M7, L1: Skeletal muscle structure

Question 1

What are skeletal muscles attached to and are they under voluntary control

Answer 1

They attached to bones via tendons and extend from one bone to the next. Yes

Question 2

How is the involuntary muscle types contractile activity controlled

Answer 2

The amplitude/frequency of contraction is modulated by nervous system input. This intrinsic activity is termed myogenic. Other involuntary muscle cells may be quiescent and their contraction is initiated by autonomic nervous system output.

Question 3

What is the structure of a myofibril: Sarcomere: I Band, A band, H zone, M line, Z disc

Answer 3

Sarcomere = Z disc to Z disc. M line is line of protein myomesin that holds adjacent thick filaments together. Thick filaments are in the A band from the middle. Thin filaments are from the Z disc, in the I band and A band but not H zone.

Question 4

What are T(ransverse)- tubules, where are they and what is their purpose

Answer 4

Deep invaginations continuous with the sarcolemma which circle each sarcomere at the junction between a and I bands. Allows action potential s to be carried deep in the muscle cell. Contains interstitial fluid.

Question 5

What is the Sarcoplasmic Reticulum, where are they and what is their purpose

Answer 5

Extensive network surrounding the myofibril that stores calcium. The terminal cisternae sit on either side of the T tubule to make a triad.

Question 6

What is the structure of a thick filament

Answer 6

Myosin has 2 subunits each with a globular head and tail, twisted to form a helix. Head is enzyme capable of hydrolysing ATP and contains binding site for actin.

Question 7

What is the arrangement of thick filament within the fibril : whats holding it down

Answer 7

Heads are arranged projecting out away from M line in a polarised fashion. Titin anchors thick filament to the Z line.

Question 8

What is the structure of thin filament

Answer 8

globular actin proteins make a double stranded helical actin chain. Troponin (bead) and Tropomyosin (braid) regulatory accessory proteins present.

Question 9

What is the sliding filament theory of muscle contraction and how do the bands change in a contraction

Answer 9

Muscle contractions happen as the sarcomere shortens sa thin filaments are pulled over thick filaments. As a result. the Z line is pulled toward the M line. The I band and the H zone become narrower but A band stays the same.

Question 10

What are the four steps of the cross bridge cycle

Answer 10

Cross bridge formation, Power stroke, detachment, energisation of the myosin head

Question 11

What is energisation of the myosin head

Answer 11

the myosin head hydrolyses ATP to ADP + Pi, and uses the energy to move to high energy (cocked) confirmation at 90 degrees to actin

Question 12

What is cross bridge formation

Answer 12

myosin head with ADP + Pi binds to actin binding site to form cross bridge

Question 13

What is power stroke

Answer 13

ADP is released and myosin head rotates to low energy state (45 degrees to actin) and pulls the thin filament over.

Question 14

What is the detachment

Answer 14

A new ATP molecule binds to Myosin, which weakens the actin-myosin bind and myosin detaches

Question 15

What are the requirements for cross bridge cycle to occur and contractions to continue

Answer 15

Ca2+ levels between 0.001-0.01 mM to come and bind to troponin which causes tropomyosin to expose the myosin binding site on actin. Also needs ATP for detachment. If none then rigor mortis

Question 16

What is the importance of Calcium

Answer 16

Calcium is the on switch for contraction when free in the intracellular space and able to interact with troponin in actin regulated muscle and calmodulin in myosin regulated muscle

Question 17

How is calcium levels regulated in muscle fibre

Answer 17

The opening of Ca2+ channels in the SR allow passive diffusion of Ca2+ ions into the cytosol. Relaxation of muscle happens when Ca2+ATPase constantly moving Ca2+ from the cytoplasm back to the SR.

Question 18

What is isometric vs isotonic contraction

Answer 18

Isometric is where the length is constant so there is no shortening of the muscle but the tension varies. (books)
Isotonic is where the muscle changes length with constant tension but variable velocity. (dumbell)

Question 19

What is the relationship between length and tension (active force) in isometric contraction

Answer 19

The tension developed is dependent on amount of cross bridges able to form: the degree of actin-myosin overlap.

Question 20

When is maximal force developed?

Answer 20

2.0-2.2 um - optimum resting length = maximum cross bridges

Question 21

At lengths less than 2.0 um what is the effect on tension- passive, active

Answer 21

active force development reduced because the ends of filaments start to collide- not enough length left to overlap

Question 22

At lengths more than 2.2 um what is the effect on tension- passive, active

Answer 22

Passive force increases as elastic connective tissue around muscle cells is stretched. The active force declines as the extent of overlap between the filaments reduces which reduces cross bridge interactions.

Question 23

At twice the length of a normal sarcomere for skeletal muscle is the overall tension more or less than at optimum

Answer 23

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