Skeletal muscle 2: Skeletal muscle contraction Flashcards

1
Q

Do skeletal muscle fibres branch?

A

No

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2
Q

Describe the connective tissue of the muscle fibres;

A

Epimysium (surrounds muscles)
Perimysium (surrounds muscle bundles or fascicles)
Endomysium (surrounds each muscle fibres)

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3
Q

Can muscles regenerate?

A

Muscle tissue contains satellite cells which can help regenerate the tissue.

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4
Q

What is the name of the cell membrane of muscle cells;

A

Cell membrane
Plasmamembrane
SARCOLEMMA

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5
Q

What are distinct features of the plasma membrane?

A

T-tubules. Deep invaginations that propagate action potentials. In continuity with extracellular space. Perpendicular to sarcolemma.

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6
Q

What is a muscle fibre?

A

One cell with many fibrils. Hence T tubule system.

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7
Q

Describe the ultrastructural appearance of skeletal muscle;

A
  • Striated appearance because of highly ordered sarcomeric proteins.
  • 100um in diam and up to severeal cm in length.
  • Multinucleated, found peripherally.
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8
Q

What is the SR?

A

Intracellular store of Ca

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9
Q

What are the terminal enlargements of the SR known as?

A

Terminal cisternae. Essential for synchronised EC coupling.

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10
Q

What are the 3 major SR-Ca regulatory proteins?

A

1) Luminal Ca binding proteins i.e Calsequestrin
2) SR Ca release channels i.e RYR1 or IP3 receptors
3) SERCA pumps

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11
Q

Describe the features of sarcomere;

A
A-Band; Thick and thin filament overlap
I-band; Only thin filaments
Z-Line; Actin filaments attach
H-band; Thick filaments only
M-line; Myosin filaments attach.
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12
Q

What are the thick filaments?

A

Myosin

Titan acts as an adjustable molecular spring during contraction.

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13
Q

What does titan contribute to?

A

Passive force and maintaining structural integrity of the filaments.

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14
Q

What comprises thin filaments?

A
  • Two strands of F-actin twisted together into a helix
  • Tropomyosin (Tm) lies along the F-actin groove.
  • Nebulin aligns the actin filaments.

Fibrous and Globular Actin

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15
Q

What is the function of tropomyosin?

A

Each tropomyosin molecule is associated with 3 other proteins which forms the troponin complex. TnT, TnC and TnL.

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16
Q

Describe the crossbridge cycle;

A
  • ATP binds myosin and is hydrolysed, forming a high free energy state. The myosin has high affinity for actin.
  • Myosin binds readily in the preferred 90 degree orientation.
  • With the released of the bound ADP-Pi, the free energy of the myosin-actin complex has a minimum at a 45degree angle.
  • The rotation from the original 90 degree orientation to the low energy 45 degree orientation (while the myosin head is attached to actin) leads to the force developed and shortening.
  • ATP binding reduces affinity of myosin for actin and crossbridge detaches. In the absence of ATP the cross bridge cannot detach and this leads to rigor.
17
Q

What is the role of Ca in the contraction process?

A
  • At rest tropomyosin prevents interaction between actin and myosin.
  • When cytosolic Ca increases, four Ca ions bind troponin C, causing the troponin complex to change conformational state. The tropomyosin molecule then shifts with respect to the actin filament, removing the steric interface to cross bridge interaction.
18
Q

What are the requirements for crossbridge cycling?

A
  • Cross bridge cycling generates force as a long as Ca remains high and ATP is present.
19
Q

What maintains cytosolic Ca levels?

A

SERCA maintains Ca at very low levels. High affinity for Ca.

Ca binds calsequestrin at its release sites in the terminal cisternae.

20
Q

What is the mechanical coupling hypothesis?

A
  • The high density of DHPR (Ca voltage gated) receptrers in the T-tubules opposite the RyRs in the SR terminal cisternae.
  • DHPR essential for EC coupling.
  • Depolarisation of the TT membrane flips the DHPR which induces a conformational change of the RyR in the terminal cisternae to a high conductance open state.

Voltage dependent calcium release.

21
Q

Whats the benefit of voltage dependant EC coupling?

A
  • Rapid kinetics
  • No dependence on current flow
  • No reliance on diffusion
  • Activation can therefore occur in the absence of extracellular Ca.
22
Q

Whats the time course of EC coupling?

A

AP (2ms)
Latent phase
Contraction-Relaxation phase 10-100ms.

23
Q

What is summation?

A

Stimuli close together, does not allow full relaxation

24
Q

What is unfused tetanus?

A

Stimuli far enough apart to allow the muscle to relax slightly whilst at maximal tension.

25
Q

What is fused tetanus?

A

Stimuli frequent enough reaching steady tetanus until fatigue sets in.

26
Q

How can the sympathetic nervous system impact the Sm contraction?

A

Directly affects the muscle fibres. Effects vary depending on fast or slow fibres.

Catecholamines;

Fast twitch; Peaked tension in response to single stimulus is increased, time course decreased.

Slow twitch; catecholamines reduce peak tension and prolong time course. Hence following stress postural muscles can feel weak.