LG 1.3 - Skeletal Muscle Phys

Question 1

Contraction of Sarcomere

Answer 1

1. Myosin heads move against actin filaments 2. sarcomere gets shorter as myosin and actin move against each other 3. Z disks get closer together as sarcomere contracts

Question 2

Outline the excitation-contraction coupling

Answer 2

1. Motor neuron in anterior (ventral) horn of spinal cord send action potential through axon to motor end plate 2. Depolarization at motor end plate generates action potential in muscle 3. Sarcolemma depolarizes 4. Depolarization transfers down transverse tubules 5. DHP receptor on T-tubule undergoes conformational change in response to depolarization 6. Causes ryanodine receptor in SR to open and Ca2+ flows out

Question 3

Locate and describe the Dihydropyridine receptor (2)

Answer 3

1. located in T-tubules 2. L-type Ca2+ channel 3. Mechanically coupled to ryanodine receptor in SR –> when DHP recetpor is depolarized, the ryanodine receptor channel is activated and the Ca2+ flows out of the sarcoplasmic reticulum

Question 4

Locate and describe the Ryanodine receptor channels (2)

Answer 4

1. located in SR 2. opening of channel allows for Ca2+ to be released from SR 3. Ca2+ release allows for muscle contraction

Question 5

Troponin I

Answer 5

strong affinity for actin

Question 6

Troponin T

Answer 6

strong affinity for Tropomyosin

Question 7

Troponin C

Answer 7

strong affinity for Ca2+

Question 8

Explain Ca2+ role in excitation-contraction coupling

Answer 8

Ca2+ binds to troponin, which leads to a conformational change in the tropomyosin and exposes myosin binding sites on the actin

Question 9

Cross-bridge cycling steps (4)

Answer 9

1. ATP binds myosin head, which dissociates the actin-myosin complex 2. Myosin head is in the released state, ATP is hydrolyzed (to ADP + Pi) causing myosin to return to resting conformation (cocked state) 3. Myosin head binds to new position on actin and cross- bridge forms 4. Phosphate is released, which causes the myosin head to change confirmation, resulting in power stroke as the filaments slide past one another 5. ADP is released and myosin head is in attached state with actin

Question 10

SERCA (3)

Answer 10

1. Sarcoplasmic and endoplasmic reticulum Ca2+ ATPase 2. Pumps Ca2+ back into SR

Question 11

Calsequestrin, Calreticulin (3)

Answer 11

Ca2+ binding proteins in the SR

Question 12

Describe the Electrical component of Excitation-Contraction Coupling (5)

Answer 12

propagation of an action potential through the muscle

Question 13

Describe the Chemical component of Excitation-Contraction Coupling (5)

Answer 13

release of Ca2+ from the sarcoplasmic reticulum

Question 14

Describe the Mechanical component of Excitation-Contraction Coupling (5)

Answer 14

development of tension in the muscle fiber

Question 15

Isometric Contraction

Answer 15

1. Muscle length does not change as muscle is active 2. Maximum isometric tension is at optimum muscle

Question 16

Isotonic contraction

Answer 16

same load, changing muscle lengths

Question 17

Twitch

Answer 17

A single action potential through the muscle will yield a given amount of Ca2+ release. That Ca2+ release will cause a contraction in the muscle = a twitch

Question 18

Tetanus

Answer 18

If the action potentials are at a high enough rate, the twitches will start to fuse. Fused tetanus is the state where the muscle does not have a chance to relax.

Question 19

Fast twitch muscle fibers

Answer 19

1. white, type II 2. extensive sarcoplasmic reticulum, more active enzymes that promote rapid release of energy

Question 20

Slow twitch muscle fibers

Answer 20

1. red, type I 2. more oxidative metabolism, myoglobin (iron containing substance similar to hemoglobin, combines with oxygen and stores it, this speeds oxygen delivery to mitochondria)

Question 21

Locate and define the Sarcomere (1)

Answer 21

basic contractile unit, that is delineated by the Z disks

Question 22

Locate and describe what composes the A band (1)

Answer 22

located in the center of the sarcomere and contain the thick (myosin) filatment

Question 23

Locate and describe what composes the I band (1)

Answer 23

located on either side of A band, contain thin (actin) filaments, intermediate filamentous proteins, and Z disks NO THICK FILAMENTS

Question 24

Locate and describe what composes the Z disks (1)

Answer 24

run down the middle of each I band, delineate the ends of each sarcomere

Question 25

Locate and describe M line (1)

Answer 25

bisects the bare zone and contains proteins that link the central portions of the thick filaments together

Question 26

Locate and describe Bare Zone (1)

Answer 26

located in center of each sarcomere. No thin filaments here, thus there can be NO overlap of thick and thin filaments or cross-bridge formation here

Question 27

Locate and describe Thin filaments (1)

Answer 27

Actin - globular protein (G-actin), which is polymerized into 2 strands that are twisted into alpha-helical structure to form filamentous actin (F-actin).

Question 28

Locate and describe Thick filaments (1)

Answer 28

Myosin

Question 29

Tension vs. Muscle length (or preload) graph (6)

Answer 29

Looking at tension created for different muscle lengths, as you stretch a muscle ("load it up with weights") you effect the length of the muscle and how much overlap of actin-myosin cross-bridge occurs

Question 30

What does the dotted orange line represent (6)

Answer 30

Active tension - represents the active force developed during cross bridge cycling

Question 31

What does the dotted green line represent (6)

Answer 31

Passive tension - tension developed by simply stretching a muscle to different lengths

Question 32

What does the purple line represent (6)

Answer 32

Total tension - tension developed when a muscle is stimulated to contract at different preloads. Sum of active and passive tension

Question 33

What is occurring at 1 (6)

Answer 33

the muscle length is too short, and thin filaments collide with each other at the center of the sarcomere --\> reduces number of cross-bridges and therefore reduces tension

Question 34

What is occurring at 2 (6)

Answer 34

This peak is created by maximal cross-bridge overlap --\> optimal muscle length to generate force

Question 35

What is occurring at 3 (6)

Answer 35

the muscle length is too long and the number of cross-bridges is decreased. Therefore, active tension is reduced

Question 36

What is occurring at 4 (6)

Answer 36

This is the total tension, and the drop is due to the minimal overlap of cross-bridge in the active tension (muscle length is too long)

Question 37

What is occurring at 5 (6)

Answer 37

The tension begins to increase again because of the passive tension created by the simple stretch of the muscle

Question 38

Velocity vs. Afterload (7)

Answer 38

Describes the velocity of shortening when the force against a muscle contracts. Velocity of shortening reflects the speed of cross-bridge cycling

Question 39

Vmax (7)

Answer 39

Velocity of shortening is maximal when the afterload on the muscle is zero. All muscle lengths have the same Vmax b/c it is dependent on how fast you can cycle cross-bridges (not dependent on the overlap). As the afterload on the muscle increases, the velocity will be decreased b/c the cross-bridges can cycle less rapidly against the higher resistance

Question 40

Describe the yellow dashed line labelled 2 (7)

Answer 40

Muscle length is shortened so there is minimal overlap. Less overlap means lower velocities. As muscle length is shortened, the afterload at V0 decreases

Question 41

Describe the green dashed line labelled 3 (7)

Answer 41

Longest muscle length = most overlap (maximal overlap in green curve). Therefore, largest initial velocities and it can lift the largest load b/c of the maximal overlap