Lecture 5 - Skeletal Muscle Contraction

Question 1

From the largest outer layer to the innermost layer, list hierarchically the components of skeletal muscle

Answer 1

Epimysium
Muscle
Perimysium
Fascicle
Endomysium
Sarcolemma
Myofiber
Myofibril
Myofilament

Question 2

Briefly Define the epimysium, muscle, perimysium, fascicle, and the endymysium

Answer 2

• Epimysium: Connective tissue surrounding entire muscle
• Muscle: Made up of multiple fascicles
• Perimysium: Connective tissue surrounding individual fascicle
• Fascicle: A bundle of myofibers
• Endomysium: Delicate connective tissue around each myofiber

Question 3

Briefly define a sarcolemma, myofiber, myofibril, and myofilament

Answer 3

• Sarcolemma (= plasmalemma): Cell membrane of muscle fiber
• Myofiber (= muscle cell): Individual multinucleated muscle cell
• Myofibril: A chain of sarcomeres within a myofiber 4
• Myofilament: Actin and myosin filaments that make up a sarcomere

Question 4

Describe the structural shape of the sarcolemma, the cellular membrane around each myofiber

Answer 4

“The sarcolemma consists of a true cell membrane, called the plasma membrane, and an outer coat made up of a thin layer of polysaccharide material that contains numerous thin collagen fibrils. At each end of the muscle fiber, this surface layer of the sarco- lemma fuses with a tendon fiber. The tendon fibers in turn collect into bundles to form the muscle tendons that then connect the muscles to the bones.”

Question 5

The sarcomere is the primary functional unit of the skeletal muscle. Describe the functional components of the sarcomere

Answer 5

• There are t-tubules within the sarcomere that are invaginations of the sarcolemma. These lie close to cisternae of the sarcoplasmic reticulum and form triads with cisternae. There are two per sarcomere.
• The sarcomplasmic reticulum has strong similarities to the endoplasmic reticulum, but modified to take in calcium.

Question 6

List the 4 bands lining the sarcomere and describe each.

Answer 6

Z discs (Z  lines): Anchor the actin filaments and are located at each end of a sarcomere 
I bands: Composed entirely of actin; their width changes during a contraction 
A bands: Composed of actin and myosin; their width does not change during  contraction 
- H bands: Composed entirely of myosin; their width changes during  contraction

Question 7

Study and memorize figure 6-3 on slides 8 and 9 in lecture 5. Make sure you know where all 4 bands are. And also be sure to know which one contracts and expands accordingly

Answer 7

DO IT!

Question 8

Which of the bands in sarcomeres change width during a contraction?

Answer 8

I-bands and H-Bands

So Not A-bands

Question 9

What are the 9 steps associated in the sliding filament contraction mechanism

Answer 9

whew, alright, ready? (1. Action potential arrives from the nerve fiber), (2. The nerve fiber opens up the voltage calcium channels (3. Synaptic vessicles release neurotransmitter Ach into the synaptic cleft), (4. Enough ligands are needed to open the Ligand-gated sodium channels in the sarcolemma), (5. Action potential generated on the sarcolemma), (6. Voltage-gated dihydropyridine channels on the T-tubule interact with ryanodine receptors on the SR membrane), (7. Opening of ryanodine-sensitive calcium-ion release channels), (8. Increase in calcium-ion concentration in cytosol), (9. Sliding filament mechanism is activated)

Question 10

What are the next 7 steps in the sliding filament mechanism?

Answer 10

(10. Released calcium ions bind to troponin.) (11. Tropomyosin uncovers myosin binding sites on actin.) (12. A second ATP binds to myosin and causes it to release actin) (13. Stored energy in myosin head causes deformation such that thick and thin filaments slide past one another.) (14.ATPase heads of myosin molecules split ATP and bind to actin) (15. Process is repeated over and over.) (16. Contraction stops when ATP-dependent calcium pump sequesters calcium ions back into SR.)

Question 11

List the ordered tier-1 steps of a skeletal muscle contraction

(Steps BEFORE it reaches the sarcolemma)

Answer 11

(1. Action potential in alpha motor neuron), (2. Calcium Ion Flux into axon terminal), (3. Exocytosis of synaptic vessicles), (4. Acetylcholine is released into the synaptic cleft), (5. Diffusion of acetylcholine (Ach) across the cleft), (6. Acetylcholine binds to acetylcholine receptors on the sarcolemma)

Question 12

List the ordered tier-2 steps in a skeletal muscle contraction

Steps up until the sarcolemma reaches an action potential

Answer 12

(1. Ligand-Gated Sodium Channels open), (2. Sodium-ion influx occurs), (3. End-Plate Depolarization), (4. Opening of the VOLTAGE-gated sodium channels occurs), (5. Sarcolemma reaches it’s own action potential)

Question 13

List the ordered Tier-3 steps in a skeletal muscle contraction

Answer 13

(1. T-tubules depolarize), (2. Conformational change in DHP receptors), (3. Conformational Change in Ryanodine Receptors), (4. Opening of ryanodine receptor calcium channels), (5. Release of Calcium from the sarcoplasmic reticulum)

Question 14

Give the ordered Tier-4 steps in a skeletal contraction

Answer 14

(1. Calcium concentration in the cytosol goes from 0.1 uMol/Liter to 10 uMol/L) (2. Calcium binds to troponin C), (3. There is a conformational change in troponin), (4. Tropomyosin is pulled away from the active sites on actin), (5. The active sites on actin are exposed), (6. The myosin heads bind to the exposed actin active sites)

Question 15

For the circular phase figure on slide 20, starting from the top, list the steps on the right half of the circle.

Answer 15

(1. Binding of myosin head (M-AD-Pi) to active site A), (loss of Pi), (2. A-M-ADP), (Loss of ADP), (3. A-M), (4. Power Stroke), (Gain of ATP), (5. ATP-MA) (Loss of Active Site, which returns to step 1)

Question 16

For the circular phase on Slide 20, starting from the bottom, give the ordered steps along with gains and losses that occur on the Left Half (Following the loss of the active site A off of (ATP-M-A))

Answer 16

(1. ATP-M), (2. M-ADP-Pi), (Loss of Heat from the dephosphorylation of ATP), (3. Recocking of the Myosin Head), (4. M-ADP-Pi) (5/1. Binding of Myosin Head (M-ADP-Pi) to active site A)

Note: Study carefully slides 20-24 in Lecture 5

Question 17

Describe the characteristics of dihydropyridine receptors

Answer 17

Voltage-sensitive L-type calcium channels arranged in quadruplets

• Located on the sarcolemma T-tubules
• Cause a conformational change in the ryanodine receptors
• A minute amount of calcium flows into the cytosol via these channels.

Question 18

Describe the characteristics of Ryanodine Receptor (RyRs or Ca(2+) release channels)

Answer 18

• Located on the cisternae of the sarcoplasmic reticulum
• Open in response to conformational change in DHP receptors
• Allow calcium into the cytosol from the SR
• SERCA* uses ATP to pump calcium back into the SR (Sarcoplasmic Reticulum Calcium ATPase)
• Calsequestrin in the SR maintains an optimum calcium concentration gradient to facilitate return of calcium to SR.

Question 19

Give a simple definition of the Preload

Answer 19

Load on a muscle in the relaxed state (before it contracts)

Question 20

What are the results of a muscle in the preload condition?

Answer 20

• Preload stretches the muscle which stretches the sarcomere.
• Preload generates passive tension in the muscle.
• The muscle resists the tension applied to it.
• The force of the resistance is measured as passive tension.
• The greater the preload, the greater the passive tension in the muscle.
• Preload keeps the muscles in a semi-tense state so they’re more prepared to stretch further later. Partial reason for muscle tone. So the relaxed state still has tension on it.

Question 21

Give the definition of afterload

Answer 21

• Load the muscle works against.

Question 22

Give the results of muscle contraction

Answer 22

• If the muscle generates more force than the afterload, an isotonic contraction occurs.
• If the muscle generates less force than the afterload, an isometric contraction occurs.
• Results in different types of contractions depending on the relationship between the afterload and the preload.
• There are 3 types of muscular tension: Passive: Which is the relaxed preload tension in the muscle, Active: Which is produced by cross-bridge cycling, and Total: sum of active and passive tension

Question 23

What is meant by cross-bridge cycling?

Answer 23

• Cross-bridge cycling starts when free calcium is available and attaches to troponin.
• Contraction is the continuous cycling of cross-bridges.
• ATP is not required to form the cross-bridge linking to actin but is required to break the link with actin.
• Cross-bridge cycling continues until:
• Withdrawal of calcium ion
• ATP is depleted

Question 24

The graph on Figure 6-9 gives the tension, and sarcomere length for a variety of different conditions. Describe scenario D (minimum tension)

Answer 24

Tension for D is very low, almost zero, and the sarcomere length is at it’s maximum relaxation (stretched out) and is about 3.5 um long. There is almost no overlap between the I band and H band.

Question 25

The graph on figure 6-9 gives a scenario C where tension is at a maximum. Describe this condition.

Answer 25

Tension is at maximum and the actin filament has overlapped all the cross bridges
- Sarcomere length = 2.2 μm

Question 26

The graph on figure 6-9 gives a scenario B. Describe this condition.

Answer 26

• Tension is at a maximum and Actin filaments are touching.
• Sarcomere length = 1.65 μm

Question 27

The graph on figure 6-9 gives a scenario A. Describe this condition.

Answer 27

• Actin filaments overlap.

- Sarcomere length

Question 28

There are 3 places where ATP is required during a muscle contraction. Name them.

Answer 28

• Most is used for the sliding filament mechanism.
• Pumping calcium ions from sarcoplasm back into sarcoplasmic reticulum.
• Pumping sodium and potassium ions through the sarcolemma to reestablish resting potential

Question 29

How much ATP is required to keep a muscle contracted for about 1-2 seconds?

Answer 29

About 4 mmol

Question 30

List the 3 sources of rephosphorylation and give characteristics of each.

Answer 30

1. Phosphocreatine: Releases energy rapidly and reconstitutes ATP. ATP & phosphocreatine provide enough energy for 5-8 seconds of contraction.
2. Glycolysis: Causes Lactic acid build-up, but can sustain contraction for 1 minute. However, an overabundance of lactic acid can cause some problems.
3. Oxidative metabolism: Provides more than 95% of all energy needed for long-term contraction. I “think” this is included in the citric acid cycle.

Question 31

Describe the differences between an isometric contraction and an isotonic one.

Answer 31

• Isometric: An isometric contraction occurs when there is an increase in tension but not in length.
• Isotonic: Muscle length changes in an isotonic contraction. There are two kinds of isotonic contractions:
  
  • Eccentric: An eccentric contraction occurs when the muscle lengthens.
  • Concentric: A concentric contraction occurs when the muscle shortens. This one is more common.
• When the muscle lengthens/relaxes, it often does so slowly and gradually to prevent tearing or over-stretching.

Question 32

There are two types of myofibers. Describe the white fiber.

Answer 32

Light, fast fibers (white fibers): Fast twitch fibers contract rapidly but have less endurance.
Characteristics include: Fewer mitochondria, Primarily use anaerobic respiration resulting in a buildup of pyruvic and lactic acids, They have Little myoglobin, but Larger concentration of ATPase

Question 33

There are two types of myofibers. Describe the Dark Fiber

Answer 33

Slow twitch fibers contract more slowly but have more endurance.
Characteristics include: More mitochondria, Primarily use aerobic respiration, More myoglobin, Smaller concentration of ATPase.

Question 34

Name a muscle that is almost entirely composed of dark fiber.

Light fiber?

Answer 34

Predominantly Dark Fiber: Soleus
Predominantly White Fiber: Gastrocnemius

Most muscles tend to use a mix of both types however

Question 35

Describe a motor unit

Answer 35

• A single nerve cell (neuron) may innervate from a few to several hundred myofibers.
• A neuron and the myofibers it innervates constitute a motor unit.
• When a neuron fires, all the myofibers in the motor unit contract.
• All-or-none really refers to a motor unit.
• In the picture on slide 48, one neruon is innervating two myofibers, and another neuron is innervating the three other myofibers.

Question 36

Give the definition of a summation and describe the mechanism behind it

Answer 36

• Electrical events occur faster than mechanical events:
• An additional spike can occur before the previous calcium ions have been returned to the SR.
• This increases the total amount of calcium ion in the cytosol and increases the rate of cycling between the myosin and actin cross-bridges.
• This increases muscle tension.
• Each additional spike adds to the effects of the previous spikes.

Question 37

Give the definition of a tetany and describe the mechanism behind it

Answer 37

• If the frequency of spikes is fast enough, there is no time for relaxation between spikes.
• The muscle remains at maximal contraction.

Question 38

There are 3 classes of muscle-lever systems

Answer 38

First-class (fulcrum is in the middle):
- Example = raising chin using sternocleidomastoids or similar muscles (fulcrum = atlas/axis complex)
- In-force and out-force move in opposite directions.
Second-class; Resistance (out-force) is in the middle:
- Example: Raising the body on the ball of the foot.
- Fulcrum = ball of foot.
- Both in and out forces are on the same side of the fulcrum.
Third-class; effort (in-force) is in the middle:
- Example: Lifting a weight in the palm of your hand
- Both in and out forces are on the same side of the fulcrum.
- Both forces move in same direction.