Skeletal Muscle Tissue

Question 1

Describe the functions of skeletal muscle tissue. [6]

Answer 1

• Produce body movement
• Maintain posture and body position
• Support soft tissues
• Guard body entrances and exits
• Maintain body temperature
• Store nutrients

Question 2

Describe the organization of skeletal muscle at the tissue level.

Answer 2

1. A skeletal muscle is a complex organ containing skeletal muscle fibers, connective tissue that harnesses the forces of contraction, blood vessels that nourish the muscle fibers, and the nerves that control the contraction.
   
   • The epimysium, a dense layer of collagen fibers, surrounds the entire muscle. It separates the muscle from surrounding tissues and organs, and is connecte to the deep fascia, a dense connective tissue layer.
2. A muscle fascicle is a bunder of muscle fibers. The fibrous perimysium layer separates the fascicles from each other.
   
   • The perimysium is a fibrous layer that divides the skeletal muscle into a series of compartments. It contains collagen and elastic fibers as well as blood vessels and nerves.
3. Individual skeletal muscle fibers are sheated in a delicate endomysium that contains capillaries, myosatellite cells, and the axons of the neurons that control the muscle fibers.
   
   • The endomysium is a thin layer of areolar connective tissue that surrounds each muscle fiber. It loosely interconnects nearby muscle fibers. Each muscle fiber contains bunders of protein filaments called myofibrils.
   • Myosatellite cells are stem cells that function in the repair of damaged muscle tissue.

Question 3

Define tendon and aponeurosis.

Answer 3

At the ends of skeletal muscle, collagen fibers of the connective tissue layers merge to form etiher a bundle known as a tendon or a broad sheet called an aponeurosis.

A tendon attaches a muscle to a specific bone.

An aponeurosis provides attachment over a broad area that may involve more than one bone.

Question 4

What special terms are used to describe the plasma membrane and cytoplasm of a skeletal muscle fiber?

Answer 4

The plasma membrane is called the sarcolemma.

The cytoplasm is called the sarcoplasm.

Note: most of the sarcoplasm consists of myofibrils (bundles of protein filaments).

Question 5

Skeletal muscle fibers contain T tubules and sarcoplasmic reticula that surround contractile myofibrils made up of sarcomeres.

Describe the structures of a sarcomere.

Answer 5

1. A myofibril is a cylindrical structure. The sarcoplasm of a single skeletal muscle fiber may contain hundreds to thousands of myofibrils.
2. Myofibrils consist of bundles of protein filaments called myofilaments. The most abundant are thin filaments composed primarily of actin and thick filaments composed primarily of myosin.
3. The myofilaments within each myofibril are arranged into repeating contractile units called sarcomeres. Each myofibril consists of approximately 10,000 sarcomeres aligned end to end.
   
   • In a resting sarcomere, the H band is a lighter region on either side of the M line. The H band contains thick filaments only.
   • The M line connects the central portion of each thick filament.
   • The dense (dark) A band is the region that contains thick filaments.
     
     • Within the A band, the zone of overlap has thin filaments arranged in a 6:1 ratio around each thick filament.
   • The light I band contains thin filaments not overlapped by thick filaments.
   • Z lines mark the boundary between adjacent sarcomeres; consist of proteins called actinins, which interconnect thin filaments of adjacent sarcomeres.
   • Transverse tubules are continuous with the sarcolemma and extend into the sarcoplasm at right angles to the cell surface, forming passageways through the muscle fiber.
     
     • Inside the sarcoplasm, T tubules encircle each sarcomere at the zones of overlap.
   • The sarcoplasmic reticulum form expanded terminal cisternae on either side of a transverse tubule.

Question 6

Within a resting skeletal muscle fiber, where is the greatest concentration of calcium?

Answer 6

The total concentration of calcium within terminal cisternae in a resting skeletal muscle fiber can be 40,000 times that of the surrounding cytosol.

A muscle contraction begins when stored calcium ions are released into the cytosol through gated calcium channels.

Question 7

The sliding of thin filaments past thick filaments produces muscle contraction.

Compare F-actin with G-actin.

Answer 7

• F-actin (filamentous) is a twisted strand composed of two rows of 300-400 individual molecules of G-actin (globular)
• Each G-actin molecule contains an active site to which myosin can bind, much like a substrate molecule binding to an enzyme’s active site.

Question 8

The sliding of thin filaments past thick filaments produces muscle contraction.

Describe the structure of thin filaments.

Answer 8

At either end of the sarcomere, the thin filaments are attached to the Z line. THe protein actinin interconnects the thin filaments there. Each filament is composed of four main proteins: F-actin, nebulin, tropomyosin, and troponin.

Question 9

The sliding of thin filaments past thick filaments produces muscle contraction.

Describe the structure of thick filaments.

Answer 9

• Thick filaments contain ~300 myosin molecules, each made up of a pair of myosin subunits twisted around one another; all arranged with tails pointing toward M line.
• Each long myosin tail is bound to other myosin molecules within the thick filament.
• The connection between the head and tail acts as a hinge that lets the head pivot at its base.
• The free head has two globular protein subunits. During contraction, the myosin heads interact with thin filaments.

Question 10

Summarize the sliding filament theory.

Answer 10

1. The H bands and I bands get smaller.
2. The zones of overlap get larger.
3. The Z lines move closer together.
4. The width of the A band remains constant.

During contraction, sliding occurs in every sarcomere along a myofibril, so the myofibril gets shorter. Because myofibrils are attached to the sarcolemma at each Z line and at either end of the muscle fiber, when myofibrils shorten, so does the muscle fiber.

Question 11

Define depolarization and describe the events that follow it.

Answer 11

1. A charge reversal begins with a small increase in sodium ion membrane permeability up to a threshold (-55mV)
2. Once the threshold is reached, voltage-gated Na⁺ channels open and positively charged sodium ions rush into the cell. The membrane potential becomes positive, and the cell is said to be depolarized.
3. The depolarization peaks at a membrane potential of -30mV, at which point the voltage-gated Na⁺ channels close and voltage-gated K⁺ open. As potassium ions move out of the cell, repolarization begins.
4. Rapid repolarization continues until the resting potential is reached, when the voltage-gated K⁺ channels begin closing.
5. As the voltage-gated K⁺ channels close, the membrane potential stabilizes at resting levels and is once again negative. After the refractory period (a time when the membrane cannot respond to another stimulus), the former concentrations of sodium and potassium ions across the plasma membrane are restored. A second depolarization cannot occur until the refractory period is over.

Question 12

Explain why the propagation of action potentials along electrically excitable membranes occurs in only one direction.

Answer 12

An action potential travels in one direction because of the refractory period, which prevents propagation back in the direction from which it initiated. Excitable membranes permit rapid communication between different parts of the cell.

Question 13

Describe the neuromuscular junction.

Answer 13

• The NMJ is made up of an axon terminal of a motor neuron, a motor end plate (a specialized region of the sarcolemma), and a synaptic cleft (an intervening space).
• The cytoplasm of the axon terminal contains vesicles filled with molecules of acetylcholine, a neurotransmitter.
• The synaptic cleft contains acetylcholinesterase, which breaks down acetylcholine.
• As the action potential sweets down each Transverse tubule and passes between the terminal cisternae, the sarcoplasmic reticulum’s permeabiltiy changes, and the calcium ions flood into the sarcomeres at the zones of overlap. This event, called excitation-contraction coupling, triggers the contraction of the muscle fiber.

Question 14

What is the stimulus for acetylcholine release from the axon of the motor neuron?

Answer 14

The arrival of an electrical impulse (i.e., action potential). This sudden change in the membrane potential travels the length of the axon and triggers exocytosis of acetylcholine into the synaptic cleft. ACh diffuses across the synaptic cleft and bind to ACh-receptor membrane channels, altering the membrane’s permeability to Na⁺. The ECF contains high concentration of Na⁺, and the ICF contains low concentrations of Na⁺, so sodium ions rush into the sarcoplasm.

AChE quickly breaks down ACh in the synaptic cleft, thus closing the ACh-receptor membrane channels.

Question 15

List the interrelated steps that occur once the contraction cycle begins.

Answer 15

1. Resting sarcomere
   
   • Each myosin head points away from the M line.
   • Cocking the myosin head requires energy obtained by breaking down ATP; thus, the myosin head acts as an ATPase.
   • At the start of contraction, the breakdown products ADP and a phosphate remain bound to the myosin head.
2. Contraction cycle begins
   
   • ​​Begins with the arrival of calcium ions within the zone of overlap in a sarcomere.
3. Active sites exposed
   
   • Calcium ions bind to troponin, weaking the bond between actin and troponin.
   • The troponin changes position, rolling the tropomyosin molecule away from the active sites on actin and allowing interaction with the energized myosin heads.
4. Cross-bridges form
   
   • ​​Once the active sites are exposed the energized myosin heads bind them, forming cross-bridges.
5. Myosin heads pivot
   
   • ​​Energy that was stored in the resting state is released as the myosin heads pivot toward the M line → this action is called the power stroke. When this occurs, the bound ADP and phosphate group are released.
6. Cross-bridges detach
   
   • ​​When another ATP binds to the myosin head, the link between the myosin head and the active site on the actin molecule is broken.
   • The active site is now exposed and able to form another cross-bridge.
7. Myosin reactivates
   
   • ​​Myosin reactives when the free myosin head splits ATP into ADP and P_i. The energy released is used to ‘recock’ the myosin head.
8. Contracted sarcomere
   
   • ​​The entire cycle repeats several times each second, as long as calcium ion concentrations remain elevated and ATP reserves are sufficient.
   • Calcium ion levels will only remain elevated as long as action potentials continue to pass along the T tubules and stimulate the terminal cisternae.
     
     • Once the stimulus is removed, the calcium channels in the sarcoplasmic reticulum close, and calcium ion pumps pull Ca²⁺ from the cytosol and store it within the terminal cisternae.
     • Troponin molecules then shift position, swinging the tropomyosin strands over the active sites and preventing further cross-bridge formation.

Question 16

What sarcomere characteristic affects the amount of tension produced when a skeletal muscle fiber contracts?

Answer 16

Tension produced at the individual muscle fiber does vary depending on the fiber’s resting length at the time of stimulation.

When sarcomeres are either stretched or compressed compared to optimal resting length, tension produced declines.

Question 17

Explain the sarcomere length-tension relationship.

Answer 17

1. Sarcomeres produce tension most efficiently within an optimal range of lengths. When resting, the maximum number of cross-bridges can form, producing the greatest tension.
2. An increase in sarcomere length reduces the tension produced by decreasing the size of the zone of overlap and the number of potential interactions.
3. When the zone of overlap is reduced to zero, thin and thick filaments cannot interact at all.
4. Tension production falls to zero when thick filaments are pressed against the Z lines and the sarcomere cannot shorten further.
5. A decrease in resting sarcomere length reduces tension because stimulated sarcomeres cannot shorten very much before thin filaments extend across the center of the sarcomere and collide with or overlap the thin filaments of the opposite side.

Question 18

Describe the events that occur during each phase of a twitch in a stimulated muscle fiber.

Answer 18

• Latent period: begins at stimulation → an action potential sweeps across the sarcolemma, and the sarcoplasmic reticulum releases calcium ions
  
  • The muscle fiber does not produce tension during the latent period because the contraction cycle has yet to begin.
• Contraction phase: tension rises to a peak → calcium ions are binding to troponin, active sites on thin filaments are being exposed, and cross-bridge interactions are occuring.
• Relaxation phase: tension returns to resting levels → calcium levels fall, active sites are covered by tropomyosin, and the number of active cross-bridges declines as they detach.

Question 19

What two factors determine the amount of tension produced by a skeletal muscle?

Answer 19

1. the amount of tension produced by each stimulated muscle fiber
2. the total number of muscle fibers stimulated at any given moment

Question 20

Compare incomplete tetanus with wave summation.

Answer 20

• Wave summation
  
  • If a second stimulus arrives before the relaxation phase has ended, a second, more powerful contraction occurs.
• Incomplete tenanus
  
  • A muscle producing almost peak tension during rapid cycles of contraction and relaxation.

Question 21

Define motor unit.

Answer 21

A motor unit is a motor neuron and all the muscle fibers that it controls. The size of a motor unit indicates how fine, or precise, a movement can be.

In the muscles of the eye, where precise control is extremely important, a motor neuron may control 4 - 6 muscle fibers.

We have much less precise control over our leg muscles, where a single motor neuron may control 1000 - 2000 muscle fibers.

Question 22

Compare isotonic and isometric contraction.

Answer 22

• Isotonic: tension rises to a constant level and the skeletal muscle’s length changes.
  
  • Concentric: muscle tension exceeds the load, and the muscle shortens.
    
    • ​Pushing up portion of a push-up
  • Eccentric: the peak tension developed is less than the load, and the muscle elongates because of the contraction of another muscle or the pull of gravity
    
    • Lowering down portion of a push-up
• Isometric: the muscle as a whole does not change length, and the tension produced never exceeds the load.
  
  • Holding a high plank

Question 23

When do muscle fibers produce lactate?

Answer 23

At peak activity levels when ATP demands are enormous and mitochondrial ATP production plateaus at a maximum rate (determined by the availability of oxygen).

At peak exertion, mitochondria only provide 1/3 of ATP needed. Glycolysis produces the rest.

When glycolysis produces pyruvate faster than it can be utilized by the mitochondria, pyruvate levels rise in the cytosol.

Under these oxygen-limited conditions, pyruvate is converted to lactate.

After only a few sections of peak activity, changes in pH will alter the functional characteristics of key enzymes so that the muscle fiber can no longer contract. Sprinters experience this type of muscle fatigue.

Question 24

What type of fatigue affects endurance athletes after hours of exertion?

Answer 24

As long as mitochondrial activity can meet the demand for ATP, the muscle will not fatigue.

The skeletal muscle relies on aerobic metabolism of pyruvate to generate ATP, and the pyruvate is provided by glycolysis, using glucose obtained from stored glycogen reserves.

When glycogen, lipid, and amino acid reserves are exhausted, the muscle will fatigue.

Question 25

What is oxygen debt?

Answer 25

Excess postexercise oxygen consumption

The amount of oxygen required to restore normal, preexertion conditions.

Question 26

What happens to the lactate produced by skeletal muscle during peak activity?

Answer 26

Much of the lactate produced during peak activity exertion diffuses out of the muscle fibers and into the bloodstream. The liver absorbs lactate and begins converting it into pyruvate.

Question 27

Contrast fast fibers with slow fibers in terms of diamter, glycogen reserves, myoglobin content, and relative abundance of mitochondria.

Answer 27

• Fast
  
  • Reach peak tension in 0.01 seconds or less
  • large diameters
  • densely packed myofibers
  • large glycogen reserves
  • few mitochondria
• Slow
  
  • Take 3x as long to reach peak tension
  • Half the diameter of fast fibers
  • contain myoglobin

Question 28

What is hypertrophy?

Answer 28

As a result of repeated, exhaustive stimulation, muscle fibers develop more mitochondria, a higher concentration of glycolytic enzymes, and largerglycogen reserves.

Hypertrophy, or an enlargement of the stimulated muscle, is the net effect.

Question 29

What is atrophy?

Answer 29

A muscle that is not regularly stimulated by a motor neuron loses muscle tone and mass.

The muscle becomes flaccid, and the muscle fibers become smaller and weaker.

= atrophy

Question 30

How does Clostridium botulinum affect skeletal muscles?

Answer 30

Toxins produced by the bacterium paralyze skeletal muscles by preventing ACh release at neurmuscular junctions.

Question 31

Explain how rigidity of a dead body can provide a clue about a murder victim’s time of death.

Answer 31

Rigor mortis = generalized muscle contraction throughout entire body.

As the SR deteriorates, calcium ions are released, and a sustained contraction begins. As ATP reserves are exhausted, the muscles become locked in the contracted state. All skeletal muscles are involved, so the individual becomes stiff as a board.

Typically begins ~ 2 - 7 hours after death and disappears after 1 - 6 days (i.e., when decomposition begins)

Timing depends on environmental factors like temperature. Forensic pathologists base time of death estimates on the degree of rigor mortis and environmental conditions.