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Excitable Cells > L15 muscle 1 > Flashcards

L15 muscle 1 Flashcards

1
Q

What is most skeletal muscle attached to?

A

Bone, via Tendons

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2
Q

What are the 2 main functions of skeletal muscle?

A
  1. Movement

2. Generation of heat

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3
Q

What are individual muscle cells called?

A

Myocytes (also known as muscle fibres)

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4
Q

What are myocytes covered with?

A

A layer of connective tissue called the endomysium. (note: it’s distinct from the muscle cell membrane, the sarcolemma).

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5
Q

How is each muscle fibre formed?

A

By the fusion of cells during development. (each cell is nucleated)

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6
Q

What are muscle fibres grouped into?

A

Bundles of fibres termed fascicles. (related to fascist which is a bundle of sticks - in the emblems). The fascicles are in turn covered with another layer of connective tissue called the perimysium).

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7
Q

What surrounds the fascicles and groups them together?

A

The perimysium covers one fascicle. The epimysium groups the fascicles together and covers everything. This is the muscle.

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8
Q

What are muscle fibres made up of?

A

Bundles of protein filaments (called myofilaments) that make up myofibrils.
Bundles of myofibrils = muscle fibre.

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9
Q

What are T tubules?

A

Tunnels which lead off the sarcomela (cell membranes). T tubules lead to the interior of the muscle fibre and mean that the membrane has a high surface area.

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10
Q

Where is the sarcoplasmic reticulum in muscles?

A

It wraps around the myofibrils.
Specialised parts of the SR, called the terminal cistemae, interact with the T tubules to form a structure called a triad.

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11
Q

What is the triad and what is it key for?

A

The terminal cistemae of the SR, interacting with the T tubules. It is key to coupling excitation of the muscle membrane to contraction.

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12
Q

From the smallest to the biggest, what is the structure of the muscle?

A

Myofilaments, myofibrils, muscle fibre/myocite, fascicles, muscle.

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13
Q

Muscle fibres are made up of repeating what?

A

Sarcomeres

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14
Q

What part of the sarcomere is actin & myosin?

A

myosin head….myosin tail… actin

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15
Q

What is the structure of myosin?

A

It is an hexamer - 2 heavy chains coiled around each other with ATPase heads, and 4 light chains.

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16
Q

What 2 forms does actin exist in?

A

A globular form - G actin

A filamentous form - F actin. (form found in muscles)

17
Q

How are Tropomyosin and Troponin involved in actin?

A

1 - Tropomyosin runs along the actin chain.
-> it consists of 2 alpha helical chains that coil around each other
-> 1 tropomyosin interacts with 7 actin monomers.
2 - Troponin is a complex of 3 proteins - Troponin T, I + C.
-> 1 troponin trimer per tropomyosin molecule
-> T- binds to tropomyosin
-> I - binds to actin and inhibits contraction.
->C - binds calcium.

18
Q

What do Tropomyosin & Troponin allow?

A

Muscle contraction to be regulated by calcium.

19
Q

What are the 6 stages of muscle movement?

A

1 - Myosin is bound to actin (attached state)
2 - Myosin head binds ATP. Detaches from actin.
3 - Myosin head hydrolyses ATP. Conformation changes and it enters “ cocked state”
4 - Myosin forms new cross-bridge with actin and actin monomers further down the chain. ( no muscle movement has happened yet)
5 - the phosphate is released and the power stroke occurs, because myosin head goes back to uncocked state.
6 - ADP is released and the system is back at start except 2 monomers movement. (the muscle has contracted).

20
Q

At rest how does the troponin trimer sit?

A

So that the troponin - I inhibits the formation of actin-myosin cross-bridge, it covers up the myosin binding site.

21
Q

What happens to troponin when [Ca 2+] increases?

A

Ca2+ binds to troponin C. This triggers a conformation change in troponin and tropomyosin. Tropomyosin moves deeper into the actin groove and troponin reveals the myosin binding sites on actin.

22
Q

How do neuromuscular junctions/end plates work?

A
  • Motor neurones release ACh into the synapse
  • ACh binds to nicotinic ACh receptors on the muscle.
  • This opens Na channels which flood into the muscle and depolarise the membrane.
  • The depolarisation triggers the calcium signal which causes the muscle contraction.
23
Q

What tunnels of plasmamembrane (sarcolemma) burrow into the muscle fibres?

A

Transverse tubules (T tubules)

24
Q

How are the sarcoplasmic reticulum and plasma membrane linked in the muscle?

A

The terminal cistemae of the SR associate with the T tubules. (2 terminal cistemae for each T tubule). This is known as a triad. (Note: the SR is not continuous with the T tubule).

25
Q

What does the T tubule do to the surface area/volume ratio?

A

Increases it. Which facilitates the spread of action potential.

26
Q

What are the [Ca2+] in the extracellular fluid, the cytoplasm and the sarcoplasmic reticulum?

A

Extracellular - a few mM
Cytoplasm - VERY LOW 1/10,000 of extracellular and sarcoplasmic reticulum. LOWEST
SR - similar to extracellular s few mM

27
Q

Why is the [Ca2+] so low in the cytoplasm (in muscle cells)?

A
  • there are calcium ATPase pumps in the SR membrane that actively pump calcium into the SR.
  • this pump is known as SERCA. -> (Sarcoplasmic/endoplasmic Reticulum ATPase)
28
Q

Why is it sensible that the cytoplasmic conc. of calcium is so low in the muscle?

A

So that the muscle isn’t constantly contracted

29
Q

How does calcium get into the cytoplasm to contract muscle?

A
  • From the SR
  • The SR has calcium channels linked to the VSCC in the T tubules via a physical connection.
  • The ryanodine receptor opens, releasing calcium into the cytoplasm.
30
Q

What are the VSCC at the T tubules called?

A
  • L-type (Voltage sensitive calcium channels - VSCC)

- Dihydropyridine receptors. -> comes from the drug that blocks the channel.

31
Q

When an action potential travels down the T tubule, what happens?

A

Changes the conformation of the dihydropyridine receptors and pulls the ryanodine receptor open.

32
Q

How do DHP receptors interact with ryanodine receptors?

A

4 DHP receptors (a tetrad) interact with each ryanodine receptor.

33
Q

How do we reach the starting point when the neuronal signal is no longer present and muscle contraction has happened?

A

SERCA pumps the calcium back into the SR. Restores the very low [Ca2+] in the cytoplasm

34
Q

Summarise the muscle contraction from nerve to end.

A

1 - Motor neurone releases acetylcholine.
2 - Activates nACh receptors
3 - Sarcolemma = depolarised. The Ap spreads to the T-tubules.
4 - DHP receptors activated. Opens ryanodine receptors on the SR.
5 - Calcium ions released from SR.
6 - Troponin C binds Ca2+ and moves out of myosin binding site on actin.
7 - Muscle contraction initiated. Myosin moves 2 actin at a time.
8 - SERCA pumps Ca2+ back into the SR.

Excitable Cells (10 decks)

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