Skeletal Contraction

Question 1

What is structure of muscle?

Answer 1

Myofilaments are either actin or myosin. Myosin is thick with two heads and actin is made of f and g actin which has tropomyosin. These make up sarcomeres

Question 2

How does myosin move along actin?

Answer 2

Question 3

What occurs in isometric muscle contraction?

Answer 3

the power-stroke is rate-limiting step because during shortening the power-stroke rate increases
proportion of post-power-stroke heads increases
post-stroke “drag bridges” counteract both sliding and force

Question 4

This inhibits attachment and force, shortening velocity slowed

Answer 4

elevated H+

Question 5

This inhibits attachment and force unloaded velocity unchanged, but contributes to fatigue

Answer 5

Elevated phosphorus

Question 6

inhibits detachment, unloaded velocity slowed, and isometric force slightly increased

Answer 6

elevated ADP

Question 7

What 3 factors influence number of fibers contracting?

Answer 7

number of motor units recruited, number of muscle fibers per motor unit, fibers available to contract

Question 8

What 5 factors influence tension of contracting fiber?

Answer 8

frequency of stimulation, length of fiber, extent of fatigue, type of fiber, and thickness of fiber

Question 9

How is excitation and contraction coupled?

Answer 9

In NMJ once ACh binds to ligand receptor causes depolarization of the cell, action potential along T-tubule causes VG Ca2+ to open SR so that the SR can also release Ca2+. They bind to troponin allowing myosin to bind actin

Question 10

What two proteins link the SR to the T tubules?

Answer 10

Dihydropyridine and ryanodine receptors. Ryanodine provides most Ca2+, and DHP goes through conformation change which pulls ryanodine receptor open

Question 11

3 events that occur during excitation

Answer 11

1. Moto end plate forms - ACh binding causes K+ out and Na+ in
2. Propagation of AP along sarcolemma
3. Release of calcium - AP arrives at SR

Question 12

What are the 3 subunits of troponin?

Answer 12

troponin T - binds to tropomyosin
toponin I inhibits myosin binding to actin
troponin C binds to calcium - needs to occur to have myofilament interaction

Question 13

What is structure of myosin?

Answer 13

Comprised of two heavy chains and two pairs of different light chains-amyosin essential light chainand amyosin regulatory light chain (RLC).
Two identical golf-club shaped subunits with interwining tails and globular heads with each head containing 1. actin binding site and 2. myosin ATPase site

Question 14

What happens in relaxed states?

Answer 14

In relaxed, unbinding of calcium from troponin C causes tropomyosin to be partially blocked and the myosin binding site on G-actin, and troponin C becoming unbounded from calcium.
During the relaxation phase, actin and myosin are not bound together, and the thick and thin filaments of the sarcomere slide back to their original position with the aid of titin.

Question 15

What happens in excited states?

Answer 15

muscle fiber is excited and Ca2+ is released which binds to troponin making troponin-tropomyosin complex, cross bridge occurs, myosin pulls thin filament inward

Question 16

what are 3 functions of ATP in contraction?

Answer 16

provides energy for power stroke from splitting ATP, necessary to break crossbridge, and necessary to actively pump calcium back into SR

Question 17

What are three mechanisms to remove calcium?

Answer 17

SERCA (SR CA2+ ATPase), Na-Ca exchanger, Ca2+ pump, Ca2+ binding proteins

Question 18

What occurs during rigor mortis?

Answer 18

ATP is no longer available the cross bridges between thick and thin filaments cannot detach. Resulting Ca2+already present in the sarcoplasm, as well as the Ca2+that continues to leak out of the sarcoplasmic reticulum, triggers a sustained contraction in the skeletal muscle fibers.

Question 19

What’s time line of rigor mortis?

Answer 19

Dead less than 3 hrs you’re still warm and not stiff, dead 3-8 hrs warm but body cooling, getting stiff, 8-24hrs room temp and stiff, and 24-36hrs room temp not stiff

Question 20

Inflammation and destruction of both type 1 and type 2 muscle fibers by cytotoxic T lymphocytes

Answer 20

Polymyositis

Question 21

Constant leak of SR Ca through abnormal ryanodine receptor which causes calcium build up in skeletal muscle, causing rigidity, lactic acidosis

Answer 21

Malignant Hyperthermia

Question 22

Muscle weakness and loss of mitochondria in core of skeletal muscle fibers

Answer 22

Central Core Disease

Question 23

Painless muscle cramping and impaired muscle relaxation during exercise, decreased activity of SERCA

Answer 23

Brody’s Disease

Question 24

How are motor units activated?

Answer 24

the more strength needed the units will be activated, strength of stimulus determines number of units recruited. Increasing the intensity of stimulation increases tension, motor unti contraction, and larger units

Question 25

Smallest motor neurons are recruited first due to their physiological properties that make them more easily excitable requires significantly less input from the brain

Answer 25

size principle

Question 26

How is frequency of stimulation intiated?

Answer 26

Tension produced by the second stimulation will be added to the first. Each pulse of Ca2+ initiates cross-bridge cycling and because the muscle has not relaxed, the mechanical force adds onto (summates) the force from the previous action potential
More time is needed for the Ca2+ to leave and for the muscle to relax.

Question 27

3 periods of muscle twitch

Answer 27

1. Latent - the time from stimulation of the muscle until shortening of the muscle begins. Consists of
   a. Depolarization
   b. Diffusion of Ca2+
   c. Establishment of actin/myosin bonding
2. Contraction Period - tension and shortening
3. Relaxation - muscle goes ack to resting, Ca2+ is actively transported into SR

Question 28

Antigravity, weight bearing, and sustained movement. Also recruited earliest

Answer 28

Slow motor units (Type I)

Question 29

Sustained locomotion

Answer 29

Fast oxidative (Type IIa)

Question 30

Burst power; white fiber

Answer 30

fast glycolytic (Type IIx)

Question 31

Type 1 vs Type 2 motor unit

Answer 31

Type 1: small, fast, high excitable, few fibers, low force, low fatigue
Type 2: large, fast, low excitability, many fibers, large diameter, high fatigue

Question 32

How do sarcomeres compare in parallel

Answer 32

at any given speed below maximal, adding units in parallel increases overall force; also, at a given load, adding units in parallel increases velocity.

Question 33

Energy Utilization over time

Answer 33

Creatine phosphate system (8-10sec) this utilizes phosphocreatine energy shuttle that uses creatine phosphokinase to make phosphocreatine for contraction, lactic acid system (1-3min), and aerobic system