Skeletal Muscle

Question 1

Where are skeletal muscles located?

Answer 1

between bones

Question 2

What are the functions of skeletal muscles? (3)

Answer 2

• body movement (locomotion)
• maintenance of posture
• respiration (diaphragm and intercostal contractions)

Question 3

Are skeletal muscle fibres striated or non-striated?

Answer 3

striated

Question 4

Describe the organization of skeletal muscle fibres.

Answer 4

• muscle contains fascicles (bundles) of muscle fibres

- muscle fibres contain myofibrils

Question 5

What are the 3 investments of skeletal muscle?

Answer 5

epimysium
perimysium
endomysium

Question 6

What is epimysium?

Answer 6

dense collagenous connective tissue that surrounds entire muscle

Question 7

What is perimysium?

Answer 7

collagenous connective tissue that surrounds fascicles

Question 8

What is endomysium?

Answer 8

fine sheath of connective tissue composed of reticular fibres that surround individual muscle fibres

Question 9

Is skeletal muscle contraction voluntary or non-voluntary?

Answer 9

voluntary

Question 10

What is the spontaneity of skeletal muscle contraction controlled by?

Answer 10

motor neurons – cell body of motor neuron is localized in ventral horn of spinal cord

Question 11

What is the basal lamina?

Answer 11

external lamina – layer of extracellular matrix secreted by epithelial cells

Question 12

Where is the basal lamina?

Answer 12

under endomysium, between plasma membrane of muscle fibre and endomysium

Question 13

What does the basal lamina contain?

Answer 13

glycoproteins and collagen

Question 14

How is the basal lamina continuous with the inside of the cell

Answer 14

basal lamina binds myofiber via dystroglycan-containing complex

Question 15

What is the dystroglycan-containing complex?

Answer 15

multi-protein complex

Question 16

What is the sarcolemma?

Answer 16

plasma membrane of muscle fibre

Question 17

What is the myotendinous junction?

Answer 17

interface between muscle and tendon, where force is transmitted between the two tissues

Question 18

What does the myotendinous junction do?

Answer 18

attaches muscles to bones

• muscle fibres end at level of tendon
• finger-like extensions at ends of muscle fibres insert into connective tissue of tendon

Question 19

Describe the structure of myofibers.

• shape
• striated or non-striated
• nuclei

Answer 19

• lond, cylindrical
• striated
• multinucleated

Question 20

Where are nuclei of myofibers?

Answer 20

at periphery, just under plasma membrane

Question 21

What are myofibrils composed of?

Answer 21

myofilaments

Question 22

How are myofibrils organized?

Answer 22

• aligned in parallel

- separated by mitochondria and SR vesicles

Question 23

What are myofibrils?

Answer 23

contractile muscle fibers

Question 24

What is the sarcoplasmic reticulum?

Answer 24

smooth ER

Question 25

Where is the sarcoplasmic reticulum and how is it organized?

Answer 25

- runs longitudinally to myofibril | - surrounds individual myofibrils

Question 26

What does sarcoplasmic reticulum form? (2)

Answer 26

- interconnected network of tubules | - chambers (lateral sacs) called terminal cisternae on either side of T-tubules

Question 27

What does the sarcoplasmic reticulum do in skeletal muscle?

Answer 27

- major role in storing Ca2+ when muscle is at rest | - releases Ca2+ into sarcoplasm when muscle is stimulated

Question 28

What are transverse tubules (T-tubules)?

Answer 28

deep invaginations of sarcolemma, perpendicular to length of myofiber

Question 29

What is a triad junction?

Answer 29

single T-tubule + 2 terminal cisternae

Question 30

What receptors do T-tubules carry?

Answer 30

dihydropyridine receptors = voltage-gated Ca2+ channels

Question 31

What receptors do terminal cisternae carry?

Answer 31

ryanodine receptors = Ca2+ release channel

Question 32

What is the sarcomere?

Answer 32

contractile unit of myofibril made of 2 myofilaments | in skeletal and cardiac muscle

Question 33

What are the 2 types of myofilaments?

Answer 33

- myosin (thick) myofilament | - actin (thin) myofilament

Question 34

What is the myosin myofilament made of?

Answer 34

myosin molecule, composed of 2 heavy chains of myosin

Question 35

What is the purpose of the myosin molecule head?

Answer 35

power stroke during contraction

Question 36

What are the two sites on the myosin molecule head?

Answer 36

- actin binding site | - myosin ATPase site: where ATP binds and hydrolyzes into ADP and Pi

Question 37

What are actin myofilaments made of?

Answer 37

- two actin filaments twisted to form helix | - proteins – tropomyosin, troponin complex

Question 38

What are actin filaments made of?

Answer 38

many g-actin (monomeric form of actin)

Question 39

What is a motor unit?

Answer 39

motor neuron and all muscle fibres it innervates

Question 40

What is the motor endplate?

Answer 40

specialized domain of sarcolemma

Question 41

What does the motor endplate do?

Answer 41

highly excitable, and is responsible for initiating AP that propagates across myofiber and causes contraction

Question 42

What is the neuromuscular junction (NMJ)?

Answer 42

junction between axon terminal of motor neuron and motor endplate (synaptic contact between presynaptic terminal and postsynaptic membrane of muscle fiber)

Question 43

What are synpatic vesicles filled with?

Answer 43

ACh (neurotransmitter)

Question 44

What are junctional folds?

Answer 44

folds in postsynaptic membrane, particular to NMJ only (otherwise, membrane is usually flat)

Question 45

What is the synaptic cleft?

Answer 45

space between presynaptic membrane and postsynaptic membrane, where ACh in vesicles is released

Question 46

Does the NMJ have a basal lamina?

Answer 46

has a prominent basal lamina

Question 47

Does the NMJ have myelin?

Answer 47

no

Question 48

What is the nerve terminal filled with?

Answer 48

organelles – mitochondria, synaptic vesicles

Question 49

Where does endplate potential (EPP) occur?

Answer 49

in motor endplate, located underneath nerve terminal

Question 50

Can myosin head bind to actin at low Ca2+ concentration?

Answer 50

no - myosin binding sites on actin are masked by tropomyosin - blocks myosin head access to myosin binding site

Question 51

Can myosin head bind to actin at high Ca2+ concentration?

Answer 51

yes - myosin binding sites on actin are not masked by tropomyosin - myosin head can access myosin binding site and bind actin

Question 52

Where are there delays in contraction? Why?

Answer 52

between AP, Ca2+ release, and degeneration of force Ca2+ needs to be released from SR, travel through sarcoplasm, find troponin complex, etc. – process takes time

Question 53

What is a single twitch?

Answer 53

muscle fiber is restimulated after it has completely relaxed – second twitch is of same magnitude as first twitch

Question 54

What is summation?

Answer 54

muscle fiber is restimulated before it has completely relaxed – second twitch is added on to first twitch

Question 55

What is tetanus?

Answer 55

muscle fiber is stimulated so rapidly that it does not have opportunity to relax at all between stimulations – maximal sustained contraction (tetanus)

Question 56

What happens when tetanus ends?

Answer 56

muscle fatigue leads to relaxation of muscle

Question 57

What happens if ACh is constantly present at synaptic cleft?

Answer 57

- muscle spasms | - ACh is degraded by AChE until another contraction needs to occur

Question 58

In skeletal muscle, are dihydropyridine receptors in contact with ryanodine receptors?

Answer 58

yes

Question 59

Skeletal Muscle Contraction Process

Answer 59

–

Question 60

What is the sliding filament model of contraction?

Answer 60

actin and myosin filaments overlap slightly actin filaments slide along myosin, resulting in greater overlap and shortening of sarcomere → contraction

Question 61

Does the length of myofilaments (thick or thin) get shorter during contraction and/or relaxation?

Answer 61

no - during contraction, actin and myosin filaments slide along each other and make the whole sarcomere shorter