E-C coupling Flashcards by Katie Angell

State the primary function of muscle

Generate force or movement in response to a physiological stimulus

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What do muscles do in order to generate a mechanical response?

Transduce a chemical or electrical signal

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What is the trigger for contraction of all 3 types of muscle?

An increase in Ca2+

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What causes skeletal muscle to contract?

Contracts in response to neuromuscular synaptic transmission

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A skeletal muscle fibre has a single NMJ where Ach receptors are located. What happens when ACh is released from the pre-synaptic nerve terminal?

It binds to nicotinic ACh receptors at the NMJ

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Are nicotinic ACh receptors selective or non-selective?

What happens when ACh binds to them?

Non-selective cation channels

Open in response to ACh binding, resulting in depolarisation of Em known as end plate potential (epp)

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What happens if the epp exceeds threshold for activating V-gated Na+ channels?

An action potential is generated

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How is ACh rapidly inactivated?

Ach-esterase

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How are skeletal muscles arranged?

Striated

Highly organised internal arrangement

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What components make up a single skeletal muscle cell?

Multiple nuclei
Abundance of mitochondria
10-100 um in diameter and up to ~75cm in length

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Define myofibrils

Specialised contractile elements that extend the entire length of the muscle fibre

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What does thick and thin myofibril mean?

Thick- special assemblies of the protein myosin

Thin- made up primarily of the protein actin

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Describe what an A band is made up of

What is the H zone?

Stacked set of thick filaments along with the portions of the thin filaments that overlap on both ends of the thick filaments

H-zone: The lighter area within the middle of the A band, where the thin filaments do not reach

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What is the M-line and where is it located?

A system of supporting proteins that hold the thick filaments together vertically within each stack

Extends vertically down the middle of the A bnad within the centre of the H zone

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What does the I band consist of and what is it visible in the middle of each I band?

The remaining portions of the thin filaments that dont project into the A band

Dense verticle line called the Z line

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What is the area between two Z-lines called and what is it?

Sarcomere

Functional unit of the skeletal muscle

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What are the characterisitics of the Z-line?

Flat, cytoskeletal disc that connects the thin filaments of the two adjoining sarcomeres

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What is the width of a sarcomere?

2.5 um

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List the levels of organisation in a skeletal muscle?

Whole skeletal muscle (organ)
Muscle fibre (single cell)
Myofibril (specialised intracellular structure)
Thick and thin filaments (cytoskeletal elements)
Myosin and actin (protein molecules)

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What do the thin filaments consist of?

Actin, tropomyosin and troponin

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Explain the backbone of the thin filaments

A double stranded alpha helical polymer of filamentous-actin (F-actin molecules

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What is F-actin associated with?

2 regulatory, actin proteins- tropomyosin and troponin

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What does tropomyosin consist of?

2 identical a-helices that coil around eachother

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head-to-tail contact between neighbouring tropomyosin molecules forms a ribbon that lies alongside the groove of the actin helix and physically covers the binding sites on actin molecules for attachment to what?

Myosin cross bridges

Prevents thin filaments attaching to cross bridges

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Each heterotrimer of troponin interacts with a single molecule of tropomyosin, which inturn interacts directly with 7 actin monomers What types of troponin are there?

Troponon T- binds to a single molecule of tropomyosin Troponin C- binds Ca2+ Troponin I - binds to actin and inhibits contraction

What are thich filaments composed of?

Multiple myosin-II molecules

Each myosin-II molecule is a double trimer composed of what?

- 2x intertwined heavy chains - 2x regulatory llight chains - 2x alkali (or essential) light chains

What are the three regions of the 2 heavy chains?

A tail, hinge and a head region

What is the tail, hinge and head portion of the thick filaments?

Tail- a-helices that intertwine Hinge- the molecule opens to form 2 globular heads Head- (S1 fragments) are the cross bridges between the thick and thin filaments of the sarcomere

What do the heads of the heavy chains in thick filaments possess?

Binding site for actin and a site for binding and hydrolysing ATP

What do the head portions of each myosin form? | What do these components do?

Forms a complex with 2 light chains, 1 alkali and 1 regulatory Alkali- stabilises the myosin head region Regulatory- regulates the ATPase activity of myosin. The activity of this chain is regulated via phosphorylation by kinases

What happens to myosin-II heads in muscle contraction? Where does the energy come from?

Bind to actin where the cross bridges become distorted and finally the myosin heads detach from actin Hydrolysis of ATP

How does Ca2+ cause contraction?

Removes the inhibition of crossbridge cycling

Troponin contains troponin C that binds what?

Ca2+

What are the 5 steps of the cross-bridge cycle?

1) ATP binding 2) Atp hydrolysis 3) Cross bridge formation 4) Release of Pi from myosin 5) ADP release

What is the sliding filament mechanism?

Cross-bridge interaction between actin and myosin bringing about muscle contraction

What filaments on each side of the sarcomere slide inward over the stationary thick filaments? What happens when they slide inwards?

Thin filaments The thin filaments pull the Z-line (to which they are attached) closer together, so the sarcomeres shorten

Do the length of the filaments shorten during contraction and why?

No | Due to interactions of the filaments sliding over one another

What happens to the A, I band, H and Z zone during contraction?

A band- Determined by thick filaments so stays the same width I band- Thin filaments not overlapping thick, so I band width decreases H zone- Within A band, thick filaments not overlapping thin so width decreases Z-line- Distance between decreases

What is rigor mortis and when does it occur?

"Stiffness of death" 3-4 hours after death and completes in ~12 hours

What happens during rigor mortis?

- Ca2+ rises after death - The Ca2+ allows the regulatory proteins aside and lets actin bind the myosin cross bridges that are charged with ATP - Dead cells cant produce more ATP so actin and myosin cannot detach

When does rigor mortis begin to reside?

The next several days after death as proteins involved start to degrade

What does E-C coupling stand for?

Excitation-contraction

What is the plasma membrane of a muscle cell also known as? | In skeletal muscle, what do they take the form of?

Sarcolemma Transverse or T tubules

Where to T tubules penetrate the muscle- two points?

Junctions of the A and I bands

What is the muscle cell equivalant of the endoplasmic reticulum and what does it serve as storage for?

Sarcoplasmic reticulum | Ca2+

What is the combination of the T tubule membrane and its two neighhbouring cisternae called? What is the structures role?

Triad | Coupling of excitation to contraction in skeletal (and cardiac) muscle

What happens when the T tubule is depolarised?

Leads to Ca2+ release from the SR at the triad

What do the V-gated channels known as tetrads do?

Function as the voltage sensor in E-C coupling

Depolarisation evokes conformational changes in each of the four L-type Ca+ channels and has two effects. What are these effects?

Conformational changes allowing: - Ca2+ to enter through the four channel pores - Induce conformation change in each of the four subunits of Ca2+ channel located in the SR membrane

Where do Ca2+ channekls cluster?

In the portion of the SR that face the T tubules

What happens upon opening of the Ca2+ release channels in the SR?

Ca2+ stored in the SR enter the cytosol leading to a rapid increase in [Ca2+]i

What is CICR and what does it play a critical role in?

Local elevations in [Ca2+] can also activate the release channel in skeletal muscle Plays a crucial role in E-C coupling in cardiac muscle

What does terminating contraction in all 3 muscle types require?

re-uptake of Ca2+ into the SR

What two ways can the cell extrude Ca2+?

- An Na-Ca exchanger (NCX) | - A Ca2+ pump at the PM (PMCA)

How is Ca2+ re-uptake mediated?

SERCA-type Ca2+ pump

Where is Calreticulin found in high concentrations? and what is it?

Within the SR of smooth muscle | Principal Ca2+ binding protein

What is the excitatory pulse used for?

If the contraction is to continue for longer periods, a series of calcium pulses must be initiated by a continous series of repetitve action potentials

Contractile activity far outlasts the electrical activity that initiated. What is the time delay called?

Latent period

When is the contraction process turned off?

When Ca2+ is returned to the SR when electrical activity stops The thin filaments then return passively to their resting position

What part of the sarcomere does not change in length?

A band - dark band | Overlap between thick and thin filaments

E-C coupling Flashcards

(61 cards)