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MNSR - MCQ > MNSR 8 - Muscular System: Ultrastructure of Striated Muscle, Physiology and Biochemistry of Movement > Flashcards

MNSR 8 - Muscular System: Ultrastructure of Striated Muscle, Physiology and Biochemistry of Movement Flashcards

1
Q

What is the cell membrane of the skeletal muscle called?

A

Sarcolemma

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2
Q

What is the contractile protein of the skeletal muscle called?

A

Myofibril

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3
Q

What is the cytosol in the skeletal muscle?

A

In muscle known as sarcoplasm
Rich in glycogen, ATP, Creatine Phosphate, glycolytic enzymes
Myofibrils comprise ⅔ the dry mass of the cells

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4
Q

What is the transverse tubules (T-tubules) in the skeletal muscle?

A

Series of membranous folds extending from plasma membrane
Transmit electrical signal

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5
Q

What is the sarcoplasmic reticulum in the skeletal muscle?

A

Flattened vesicles which surround each myofibril
Sequester calcium

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6
Q

Myofibrils are…

A

1µm in diameter and divided by conspicuous light and dark bands.

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7
Q

The light band is called the ____________ (isotropic); the dark band the __________ (anisotropic)
in the middle of the I-band is a dark line called the ___________

A

I-band, A-band, Z-line.

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8
Q

The unit of contraction, the ______________, extends between Z-lines. Gives skeletal muscle the striated appearance under the light microscope.

A

sarcomere

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9
Q

What is the Z line/disc component of the sarcomere?

A

It separates one sarcomere from the next.
Thick and thin filaments overlap one another.

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10
Q

What is the A band component of the sarcomere?

A

Darker middle part of the sarcomere
Thin and thick filaments overlap

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11
Q

What is the I band component of the sarcomere?

A

Lighter, contains thin but no thick filaments
Z discs passes through the center of each I band

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12
Q

What is the H zone component of the sarcomere?

A

Center of each A band which contains thick but no thin filaments

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13
Q

What is the M line component of the sarcomere?

A

Supporting proteins that hold the thick filaments together in the H zone

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14
Q

2 main filamentous components:

A

Actin (thin filaments)
Myosin (thick filaments)

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15
Q

What do contractile proteins (actin and myosin) do?

A

Generate force during contraction
* Myosin (Thick filaments) - Thread-like filament
* Actin (thin filaments) -Thread-like filament (like microfilaments)

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16
Q

What do regulatory proteins (troponin and tropomyosin) do?

A
  • Switch the contraction process on and off
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17
Q

What do structural proteins (titin and dystrophin)* do?

A
  • Align the thick and thin filaments properly
  • Provide elasticity and extensibility
  • Link the myofibrils to the sarcolemma
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18
Q

What does the myosin filament consist of?

A

A compact ‘head’ region which can move (needs ATP)
A long ‘tail’ region composed of two alpha-helices

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19
Q

The tails in myosin pack together to form the…

A

thick central portion of the myosin filament

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20
Q

The heads stick out to form…

A

‘cross bridges’ (or attachments) with the actin filaments.

21
Q

Most of the tail is called…

A

light meromyosin

22
Q

The head and some of the tail is composed of…

A

heavy meromyosin

23
Q

Myosin forms the

A

A-band

24
Q

What is the molecular structure of actin filament?

A

Built from actin monomers (G actin) to form actin polymer (F actin) or filamentous actin. It is double stranded and forms a double helix.

25
Q

What is tropomyosin?

A

Part of the thin filament

Double stranded alpha-helical protein molecule which lies in the groove of the F-actin helix

In relaxed muscle the myosin-binding site on actin is blocked by tropomyosin

26
Q

What is troponin?

A

Part of the thin filament

A complex consisting of 3 globular proteins (TnC, TnI, TnT) attached to the tropomyosin strand

Activated by calcium

Moves the tropomyosin away from actin – unblocking the binding sites for myosin and initiating muscle contraction

27
Q

In summary, the thin filament is made up of three different components:

A

F-actin
Tropomyosin
Troponin complex (TnI, TnT and TnC)

28
Q

In the resting state the tropomyosin strands…

A

cover the myosin binding sites on actin.

29
Q

TnC binds to…

A

Calcium (Ca2+) ions

30
Q

TnT binds to…

A

tropomyosin

31
Q

TnI binds to…

A

F-actin (it inhibits myosin binding to actin)

32
Q

Each myosin head behaves as an ATPase enzyme in which it is…

A

binding to ATP, hydrolysing it and using the energy derived from the ATP’s high energy phosphates to drive the contraction process.

33
Q

What does titin do?

A
  • Stabilizes the position of myosin
  • Accounts for much of the elasticity and extensibility of myofibrils
34
Q

What does dystrophin do?

A
  • Links thin myofibrils to the sarcolemma
35
Q

What does the sliding filament theory state?

A

Contraction is due to the actin and myosin filaments sliding between each other

The force of contraction is caused by the movement of the cross-bridges

36
Q

During muscle contraction:
The _____ remains the same length
The _____ shortens
The _____ is reduced or disappears

A

A-band, I-band, H-zone

37
Q

What is the ratchet mechanism of contraction?

A

Describes the biochemical and biophysical events which occur during contraction

Also known as cross-bridge cycle

Conversion of the energy of ATP molecules into the physical work of the displacement of the actin and myosin relative to each other

Formation of cross-bridges between the myosin head and the myosin binding site on the actin filament

38
Q

Ratchet Mechanism in an overview:

Attachment of…

‘Contraction’ (or power stroke)…

Breaking of…

Re-attachment…

A

myosin head to actin binding site

of myosin head
- Conformational change occurs in the myosin head which translates chemical energy into physical work
- The powerstroke drags the actin filament along some 5 - 12nm towards the M-line or centre of the myosin filament

actin/myosin interaction and return of myosin head to resting conformation

further along the actin filament at the next active site

39
Q

One ATP molecule is used per myosin head for…

A

each power-stroke achieved.

40
Q

In resting muscle, most of the _______ is in the Myosin-ADP - Pi form and the myosin binding site on _______ is inhibited by _________ and ___________

A

myosin, actin, troponin, tropomyosin

41
Q

On a separate sheet of paper, briefly go over the ratchet mechanism steps and compare and contrast what you might be missing with the answer.

A
  1. Myosin binding site on Actin is uncovered through the binding of Ca2+ to troponin. The Myosin-ADP - Pi complex binds to actin (i.e. attached).

2&3. The myosin head changes conformation - so called ‘ratchet action’ or ‘powerstroke’ pulling the actin filament towards the centre of the sarcomere. ADP and Pi is released.

  1. Immediately another ATP molecule replaces ADP in the myosin head, binding to the actomyosin complex to form an A-M-ATP complex. The actin is released (i.e. detached)

5&6. Myosin partially hydrolyzes the ATP and some of the energy is used to return the myosin head to its resting state. Myosin binding site on actin is inhibited by troponin and tropomyosin. (however if calcium is till elevated, the troponin & tropomyosin will uncover binding sites on the actin, and myosin which will undergo another power stroke).

42
Q

Excitation Contraction Coupling is the…

A

mechanism by which stimulation of a motor neuron leads to skeletal muscle contraction.

43
Q

How does ECC work?

A

Arrival of Action Potential/impulse at synaptic ending of motor neuron leads to release of the neurotransmitter Acetylcholine (ACh). The ACh binds to ACh receptors on the motor end plate and depolarizes it.

44
Q

In ECC, depolarization of motor end plate leads to…

A

depolarization of sarcolemma along surface and inward via the T-tubule system.

45
Q

What results in This results in muscle contraction and ATP hydrolysis during ECC?

A

Ca2+ binds to TnC which in turn affects Troponin-I TnI, bound to F-actin.
The TnI disengages from actin thereby causing a conformational change in tropomyosin, uncovering the active sites on F-actin.

46
Q

How do depolarizing muscle relaxants work?

A

Cause contraction of the muscle once, but prevent further contractions. Depolarizing muscle relaxants acts as ACh receptor agonists.
* They bind to the ACh receptors and generate an action potential. However, because they are not metabolized by acetylcholinesterase, the binding of this drug to the receptor is prolonged resulting in an extended depolarization of the muscle end-plate. As the muscle relaxant continues to bind to the ACh receptor, the end plate cannot repolarize. E.g. Suxamethonium

47
Q

How do non-depolarizing muscle relaxants work?

A
  • Prevent muscle from contracting
  • Nondepolarizing muscle relaxants act as competitiveantagonists for the Ach receptor. They bind to the ACh receptors but are unable to induce ion channel openings. They prevent ACh from binding and thus end plate potentials do not develop E.g. Tubocurarine
48
Q

How does Botulinum toxin work? (Botox)

A
  • Blocks release of ACh from synaptic vesicles (i.e. it is a neurotoxin and muscle relaxant)
  • May be found in improperly canned foods
  • A tiny amount can cause death by paralyzing respiratory muscles

MNSR - MCQ (5 decks)

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