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A2 Biology > Chapter 15: Muscles (Part II) > Flashcards

Chapter 15: Muscles (Part II) Flashcards

1
Q

Describe the action of muscles

A
  • muscles act in antagonistic pairs against an incompressible skeleteon to create movement
  • can be automatic as part of a reflex response or controlled by concious
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2
Q

What does the phrase ‘antagonistic pair of muscles’ mean?

A

Muscles can only pull, so they work in
pairs to move bones around joints.
Pairs pull in opposite directions: agonist
contracts while antagonist is relaxed
- e.g. when the triceps relax, biceps contract to lift the arm

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3
Q

Name the 3 types of muscle in the body and where they are located

A 
● Cardiac: exclusively found in heart.
● Smooth: walls of blood vessels and
intestines (the gut)
● Skeletal: attached to incompressible
skeleton by tendons.
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4
Q

what are the role of tendons and ligaments?

A

tendons: tendons attach bones to skeletal muscle
ligaments: attach bones to other bones (hold them together)

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5
Q

which muscle is the antagonist?

A

the one relaxing

chillin’ (i.e. relaxing) like a villan (i.e. antagonist)

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6
Q

which muscle is the agonist?

A

the one contracting

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7
Q

skeletal muscle is made up of what?

A
  • large bundle of cells called muscle fibres
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8
Q

cell membrane of muscle fibre cells is called what?

A

sacrolemma

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9
Q

describe the structure of sarcolemma (muscle fibre cell membrane)

A
  • fold inwards of sarcoplasm (muscle fibre cytoplasm) at certain points
  • these inwards fold called transverse (T) tubules (they help spread electrical impulses throughout sarcoplasm so they reach all parts of muscle fibre)
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10
Q

what are the sarcoplasm reticulums (SR) ?

A
  • network of internal membranes that run through sarcoplasm

- SR stores and releases Ca2+ ions needed for muscle contraction

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11
Q

why do muscle fibres contain a lot of mitochondria?

A
  • to provide necessary amount of ATP needed for muscle contraction (which is a lot!!)
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12
Q

muscle fibres are multinucleate, what does this mean?

A

they contain many nuclei

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13
Q

what are myofibrils?

A
  • cylindrical organelles that run along length of mucle fibres
  • also the site of muscle contraction
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14
Q

Describe the gross structure of skeletal muscle

A
  • Muscle cells are fused together to form bundles of parallel muscle fibres (myofibrils).
  • A single muscle fiber is made up of myofibril. The myofibril is made up of two proteins called actin and myosin
  • Arrangement ensures there is no point of weakness between cells.
  • Each bundle is surrounded by endomycium: loose connective tissue with many capillaries.
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15
Q

Describe the microscopic structure of skeletal muscle.

A

Myofibrils: site of contraction.
Sarcoplasm: shared nuclei and cytoplasm with lots of mitochondria & endoplasmic reticulum.
Sarcolemma: folds inwards towards sarcoplasm to form transverse (T) tubules.

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16
Q

describe the features of a muscle fibres

A

sacroplasmic reticulum: organelle in sarcoplasm that stores Ca2+ ions
mitochondria: organelle that provides lots of ATP to power muscle contraction
myofibril: cylindrical organelles that run along length of muscle fibres
transverse tubules: areas where sarcolemma folds inwards towards sarcoplasm
sarcolemma: membrane of muscle fibres

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17
Q

myofibrils contain bundles of what?

A
  • thick and thin myofilaments that move past eachother to make muscles contract
  • thick myofilaments: made up of proteins called myosin
  • thin myofilaments: made up of proteins called actin
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18
Q

briefly explain what you will see of you look at myofibril under electron microscope

A

alternating dark and light bands:

  • dark bands contain thick myosin filaments and some overlapping thin actin filaments (called A-bands)
  • light bands contain thin actin filaments only (called I-bands)
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19
Q

myofibrils are made up of many short units called what?

A

sarcomeres

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20
Q

briefly describe the structure of sarcomeres (short units that make up myofibrils)

A
  • ends of each sarcomere marked with a Z-line
  • middle of each sarcomere is an M-line (M-line is Middle of Myosin filament)
  • around M-line is H-zone, H-zone only contains myosin filaments
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21
Q

describe the structure of myosin filaments

A
  • myosin and actin filaments arranged in alternating patter in sarcomeres
  • thick myosin filaments overlap with thin actin filaments at each end
    overlapping region = A-band
    region w/ only myosin = H-zone
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22
Q

describe the structure of actin filaments

A
  • thin actin filaments only overlap w/ myosin filaments in middle of sarcomere (middle is the M-line)
  • region w/ only actin filament is I-band
23
Q

describe the features of myofibrils

A
  • myosin filaments: thick myofilaments that overlap with thin actin filaments
  • actin filaments: thin myofilaments that overlap with thick myosin filaments
  • sarcomere: units that run along myofibrils, these units are site of muscle contraction
24
Q

Draw a diagram to show the ultrastructure of a myofibril.

A
  • check image on google/PMT flashcards/CGP textbook *

Z-line: boundary between sarcomeres
I-band: only actin
A-band: overlap of actin & myosin
H-zone: only myosin

25
Q

How does each band appear under an optical microscope?

A

I-band: light

A-band: dark

26
Q

Where are slow and fast-twitch muscle fibres found in the body?

A

Slow-twitch: sites of sustained
contraction e.g. calf muscle
Fast-twitch: sites of short-term, rapid,
powerful contraction e.g. biceps

27
Q

Explain the role of slow and fast-twitch muscle fibres

A

Slow-twitch: long-duration contraction;
well-adapted to aerobic respiration to prevent lactate buildup.
Fast-twitch: powerful short-term contraction; well-adapted to anaerobic respiration

28
Q

Explain the structure and properties of slow-twitch muscle fibres

A

● Glycogen store: many terminal ends can be hydrolysed to release glucose for respiration.
● Contain myoglobin (bright red molecule): higher affinity for oxygen than haemoglobin at lower partial pressures.
● Many mitochondria: aerobic respiration produces more ATP.
● Surrounded by many blood vessels: high supply of oxygen &
glucose

29
Q

Explain the structure and properties of fast-twitch muscle fibres

A

● Large store of phosphocreatine
● Thicker AND more myosin filaments.
● High concentration of enzymes involved in anaerobic respiration.
● Extensive sarcoplasmic reticulum: rapid uptake & release of Ca2+

30
Q

Explain the role of phosphocreatine in muscle contraction.

A
  • means of generating ATP anaerobically is required bc active muscle demand for ATP is greater than rate at which blood can supply O2
  • Phosphorylates ADP directly to ATP
    when oxygen for aerobic respiration is
    limited e.g. during vigorous exercise
    (so provided energy for muscle contraction)
31
Q

How is muscle contraction stimulated?

A
  1. Neuromuscular junction: action potential = voltage-gated Ca2+ channels open.
  2. Vesicles move towards & fuse with presynaptic membrane.
  3. Exocytosis of acetylcholine (ACh), which diffuses across synaptic cleft.
  4. ACh binds to receptors on Na+ channel proteins on skeletal muscle cell membrane (causes them to open)
  5. Influx of Na+ = depolarisation
32
Q

Explain the role of Ca2+ ions in muscle contraction.

A
  1. Action potential moves through T-tubules in the sarcoplasm = Ca2+ channels in sarcoplasmic reticulum open.
  2. Ca2+ binds to troponin, triggering conformational change in tropomyosin.
  3. Exposes binding sites on actin filaments so actinomyosin bridges can form.
33
Q

Describe the roles of calcium ions and ATP in the contraction of a myofibril.

A
  1. Calcium ions diffuse into myofibrils from (sarcoplasmic) reticulum;
  2. (Calcium ions) cause movement of tropomyosin (on actin);
  3. (This movement causes) exposure of the binding sites on the actin;
  4. Myosin heads attach to binding sites on actin;
  5. Hydrolysis of ATP (on myosin heads) causes myosin heads to bend;
  6. (Bending) pulling actin molecules;
  7. Attachment of a new ATP molecule to each myosin head causes myosin heads to detach (from actin sites);
34
Q

what happens in the sliding filament theory (briefly)?

A
  • myosin and actin filaments slide over eachother to make sarcomeres contract (myofilaments don’t themselves contract)
  • simultaneous contractions of lots of sarcomeres means myofibrils and muscle fibres contract
  • sacromeres return to original length as muscle relaxes
35
Q

describe how the lengths of the different bands in a myofibril change during muscle contraction

A
  • A-bands stay same length during muscle contraction

- I-bands get shorter

36
Q

Sliding filament theory takes place due to what?

A
  • globular heads on myosin filaments

- globular head allows myosin and actin filaments to bind together and slide past each other

37
Q

what two sites are there on every myosin head?

A
  • one site for binding to actin
  • one site for binding to ATP
    ( also binding site on actin for myosin filaments called actin-myosin binding site).
38
Q

When muscles are resting what us happening to tropomysoin?

A
  • tropomyosin is blocking the actin-myosin binding site
39
Q

Outline the ‘sliding filament theory’

A
  1. ) A nerve impulse arrives at the NMJ releasing acetylcholine. Depolarisation of sarcolemma continues down the t-tubules, causing Ca2+ release (the A.P. open Ca2+ ion channels on endoplasmic reticulum and Ca2+ diffuse into sarcoplasm down CG)
  2. ) Ca2+ binds to troponin, altering the shape of troponin, causing tropomyosin to move off actin binding sites. Myosin heads are now able to bind to the exposed actin-binding sites, forming a cross-bridge. (ADP attached to myosin heads mean they’re in state to bind to actin filament and form cross-bridge)
  3. ) Head of myosin changes angle, moving actin filament along as it does (creates overlap). The ADP molecule is released. This is muscular contraction (shortening).
  4. ) ATP binds to myosin heads, causing the cross-bridge to be broken. Once the ATP is hydrolysed (by ATPase) it can bind to another actin binding site, further down actin. Allowing contraction to continue.
  5. ) When ATP and Ca2+ are depleted in the muscle contraction terminates. Actin binding sites are covered again with tropomyosin.
40
Q

what does the presence of Ca2+ ions do? Whats the synoptic link?

A
  • changes the environment of protein tropomyosin leading to change in its tertiary structure
  • synoptic link: structure of proteins related to their function
41
Q

How does sliding filament action cause a

myofibril to shorten?

A
  • myosin heads flex in opposite directions = actin filaments are pulled towards each other.
  • Distance between adjacent sarcomere Z lines shortens.
  • Sliding filament action occurs up to 100 times per second in multiple sarcomeres.
42
Q

State 4 pieces of evidence that support the sliding filament theory.

A

● H-zone becomes narrower
● I-band becomes narrower
● Z-lines get closer (sarcomere shortens)
● A-zone remains same width (proves that myosin filaments do not shorten)

sarcomere also contracts/narrows

43
Q

describe the structure of myosin

A
  • made up two types of protein:
  • a fibrous protein arranged into filament made up of several hundred molecules (the tail)
  • a globular protein formed into two bulbous structures at one end (the head)
44
Q

describe the structure of actin

A

globular protein whose molecules are arranged in long chains that are twisted around each other, to form helical strand

45
Q

describe the structure of tropomyosin

A
  • form long thin threads that are wound around actin filaments
46
Q

What happens during muscle relaxation?

A
  1. Ca2+ is actively transported back into
    endoplasmic reticulum (using energy from hydrolysis of ATP)
  2. Tropomyosin once again blocks actin
    binding site.
47
Q

Muscle contraction requires considerable energy. This is supplied by what?

A
  • they hydrolysis of ATP to ADP and Pi
48
Q

What is the energy released (in ATP hydrolysis) for muscle contraction needed for?

A
  • movement of myosin heads

- reabsorption of Ca2+ ions into endoplasmic reticulum by active transport

49
Q

how is phosphocreatine replenished?

A
  • using phosphate from ATP when muscle is relaxed
50
Q

How could a student calculate the length of one sarcomere?

A
  1. View thin slice of muscle under optical
    microscope.
  2. Calibrate eyepiece graticule.
  3. Measure distance from middle of one light band to middle of another
51
Q

What is a motor unit?

A

consists of one motor neuron and all the muscle fibers it innervates or supplies

52
Q

explain how the shape of myosin molecule is adapted to its role in muscle contraction

A

fibrous protein:
- long and thin in shape which allows it to combine w/ others to form a long thick filament which actin can move along
globular protein:
- forms two bulbous structures, this shape allows it to fit and attach to actin molecule, its shape also means it can be moved at an angle, this allows it to change its angle when attached to actin and so move it along, causing muscle to contract

53
Q

What happens when the action potential stops arriving?

A
  • the calcium ions are actively pumped back into the sarcoplasmic reticulum
  • the tropomyosin returns to its original position
54
Q

What is the role of phosphocreatine (PC) in providing energy during muscle contraction?

A
  1. (Phosphocreatine) provides phosphate / phosphorylates;

2. To make ATP;

A2 Biology (13 decks)

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