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neuromembrane biology > Lecture 7: striated muscle excitation - contraction coupling > Flashcards

Lecture 7: striated muscle excitation - contraction coupling Flashcards

1
Q

What are the 3 types of muscle?

A
  • skeletal muscle- attaches to bone and is involved in moving skeleton
  • cardiac muscle
  • smooth muscle
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2
Q

Describe the **structure **of skeletal muscle

A
  • has striated appearance - due to alternating dark and light bands
  • multiple nuclei
  • elongated shape
  • each skeletal muscle cell is called a muscle fiber
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3
Q

**What is a myofibril?

A
  • skeletal cells are composed of many long tubular cells called muscle fibers
  • these muscle fibers contain many chains of myofibrils (rod like organelle of muscle fiber)
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4
Q

What is the striated appearance of skeletal muscle due to?

A
  • the** repeated pattern of dark and light bands** due to the arrangement of the thin (actin) and thick (myosin) filaments
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5
Q

How are the filaments in a skeletal muscle fiber organised?

A
  • Actin containing filaments/ thin filaments are anchored to the Z lines at each end of the sarcomere
  • the actin filaments free ends wrap around the myosin / thick filaments in the A band in the centre of the sarcomere
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6
Q

What is a sarcomere?

A
  • Many sarcomeres located in 1 myofibril
  • Sarcomere - the contractile/structural unit of a myofibril consisting of a dark A band and a light I band
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7
Q

What is a sarcomere composed of?

A
  • 2 filaments - thick (myosin) and thin (actin)
  • A band (dark) - myosin, I band (light) - actin , H zone (part between 2 actins) etc
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8
Q

Describe the **structure of the thin filament **in a sarcomere

A
  • Actin
    *in sarcomere, contains 2 other proteins called troponin and tropomysosin which regulate contraction
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9
Q

Describe the structure of the thick filament

A
  • Myosin
  • Composed of heavy chain & light chain
  • These** chains** combine to form molecule with 2 globular heads -cross bridges
  • contains actin binding site and** ATP binding site**
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10
Q

What happens to the** I band** in sarcomere when a skeletal muscle contracts?

A
  • the I band/ actin filaments gets smaller (towards centre) when contracted
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11
Q

Describe the events of the cross bridge cycle/ sliding filament mechanism

A

1.** binding of Ca2+** to troponin on myosin, this activates the cross bridge cycle, tropomyosin moves out of way from binding site &myosin binds to actin
2. cross bridge moves/ the power stroke , **ADP and Pi are released
3. another ATP** molecule comes and **breaks the cross bridge link **and binds to ATP binding site on myosin
4. ATP is then hydrolysed by myosin ATPase which refroms the energised state of myosin
5. cycle can begin again

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12
Q

What is the role of ATP in the cross bridge cycle?

A
  1. ATP binds to myosin and breaks cross bridge link between actin and myosin- allowing another cycle to begin
  2. ** ATP hydrolysis releases energy** which is then** used as energy for the cycle**
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13
Q

What is contraction in skeletal muscle initiated by?

A
  • an increase in cytosolic Ca2+ concentration
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14
Q

What is the sarcoplasmic reticulum?

A
  • same as the endoplasmic reticulum in most cells
  • it functions to store Ca2+ ions
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15
Q

What structure is present at the end of each segment of the SR?

A
  • terminal cisternae / lateral sacs
  • they contain the Ca2+ binding protein calsequestrin which allows the storage of a large amount of Ca2+
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16
Q

What are **Transverse / T tubules **?

A
  • they are tubular structures that surround the myofibrils
17
Q

What is the NMJ?

A
  • the neuro muscular junction- a synapse between an alpha motor neuron and a muscle cell/ muscle fiber
18
Q

what is the function of the NMJ?

A
  • it allows a motor neuron to transmit an AP to a muscle fiber, which later causes a contraction
19
Q

Describe the sequence of events of the NMJ

A
  1. AP reaches axon terminal causing **depolarisation **
  2. depolarisation leads to opening of voltage gated Ca2+ channels
  3. Influx of Ca2+
  4. Ca2+ binds to Synaptotagmin
  5. This causes Ach vesicles to fuse with membrane and be** released** into synaptic cleft
  6. Ach binds to nicotinic receptors
  7. binding allows Na+ and K+ entry into muscle fiber - EPP
  8. depolarisation exceeds threshold and all voltage gated Na+ channels open in fiber
  9. AP propagates through muscle fiber
20
Q

How is the AP propagated along the muscle fiber ?

A
  • it is propagated into the** interior of the muscle fiber** along the transverse tubules to the region of the sarcoplasmic reticulum, where it produces a release of Ca2+ ions from the reticulum
21
Q

how does the relaxation of a contracting muscle fiber occur?

A
  1. relaxation occurs as a result of active transport of cytosolic Ca+ bacj into the sarcoplasmic reticulum
  2. calcium is removed from troponin which allows tropomysoin to move back to its original place and preveent actin-myosin binding
22
Q

what is the **triad junction **in the skeletal muscle?

A
  • 2 ** terminal cisterns** (at the end of the sarcoplasmic reticulum segments) in close association to the T tubule
  • occurs at every Z line
23
Q

what type of membrane protein is present on the T tubule membrane ?

A
  • **DHPR **- dihydropyridine receptor - which is a modified version of L type Ca2+ channel in T tubule
  • functions as an voltage sensor (ie senses the AP) - allows T tubules to bring action potentials into the interior of muscle fiber
24
Q

What protein is embedded in the SR membrane?

A
  • the** ryanodine receptor** (RYR1)
  • contains foot processes that connect to the DHP receptor
  • forms a** Ca2+ channel**
25
Q

Describe the mechansim of the cytosolic increase in Ca2+ performed by T tubules/ SR

A
  • during a T tubule AP, charged amino acid residues within the DHP receptor protein** induce a conformational change**
  • this acts as the foot process to open the ryanodine receptor channel
  • Ca2+ is then released from the terminal cisternae of the SR into the cytosol where it can bind to troponin
26
Q

What is the SERCA pump?

A
  • Sarcoplasmic Endoplasmic ATPase
  • Ca2+ ATPase pump
  • transfers Ca2+ from cytosol to sarcoplasm
27
Q

What are the 4 main proteins responsible for Ca2+ cycling and storage within the SR?

A
  • DHPR
  • RYR1
  • SERCA pump
  • calsequestrin
28
Q

what is calsequestrin?

A
  • high capacity Ca2+ binding protein found in the terminal cisternae of the SR

neuromembrane biology (14 decks)

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