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cell physio > Muscle contraction > Flashcards

Muscle contraction Flashcards

1
Q

General Function of muscle

A

Contraction
Generation of force
Production of movement
All types have contractile proteins (actin & myosin)

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2
Q

3-Types of muscle

A

(a) Skeletal: voluntary, striated
(b) Cardiac: involuntary, striated
(c) Smooth: involuntary, non-striated

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3
Q

Skeletal Muscle

A

40-50% of body mass (largest structural component in the body)
Function: Movement of the skeletal system
○Many exceptions: tongue (lingual muscle), pharynx, esophagus, vocalis, diaphragm, extraocular muscles, facial muscles….
Generally, attached to the skeletal system via tendons (many exceptions)
○Tendon of origin (bone fixed in place, towards midline)
○Tendon of insertion (moveable bone, towards periphery)
Muscle cells = muscle fibers = myofibers
Long cylindrical-like cells
Typically, myofibers arranged in parallel
Whole muscles have myofibers arranged in various patterns:
-parallel, pennate, bipennate, multipennate, radiate,…
Although we consider muscle as a tissue, each muscle is an organ (a collection of tissues)

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4
Q

CT Components

A

(Allow for blood vessels and nerves to enter the whole muscle)

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5
Q

Epimysium

A

Outer connective tissue covering,

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6
Q

Perimysium

A

Surrounds a fascicle (group of myofibers)

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7
Q

Endomysium

A

Surrounds a myofiber

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8
Q

Cross-section (whole muscle)

A

CT Components (Allow for blood vessels and nerves to enter the whole muscle)
Epimysium
Perimysium
Endomysium

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9
Q

Longitudinal-section (whole muscle)

A
  • Series of parallel myofibers with cross striations

* Multinucleated myofibers

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10
Q

Skeletal Muscle Fibers

A

Long Cylindrical-like Cells (up to 1 cm in length)
○Large diameter: 10 – 100 mm
○Multinucleated: hundreds to thousands of myonuclei
-Located at periphery of fiber (just within the cell membrane)
-“True syncitium”
○Packed with long cylindrical structures: myofibrils (~1 mm diam.)
-Each myofibril contains contractile proteins (filaments)
-Responsible for cross striation pattern
○Usually, each fiber has a single neural connection (neuromuscular junction = NMJ is a chemical synapse)

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11
Q

Sarcolemma

A

cell membrane of muscle cell

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12
Q

Sarcoplasm (myoplasm)

A

cytoplasm of muscle cell

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13
Q

Sarcoplasmic reticulum (SR)

A

modified ER of muscle cell (stores Ca2+)

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14
Q

T-tubule (transverse tubule)

A

long tubular invaginations of the sarcolemma

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15
Q

T-tubules (transverse tubules

A

thin tubules that extend from the sarcolemma and penetrate into the fiber and communicate with the SR

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16
Q

Triad

A

region where one T-tubule meets 2 SR (one on either side)

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17
Q

Myoplasm contains..

A

mitochondria, ribosomes, glycogen, enzymes, etc.…

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18
Q

Myofibril

A

Long cylindrical proteinaceous structures (span the length of the fiber) (~1 mm in diameter) (many per fiber)
Contain the contractile proteins (actin & myosin)
Consist of alternating & overlapping protein filaments in the form of repeating subunits known as sarcomeres (thousands per myofibril)

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19
Q

Sarcomere

A

Functional unit of a muscle fiber (where contraction/force generation occurs)
•Extends from z-line to z-line (z-disk)
•Thin filaments attached to z-lines
•Thick filaments in the middle (A-band)
•Area without thick filaments = I band
•Thick and thin filaments overlap in the A-band
•Region of no overlap in the middle of the A-band = H-zone
•M-line in middle of H-zone
Thick filament contains myosin (molecular motor of muscle)
Thin filament contains actin (binds to myosin) & regulatory proteins (troponin & tropomyosin)

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20
Q

Z-line (disc)

A

Z = Zwischen (German for “in between”, “inter”)

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21
Q

I-band:

A

I = Isotropic (reflects polarized light equally in all directions)

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22
Q

A-band:

A

A = anisotropic (reflects polarized light non-uniformly)

23
Q

H-zone

A

H = Heller (German for “clearer or brighter”)

24
Q

M-line:

A

M = Mittel (German for “middle”)

25
Q

Triad

A

region where t-tubule interacts with 2 regions of SR

26
Q

When does physiological contraction occurs?

A

when thick and thin filaments interact, in response to elevated myoplasmic Ca2+ (released from SR) following an action potential)

27
Q

A-band (Thick and thin filament overlap)

A

Each thick filament is surrounded by 6 thin filaments at 60o angular spacing

28
Q

Hierarchical Structure of Skeletal muscle

A
  • Whole Muscle (Organ)
  • Myofiber (Cell)
  • Myofibril (Subcellular structure)
  • Myofilaments
  • Thick filaments
  • Thin filaments
  • Individual proteins (Macromolecules)
29
Q

Proteins associated with Sarcomere

A
  • Thick filament: Myosin (heavy and light chains), C-protein (myosin binding protein C).
  • Thin filament: actin, troponin, tropomyosin.
  • Z-line: desmin, alpha-actinin, CapZ, several others.
  • M-line: M-line creatine kinase, M-line protein.
  • Others
30
Q

Thick Filament

A

~1.5 mm long
Primarily composed of:
Myosin = ~470 kDa (big) protein = molecular motor of muscle
The molecular motor protein of muscle (Class II myosin)
Myosin molecule is a hexamer (6 protein subunits)
2 Myosin heavy chains (MyHC or MHC) (200 kDa each)
4 Myosin light chains (MyLC or MLC) (15-25 kDa each)
Fibrous portion (Rod or tail)
Globular Portion (Head)

31
Q

Myosin

A

Myosin Head (on the heavy chain):
1.ATPase activity: chemical breakdown of ATP into ADP + Pi (inorganic phosphate)
ATP binding site
2.Ability to transduce chemical energy into mechanical energy
Chemical energy released from ATP hydrolysis causes conformation change (kinetic energy)
3.Actin binding domain
Binding of the myosin head to actin also causes a conformation change
Myosin light chains modify the heavy chain function

32
Q

Thick filament:

A

is formed by many myosin molecules assembling into a filamentous structure in a “parallel” and “anti-parallel” arrangement.

33
Q

Thin Filament Structure

A
  • Thin filament = ~ 1 mm in length extending from the z-line
  • g-actin = globular actin
  • f-actin = filamentous actin - formed by many g-actin molecules polymerizing into a long helical chain.
  • The core of the thin filament is composed of 2 f-actin helices intertwined to form a double helix.
  • Wrapping around the actin double helix are 2 helices of tropomyosin.
  • Tropomyosin blocks the site on actin to which the myosin head can bind.

•At regular intervals (~ every 6 - 8 g-actins) a globular protein known as troponin is bound to tropomyosin and actin.

34
Q

Tropomyosin

A

Rod like protein
Binds to actin
Blocks the site on actin to which the myosin head can bind
Each molecule spans ~7 g-actin units
Form a continuous double helical structure
Two double helical structures per thin filament (one for each of the globular-actin helices)

35
Q

3 subunits of troponin(TN)

A

TN-1
TN-C
TN-T

36
Q

TN-1

A

Inhibitory subunit (loosely binds with actin)

Holds troponin to the actin filament

37
Q

TN-C

A

Ca2+ binding subunit
Four EF-hand structures (Ca2+ binding domains)
Upon binding Ca2+ will undergo a conformational change
“Switch” that initiates force development

38
Q

TN-T

A

Tropomyosin binding subunit
Binds the troponin to tropomyosin
Helps to position tropomyosin (allowing tropomyosin to block the site on actin to which myosin can bind)

39
Q

Skeletal Muscle Contraction Phases:

A

1) Excitation (initiated at NMJ; action potential at sarcolemma & T-tubule)
2) Excitation-Contraction coupling (T-tubule, sarcoplasmic reticulum, Ca2+)
3) Active Site Exposure, or Ca2+ activation (thin filament)
4) Cross-Bridge Formation (thick and thin filament)
5) Power Stroke (myosin)
6) Cross-Bridge Detachment (thick and thin filaments, ATP binding)
7) Myosin Reactivation (myosin, ATP hydrolysis)
8) Relaxation (parvalbumin, sarcoplasmic reticulum)

•Sliding Filaments due to the Cross-Bridge Cycle (steps 4 – 7 repeated) results in the sliding of filaments (shortening of the sarcomere); hence the Sliding Filament Theory of Contraction

40
Q
  1. Fiber Excitation
A

Muscle is an ‘excitable’ tissue
Has the capacity to fire an action potential (AP)
The AP is initiated at the NMJ (nerve-muscle synapse)
○The a-motoneuron is the pre-synaptic cell.
○The muscle fiber is the post-synaptic cell.
○Acetylcholine (Ach) is the neurotransmitter
○The nAch receptor is a ligand-gated Na+ channel
What would happen when Acetylcholine is released at the synapse?
The muscle fiber will contract (generate force) in response to the AP

41
Q

Basic NMJ Function

A
  1. AP in presynaptic cell (a-motor neuron) initiates the release of acetylcholine (Ach, neurotransmitter) from synaptic vesicles into the synaptic cleft (at presynaptic membrane of terminal bulb).
  2. Ach diffuses across the cleft and binds to its receptor (nAch Receptor) on the postsynaptic cell membrane (muscle fiber).
  3. The Ach receptor (nAchR): is a ligand – gated Na+ Channel.
  4. When bound to Ach, the nAchR Na+ Channels open, initiating a depolarization (EPSP)
  5. The depolarization is almost always sufficient to cause an AP.
42
Q

NMJ Summary of events

A

1.Action potential in a-motoneuron
2.Action potential causes voltage-gated Ca2+ channels to open in the axon terminal
3.Ca2+ diffuses into axon terminal and by interacting with the vesicles causes them to fuse with the presynaptic membrane
4.Release of Ach into synaptic cleft
5.Diffusion of Ach to muscle fiber membrane
6.Ach binds to Ach receptor
7.Opening of Na+ channel
8.Depolarization of muscle fiber
9.Depolarization leads to an action potential in muscle fiber.
10.The AP occurs along the entire sarcolemma and t-tubule membranes.
RESULTS IN ACTIVATION & THEN CONTRACTION OF THE FIBER

43
Q

SUSTAINED CONTRACTION

A

If neuron stimulation continues, the fiber will undergo a series of action potentials.

44
Q

RELAXATION

A

When nerve stimulation stops, acetylcholinesterase (in the synaptic cleft) will cause the hydrolysis (breakdown) of Ach into acetate and choline.

45
Q

Excitation-Contraction Coupling (E-C coupling)

A

•Is initiated by the action potential in the T-tubules
•Results in an elevated Ca2+ in the (myoplasm)
•At rest, [Ca2+] in the myoplasm is typically ~50 - 100 nM
•During a contraction [Ca2+] can rise to ~1-20 mM
•Thus, [Ca2+] in the myoplasm can increase from 20 to 400X
•This calcium transient can occur in ~2-5 ms
•The increase in [Ca2+] in the myoplasm acts as a switch to ‘turn on’ muscle contraction
•E-C coupling = coupling between the action potential in the T-tubule and the release of Ca2+ from the SR.
Occurs at the Triad:
T-tubule & SR

46
Q

Sarcoplasmic Reticulum Ca2+

A

•At rest, total Ca2+ in the SR is 40,000X the [Ca2+] in the myoplasm.
•Free [Ca2+] inside the SR is ~200 mM (~4,000 x myoplasmic [Ca2+])
.Huge gradient for Ca2+ diffusion from SR to myoplasm
•Source for the elevated myoplasmic Ca2+ is the SR Ca2+

47
Q

How is calcium release from the SR signaled?

A 
Involves a couple of different proteins:
1.Dihydropyridine receptor (DHPR) = voltage sensor
Large (400 kDa) protein
Senses the action potential
Located in the t-tubule membrane
  1. Ryanodine receptor (RYR) = Ca2+ release channel
    Huge protein complex (over 2,000,000 kDa);
    Four protein subunits (560,000 kDa each) protein complex
    = SR foot protein (mentioned in text)
    Located in the SR membrane & spans across to the t-tubule
    Appears to anchor (attach) the t-tubule to the SR at the triad
    Mechanical linkage between the Ryanodine receptor and the DHP receptor
48
Q

Active Site Exposure (or Ca2+ Activation, or Thin Filament activatin)

A
  • Occurs at the thin filament
  • Myoplasmic Ca2+ binds to TN-C
  • Causes a conformational change in TN and movement of tropomyosin
  • Exposes the binding site on actin (1 on each g-actin in the chain) for the myosin head
  • Allows for the myosin head to automatically bind to actin and form the Cross Bridge (strong binding state, or rigor state) between thick and thin filaments
49
Q

Cross-Bridge Cycle

A

1)Following Ca2+ activation, the myosin head is bound to actin (Cross-Bridge Formation). ADP and Pi are bound to the myosin head.
2)Automatically, the Pi is released, after which the myosin head then swivels (~45o) = Power Stroke.
Then ADP is released.
Also at the same time, since the myosin head is attached to the thin filament, it pulls (slides) the thin filament relative to its position a distance of ~ 6 nm.
3)An ATP molecule will then bind to the myosin head. This causes the Cross-Bridge to break (“weak binding state”). This is called Cross-Bridge Detachment.
4)The ATP molecule is then hydrolyzed to ADP and Pi (which remain bound to the myosin head). The energy released from the hydrolysis of ATP is then used to reposition the myosin head back to its original place. This is called Myosin Reactivation.
5)Once the myosin head is moved back to its original position, it will automatically reform the Strong Binding State (Cross-Bridge Formation).

Provided that myoplasmic Ca2+ remains elevated, the cycle of events will continue and the sarcomere will shorten.

50
Q

The Sliding-Filament Model Explains Muscle Contraction

A

The sliding filament model was proposed in 1954

According to the model, muscle contraction is due to thin filaments sliding past thick filaments, with no change in length of either

51
Q

Relaxation

A

Two specialized proteins:
Sarco(endo)plasmic reticulum Ca2+ ATPase pump (SERCA)
Located in the SR membrane
Pumps Ca2+ into the SR against a concentration gradient (Active Transport)
Parvalbumin
Ca2+-Binding protein in the myoplasm of fast fibers
Steps:
1.Stop excitation (at neuron)
2.Ca2+ released from TN-C
3.Parvalbumin binds up the Ca2+ (might act as a shuttle)
4.SERCA pumps the Ca2+ back into the SR

52
Q

Rigor

A

the state when the myosin head is still tightly bound to actin after the power stroke and before a new ATP comes in to cause cross bridge detachment

53
Q

Rigor mortis

A

Shortly after death, muscles become rigid (hence, the term “stiff”).

54
Q

Rigor mortis = Shortly after death, muscles become rigid (hence, the term “stiff”). Why?

A

Due to Ca2+ and lack of ATP.
1.Lack of ATP (ATP synthesis eventually stops after death)
2.SR pumps stop
3.Ca2+ leaks out of SR
4.Formation of cross-bridges
5.Not broken because no ATP to bind to cause cross-bridge detachment
6.Muscles are maintained in “rigor state” and become stiff
Begins ~2-3 hours after death and lasts ~72 hours

cell physio (5 decks)

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