Unit 3 Chapter 13: The Muscular System: Activation and Sliding Filament Theory

What is a skeletal muscle cell?

Is very specialized to shorten in length, on command, and produce force by moving your skeleton.

What is an example of the plasma membrane of a skeletal muscle cell?

It’s called the sarcolemma.

What are the two important proteins (and their specific names) that are bundled into those intracellular myofibrils?

Actin and Myosin (important for muscle contraction).

Why are there so many mitochondria in a skeletal muscle cell?

To produce the large amounts of ATP (energy) continuously required to power muscle contraction and relaxation.

What is the function of the transverse tubules?

To rapidly conduct electrical impulses (action potentials) from the sarcolemma deep into the cell interior, triggering the release of calcium ions from the sarcoplasmic reticulum for coordinated muscle contraction.

What structure surrounds each myofibril?

The sarcoplasmic reticulum.

What is the function and purpose of the sarcoplasmic reticulum?

To store and release

What are transverse tubules?

Is an extension if the sarcolemma, running deep into the cells interior.

associate with the ends (terminal cisternae)

What is the sarcoplasmic reticulum?

• Contains calcium.

• Important intracellular second messanger.

• The SR covers (is wrapped around) each myofibril.

Why are the Transverse Tubules and the Sarcoplasmic Reticulum positioned right next to each other?

It’s important for signal transduction (electrical signal converted to a chemical signal) in our muscle contraction story.

What are somatic motor neurons?

• Originate in the CNS (cell bodies are in the brainstem or the spinal cord)

• Many myelinated axons collect in motor nerves.

Where do somatic motor neurons terminate?

At the neuromuscular junction.

Somatic motor neurons at their terminus:

• Their axons branch close to target, forming multiple synapses on muscle cell(s).

• These axon terminals, along with motor end plate and Schwann cell sheath, form a neuromuscular junction.

What makes up the neuromuscular junction?

Consists of axon terminals, motor end plates on the muscle membrane, and schwann cell sheathes.

What is the motor end plate?

Is a region of muscle membrane that contains high concentrations of ACh receptors.

Where are neurotransmitters stored at the somatic motor neurons axon terminal?

In vesicles

What is the neurotransmitter?

A chemical messenger released by neurons to communicate with muscle cells, primarily acetylcholine (ACh) in the context of muscle activation.

What is the process of secretion called? Is it active or passive?

The process of secretion is called exocytosis, and it is an active process that requires energy.

Influx of what ion stimulates that secretion process?

Calcium

What is the specific name of receptor for ACh on the motor end plate?

Nicotinic Acetylcholine receptor channel

Is ACh always excitatory at the NMJ?

Yes,

When it is released and binds to its receptors, the muscle cell will contract fully.

The muscle cell will relax once ACh resale stops and it is inactivated.

What enzymes inactivates ACh at the NMJ?

Acetylcholinesterase breaks down ACh at the NMJ, terminating its action and allowing muscle relaxation.

Nicotinic receptors are ———— ——— —————-?

monovalent cation channels.

Steps of excitation-contraction coupling:

• ACh is realsescd from the somatic motor neuron’s axon terminal.

• ACh binds to its receptor channels and they open.

• The end plate potential always reaches threshold.

When ACh binds to its receptor channels an they open:

Name of channel?

Type of gated channel?

Causes influx or efflux of what?

ACh binds to nicotinic receptors; these are ligand-gated ion channels. This causes an influx of sodium ions (Na+).

What takes place when the end plate potential reaches threshold?

• Voltage gated Na+ and K+ channels open on the surrounding sarcolemma.

• The sarcolemma is the trigger zone for a muscle cell action potential.

The muscle cell action potential spreads all along the ———- by propagation.

sarcolemma.

Where does the action potential spread down into?

Any nearby traverse tubules.

How is that electrical signal transducer into a second messenger signal inside the muscle cells?

When the action potential travels along the sarcolemma and down the transverse tubules, it triggers the release of calcium ions from the sarcoplasmic reticulum, initiating muscle contraction.

What are dihydropyridine receptors (DHP)?

A voltage-sensitive protein that is mechanically linked to Ca2+ channels on the sarcoplasmic reticulum called ryanodine receptors.

What are ryanodine channels (RyR)?

They allow for the release of Ca2+ from sarcoplasmic reticulum, which acts as an intracellular second messenger.

Where does calcium bind in excitation-contraction coupling?

Binds to troponin.

When calcium binds to troponin, it enables myosin and actin to bind and preform what action?

The action of cross-bridge cycling.

What is the ultimate cell response in excitation-contraction coupling?

muscle contraction

What happens when the somatic motor neuron stops releasing ACh?

Relaxation occurs by removing Ca2+ from the myofilaments, so myosin and actin can’t interact.

How does the process of removing Ca2+ from the myofilaments happen?

Type of transporter?

What direction/number of ions moving through?

Energy requirements?

Calcium is removed from the myofilaments primarily through the action of calcium pumps, specifically the sarcoplasmic reticulum calcium ATPase (SERCA). This process moves Ca2+ ions back into the sarcoplasmic reticulum against their concentration gradient, which requires ATP for energy. 6

What is a myofibril?

Is a cylindrical bundle if protein filaments inside a skeletal muscle cell.

What is a sarcomere?

The repeating functional unit of myofibrils.

What is the length of the thick filaments and also defines the area where thick and thin filaments overlap?

A band

What structures only contains thin filaments in a relaxed muscle?

I band.

What is the structure that defines each end of a sarcomere?

Z line

What runs right down the middle of the sarcomere?

M-line

What contains only thick filaments in a relaxed muscle?

H zone.

Qualities of thick filaments:

• Mostly made up of myosin molecules.

• Anchored at center of the sarcomere at the M-line.

• Thick filaments are held in place (stabilized) by protein titan.

What does titan bind to?

The Z-disc and M-line

Qualities of thin filaments:

• Mostly made up of three proteins (actin, troponin, tropomyosin).

• Ends of the thin filaments are anchored to Z-disk.

• Arrangement of thin filaments in the sarcomere is stabilized by the protein nebulin.

What makes up thick filaments?

About 250 myosin molecules and a Titian molecule ringing down the core.

In the tail region of the myosin molecule how many tails are there?

around 250

Each myosin has two flexible, globular heads that:

1. Can bind to actin

2. Can bind to ATP

3. Function as ATPases

Myosin ATPases catalyzes what reaction?

The hydrolysis of ATP to ADP and inorganic phosphate.

The part of myosin that extends up from the tail and towards the thin filament is called:

The myosin cross bridge or myosin heads.

What happens when the cross bridge portion of myosin binds to actin?

The heads change structure (they tilt), pulling the thin filament towards the M-line.

What is the pulling action?

Power stroke.

What does the power stroke induce?

Mechanism that causes force generation and contraction of the muscle cell.

What are thin filaments made of?

Made up of 3 functional proteins, plus nebula to hold it straight.

What is the actin chain made up of?

(Two f-actin polymers twisted in a double helix)

• A polymer or long chain of globular actin molecules (G-actin).

• Has bing sites for myosin (actin “active sites”)

What is troponin made up of?

(Three globular sub-units each with a specific binding site)

• 1 binding site for actin

• 1 binding site for tropomyosin

• 1 binding site for Ca2+

What is tropomyosin made up of?

• A long protein strand that physically covers the myosin binding sites on actin in the relaxed muscle cell.

• Attached to troponin, which can pull it away from the myosin binding site on actin.

What is the name of the tilting or pulling motion that imitates when myosin heads attach to actin?

The power stroke, a key action in muscle contraction.

In a relaxed state what is the interaction between actin and myosin?

Myosin heads cocked. Tropomyosin partially blocked ending site on actin. Myosin is weakly bound to actin.

What is the initiation of a contraction between actin and myosin interaction?

A calcium signal initiates contraction.

What are the the steps of actin-myosin interaction?

1) Ca2+ levels increase in the cytosol.

2) Ca2+ binds to troponin (TN).

3) Troponin-Ca2+ complex pulls tropomyosin away from actin’s myosin-binding site.

4) Myosin binds strongly to actin and completes a power stroke.

5) Actin filament moves.

What are the steps of the sliding filament theory?

1. Actin binds to myosin. Myosin releases actin.

2. Myosin hydrolyzes ATP. Energy from ATP rotates the myosin head to the cocked position. Myosin binds weakly to actin.

3. Powerstroke begins when tropomyosin moves off the binding site.

4. Myosin releases ADP at the end of the power stroke.

Protein Interaction: The contraction:

1. Myosin binds ATP and detaches from actin active site.

2. Myosin-ATPase activates: ATP to ADP + Pi + energy

3. That energy is used to “cock” back the myosin head (stores potential energy)

4. Myosin binds tightly to actin

5. Inorganic phosphate is released from myosin head

6. Power stroke occurs.

7. ADP is released from myosin head.

What protein senses the Ca2+ second messenger signal?

Troponin

What does the protein do to enable interaction between the thick and thin filaments?

Troponin changes shape and moves tropomyosin away from the myosin binding sites on actin, allowing myosin to bind.

What specific part of myosin binds to the thin filament? And what protein does it bind to?

The myosin head. It binds to actin.

What role does ATP play in the sliding filament theory?

Binds to myosin, causing it to detach from actin. ATP hydrolysis provides energy to cock the myosin head.

What role does ADP play in the sliding filament theory?

Remains bound to myosin after ATP is hydrolyzed and is released during the power stroke.

What role does Pi play in the sliding filament theory?

Its release helps trigger the power stroke, pulling the thin filament toward the center of the sarcomere.

Energy from ATP is actually needed at 3 primary locations in a skeletal muscle cell. Describe each of those locations.

Myosin heads: ATP provides energy for cross-bridge cycling and muscle contraction.

Sarcoplasmic Reticulum: ATP powers calcium pumps that return Ca2+ to the sarcoplasmic reticulum, allowing the muscle to relax.

Cell membrane (sarcolemma): ATP powers ion pumps, especially the Na+/K+ pump, to restore ion gradients needed for muscle cell excitability.

Where is smooth muscle found?

Along the walls of hallow organs (blood vessels, bronchioles, digestive tract, etc).

What is smooth muscle controlled by?

Autonomic nervous systems.

How can you describe smooth muscle cell contraction?

It’s graded or has a range of lengths. Not “all or nothing.”

Is a within smooth muscle slower or faster than skeletal muscles?

Much slower and requires less ATP. Are largely fatigue resistant.

Are contractile proteins arranged in sarcomeres in smooth muscle?

No.

What does smooth muscle lack?

Lacks specialized receptor regions like the motor endplates on the skeletal muscle cells. Receptors are scattered.

What are the steps of muscle contraction in smooth muscle?

1. Intracellular Ca2+ concentrations increase when Ca2+ enters cell and is released from the SR.

2. Ca2+ binds to calmodulin (CaM).

3. Ca2+ -calmodulin activates myosin light chain kinase (MLCK).

4. MLCK phosphorylates light chains in myosin heads and increases myosin ATPase activity.

What are the steps to lead to relaxation in smooth muscle?

1. Free Ca2+ in cytosol decreases when Ca2+ is pumped out of the cell or back into the sarcoplasmic reticulum.

2. Ca2+ unbinds from calmodulin (CaM), MLCK activity decreases.

3. Myosin phosphatase (MLCP) removes phosphate from myosin light chains, which decreases myosin ATPase activity.

4. Less myosin ATPase activity results in decreased muscle tension.

Qualities of slow-twitch oxidative muscle fibers:

Smaller diameter, darker color due to myoglobin. Fatigue-resistant.

Qualities of fast-twitch glycolic muscle fibers:

Larger diameter, pale color. Easily fatigued.

Which contracts the slowest?

Slow-twitch oxidative

Which has the most glycogen stored in it?

Fast-twitch glycolic.

• Glycogen is a long chain of glucose monomers.

• It is broken down through glycolysis and can support anaerobic ATP production.

Which one would be considered the most “aerobic”?

Slow-twitch oxidative.

• It has lots of mitochondria to produce ATP using oxygen.

Which one produces the most lactic acid?

Fast-twitch glycolytic.

• It relies heavily on anaerobic glycolysis, which produces lactate.

Which one contains the most myoglobin?

Slow twitch oxidative.

Which ine has the most capillaries around it?

Slow-twitch oxidative.