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Structure and Function 1- Exam 2 > Skeletal Muscle Physiology > Flashcards

Skeletal Muscle Physiology Flashcards

1
Q

Isotonic contraction

A

—Isotonic: Once the muscle has developed enough tension to move the load, the tension remains constant and muscle shortens.

  • Shortening of muscle
  • Movement of load
  • Tension > load
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2
Q

List skeletal muscle types by increasing fiber diameter

A

Slow oxidative (small)

Fast oxidative (Intermediate)

Fast glycolytic (large)

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3
Q

What are the components of a triad?

A

1 T-tubule + 2 terminal cisterns of the sarcoplasmic reticulum

*The triad association is important for the contraction of the skeletal muscle because it is couple to the release of calcium.

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4
Q

What activities are each skeletal muscle type most suited for?

A

Slow oxidative

  • Endurance type activities

Fast oxidative

  • Sprinting, walking

Fast glycolytic

  • Short term interse or powerful movements
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5
Q

What does a motor unit consist of?

A

A somatic motor neuron plus all the skeletal muscle fibers it stimulates.

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6
Q

What are the two methods of generating graded responses?

A

—Temporal summation: An increase in the tension developed in a skeletal muscle fiber caused by an increase in the frequency of action potentials.

—Motor unit recruitment

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7
Q

Molecules found in skeletal muscles that contain enzymatic material for glycolysis

A

Glycosome

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8
Q

What is troponin?

A

A complex of 3 proteins that bind to actin, tropomyosin, and calcium

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9
Q

What are the stabilizing filaments of muscle?

A

Titin

Nebulin

Myomesin

Dystrophin

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10
Q

Each axon terminal innervates _______ muscle fiber.

A

One

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11
Q

Bulbous end regions of the sarcoplasmic reticulum are called_____________

A

Terminal cisterna

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12
Q

Myasthenia gravis

A
  • Autoimmune disease: antibodies to nicotinic acetylcholine receptors inhibit contraction.
  • Chronic inflammation of MG causes structural changes of the neuromuscular junction.
    • flattening out of the junctional folds
      • Don’t function well
    • spreading out of AChR and Acetylcholinesterase
    • 66% decrease in number of AChR
    • an increased junctional gap.
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13
Q

Once the acetycholine is released, and it binds to its receptors, ion channels are opened. Are these ion channels specific? What do they allow for?

A

Ion channels are not specific. It allows sodium and potassium ions to flow through and down their gradients. Sodium flows into the cell and potassium flows out.

NOTE: Sodium flows much faster than potassium, so this will lead to depolization at the motor end plate.

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14
Q

Why is it that another AP can be generated shortly after the muscle has started to contract?

A

Muscles AP lasts for a short time and it’s almost finished before the muscle even begins to twitch, meaning that another AP can be generated shortly after the muscle has started to contract.

* These conractions can therefore be summated in skeletal muscles.

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15
Q

When you contract the sarcomere the ___________ disappears.

A

H-zone

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16
Q

End plate potential .vs. action potential

A

—EPP (generator potential)

  • Ligand gated channels
  • Charge carriers are Na+, K+
  • Passive repolarization

—AP

  • Voltage gated channel
  • Charge carrier for depolarization is Na+
  • Repolarization due to increased K+ conductance
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17
Q

How do you generate a stronger force using motor unit recruitment?

A
  • Each motor unit requires a different stimulus. If you have a stimulus that stimulates all at once, you have a stronger force opposed to a stimulus that only stimulates one motor unit.
  • Increasing the amount of stimulus coming from upper motor neurons increases the amount of lower motor neurons that are activated.
    • This increases the amount of cells that contract and increase the overall contraction of the muscle.
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18
Q

How is ATP regenerated?

A

Phosphorylation of ADP by creatine phosphate (CP)

—Generates 1 ATP per CP

—Does not require oxygen

—Provides 15 sec. of energy

Glycolysis

—Generates 2ATPs per glucose

—Does not require oxygen

—Fast process

*Fast twitch fibers tend to use these two methods

Aerobic respiration in mitochondria (citric acid cycle)

  • Generates 30-32 ATPs per glucose or FA
  • Slow process

•Dependent on oxygen

*Slow twitch fibers use this method

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19
Q

Basic contractile unit of skeletal muscle

A

Sarcomere

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20
Q

What properties of muscle fibers determine twitch activity?

A
  • Muscle properties such as:
    • # of blood vessels
    • Whether or not they have mitochondria
    • Whether or not they use aerobic respiration

*There are different variants of myosin in our bodies that are genetically determined

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21
Q

What is the function of tropomyosin?

A

Blocks the myosin-binding site, when the muscle is at rest

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22
Q

What are three factors which affect the development of muscle tension?

A

—1. Frequency of stimulation

—2. Number of motor units recruited

—3. Degree of muscle stretch

NOTE: We don’t need to generate maximum force in our muscles at all times. Less force for lighter things. More force for heavier things.

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23
Q

Strings of G- actin produce _____________

A

F- actin

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24
Q

The synaptic cleft of the neuromuscular junction is rich in __________ and ____________.

A

Glycoproteins and collagen fibers.

*The glycoproteins help keep neurotransmitters in the cleft just a little bit longer before being degraded by acethylcholinesterase.

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25
Q

What is treppe?

A

A gradual increase in tension due to a certain stimulus frequency

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26
Q

Adaptations to aerobic exercise leads to…

A
  • Leads to increase in muscle capillaries, number of mitochondria and myoglobin synthesis.
  • Results in greater endurance, strength, and resistance to fatigue
  • May convert fast glycolytic fibers into fast oxidative fibers

* You can’t change slow to fast twitch becuase you can’t change the myosin activity

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27
Q

Is their an axon hillock in muscle cells?

A

No

*Outside of the motor end plate is where you’ll find the sodium and potassium voltage-gated channels

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28
Q

Active tension is due to binding between _____ and ________. Force generated can be limited by the __________.

A

Actin

Myosin

starting point

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29
Q

Explain steps to propagation of action potential

A
  1. An end plate potential is generated at the neuromuscular junction
  2. Depolarization: Generating and propagating an action potential (AP). The depolarization current spreads to adjacent areas of the sarcolemma. Voltage-gated Na+ channels open. Na+ enters cell and initiates the AP.
  3. Repolarization occurs as Na+ channels close (inactivate) and voltage-gated K+ channels open. K+ diffuses rapidly out of the muscle fiber.
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30
Q

What is the function of synaptotagmin?

A
  • Binds Ca2+
  • Initiates fusion of vesicles with the docking areas/ SNARES, allowing for release of acetylcholin
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31
Q

At what point does the force generation reach its peak?

A

When all of the troponin C molecules are bound to calcium.

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32
Q

Each muscle fiber has specialized region of sarcolemma with junctional fold containing ____________.

A

Ach receptors

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33
Q

What is the relationship between length and tension, in active tension?

A
  • If you overcontract the muscles, the actin in the middle begins to overlap. When they overlap they begin to cover up myosin-binding sites.
  • If you stretch the muscle, we get an “overlap” between myosin and actin that is no longer ideal becuase you have myosin heads that no longer have actin to bind to.
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34
Q

Why can each muscle cell only be innervated by one motor neuron?

A

In skeletal muscle each muscle cell is innervated by one neuron because it only has one motor end plate, so there’s only ONE region on the skeletal muscle cell that is innervated.

*That one end plate has to be enough to activate that cell at that one muscular junction

NOTE: This is very different from neurons that can have inhibitory post synaptic junctions and excitatory post synaptic junctions.

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35
Q

How is a skeletal muscle fiber stimulated to contract?

A

Glutamate is released from upper motor neurons to stimulate lower motor neurons. Once stimulated lower motor neurons secrete acetylcholine.

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36
Q

At shortened lengths, what contributed completely to total tension?

A

Active tension

*All force is generated actively

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37
Q

The ___________ receptor is found on the T-tubule and the ____________ is found on the SR.

A

Dihydropyridine; ryanodine

*The dihydropyridine and ryanodine receptors interact when an AP is proprogated. A conformational change happens with DP receptors that causes RD receptors to open. This allows for the release of Ca2+ into the cytosol.

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38
Q

Total tension=__________

A

Active tension + Passive tension

(a)At shortened lengths all force is generated actively.

Total tension = active tension

(b) As the muscle fiber is stretched beyond its resting length, passive tension begins to contribute to the total force.

Total tension = Active + passive

(c)If the muscle is further stretched, passive tension accounts for most of the total force.

Total tension = passive tension (almost)

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39
Q

What steps follow the formation of actin-myosin cross bridge?

A
  1. Cross bridge formation. Energized myosin head attaches to an actin myofilament, forming a cross bridge.
  2. The power (working) stroke. ADP and Pi are released and the myosin head pivots and bends, changing to its bent low-energy state.
    * As a result it pulls the actin filament toward the M line.
  3. Cross bridge detachment. After ATP attaches to myosin, the link between myosin and actin weakens, and the myosin head detaches (the cross bridge “breaks”).
  4. Cocking of the myosin head. As ATP is hydrolyzed to ADP and Pi, the myosin head returns to its prestroke high-energy, or “cocked,” position. *
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40
Q

The voltage stimulus that produces the initial contraction is called the ______________

A

threshold stimulus.

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41
Q

____________ is a low-affinity, high capacity Ca2+ binding protein, within the terminal cisternae, keeps [Ca2+] in the SR.

A

Calsequestin

*Most of Ca2+ is stored throughout the entire SR but there is a higher concentration in the terminal cisterna becuase of the presence of calsequestrin, which binds to Ca2+

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42
Q

What happens once Ca2+ is released into the myoplasm?

A

Calcium binds to troponin- C in the skeletal muscle cell. Once Ca2+ binds it causes a conformational change in troponin C that causes tropomysosin to move from the binding site (it was covering up actin). Now myosin can bind.

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43
Q

What is muscle fatigue? How and why does it occur?

A

Muscle fatigue is the physiological inability to contract despite continued stimulation.

  • Occurs when
    • Ionic imbalances (K+, Ca2+, Pi) interfere with E‑C coupling
      • Decreased release of calcium from SR
    • ATP generation via glycolysis produces increases in [H+]. Decrease pH has been demonstated to:
      • reduce the affinity of troponin for calcium.
      • Reduce myosin ATPase activity

NOTE: Muscle fatigue is NOT about using up ATP

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44
Q

Are smaller or larger motor units more excitable?

A

Small motor units (small fibers) are more excitable (lower threshold stimulus) than large motor units.

—Smallest motor units have slow oxidative fibers (slow twitch)

—Larger motor units have fast glycolytic fibers (fast twitch)

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45
Q

What is the difference between an AP of skeletal muscle fiber and cardiac muscle fiber?

A

Skeletal muscle fiber

  • AP pretty much done before muscle contracts
  • Refractory period is short
    • This means that skeletal muscle can undergo summation and tetanus, via repeated stimulation

Cardiac muscle fiber

  • Long AP
  • Tension in muscle develops way before action potential is over
  • AP repolarization and muscle relaxation occur almost at the same time
  • Cardiac muscle CAN NOT sum action potentials or contractions can’t be tetanized
46
Q

What is passive tension?

A

Our muscles also have passive tension. This is due to titin molecules (that have an elastic recoil force).

  • Titin helps to stabilize and prevent overstretching of sarcomere.
47
Q

A stimulus strength that recruits all motor units is the ____________

A

maximum stimulus

48
Q

What is the function of myomesin?

A

Binds myosin and titin at M line

*The M line is the midline of the sacromere

49
Q

What are the 3 phases of a muscle twitch?

A

—Latent period:

  • follows stimulation
  • Excitation-contraction coupling begins
  • No measurable muscle tension developed

—Period of contraction:

  • Cross bridge formation occurs
  • Tension develops

—Period of relaxation

—Ca2+ reentry into the SR

—Tension declines

50
Q

What is needed for muscles to relax?

A
  • Ca2+ needs to reenter the SR
    • This is done by ATP-DEPENDENT sacro pumps on the surface of the SR
    • Once the calcium is taken back in and its no longer in the cytosol, the muscle can relax

*There are also pumps on the surface of muscle cell membrane (sarcolemma) that can pump Ca2+ out into ECM

51
Q

The tension that you feel when you try to stretch your muscles is due to which protein?

A

Titin

  • Titin trys to pull sacromere back to ideal length
52
Q

Incomplete tetanus

A

Maximum contractions, but, muscle relaxes slightly between contractions creating a quivering effect

*Muscle fibers do not completely relax before the next stimulus because they are being stimulated at a fast rate; however there is a partial relaxation of the muscle fibers between the twitches

53
Q

What are the components of the thin filament?

A

Actin

Tropomyosin

Troponin

54
Q

in the absence of ATP, myosin heads will not detach from actin, causin ____________.

A

Rigor mortis

55
Q

During muscle shortening, the heads of the myosin filaments pull the actin filaments inward toward the __________. This action____________ (increases/ decreases) the area of overlap between the thick (myosin) and thin (actin) filaments.

A

M line; increases

56
Q

Explain the steps to excitation at neuromuscular junction

A
  1. Action potential arrives at axon terminal of motor neuron.
  2. Voltage-gated Ca2+ channels open. Ca2+ enters the axon terminal moving down its electochemical gradient.
  3. Ca2+ entry causes ACh to be released by exocytosis.
  4. ACh diffuses across the synaptic cleft and binds to its receptors on the sarcolemma.
  5. ACh binding opens ion channels in the receptors that allow simultaneous passage of Na+ into the muscle fiber and K+ out of the muscle fiber. More Na+ ions enter than K+ ions exit, which produces a local change in the membrane potential called the end plate potential.
57
Q

Explain the interaction between dihydropyridine and ryanodine after an action potential is fired.

A

AP causes a conformational change to receptors in T-tubule (dihydropyridine) because of voltage. Because DP and RD receptors are closely related, a conformational change in DP causes a conformation change in RD that causes it to open.

58
Q

What is the function of nebulin?

A
  • Binds with and stabilizes F-actin
  • Attaches to Z-disc
59
Q

How do you get a stepwise increase in peak tension of individual fibers?

A
  • When a muscle has been at rest for a long time and then its been stimulated to contract, you get a little bit of Ca2+ come in from the SR to get a contraction.
  • If you stimulate again right after that, all of the Ca2+ thats in the cytoplasm has not been pumped out to ECM or pumped back into SR yet, so this lingering Ca2+ in addition to the bolus of calcium coming in upon the second stimulus will cause an increase in tension.
  • Each time there’s a stimulus right after relaxation, you build upa residual calcium, until you reach a maxpoint/ plateau, where you have enoigh Ca2+ to where all of troponinc C molecules are attached to calcium. This is the largest force you can get for that single motor unit.
  • If you want to get that muscle to contract even more you have to stimulate additional motor units.
60
Q

________________ breaks down Ach.

A

Acetylcholinesterase

61
Q

What is different in the innervation of muscle cells and neurons?

A
  • When it comes to neurons, they can innervate and be innervated by several neuron.
  • When it comes to muscle cells, they can only be innervated by one neuron.
    • However when it comes to motor units, neurons can innervate several muscles cells.

*So you can have an upper motor neuron that innervates mor than one lower motor neuron. One lower motor neuron can be innervated by several different upper motor neurons.

62
Q

What is the importants of the essential component of the light chain?

A

The essential component can be phosphorylated and can direct how the head moves in addition to how quickly it moves.

*This regulation is important in cardiac and smooth muscle, not so much in skeletal

63
Q

Complete tetanus

A

Contractions maximal, smooth and sustained

*No relaxation of the muscle fibers between stimuli and it occurs during a high rate of stimulation.

*A fused tetanic contraction is the strongest single-unit twitch in contraction.

64
Q

Lower motor neurons innervate _____________, while upper motor neurons innervate___________.

A

Skeletal muscle; one or mor lower motor neurons in the spinal cord

65
Q

Define tension and load.

A

Muscle tension:

—The force generated by a contracting muscle

Load:

—The force opposing the contracting muscle

66
Q

As a muscle is stretched beyond a critical length (ideal length) tensions builds as an ___________ function.

A

exponential

67
Q

Where are upper motor neurons located? What about lower motor neurons?

A

Upper motor neurons are located in the motor cortex. Lower motor neurons are primarily located in the spinal cord (ventral horn)

68
Q

Why are slow twitch fibers red?

A
  • High levels of myoglobin
  • High levels of blood vessels
69
Q

We have muscle tone in all of our muscles because of ___________

A

Spinal reflexes

*There is a sensory input from muscle stretch receptors. Groups of motor unit are activated in response to input from stretch receptors.

70
Q

Why are small motor units more excitable than large motor units?

A

Because they have a lower threshold stimulus

REMEMBER: The threshold stimulus is the voltage stimulus that produces the initial contraction, so small motor neurons require a small voltage to illicit a contraction

71
Q

Where are the cell bodies of lower motor neurons located? Where do the axons of lower motor neurons exit?

A
  • Cell bodies of lower motor neurons are located in the ventral horn of the spinal cord.
  • Axons exit the spinal cord via the ventral root and project in spinal nerves to innervate target muscles.
72
Q

What effect does the load have on duration and velocity of a contraction?

A

The greater the load, the less the muscle shortens and the shorter the duration of contraction

73
Q

In which type of contraction is tension less than load?

A

Isometric

74
Q

What is the function of titin?

A
  • Connects myosin to Z line
  • Stabilizes myosin and protects from over stretch
  • Responsible for muscle flexibility

*Titin is elastic and can extend from the Z- disc and run through the core of the myosin filament

75
Q

In ___________ contraction the muscle shortens. In ____________ contraction the muscle lengthens.

A

Concentric; eccentric

76
Q

What is the recruitment order for the three skeletal muscle types?

A

1st: Slow oxidative
2nd: Fast oxidative
3rd: Fast glycolytic

77
Q

How does the end plate potential undergo passive repolarization?

A

The ligand gated channels don’t stay open forever and the Ach also gets metabolized

78
Q

What is the function of titin?

A

It stabilizes myosin to the Z line

79
Q

Skeletal muscles are innervated by motor neurons that emanate from the spinal cord. Can our skeletal muscles contract without that innervation?

A

No!

*Other muscles can contract without innervation but our skeletal muscles cannot.

80
Q

Calcium binds to what component of the SNARE complex?

A

Synaptotagmin

81
Q

What is active tension?

A

Muscles operate with greatest active tension when close to an ideal length (often their resting length).

  • When stretched or shortened beyond this , the maximum active tension generated decreases.
  • This decrease is minimal for small deviations, but the tension drops off rapidly as the length deviates further from the ideal.
82
Q

What is the importance of muscle tone?

A
  • Responsible for balance and posture
  • Keeps muscles firm, healthy and ready to respond

*Muscle tone is the constant, slightly contracted state of muscles

83
Q

What steps occur to lead to a contraction, after the action potential is fired?

A
  1. AP propagates along the sarcolemma and down the t-tubules.
  2. Receptor coupling t-tubule (Dihydropyridine receptor) to SR changes form and opens Ca2+ channels (Ryanodine receptor).
  3. Ca2+ is released into the myoplasm.
  4. Ca2+ binds to troponin releasing the blocking action of tropomyosin, uncovering myosin binding sites .
  5. Myosin head forms binds to actin creating cross bridge.
84
Q

What is the difference between the AP for skeletal muscle cells compared to neurons?

A

There is NO hyperpolarization in skeletal muscles because the resting membrane potential is lower/ more negative than potassium’s equilibruim potential.

85
Q

Z line represents ___________

A

End of sarcomere

86
Q
  • 25-40% of energy released in muscle activity useful as ________
  • Remaining energy (60-75%) is given off as _________
A

work

heat

87
Q

As the muscle fiber is stretched beyond its resting length, __________ tension begins to contribute to the total force.

A

Passive

NOTE: If the muscle is further stretched, passive tension accounts for most of the total force.

88
Q

I band represents ___________

A

Actin

89
Q

Isometric contraction

A

Isometric: muscle develops tension, but muscle length does not change.

—Muscle does not shorten

—Load is not moved

—Tension < load

*Tension isn’t enough to move the load

90
Q

Large motor units have __________ fibers.

A

Fast glycolytic

91
Q

What is the function of dystrophin?

A
  • Maintains structural integrity
  • Achors actin to the sarcolemma

*Defienciencies and mutations linked to muscular dytrophies

92
Q

The neurotransmitter that is released from our lower motor neuron is ____________; the receptors it binds t on skeletal muscle is ____________. The neurotransmitter that is released from upper motor neurons is ___________; the receptor that it binds to on lower motor neurons is _____________.

A

Acetylcholine; Nicotinic

Glutamate; glutamatergic

93
Q

Thick filaments: ___________

Thin filaments: __________

A

Myosin

Actin

94
Q

What is considered a muscle twitch?

A
  • Motor unit’s response to a single action potential of its motor neuron
  • Simplest contraction observable in lab (recorded as myogram)
95
Q

What happens after the membrane is depolarized?

A

If the depolarization is strong enough to cross the threshold, it will lead to the firing of an action potential.

NOTE: The threshold is the point at which voltage-gated channels are activated. You won’t get an AP with ligand gated channels. You will only get an AP with voltage-gated channels

96
Q

Characteristics of Skeletal, Cardiac, and Smooth Muscle

A

—Skeletal

  • Striated & Voluntary
  • Requires somatic nervous system stimulation
  • Rapid contractibility & high fatigability

—Smooth

  • Smooth & Involuntary
  • Stimulated by ANS, hormones, stretch & pacemaker cells
  • Slow, sustained contractions & low fatigability

—Cardiac

  • Striated & Involuntary
  • Self initiated stimulation & ANS modification
  • Continuous contraction/relaxation & No fatigability
97
Q

What are the two myosin protein chains? What are characteristics of each?

A

—Myosin has 2 types of protein chains.

  • Heavy chains
    • Has ATPase activity
    • Binds to actin
  • Light chains
    • Essential
    • Regulatory
98
Q

O2 storage unit in muscles

A

Myoglobin

99
Q

Skeletal muscles use ATP for contraction and relaxation. In what way?

A
  • Cross bridge attachment and detachment
  • Calcium pumps in the sarcoplasmic reticulum
  • Na+/K+ pumps that return sarcolemma to resting conditions.

NOTE: Cellular stores of ATP are limited and depleted within 6 seconds of contraction.

100
Q

How does botulinum toxin work?

A
  • Works by blocking the exocytosis of Acetylcholine
    • ​disrupts the function of SNARE proteins
    • It degenerates/ cleave the SNARE proteins, thereby inhibiting the release of the vesiscles.

*Short-lived because SNARE proteins just grow back

101
Q

What is the relationship of action potention to tension development in skeletal muscle?

A

The AP you trigger the more fibers that will be stimulated, which will increase the tension development of the WHOLE muscle

*Each time a skeletal muscle is stimulated it builds tension until it reaches a plateau, at which point you won’t get an increase in tension with the same level of frequency of stimulation.

102
Q

What is excitation- contraction coupling?

A

E-C coupling is the sequence of events by which the initiation of an action potential along the sarcolemma leads to the contraction of myofilaments.

103
Q

A band represents____________

A

Myosin

104
Q

Cardiac muscle cannot sum action potentials or contractions and can’t be tetanized. Why is this important?

A

The heart keeps the rhythm, fils up with blood, and then pumps it out. The heart needs to be able to refill with blood and then supply the body with blood

105
Q

Contratile units of muscle

A

Myofibrils

106
Q

What are the three skeletal muscle types?

A

Slow oxidative fibers

Fast oxidative fibers

Fast glycolytic fibers

107
Q

Small motor units have _________ fibers.

A

Slow oxidative

108
Q

Duchenne Muscular Dystrophy

A
  • X- linked, recessive disease
  • Caused by a mutation in the dystrophin gene (largest gene on the X-chromosome).
  • Causes loss of structural stability of myocytes.
109
Q

In which type of contraction is tension greater than load?

A

Isotonic

110
Q

What are some characteristics of motor unit fibers?

A
  • Fibers of an individual motor unit are dispersed throughout the muscle.
  • Stimulation of a single motor unit causes a weak contraction of the entire muscle.
  • Fibers of a single motor unit contract in unison.
  • Fibers of single motor unit are all the same type.

NOTE: One muscle may have many motor units of different fiber types

111
Q

It is at the _____________ that a motor neuron is able to transmit a signal to the muscle fiber, causing muscle contraction.

A

neuromuscular junction

112
Q

Which components of the sarcomere shorten with contraction?

A

H zone and I band

Structure and Function 1- Exam 2 (18 decks)

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