L3 Striated Muscle Physio

Question 1

Layers in skeletal muscle

Answer 1

Epimysium (surrounds entire muscle)
Perimysium (surrounds each fascicle)
Fascicle (group of muscle fibers)
Endomysium/Sarcolemma (surrounds ind. fibers)
Myofiber/muscle cell
Myofibrils (repeating sarcomeres of thick and thin filaments)
Myofilaments (M-A filaments, 40 make up a myofibril)
*All connective tissue ends up in the tendon

Question 2

Functional characteristics of skeletal muscle

Answer 2

Striated muscle, multi-unit

Question 3

Physical characteristics of smooth muscle

Answer 3

2

Question 4

Functional characteristics of smooth muscle

Answer 4

Single and multi-unit

Question 5

Physical characteristics of cardiac muscle

Answer 5

2

Question 6

Functional characteristics of cardiac muscle

Answer 6

Striated muscle, single-unit

Question 7

Characteristics of myosin

Answer 7

Binds actin (at head)
Has ATPase activity (at head)
2 heavy chains make tail with 4 light chains at 2 heads (crossbridges)

Question 8

What happens at the NMJ?

Answer 8

• AP impulse arrives
• Depolarization causes vesicles to fuse to end plate
• ACh release and diffuses across cleft
• ACh binds at post-synaptic receptor sites in the muscle
• Na channels open and Na influx occurs
• Depolarization of muscle cell
• If threshold reached, AP occurs, travels down sarcolemma and muscle contract

Question 9

Chemical steps in crossbridge cycle

Answer 9

A + M, ADP, Pi (high actin affinity)
Binding
A-M, ADP, Pi
Product release (-ADP, -Pi)
A-M (cycle stops here if no ATP= rigor mortis)
ATP binding (+ATP)
A-M, ATP (low actin affinity)
Dissociation p and partial p hydrolysis
A + M, ADP, Pi (cycle stops here in relaxed living muscle)

Question 10

Mechanical steps in crossbridge cycle

Answer 10

Ca binds to troponin-C
Myosin binds to actin
Powerstroke
Myosin hydrolyzes ATP and releases

Question 11

How does crossbridge cycle result in shortening of muscle fiber?

Answer 11

Actin filaments are attached to Z line, bend in myosin pulls actin filaments towards H zone, sarcomere shortens and muscle contracts (sliding filament model)

Question 12

What is the Length-Tension relationship?

Answer 12

If sarcomere is too short- there is steric hindrance between actin and myosin heads
If sarcomere is too long- there aren’t enough overlapping crossbridges, so there is less force
-There is an ideal length where there isn’t steric hindrance of A-M heads, but there is still crossbridge overlap to produce the optimal force
-Heart fxns at this optimal level- becomes stronger muscle as it is filling with blood

Question 13

Explain curve shape of L-T diagram

Answer 13

As sarcomere length increases, stress on muscle cell increases until the length becomes too long and stress drops due to lack of available crossbridges

Question 14

Force-Velocity relationship

Answer 14

Contracting a muscle without changing length, V=0
-Lifting a weight that = max. force of muscle
-Depends on # of A-M interactions
As weight drops, muscle can shorten with faster V
-No load= max V, depends on A-M ATPase activity
As weight increases, muscle lengthens with faster V

Question 15

What is isometric muscle contraction?

Answer 15

Muscle is actively help at a fixed length

Question 16

What is isotonic muscle contraction?

Answer 16

Muscle is actively contracting at constant force/load,

• No change in tension, muscle’s length changes
• Heart, lifting an object, walking, running

Question 17

Explain differences in EC coupling in skeletal vs. cardiac muscle

Answer 17

Skeletal muscle: mechanical EC coupling

• DHPR is voltage sensor in T tubule, mechanically linked to RyR in SR, opens RyR when AP arrives and Ca released
• Every skeletal muscle cell has nerve connected to it- can control which fibers response to AP

Cardiac muscle: CICR

• DHPR is Ca channel that opens when AP arrives, Ca influx stimulates opening of RyR on SR and Ca release
• There isn’t a nerve for each muscle cell, controlling how much Ca goes through allows for regulation of force/which cells are activated

Question 18

What is the sarcomere?

Answer 18

Contractile unit of the muscle cell

1 sarcomere= Z line to Z line

Question 19

What makes up a sarcomere?

Answer 19

• A band= dark, thick myosin filaments with some overlapping thin actin filaments
• I band= light, thin actin filaments
• Z line= where actin filaments attach, connects to EC matrix and transmits force of contraction
• M line= proteins that anchor myosin filaments together

Question 20

Characteristics of thin actin filament

Answer 20

F-actin make double strand helix
Made up of actin, tropomyosin and troponin- T, I, C
-T binds tropomyosin and actin
-I blocks myosin binding site, prevents muscle contraction
-C binds Ca: Ca binding makes A-M binding site free

Question 21

What is the Transverse tubule?

Answer 21

T-tubules are dips in sarcolemma membrane that go deep into muscle, carry AP down into muscle to DHPR voltage sensor for Ca in SR

Question 22

What is the sarcoplasmic reticulum?

Answer 22

SR is ER in the muscle that contains Ca stores, RyR mechanical Ca channel is connected to DHPR of T-tubules and is opened with AP arrives

Question 23

What is the muscle triad?

Answer 23

1 T-tubule and 2 lateral SR sacs

Question 24

What is SERCA?

Answer 24

Ca ATPase that pumps Ca back into SR to restore the Ca gradient

Question 25

What is EC coupling?

Answer 25

Excitation-contraction coupling is where electrical membrane depolarization is transformed into chemical signals to initiate muscle contraction
-Ca is the link

Question 26

Steps in EC coupling

Answer 26

1. AP travels into T tubule
2. Depolarization of DHPR
3. Mechanical link to RyR causes conformational change
4. RyR channel opens and Ca released from SR
5. Ca initiates muscle contraction
6. SERCA pumps Ca back into SR (muscle relaxes)

Question 27

Steps in muscle contraction

Answer 27

1. EC Coupling
2. Ca binds troponin-C
3. Troponin-I and tropomyosin move into actin groove
4. Myosin binds to actin
5. Crossbridge cycle/ powerstroke
6. Ca sequestration= relaxation

Question 28

When does myosin have the highest affinity for actin?

Answer 28

Right after the hydrolysis of ATP

Question 29

When are ADP and Pi released during the cross-bridge cycle?

Answer 29

After myosin and actin bind

Question 30

What things help to regulate strength of muscle contraction?

Answer 30

1. Twitch summation
2. Recruitment of additional motor units
3. Muscle fiber thickness
4. Length of fiber at onset of contraction (length-tension relationship)

Question 31

What is twitch summation?

Answer 31

Length of twitch is longer than a single AP, so multiple APs can fire during a single muscle twitch= cumulative increase in Ca= more available actin filaments= more muscle contraction

Question 32

What is recruitment of motor units?

Answer 32

Picking additional fibers to turn on and fuse their APs together to get a stronger contraction

Question 33

How does muscle fiber thickness affect contraction?

Answer 33

More sarcomeres in parallel have a summative force

More sarcomeres perpendicular shorten more

Question 34

Power-stress curve

Answer 34

Heart functions at the peak of the power-stress curve

Question 35

What are the two types of isotonic contraction?

Answer 35

Concentric- muscle actively shortening, tension rises to meet resistance
Eccentric- muscle actively lengthening (damaging, muscle injury and soreness, increases muscle strength), lengthening due to resistance being greater than force muscle produces

Question 36

What are 3 sources of muscle ATP?

Answer 36

1. Creatine phosphate: Creat-P + ADP –> Creat + ATP
   - first energy store used and depleated
2. Oxidative phosphorylation
   - requires O2, muscles w/ high demand = red b/c of myoglobin
3. Glycolysis
   - anaerobic, less efficient

Question 37

What muscle fiber types are there?

Answer 37

Type 1- Slow fibers (endurance)
Type 2- Fast fibers
-2A= Red, fatigue resistant
-2B= White, fatiguing