Muscle Structure and Contraction

Question 1

What types of muscle are ‘striated’?

Answer 1

skeletal muscle or ‘voluntary muscle’ looks banded or ‘striated’ under a microscope

heart muscle is also striated but is not voluntary

Question 2

where do we find smooth muscle?

Answer 2

organs - note that it’s not striated muscle, it’s smooth muscle

Question 3

skeletal muscles are surrounded by a connective tissues called the …..?

Answer 3

endomysium

Question 4

muscle cells are grouped in fascicles surrounded by … ?

Answer 4

perimysium

Question 5

Fascicles are grouped together to form the muscle which is surrounded by… ?

Answer 5

epimysium

Question 6

what percent of body weight is made up of skeletal muscle?

Answer 6

40% of body weight

Question 7

how would you describe the structure of skeletal muscle?

Answer 7

multinucleate syncytial structure = the whole thing contracts at once, it’s not seperated in function

Question 8

Describe on the photo below what we’re looking at

Answer 8

• thick filaments, with thin overlapping them.
• A band (just where they overlap) and that creates a ‘dark’ bank
• either side of the A band you have thin filaments ( so it looks light in color)
• the Z line bisects the I band, acts as an anchoring structure holding the thin filaments together
• notice the A band where there is an overlap - the thin filaments overlap to some extent but they don’t come into the middle.
• The H zone is a light band in the middle where the Thick filaments solely exist ( no thin overlap) -
• Also in the middle of the H zone (not shown on the diagram) = M line - giving the thick filaments a certain amount of achoring.
• This whole thing is a sarcomere- which is repeated over and over again in the cell - the Sarcomere is the unit between two Z lines.

Question 10

The Z band bisects what structures?

Answer 10

thin filaments

Question 11

the M line bisects the….?

Answer 11

H zone (middle of sarcomere)

Question 12

the H zone is in the middle of what zone?

Answer 12

the A zone

Question 13

the M and Z band hold the thick and thin filaments together - what are these bands composed of?

Answer 13

they are accessory proteins - which maintain the structure such as alpha -actinin which maintains the actin lattice and dystrophin which anchors actin filaments to the sarcolemma

Question 14

Patients with a deficiency in dystrophin… what happens?

Answer 14

their accessory proteins which hold together the thick and thin filaments cannot anchor the actin filaments to the sarcolemma - leads to destruction of the muscles

Question 15

the action potential is carried through what structure in the muscle cell?

Answer 15

the ‘transverse tubule’ which is a sarcolemma - carries the action potential into the interior of the cell

Question 16

does the sarcoplasmic reticulum share a membrane with the transverse tubule?

Answer 16

no the sarcoplasmic reticullum is an intracellular organelle - so even though the membrane is very close to the transverse tubule, they are seperate membranes -

Question 17

how does an action potential cause the terminal cysterna to release Ca2+?

Answer 17

it’s the RyR (ryanodine receptor Ca2+ channel) and DHPR (dihydropyridine receptor) - when the action potential is detected by the DHPR, you get a conformational change and the RyR opens (a calcium channel) because they are physically linked and calcium is allowed to rush into the sarcoplasm (down its concentration gradient)

Question 18

how many triads are in each sarcomere?

Answer 18

each sarcomere has two triads (triads are a source of calcium)

Question 19

the cross bridges are conveniently located by what structure?

Answer 19

they are conveniently located near the triads (which are the sarcomere’s source of calcium)

Question 20

what part of myosin forms the bridges between thick and thin filaments?

Answer 20

the myosin head - forms bridges - these heads stem from the thick filaments

Question 21

what proteins are associated with the thin filaments?

Answer 21

troponin and tropomyosin

Question 22

the sarcomere is a unit of myofibril contained between what two lines?

Answer 22

the Z lines

Question 23

in skeletal muscle, triads are located at the junction of which two bands?

Answer 23

triads are located at the junction of the A and I bands

Question 24

describe how an action potential propagates and leads to muscle shortening

Answer 24

1. Na+ entry initiates an action potential which is propagated along the sarcolemma and down the T tubules
2. action potential in the T tubule activates voltage sensitive receptors which in turn trigger Ca2+ release from terminal cisternae of SR in cytosol
3. caclium ions bind to troponin- troponin changes shape, removing the blocking action of tropomyosin (pulls the tropomyosin deep into the groove of the filament which exposes the active site) actin activates sites exposed
4. contraction- myosin heads alternatively attach to actin and detach, pulling the actin filaments toward the center of the sarcomere - release of energy by ATP hydrolysis powers the cycling processes
5. removal of Ca2+ by active transport into the SR after the action potential ends
6. tropomyosn blockage restored, blocking myosin binding sites on actin - contraction ends and muscle fiber relaxes

Question 25

describe the steps of the myosin head binding and dissociating from the thin filament

Answer 25

1. myosin head attaches to the actin myofilament forming a cross bridge -generates Pi
2. inorganic phosphate generated in the previous contraction cycle is released initiating the power stroke - the myosin head pivots and bends as it pulls on the actin filament sliding it toward the M line - then ADP is released
3. as new ATP attaches to the myosin head, the link between myosin and actin weaknes, and the cross bridge detaches
4. as ATP is split into ADP and Pi the ysoin head is energized (cocked into the ‘high energy’ conformatio

*ATP bound = low energy conformation

*ADP + Pi bound = High energy conformation

Question 26

what is the sliding filament hypothesis of Huxley?

Answer 26

• muscle shortens by interdigitation of actin and myosin
• the sarcomere shortens
• the I band shortens
• the H zone shortens
• the A band stays constant in length

Question 28

what zones of the sarcomere shorten with contraction? Which zones stay the same?

Answer 28

shorten

• I band
• H zone
• Overall sarcomere

Stay the same ?

• the A band