Muscle Selflab

7/31/2019 Muscle Selflab

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Interactive Physiology 2

Label this diagram:

Internal Structure of a Fascicle Important Points About Endomysium:

Made of connective tissue.

Surrounds individual muscle cells.

Functions to electrically insulates muscle cells from one another. Three connective tissue layers of the muscle (endomysium, perimysium, and epimysium):

Bind the muscle cells together.

Provide strength and support to the entire muscle.

Are continuous with the tendons at the ends of the muscle.

Label this diagram:


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Internal Structure of a Skeletal Muscle Cell Label this diagram:

Muscle fibers: Alternative name for skeletal muscle cells.

Nucleus: Contains the genetic material.

Sarcolemma: Plasma membrane of the muscle cell. Sarcoplasmic reticulum (SR): Interconnecting tubules of endoplasmic reticulum that surround each

myofibril.

Terminal cisternae: Sac-like regions of the sarcoplasmic reticulum that contain calcium ions. T tubules: Invaginations of the sarcolemma that project deep into the cell.

Cytosol: Intracellular fluid.

Mitochondria: Sites of ATP synthesis. Myofibril: Contains the contractile filaments within the skeletal muscle cell.

** Student should be very familiar with terms listed above.

Structure of a Myofibril Myofibrils: Contractile units within muscle cells.

Made of myofilaments called thin filaments and thick filaments.

Thin and thick filaments are made mainly of the proteins actin and myosin.

Arrangement of Myofilaments The activity goes into some detail on A bands, I bands, H zones etc, but I will only test you on whetheryou know what the following are.

Z-lines and Sarcomeres


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A bands: Dark areas that correspond to the areas where thick filaments are present. I bands: Light areas that contains only thin filaments.

Z line: A protein disk within the I band that anchors the thin filaments and connects adjacent

myofibrils.

H zone: Located in the middle of each A band, this lighter stripe appears corresponding to the regionbetween the thin filaments.

M line: Protein fibers that connect neighboring thick filaments.

Sarcomere: The region of the myofibril between two Z lines.

The Neuromuscular Junction

Introduction

Motor neurons stimulate muscle cells to contract at the neuromuscular junction.

Role of Motor Neuron

Axons of motor neurons innervate skeletal muscle cells at the neuromuscular junction.Anatomy of a Neuromuscular Junction

The following parts of a neuromuscular junction and skeletal muscle cell are described:Axon terminal Synaptic Vesicles Synaptic Cleft

Motor End Plate T Tubule Sarcolemma

Terminal Cisternae & Sarcoplasmic Reticulum Sarcomere


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Label this diagram:

** Make sure you could label this diagram in a quiz.

Overview of Neuromuscular Junction Activity

The muscle cell, including the T Tubules are polarized. Stimulation of the motor end plate on a muscle

cell by acetylcholine triggers depolarization resulting in contraction of the sarcomeres.

Summary

Each skeletal muscle cell is individually stimulated by a motor neuron. The neuromuscular junction is the place where the terminal portion of a motor neuron axon meets a

muscle cell membrane, separated by a synaptic cleft.

An action potential arriving at the axon terminal brings about the release of acetylcholine, which leadsto depolarization of the motor end plate.

Depolarization of the motor end plate triggers an action potential that propagates along the sarcolemmaand down the T Tubules.

Sliding Filament TheoryIntroduction

When a muscle cell contracts, the thin filaments slide past the thick filaments, and the sarcomereshortens.

Molecular Participants

The chemical players in muscle contraction are:

1. myosin (protein)

2. actin (protein)


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3. tropomyosin (protein)4. troponin (protein)

5. ATP (nucleotide)

6. calcium ions

Myosin

Myosin is a protein molecule found in the thick filaments.

Myosin Molecule with Hinged Head Myosin has a tail and two heads (called cross bridges) which will move back and forth, providing the

power stroke for muscle contraction.

Myosin Molecule with Hinged Head and Tail

The tail of myosin has a hinge which allows vertical movement so that the cross-bridge can bind to

actin.

Myosin ATP Binding Site

The cross bridge (head) of myosin has a binding site for ATP. Myosin is in its low energy conformation when the cross bridge is in this position:

**Note: The term "conformation" is often used to indicate the shape of a protein. Proteins often change their shape or conformation asthey function.

Energized Cross Bridge

ATP is a molecule with a high chemical energy. ATP binds to myosin heads when they are tilted backin their low energy position. When ATP is hydrolyzed into ADP and phosphate, the energy is

released and transferred to the myosin head. Note that the ATP is glowing yellow indicating that it's in a high energy state. After the ATP has beenhydrolyzed to ADP and phosphate, the energy is transferred to the myosin head. Now the head is

glowing to show that it's high energy. When the myosin head is pointing up, it is in a high energy

state.

As myosin functions within muscle cells, it undergoes the following four steps:


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**This may seem like a lot of detail at this point, but later on this will be very important. At that time you may want to come back to thito review.

Actin Binding Site on Myosin

There are two binding sites on each myosin head, one for ATP and one for actin.

Thin Filaments of the Sarcomere

Thin filaments are made of these three protein molecules:

1. actin 2. tropomyosin 3. troponin

Actin

The major component of the thin filament, actin is composed of a double strand of actin subunits eachof which contain myosin binding sites.

Tropomyosin The regulatory protein, tropomyosin, is also part of the thin filament. Tropomyosin twists around the

actin. When the sarcomere is not shortening, the position of the tropomyosin covers the bindingsites on the actin subunits and prevents myosin cross bridge binding.

Troponin

Troponin, which is found periodically along the tropomyosin strand, functions to move thetropomyosin aside, exposing the myosin binding sites.

Calcium Ions

Role of Calcium in Muscle Contraction:

Action Potential Occurs

Calcium Ions are Released from the Terminal Cisternae

Calcium Ions then Bind to Troponin

Tropomyosin Moves Away from the Myosin Binding Sites on Actin


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There has been a transfer of energy from the myosin head to the movement of the thin filament.

Step 4: Disconnecting the Cross Bridge

This animation shows ATP binding to the cross bridge, allowing the cross bridge to disconnect from

the actin.

**Note that even though the ATP has bound, the energy has not yet been transferred from the ATP to the cross bridge since the head isstill tilted backward.

Step 5: Re-energizing the Cross Bridge

In this animation, ATP is hydrolyzed into ADP and phosphate. The energy (yellow glow) is

transferred from the ATP to the myosin cross bridge, which points upward.

Step 6 Removal of Calcium Ions

Detail of steps in this animation:Calcium ions fall off the troponin.

Calcium is taken back up into the sarcoplasmic reticulum.

Tropomyosin covers the binding sites on actin.

**Note: Since the terminal cisternae and the sarcoplasmic reticulum are continuous, the pumping of calcium ions back into thesarcoplasmic reticulum causes an increase in calcium ions within the terminal cisternae as well. This animation shows the calcium

ions being pumped back into the terminal cisternae, but they are really pumped back in along the length of the sarcoplasmic reticulum

Calcium Pumps

This animation shows how calcium ions are pumped back into the sarcoplasmic reticulum. Twocalcium ions enter the pump (transport protein embedded in the membrane of the SR). ATP binds tothe pump and is hydrolyzed into ADP and Pi. The energy released from ATP hydrolysis is used to

change the conformation of the pump, allowing the calcium ions to move into the lumen of the SR.

ADP and Pi fall off the pump, allowing it to return to it's original conformation.

In a relaxed muscle cell, the concentration of calcium ions about 10,000 lower in the cytosol than inthe SR. During a muscle contraction, the concentration of calcium in the cytosol increases, but it is

still higher inside the SR. To move the calcium against the gradient, from the lower concentration in

the cytosol to the higher concentration inside the SR, Active transport is needed.


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Try to pick out these six steps:

Step 6. The transport of calcium ionsback into the sarcoplasmic reticulum.

Step 1. The infux of calcium,triggering the exposure of

binding sites on actin.

Step 5. The hydrolysis of ATP,whichleads to the re-energizingand repositioning of thecross bridge.

Step 2. The binding ofmyosin to actin.

Steps 2-5repeat

over and overbeforestep 6

occurs.

Step 4. The binding ofATP to the cross bridge,

which results in the crossbridge disconnecting fromactin.

Step 3. The power stroke ofthe cross bridge that causes thesliding of the thin filaments.

Multiple Cross Bridge Cycles

During the contraction of a sarcomere about half of the cross bridges are attached to actin and about

half are bound at any given time. If all the cross bridges detached at the same time, then the thin

filament would slide back on the thick filament.

**Note: Many myosins (about 200) make up each thick filament - so there are many myosin heads (cross bridges) available to bind thethin filament. In this animation you see only four of these cross bridges, but keep in mind there are many more on each thick filament


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Multiple Myofilaments Many power strokes occur to bring the Z lines of the sarcomere closer together during the contraction

of a muscle cell. During relaxation, the myosin heads detach from the actin and the thin filaments

slide back to their resting position. The width of the H zone decreases during a contraction and increases during relaxation.

The length of the sarcomere shortens during a contraction, but the thin and thick filaments do not

shorten, they just slide by each other.

**This is an important animation. View it several times to make sure to understand the theory here. Remember as long as calcium ionand ATP are present, the thin filaments slide toward the center of the sarcomere. When the calcium is taken back up into the SR, thenall the cross bridges let go and the thin filaments passively slide back to their resting position.

Review of the Role of ATP

Summary of the role that ATP plays in the contraction of muscle:1. ATP transfers its energy to the myosin cross bridge, which in turn energizes the power stroke.

2. ATP disconnects the myosin cross bridge from the binding site on actin.

3. ATP fuels the pump that actively transports calcium ions back into the sarcoplasmic reticulum.

Summary

The sequence of events in a single cross bride cycle includes:

1. In influx of calcium, triggering the exposure of binding sites on actin.

2. The binding of myosin to actin.3. The power stroke of the cross bridge that causes the sliding of the thin filaments.

4. The binding of ATP to the cross bridge, which results in the cross bridge disconnecting fromactin.

5. The hydrolysis of ATP, which leads to the re-energizing and repositioning of the cross bridge.

6. The transport of calcium ions back into the sarcoplasmic reticulum. Multiple cross bridge cycling is coordinated sequentially to prevent all cross bridges from either being

connected or disconnected at the same time.


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Muscle Physiology Lab Name:_____________________________

Anatomy Review: Skeletal Muscle Tissue1. (Page 3)Match the following types of contractile cells to their shape (branching, elongated, spindle-shaped):

___________________ a. Skeletal muscle cells

___________________ b. Cardiac muscle cells

___________________ c. Smooth muscle cells

2. (Page 3.) Match the following types of contractile cells to the characteristics of their nuclei and presenceor absence of striations:

Cardiac Muscle Cells Smooth Muscle Cells Skeletal Muscle Cells

___________________ a. presence of visible striations & single, centrally-located nuclei

___________________ b. presence of visible striations & multiple peripheral nuclei

___________________ c. absence of visible striations & single, centrally-located nuclei number of

nuclei

3. (Page 4.) What is the name of the structure that attaches skeletal muscles to bones?

4. (Page 5.) Bundles of skeletal muscle cells are called ________________.

5. (Page 5.) The connective tissue which immediately surrounds a muscle is called _______________ and

the connective tissue around the fascicles is called ________________.

6. (Page 6.) What is the function of endomysium?

7. (Page 7.) Match these terms to their description:

TriadT tubules

Terminal cisternae

Sarcolemma

Muscle fibersMitochondria

Sarcoplasmic reticulum

Myofibril

___________________ a. Sac-like regions of the sarcoplasmic reticulum that contain calciumions.

___________________ b. Sites of ATP synthesis.

___________________ c. Plasma membrane of the muscle cell.

___________________ d. Alternative name for skeletal muscle cells.


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___________________ e. Interconnecting tubules of endoplasmic reticulum that surround eachmyofibril.

___________________ f. A group of one T tubule lying between two adjacent terminal cisternae.

___________________ g. Invaginations of the sarcolemma that is projecting deep into the cell.

___________________ h. Contains the contractile filaments within the skeletal muscle cell.

8. (Page 8.) What are the names for the two types of filament in a myofibril?

9. (Page 9.) What creates the skeletal muscle cell's striated appearance?

10. (Page 10.) Arrange the following from smallest structure to largest structure:Muscle cell or muscle fiber

Fascicle

MyofilamentsWhole skeletal muscle

Myofibril

THE NEUROMUSCULAR JUNCTION

11. (Page 1 in) What causes skeletal muscle cells to contract?

12. (Page 1.) What is the place called where a motor neuron stimulates a muscle cell?

13. (Page 3.) How are skeletal muscle cells electrically insulated from each other?

14. (Page 3.) What is a motor neuron?

15. (Page 4.) Match the following terms to their description:

Axon terminal Synaptic Vesicles Synaptic CleftMotor End Plate T Tubule Sarcolemma

Terminal Cisternae & Sarcoplasmic Reticulum Sarcomere

________________________ a. Invaginations of the sarcolemma penetrating deep into the interior of

the muscle cell.

________________________ b. The space between the axon terminal and the motor end plate.

________________________ c. The swollen distal end of the motor neuron axon.

________________________ d. The muscle cell membrane.

________________________ e. Structures within the axon terminal that contain the neurotransmitter

acetylcholine.________________________ f. The contractile unit of the muscle cell that extends from one Z line to

the next.

________________________ g. Structures within skeletal muscle cells that serve as reservoirs ofcalcium ions.

________________________ h. A folded region of the sarcolemma at the neuromuscular junction.

16. (Page 5.) What does an action cause to be released at the axon terminal of a motor neuron?


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17. (Page 5.) After an action potential begins in the muscle, it propogates along the muscle and down the

___________________________.

18. (Page 5.) List the following events in the order they occur:

_____ a. The motor end plate is depolarized.

_____ b. The sarcomeres contract.

_____ c. Acetyl choline is released from the axon terminal into the synaptic cleft.

_____ d. The depolarization triggers an action potential which propagates along the sarcolemma and the

T tubules.

_____ e. An action potential arrives at the axon terminal

19. (Page 6.) Place the following events in their proper sequence:

_____ a. Acetyl choline is released into the synaptic cleft._____ b. Action potential propagates along the sarcolemma and down the T Tubules.

_____ c. Synaptic vesicles fuse to membrane of axon terminal.

_____ d. Motor end plate becomes depolarized.

_____ e. Action potential is initiated on the sarcolemma.

_____ f. Action potential arrives at the axon terminal.

_____ g. Calcium ions are released from the terminal cisternae.

_____ h. Acetylcholine binds to receptor sites on the motor end plate.

_____ i. The muscle cell contracts.

_____ j. Calcium ions enter the axon terminal.

20. (Page 1 in the sliding filament theory) According to the sliding filament theory, when a muscle cellcontracts, the __________ filaments slide past the ___________ filaments and the ____________ shortens.

22. (Page 5.) Where is myosin found in skeletal muscle cells?a. in the thin filaments

b. in the thick filaments

c. in the sarcoplasmic reticulumd. in the terminal cisternae

e. in the T tubules


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23. (Page 9.) Which of these are high energy conformations of myosin? Why?

24. (Page 10.) What binds at each of these sites on myosin?

25. (Page 13.) What is the function of tropomyosin?

26. (Page 14.) What is the function of troponin?

27. (Page 14.) Identify the parts of the thin filament below.

28. (Page 15.) What causes the tropomyosin to move away from the myosin binding sites on the actin?

29. (Page 16). a. Which of the following will attach to myosin?actin tropomyosin troponin ATP calcium ions

b. Which of the following will attach to actin?

actin tropomyosin troponin ATP calcium ions

c. Which of the following will attach to troponin?

myosintropomyosin actin ATP calcium ions

30. (Page 19.) What causes the release of calcium ions into the cytosol from the terminal cisternae?

31. (Page 19.) What causes the myosin binding sites on actin to be exposed?

32. (Page 20.) What happens after the tropomyosin moves over, exposing the binding sites on the actin?

33. (Page 22.) What causes the myosin heads (cross bridges) to disconnect from the actin?

34. (Page 24.) What causes the tropomyosin to cover back over the actin binding sites?

35. (Page 25.) What is required to move the calcium ions from the cytosol back into the sarcoplasmicreticulum?

36. (Page 26.) List the following steps in the order they would occur in a single cross bridge cycle.

_____ a. ATP binds to the cross bridge and the cross bridge disconnecting from actin._____ b. Myosin bind to actin.

_____ c. Calcium ions are transported back into the sarcoplasmic reticulum.

_____ d. Presence of calcium ions in the cytosol trigger the exposure of binding sites on actin._____ e. The power stroke occurs.

_____ f. ATP is hydrolyzed, leading to the re-energizing and repositioning of the cross bridge.


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37. (Page 27.) What would happen if all the myosin cross bridges were synchronized (doing the same thing

at the same time)?

38. (Page 29.) Which of these is not a role of ATP in muscle contraction?

a. Allows the tropomyosin to move over, exposing the myosin binding sites on actin.b. Actively transports calcium ions into the sarcoplasmic reticulum.

c. Energizes the power stroke of the myosin cross bridge.

d. Disconnects the myosin cross bridge from the binding site on actin at the conclusion of a power

stroke.

39. Was there anything that you would report as being particularly good about this activity?

40. Was there anything that you found particularly unhelpful about this activity?

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Muscle Selflab