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258 - physiology > lecture 4: skeletal muscle contraction > Flashcards

lecture 4: skeletal muscle contraction Flashcards

1
Q

What are the 2 types of filaments involved in contraction of skeletal muscles

A

actin and MYOSIN

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2
Q

What are skeletal muscles and what are they composed of?

A

the muscles that surround the skeleton (bones)

are composed of numerous muscle fibers (or muscle cells)

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3
Q

what are muscle fibers and what are they innervated by

A

are excitable cells: generate and propagate action potentials

are innervated by nerve fibers
(to cause muscle contraction)

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4
Q

TRUE or false: One nerve fiber usually innervates multiple muscle fibers

A

TRUE

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5
Q

What is it called when one nerve motor innervates multiple msucle fibers?

A

MOTOR UNIT

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6
Q

Which disease is characterized by the degeneration of alpha motoneurons in the spinal cord and brain stem?

A

Amyotrophic lateral sclerosis

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7
Q

What does each muscle cell fiber contain?

A

Each muscle fiber (cell) contains several hundred to several thousand Myofibrils (tiny tubes which contain myson and actin)

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8
Q

What are the 2 contractile elements of a myofibril?

A

myofilaments (proteins):
Thick filament: Myosin
Thin filament: Actin

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9
Q

Which contractile element is thick

A

myosin

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10
Q

Which contractile unit is thin

A

actin

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11
Q

what is the sarcolemma ?

A

Sarcolemma is the membrane of the muscle fiber that surrounds myofibrils

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12
Q

explain sarcomeres

A

Sarcomeres are the smallest contractile unit of muscle fibers (2 micrometers during contraction)

Where the contraction occurs

They are aligned end-to-end in myofibrils. Separated by Z discs

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13
Q

What causes the actin filaments to slide inward on the myosin

A

Forces generated by interaction between the two filaments: Power strokes

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14
Q

During muscle contraction, the actin filaments are pulled by the myosin filament inward or outward toward the center of the sarcomere

A

INWARD

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15
Q

What is myosin composed of and be able to label it

A

Myosin: Thick filament

Composed of the body (stem) and cross-bridges (rowers) (head & arm)

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16
Q

What are actin filaments composed of ?

A

Actin: Actin filaments are composed of Actin, Tropomyosion, and Troponin

Tropomyosin molecule: hides active sites (long filament during relaxion)
Troponin molecule: moves tropomyosin from active sites

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17
Q

What is the function of the tropomysin molecule?

A

Tropomyosin molecule: hides active sites (long filament during relaxion)

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18
Q

What is the function of troponin

A

Troponin molecule: moves tropomyosin from active sites

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19
Q

What is the interaction between actin and myosin in the relaxed state>

A

NO INTERACTION
In the relaxed state
Active sites on the actin filament are covered by tropomyosin

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20
Q

What is the interaction between actin and myosin in the muscle contraction state?

A

Ca2+ is released and Ca2+ ions bind on troponin

Troponin undergoes a conformational (i.e. shape) change, moving away the tropomyosin and uncovering the active sites

Myosin cross-bridges (heads) are attracted to the active sites of the actin

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21
Q

What are the 4 steps of cross bridge cycling

A

1) ATP binding
2) uncovering of active sites
3) power stroke
4) detachment

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22
Q

explain the first step of the cross bridge cycling

A

ATP binds on the head of the myosin and splits into ADP & Pi (stored energy),

The activated head extends perpendicularly toward the actin filament (not attached yet)

ATP STORED AND AVAILABLE

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23
Q

explain the 2nd step of the cross bridge cycling

A

Ca2+ causes the Troponin to move Tropomyosin and uncovering the active sites of the Actin.

Myosin head attaches to the active site of the actin filament.

Active sites are uncovered

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24
Q

explain the 3rd step of the cross bridge cycling (powerstroke)

A

Liberation of the stored energy (ADP & Pi) forces the head to pivot toward the arm and thus pulling the actin filament and sliding it toward the center line, Power Stroke.

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25
Q

explain the 4th step of the cross bridge cycling

A

A new ATP attaches to the myosin head and causes detachment of the head from the actin

recovering the active sites

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26
Q

what is. a muscle twitch

A

A muscle twitch (contraction) is the tension developed in response to one nerve stimulation

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27
Q

explain sumamtion of muscle twitches

A

Summation means the adding together of individual muscle twitches to increase the intensity of overall muscle contraction

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28
Q

What are the 2 ways that summation occurs

A

1) multiple fiber summation

2) frequency summation

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29
Q

explain multiple fiber summation

A

Increasing # of motor units contracting at the same time

recruitment of muscle

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30
Q

explain frquency summation

A

Increasing the frequency of stimulation of one motor unit

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31
Q

explain the size principle and what type of summation doe it relate to

A

Size Principle: Depending on the intensity of the stimulation, motor units are recruited in an orderly fashion according to their size:
Smallest motor units are recruited first (for weak stimulation)
Largest motor units are recruited last (for strong stimulation)

MUSCLE FIBER SUMMATION

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32
Q

small motor units are recruited first or last and what type of stiumlation occurs

A

first and weak

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33
Q

What does the size principle allow?

A

It allows the graduation of muscle force from small steps (weak contraction) to great steps (strong contraction)

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34
Q

EXplain incomplete tetanus

A

As the frequency of the signal increases, individual twitches summate (no recovery form previous contactions)

35
Q

explain tetanization

A

If the frequency reaches a critical level, the summated twitches fuse together to form one continuous contraction, TETANIZATION

Contraction reaches a Max level, no response to further stimulations

Ca2+ ions are maintained in the sarcoplasm, the contractile state of the muscle is sustained (myosin & actin stay attached).

36
Q

be able to explain frequency summation

A

LOOK IN SLIDES

37
Q

Do small muscles that require precision have high or low innervation ration?

A

Low innervation ration (ie. one nerve innervates only 2-3 muscle fibers)

38
Q

Do Large muscles that do not require precision (e.g. gastrocnemius) have a low or high innervation ration

A

-High innervation ratio (ie. one nerve innervates 1000-2000 muscle fibers)

39
Q

What are the 2 types of muscle fibre classifications

A

1) slow fibers (type 1, red muscle)

2) fast fibers (type 2a, 2b, white msucle)

40
Q

Explain slow fibers

A

Smaller than fast fibers
Innervated by smaller nerve fibers
Have a more extensive blood vessel system and more capillaries to supply extra amounts of O2
Have more mitochondria to support high levels of oxidative metabolism

41
Q

explain fast fibers

A

Larger for great strength of contraction
An extensive sarcoplasmic reticulum is present for rapid release of Ca2+ to initiate contraction
Large amounts of glycolytic enzymes are present for rapid release of energy (do not last as long)
Less extensive blood supply and fewer mitochondria vs. slow fibers

42
Q

True or false: The mixture of our slow and fast fibers is determined by training?

A

faslse its genetically determined

43
Q

In most muscles used for movement:

what is the average percentage of slow and fast fibers

A

50% slow fibers

50% fast fiber

44
Q

know the graph for Speed and Strength of Contraction of Muscle Fibers

A

check slides

45
Q

Can Exercise Training Change Muscle Fiber Types?

A

yes but very limited changes

-Fibers can change within their own type—IIa can convert to IIb and vice versa

46
Q

what are the unknown factors when it comes to muscle fibers (3)

A

unknown if:

1) muscle types are more malleable early in life
2) certain muslces like the biceps, are more adept at changing fiber types than others
3) muscle fibers can truly change between type 1 and 2 and how long it takes to do so

47
Q

Why do you think soleus & gastrocnemius muscles have a greater % of slow twitch vs. fast twich fibers?

A

ANSWER:To sustain prolonged periods of standing and walking

48
Q

What is another name for the neuromuscular junction

A

MOTOR END PLATE

49
Q

What is The released neurotransmitter in muscular contraction ??

A

acetylcholine

50
Q

Explain the neuromusclar junction

A

Contact between nerve terminals (branching from motor neuron axon) and muscle fiber plasma membrane (Sarcolemma)

Equivalent of chemical synapse in the CNS

Nerve terminal invaginates into the surface of the muscle fiber

51
Q

explain the steps at the neuromuscular junction

A
  1. AP arrives at the presynaptic terminal
  2. Local depolarization opens voltage-gated Ca2+ channels and Ca2+ ions flow into terminal
  3. Ca2+ ions trigger the fusion of synaptic vesicles with the pre-synaptic membrane
  4. Release of transmitter onto the synaptic cleft (exocytosis)
52
Q

What does the muscle fiber membrane contrain

A

Muscle fiber membrane has acetylcholine receptors that contain cation channels (Na+, K+).

53
Q

explain what happens at the neuromuscular junction

A

Acetylcholine (ACh) molecules attach to binding sites in the muscle fiber membrane

Cation channels open:
(Positive ions (Na+, K+, Ca++) flow through channels
BUT far more Na+ ions flow inside
 1 Na+ channel can transmit: 
  (15 000 to 30 000 Na+ ions 1 ms!))

Entry of Na+ ions creates an end plate potential (equivalent of EPSP).

End plate potential initiates action potential (Threshold approx. -40 mV)

54
Q

What is the neuromuscular. junction equivalent to in the CNS

A

gthe chemical synapse

55
Q

be able to label the sarcolemma, sarcoplasmic reticulum, t tubles, myofibirls, terminal cisternea

A

docs

56
Q

Where is the SR location and what does it contain

A

Surrounds myofibrils (as a net)

Contains Ca2+ (necessary for muscle contraction process)

Contains voltage-sensitive Ca2+ receptors(diffusion of Ca2+ into the sarcoplasm)

Contains a Ca2+ pump to remove the Ca2+ from the sarcoplasm

57
Q

Where is the T tubles located and what are the fucntion

A

Begin at cell membrane (sarcolemma) and penetrate all the way from one side of the membrane to the opposite side
Run transverse to myofibrils

For easy propagation of Aps inside the muscle fiber (rich with Na+/K+ channels)

58
Q

how many tranverse tubules per sarcomere?

A

2 transverse tubules per sarcomere

59
Q

Where is the Ca2+ stored and what does it do

A

Terminal cisternae

increasing the capacity of SR to release Ca2+ rapidly

60
Q

be able to explain the excitation cotnraction couple grpahic

A

slides

61
Q

be able to describe MS

A

Antibodies block/destroy Ach receptors

Symptoms: muscle weakness, that develops progressively across the day (worse towards the end of the day)

Affects facial muscles (ex: eyelid drooping), limb muscles and respiratory muscles etc.

Immunosuppressant treatments
Symptomatic treatment: Acetylcholinesterase inhibitors

62
Q

What is another name for Rigor Mortis aand what is it and how long does it last

A

postmordem rigidity
Sustained muscle contraction after death
Occurs within the first day (3-12 hours), and decreases gradually (depending on the ambient temperature: cold slows down)

63
Q

Why does Rigor Mortis occur?

A

LACK OF ATP

Ca2+ pump cannot transport Ca2+ back into the sarcoplasmic reticulum, thus Ca2+ keeps the actin active sites continuously exposed

Myosin head cannot detach from actin (needs ATP)

64
Q

be able to recognize the tension graph vs lengthof sarcomere

A

graph

65
Q

What point on the tension/lengh graph has the least tension and why

A

Point D: Actin filament has pulled all the way out to the end of myosin filament

66
Q

what points on the graph have the most tension and why

A

Point C: Sarcomere shortens and actin filament begins to overlap myosin.

Tension ↑ until sarcomere reaches 2.2 micrometers (μm)
Point B: sarcomere maintains full tension (length = 2.0 μm)

67
Q

explain point A on the tension/length graph

A

The end of actin filaments overlap each other. As the sarcomere length ↓ from 2.0 μm down to 1.65 μm, the strength of contraction ↓ rapidly

68
Q

explain passive, active and total tension

A

Passive tension: Tension/force generated in absence of energy (no ATP) when muscle fibers are pulled apart
(ex: stretching fingers after stroke, MS)

Active tension: Tension/force when muscle is stimulated to contract
Due to overlapping of actin and myosin heads
Involves energy (ATP)

Total tension: Summation of passsive and active tension

69
Q

be able to reconize passive, active and total tension on a graph

A

slides

70
Q

what are the passive structures that elongate in passive tension

A

TITIN (inside the muscle)

COLLAGEN (surrounding muscle cell

71
Q

explain. Titin:

A

Protein believed to be the greatest contributor to passive force throughout normal ranges of motion

Connects the Z disk to the M line in the sarcomere

Limits the range of motion of the sarcomere in tension, thus contributing to the passive stiffness of muscle

72
Q

true or false: titin is believed to be the greatest contributor to passive force throughout normal ranges of motion

A

.true

73
Q

what determines tension in the muscle

A

Interaction between myosin heads and actin filaments determines the tension

74
Q

what is the resting length of a muscle

A

the length at rest from which a muscle develops MAXIMUM isometric tension

75
Q

what is muscle tension dependent on?

A

Muscle tension is dependent on muscle length

76
Q

When a muscle becomes more flexible (as a result of stretching), what do you think happen to the passive tension curve?

A

moves to the right (longer length becfore tension occurs)

77
Q

What is maximum strenght of xontraction of a muscle

A

Maximum strength of contraction of a muscle operating at a normal length averages 3.5 Kg/cm2 (sq. cm) of muscle

78
Q

Does occular movement muscle contraction have to be slow or fast and why

A

Must be extremely rapid to

maintain fixation of the eyes on specific objects to provide accuracy of vision

79
Q

Does gastrocnemius movement muscle contraction have to be slow or fast and why

A

Gastrocnemius must contract moderatly rapidly to provide enough velocity of limb movement for running and jumping

80
Q

Does soleus muscle contraction have to be slow or fast and why

A

Soleus is concerned mainly with slow contraction for continual, long-term support of the body against gravity

81
Q

what is muscle hypertrophy

A

increase in total muscle mas

82
Q

How does muscular mypertrohy happen?

A

Results from an ↑ in number of actin and myosin filaments in each muscle fiber, causing enlargement of the individual muscle fibers

83
Q

true or false: Only a few strong contractions each day are required to cause significant hypertrophy within 2-4 weeks

A

false: 6-8 weeks

84
Q

what is the effect of training? (2)

A

1) Number of actin and myosin filaments in myofibrils can increase by as much as 50%!
2) Myofibrils can split to form new myofibrils and ↑ fiber & muscle size

258 - physiology (21 decks)

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