP to ADP to reach high energy state
3) Myosin head binds to actin to form cross-bridge
4) After releasing ADP+P (power stroke) myosin returns to low-energy state
5) Binding of new ATP releases myosin

Question 11

How is muscle contraction regulated?

Answer 11

• Troponin-tropomyosin complex prevents actin-myosin interaction
• Ca2+ binding causes a conformational change

Question 12

What are the structural properties of troponin and tropomyosin?

Answer 12

tropomyosin - double stranded protein covering myosini binding sites
troponin complex - 3 main binding sites where I binds to actin, T binds to tropomyosin and C binds to Ca2+

Question 13

How is a contraction initiated?

Answer 13

Excitation-contraction coupling

• Motorneuron stimulation propagates along the sarcolemna
• V dependent channels cause membrane to spike, which spreads down T-tubules and into the fibre
• Increase in Ca2+ concentration
• Each muscle spike causes a contraction which can summate to form a tetanic contraction

Question 14

What are the characteristics of the sarcoplasmic reticulum?

Answer 14

• SER which surrounds each myofibril
• SR tubules enlarge to form chambers (terminal cisternae) on either side of the T-tubules
• Ca2+ is stored in the SR and pumped by ATP-dependent Ca2+ pump

Question 15

How are the T-tubules and SR linked?

Answer 15

• In t-tubules: dihydropyridine receptor (DHPR)
• In SR: ryanodine receptor (RyR1: intracellular calcium channel)
  These are coupled. DHPR senses deplarisation and undergoes conformational change causing Ca2+ release form RyR1

Question 16

What are the 3 things the duration of a muscle contraction depends on?

Answer 16

• Period of stimulation at neuromuscular junction
• Availability of free Ca2+ in sarcoplasm
• ATP availability

Question 17

How is a contraction terminated?

Answer 17

1. Ach broken donw by AChE
2. SR actively reabsorbs Ca2+ (pump)
3. Troponin-tropomyosin complex returns to normal position
4. muscle returns to resting length