Lecture 11 - Muscle Contraction Flashcards
1
Q
Motor unit
A
The motor neuron and the muscle fibers it innervates
2
Q
How can force be increased during muscle contraction?
A
- Recruiting more motor units
- Increase frequency
3
Q
What changes occur when the frequency of muscle contraction increases?
A
more action potentials = more muscle twitches = more force
4
Q
What changes occur when more motor units are recruited?
A
more motor units = more muscle fibers activated = more force
5
Q
All fibers in a muscle unit contract _____.
A
Simultaneously
6
Q
What is the most important means of controlling muscle tension?
A
Recruitment of motor units.
7
Q
All fibers in a motor unit are the same ____.
A
Fiber type.
7
Q
What coordinates gradation of force in skeletal muscles?
A
The nervous system
7
Q
The smallest amount of muscle that can be activated voluntarily is called
A
Motor unit.
8
Q
Slow motor units contain ____.
A
Slow fibers.
9
Q
Fast motor units contain ____.
A
Fast fibers.
10
Q
Describe the physiological profile of slow motor units.
A
- ATP is used at a slow rate, myosin has a long cycle time.
- Many mitochondria are present, large ability to replenish ATP.
- Force is maintained economically during isometric contractions. There is efficient performance of repetitive slow isotonic contractions.
11
Q
Which type of motor unit can manage with anaerobic conditions?
A
Type 2B
12
Q
Describe the physiological profile of fast motor units.
A
- Myosin heads have a rapid cycling rate.
- Produces higher power for when isometric force
produced by slow motor units is insufficient. - 2A fibers sustain power, 2B fibers are faster.
13
Q
What is the function of 2A fibers?
A
Fast fibers - adapted for producing sustained power.
14
Q
What is the function of 2B fibers?
A
Faster fibers - fatigue rapidly due to being non-oxidative.
15
Q
Isometric contraction
A
The development of tension without muscle contraction (shortening)
16
Q
How do motor neurons compare to other neurons?
A
Motor neurons are larger than other neurons.
17
Q
The neuromuscular junction is a _____.
A
Specialized synapse.
18
Q
What is released at the NMJ?
A
Acetylcholine
19
Q
Where is the neuromuscular juntion?
A
Where the motor neuron attaches to the muscle fiber.
20
Q
Where does ACh bind in the NMJ?
A
Receptors on the muscle fiber (there are many different types of ACh receptors).
21
Q
List the steps that occur at the NMJ. (i’m so sorry this card is so long)
A
- Transmitters are synthesized and stored in vesicles.
- The action potential reaches the synaptic terminal of the pre-synaptic motor neuron.
- The synaptic terminal is depolarized, causing the opening of voltage gated calcium channels.
- There is an influx of calcium into the motor neuron through calcium channels.
- Calcium causes the fusion of vesicles with the synaptic membrane.
- Vesicles are retrieved from the plasma membrane and returned into the cell.
- Transmitters are released into the synaptic cleft through exocytosis.
- The transmitter binds to receptor molecules on the postsynaptic cell membrane.
- Postsynaptic channels open and close.
- Postsynaptic current causes excitatory/inhibitory postsynaptic potential that changes the excitability of the postsynaptic cell.
22
Q
What is the effect of clostridium botulinum toxin on acetylcholine?
A
Botox works on the pre-synaptic side of the NMJ. It reduces the efficacy of calcium channels which prevents the release of acetylcholine into the synapse.
23
How does clostridium botulinum toxin cause death?
Paralysis of respiratory muscles.
24
Describe acetylcholine metabolism.
Acetyl CoA is converted to CoA which releases an acetyl group. At the same time, Choline is converted to acetylcholine through the addition of an acetyl group. The enzyme for this reaction is choline acetyl transferase.
25
What are the uses of clostridium botulinum?
Reduce wrinkles (Botox), control strabismus (crossed eyes),
blepharospasm (uncontrolled blinking)
26
What is the effect of curare on acetylcholine?
Curare can bind to acetylcholine receptors, preventing ACh binding. It specifically blocks nicotinic receptors which then prevents ACh induced muscle contractions.
27
How are curare derivatives used?
A muscle relaxant prior to surgery.
28
What does myasthenia gravis cause?
It is an autoimmune condition. Causes reduced ACh receptor function.
29
How is myasthenia gravis treated?
With Neostigmine - an AChE inhibitor. This results in additional ACh to remain in the synapse and increases the binding to the receptor.
30
How do AChE inhibitors work?
Inhibit the enzyme acetylcholine esterase which normally breaks down acetylcholine into choline and acetate in the synapse. By inhibiting this enzyme, there is increased ACh that remains in the synapse.
31
How is acetylcholine regenerated in the cell?
Acetylcholine in the synapse is broken down into acetate and choline by acetylcholine esterase. Choline is then brought back into the cell through a choline carrier. Choline is then converted to acetylcholine by choline acetyl transferase.
32
Describe excitation-contraction coupling.
Depolarization of the motor end-plate is coupled to muscle contraction.
33
List the steps of excitation-contraction coupling.
1. Action potential propagates along sarcolemma and down the T tubules
2. Depolarization causes a conformational change in ‘DHP’ (Dihydropyridine) receptor in T tubule
3. DHP opening linked/gated to opening of ‘Ryanodine’ receptor on SR
4. Calcium is released from the SR
5. Ca2+ binds to troponin → causes position change in tropomyosin → active sites on actin exposed
6. Formation of “cross bridges” between myosin and actin
7. Calcium removed back to SR
8. Tropomyosin block on active sites for actin is restored → contraction ends (troponin and calcium are no longer bound)
34
What is the difference in the DHP receptor between different muscle types?
It is voltage sensitive in skeletal and a Ca2+ channel in cardiac.
35
Conformational change in DHP receptors allows for ____.
Rapid release of calcium from the SR.
36
What is the H zone?
The area of the sarcomere with only myosin (no actin overlapping)
37
What is the I band?
The area of the sarcomere with only actin.
38
What is the A band?
All areas of the sarcomere with myosin (areas with and without overlap).
39
What happens to the H zone during contraction?
It shortens.
40
What happens to the I band during contraction?
It shortens.
41
What happens to the A band during contraction?
It stays the same width.
42
What is the power stroke?
The myosin head binds to actin and pulls it towards the M line.
43
What is the M line?
The line in a sarcomere that myosin binds to.
44
What is the Z line?
A zig-zag line where actin binds. This line defines the end of a sarcomere.
45
What factors affect skeletal muscle contraction?
- Muscle length and tension
- Energy supply
46
What is the physiological range in the length-tension relationship?
The rate of length changes that can occur in the body. Here, there is the best amount of overlap in the sarcomere which allows the maximal amount of tension.
47
What occurs if the muscle is shortened past the physiological range?
Actin begins to overlap, the Z lines cannot move any closer together to contract.
48
What occurs if muscle is stretched beyond the physiological range?
There is limited overlap between actin and myosin which limits the force due to less actin-myosin binding. When the muscle is stretched to a certain point, there can be no overlap between actin and myosin and cross bridges will be unable to form.
49
What percentage of muscle length compared to resting is the physiological range?
70-130%
50
At what percentage of muscle length compared to resting do myosin and actin not overlap?
170%
51
When does the muscle have the ability for maximal tension?
At resting muscle length.
52
Isotonic contractions
Same tension - shortened muscle.
53
Describe the force-velocity relationship.
The capacity to generate force depends on the velocity of muscle contraction. These factors are inversely proportional, with maximal velocity occuring at zero load and maximal force at zero velocity (isometric contraction).
54
At what velocity is force at a maximal load?
Zero velocity.
55
What does increased frequency of muscle contraction result in?
Endurance
56
What is meant by muscle is plastic?
Muscle can adapt to meet the habitual level of demand placed on it.
57
What does increased load during muscle contraction result in?
Increased strength.
58
What happens when muscle use is decreased?
Decreased muscle mass.
59
What factors can lead to muscle use being decreased?
- Limb casting
- Denervation
- Aging
60
The level of physical activity is determined by _____.
the frequency of recruitment and the load.
61
How does the body and muscle adapt to endurance training?
- Cardiovascular adaptations
- Increased mitochondrial density
- Enhanced fuel storage
62
How does the body and muscle adapt to strength training?
- Muscle hypertrophy
- Neural adaptations
- Muscle fiber type changes
63
What happens during muscle hypertrophy?
The muscle cells gain myofibrils (there is not an increase in muscle cells).
64
What are the types of isotonic contractions?
- Concentric
- Eccentric
65
Concentric contraction
A decrease in muscle length due to the formation of cross bridges.
66
Eccentric contraction
An increase in muscle length due to the tension developed being less than the load on the muscle. Ex - Walking down a steep incline requires control of rate of elongation.
67
Is there any shortening during isometric contractions?
Yes, there is a small amount of internal shortening that occurs (the sarcomeres shorten when tension is developed).
68
Muscle agonist
The muscle doing the desired movement.
69
Muscle antagonist
The muscle lengthening during a movement.
70
What causes muscle relaxation?
- Elastic forces
- Opposing muscles
- Gravity
*these factors return muscle to resting length
71
Muscle relaxation is a ____ process.
Passive.
72
What are the stages of muscle twitch?
1. Latent phase.
2. Contraction phase.
3. Relaxation phase.
73
In which phase of muscle contraction is calcium sequestered?
The relaxation phase.
74
What is the latent phase of muscle twitch?
A short delay from the time when the action potential reaches the muscle until tension can be observed in the muscle.
75
What is the contraction phase of muscle twitch?
Occurs while the muscle is developing tension and forming cross bridges.
76
What is the reason for the latent phase of a muscle twitch?
It is the time required for calcium to diffuse out of the SR, bind to troponin, the movement of tropomyosin off of the active sites, formation of cross bridges, and taking up any slack that may be in the muscle.
77
What is the relaxation phase of muscle twitch?
There is a decrease in muscle tension.
78
What is treppe?
Occurs when the muscle is stimulated again immediately after the relaxation phase of the previous twitch. It causes an increase in tension in a staircase like way.
79
What occurs during the relaxation phase of muscle twitch.
- Calcium is actively transported back into the sarcoplasmic reticulum using ATP.
- The troponin moves back into position blocking the myosin binding site on the actin.
- The muscle passively lengthens.
80
What causes treppe?
A gradual increase in calcium due to the inability of the SR pumps to sequester all of the calcium before next twitch.
81
What is wave summation?
Occurs when the muscle is stimulated again before the relaxation phase is complete. The twitches are superimposed which results in a smooth, continuous muscle contraction.
82
What is incomplete tetanus?
Individual twitches are distinguishable, but the fluctuations are not as pronounced as treppe or summation. There are small peaks and valleys at maximal muscle contraction.
83
What is complete tetanus?
Individual twitches indistinguishable because of high frequency of stimulations. There is no relaxation phase tension.
84
What are the types of muscle energy metabolism?
- Creatine phosphate
- Glycolysis
- Oxidative metabolism
85
Is creatine phosphate aerobic or anaerobic?
Anaerobic.
86
Is glycolysis aerobic or anaerobic?
Anaerobic.
87
Is oxidative metabolism aerobic or anaerobic?
Aerobic.
88
Which type of energy metabolism has slow ATP production?
Glycolysis.
89
Which type of energy metabolism has rapid ATP production?
Creatine phosphate.
90
Explain creatine phosphate as energy metabolism for muscle.
- Excess energy is stored as creatine phosphate exclusively in muscle.
- Creatine phosphate is converted to creatine by the enzyme creatine kinase.
- During the reaction, the phosphate group is transferred to ADP to produce ATP.
91
What is the energy concentration in creatine phosphate compared to ATP?
Concentration is 3-6 times more in CP than in ATP.
92
What is the enzyme responsible for the phosphorylation of creatine phosphate?
Creatine kinase.
93
How long can ATP and creatine phosphate provide energy to a muscle?
10-15 seconds.
94
What are the products of the creatine phosphate reaction?
- Creatine
- ATP
95
How much ATP is produced by glycolysis?
2 ATP molecules per glucose molecule.
96
How long can the products of glycolysis provide energy?
2 mins
97
What muscle fibers use glycolysis?
Type IIB fibers.
98
What percentage of muscle weight is glycogen?
1%
99
Which metabolic process in muscles produces lactate?
Glycolysis - pyruvate is converted to lactate.
100
How much ATP is produced by oxidative metabolism?
36 ATP molecules per glucose.
101
Which muscle fibers use oxidative metabolism?
Type I fibers.
102
What processes are involved in oxidative metabolism?
- TCA cycle
- Oxidative phosphorylation
103
When are lipids used in muscle energy metabolism?
Oxidative metabolism switches from glucose to lipids during prolonged muscle use (ex. endurance training)
104
Where does oxidative phosphorylation occur?
Mitochondria.
