Chapter 6- Skeletal Muscle

Question 1

• How is Ca2+ released for skeletal muscle contraction?

Answer 1

1. An action potential travels along the Aα-motor neuron to the synapse.
2. Acetylcholine is released from the neuron
3. Ach binds to Ach-channels which allows large amounts of Na+ to flow into the muscle cell
4. If the depolarization on the end plate is above membrane threshold, an action potential travels along the muscle fiber much like it does in a nerve axon.
5. The depolarization travels down invaginations of the plasma membrane called T-tubules
6. There are voltage-senstative proteins called dihydropyridine (DHP) receptors, and the + voltage causes a conformational change in the receptors.
7. Since these receptors are linked to the ryanodine receptor channels on the sarcopasmic reticulum, they pull on the ryanodine receptors.
8. The opening of the ryanodine receptors causes the sarcoplasmic reticulum to release its stores of Ca2+

Question 2

• What is the role of Ca2+ in muscle contraction?

Answer 2

o Calcium initiates the contraction process by binding to troponin C (TnC). This causes a conformational change in Troponin I (TnI), which relieves the inhibition of Troponin T (TnT) from binding to tropomyosin.

Question 3

What are the sliding filament theory steps?

Answer 3

1. Myosin heads bind with ATP. (A)
2. The ATPase on the myosin heads immediately cleaves the ATP and leaves the ADP + Pi bound to the head. This provides the energy for the myosin head to assume a “cocked” position. (B)
3. The myosin head binds to the freed actin filament and immediately causes the myosin head to tilt toward the arm of the cross-bridge. This moves the actin filament towards the middle of the sarcomere. (C and D)
4. When the head tilts, the ADP and Pi molecule are released.
5. Once a new molecule of ATP binds to the myosin head, then the myosin head is released from the actin filament.

6. The new dissociated myosin-ATP complex can now run through steps 1-5 and cause a power stroke on the next actin filament.

Question 4

How is ATP hydrolysis used for muscle contraction?

Answer 4

1. ATP hydrolysis is best described in the previous objective, where the myosin ATPase converts ATP  ADP + Pi and utilizes that energy for the “cocking” of the myosin head.

Question 5

Why is ATP hydrolysis used for Ca++ regulation?

Answer 5

1. When calcium is released from the sarcoplasmic retiuculum in response to sarcoplasm depolarization, it causes muscle contraction. This is good when we need to contract our muscles but we need to get the calcium out of the muscle cell to get back to a normal state.
2. On the sarcoplasmic reticulum there is an ATP-dependent Ca2+-pump, called SERCA, which pumps Ca2+ ions back into the SR against its concentration gradient. This lowers [Ca2+]cytoplasm and returns the inhibition of troponin on tropomyosin by a lack of binding of Ca2+ to TnC.

Question 6

Why does rigor mortis happen?

Answer 6

2. Several hours after death the body no longer makes ATP. A fresh ATP molecule is required to cause the separation of the cross-bridges from the actin filaments during the relaxation process. Without the ATP molecule, the myosin filaments remain bound to the actin, thus keeping the muscles in a state of contracture “rigor.”
3. Eventually rigor mortis subsides 15-20 hours later, when autolysis from the enzymes of lysosomes breaks down the muscle fibers entirely.

Question 7

What is the length-tension relationship of active muscle contraction?

Answer 7

the tension a muscle fiber can generate is directly proportional to the # of crossbridges formed between the thick and thin filament.

Question 8

What is the relationship between active and passive tension?

Answer 8

1. This describes the same concept as the prior graph, in that lengthening the muscle will produce more passive tension but will reduce active tension.
2. Passive tension= the tension that develops in the muscle as a result of the passive “stretch” of the muscle that occurs prior to contraction.
3. Active tension= the tension of the muscle that develops as a result of contracting the muscle.

Question 9

What is the realtionship between load and velocity of contraction?

Answer 9

• This is essentially how fast you can contract your muscle given different amounts of weights. Generally, the lighter the weight (load) the faster you can contract your muscles.
• When loads are applied, the velocity of the contraction becomes less as the load increases. This occurs up until the load is so much that it is ≥ maximum force that the muscle can generate. At that point, the velocity will be 0 and the muscle will not shorten despite contraction.

Question 10

How is phosphocreatine used as an ATP source?

Answer 10

 This molecule carries a high energy phosphate bond similar to ATP, and reconverts ADP  ATP.

 As a side note: phosphocreatine is made from the reaction of creatine + ATP –> phosphocreatine + ADP. Phosphocreatine is made and stored before muscle contraction occurs. Kinda like a backup system.

Question 11

How long can phosphocreatine cause muscle contraction?

Answer 11

 However, the total amount of phosphocreatine is very little, so it can only cause muscle contraction for 5-8 seconds.

Question 12

How is glycogen used as an ATP source?

Answer 12

 The glycogen stores are broken down into free glucose molecules and G1P. These enter the glycolysis pathway immediately and subsequently ATP is produced. The ATP is used for the energize additional muscle contraction and re-form phosphocreatine. Subsequently, pyruvic and lactic acid are produced.
 This is important because this is an anaerobic pathway- muscles can contract for many seconds, and sometimes up to a minute, in the absence of oxygen.

Question 13

How long can glycogen stores cause muscle contraction?

Answer 13

~1.5 minutes

Question 14

How is oxidative metabolism used as an ATP source?

Answer 14

 The stuff we eat (carbs, fats, and protein) are broken down and combined with oxygen to enter the oxidative phosphorylation pathway.
 This is the major source of muscle energy. More than 95% of all energy used by muscles for sustained, long-term contraction is derived from this source.

Question 15

How long can oxidative metabolism be used for muscle contraction?

Answer 15

hours and hours

Question 16

What is an isometric contraction?

Answer 16

1. Muscle contraction is said to be isometric when the muscle doesn’t shorten during contraction.
2. This often occurs when we try to lift something that is too heavy for us to pick up. We will contract our muscles trying to lift it but our muscles will never shorten.

Question 17

What is an isotonic contraction?

Answer 17

1. Muscle contraction is said to be isotonic when the muscle shortens but the tension on the muscle remains constant throughout the contraction.
2. This often occurs when we lift really light things. Our muscles need to generate a small, consistent amount of force to lift a 1 lb weight and our muscles shorten appropriately.

Question 18

What are the characteristics of slow fibers?

Answer 18

1. AKA “Type 1” or “red” fibers
2. They are smaller fibers and are innervated by smaller nerve fibers.
3. Have an extensive blood vessel system.
4. Have an increased # of mitochondria to support high levels of oxidative metabolism.
5. Contain large amounts of myoglobin- stores O2 and speeds its transport into the mitochondria. The redness of the myoglobin give the red appearance to the muscle.

Question 19

What are the characteristics of fast fibers?

Answer 19

1. AKA “Type 2” or “white” fibers
2. Large fibers for great contraction strength
3. Extensive sarcoplasmic reticulum for rapid release of Ca2+ to initiate contraction
4. Large amounts of glycolytic enzymes for rapid release of energy by the glycolytic pathway.
5. Lesser blood supply, mitochrondria and myoglobin than red fibers.

Question 20

What is summation?

Answer 20

Summation is means adding together the individual twitch contractions to increase the intensity of overall muscle contraction. In a single action potential from a single motor neuron, the muscle will contract (twitch) and then return back to its resting state. When you increase the frequency of motor neuron firings, so that the muscle cannot return back to its resting state, the persistence of calcium and ATP will cause the force of contraction to increase.

Question 21

Why does tetanus occur?

Answer 21

• Eventually, the frequency of the action potentials will be increased so much that the muscle will remain in a continuous contracted state. This will create tetanus (Greek for “taut”), where the force of contraction will plateau and the muscle remains rigid.

Question 22

Why do muscles fatigue?

Answer 22

1. Fatigue occurs when muscle glycogen stores are depleted. When they are depleted there is an inability of the contractile and metabolic processes of the muscle fibers to continue to supply the same work output.

Question 23

What is the skeletal muscle lever system?

Answer 23

• Muscles operate by applying tension to their points of insertion into bones. Muscles are designed so that they act on a lever system to move different bones at joints.
• The lever system depends on many things: the point of muscle insertion, its distance from the fulcrum of the lever, the length of the lever arm, and the position of the lever.

Question 24

What is an example of the skeletal muscle lever system?

Answer 24

1. For instance, if the arm is at 90^o, the biceps tendon is far from the fulcrum (elbow joint) of the arm. This creates a maximal force generated.
2. If the arm is in full extension, the tendon is close to the fulcrum and less force can be generated from the hand.

Question 25

What is the arrangement of SkM from muscle –> actin/myosin?

Answer 25

• Skeletal muscle is composed of muscle fibers. Muscle fibers are surrounded by the sarcolemma. Muscle fibers are made up of myofibrils which are arranged into sarcomeres composed of actin and myosin

Question 26

What is the sarcoplasm?

Answer 26

fills the space between the myofibrils that make up the muscle fiber. It contains large quantities of potassium, magnesium, phosphate, and proteins. Large amount of mitochondria are present also to allow from the large amount of ATP needed for muscle contraction.

Question 27

What is the sarcoplasmic reticulum?

Answer 27

found in the sarcoplasm. It is the internal tubular structure that is the site of Ca2+ storage and release for muscle contraction.

Question 28

What are t-tubules?

Answer 28

are an extensive tubular network, open to the extracellular space that carry the depolarization from the sacrolemmal membrane to the cell interior. Located at the junctions of A band and I band

Question 29

What is titin?

Answer 29

extends from the z disc of the sarcomere and connects to the thick filament. The side by side relationship of between myosin and actin is achieved by titin.

Question 30

What is the I-band?

Answer 30

surrounds the z disc and where only the thin filament actin is present

Question 31

What is the A band?

Answer 31

contains the entire length of the thick filament myosin

Question 32

What is the H-band?

Answer 32

within the A band the area of myosin where no actin is present

Question 33

What is the M-line?

Answer 33

found in the H band formed by cross connecting elements of cytoskeleton

Question 34

During contraction, what happens to the I, A, and H bands, as well as the M line and Z dics?

Answer 34

o I band-shortens
o A band- does not change
o H band-shortens
o M line-doesn’t change
o Z discs-come closer together

Question 35

What is the structural anatomy of actin?

Answer 35

is made up of a double stranded F-actin protein molecule. Two strands are wound in a helix. Each strand of the double F-actin helix is composed of polymerized G-actin molecules. ADP is attached to each G-actin molecule. Believe that the ADP molecules are the active sites on the actin filament with witch the myosin cross-bridges with to cause muscle contraction.

Question 36

What is the structural anatomy of tropomyosin?

Answer 36

each molecule is wrapped spirally around the sides of the F-actin helix. In resting state tropomyosin lies on top of the active sites of actin so actin myosin interaction cannot take place to cause muscle contraction

Question 37

What is the structural anatomy of troponin?

Answer 37

is the regulatory protein that permits cross-bridge formation when it binds with Ca2+

Question 38

What is the structural anatomy of the myosin molecule?

Answer 38

o Myosin molecule- composed of 6 polypeptides-two heavy chains and four light chains. The two heavy chains wrap spirally around each other to form a double helix called the tail of the myosin molecule. One end of each chain forms a myosin head. The four light chains are also part of the head two to each head. Help to control head during contraction. The tails of the myosin molecules bundle together to from the body. Part of the body of the myosin filament that hangs to the side with the head is called the arm. The arm and head make the cross –bridges. Each cross bridges has a hinge which allows the heads to be either extended far outward from the body of the myosin filament or be brought close to the body. Hinges are located at two points: between arm and myosin body and between arm and head. Also help to allow contraction to take place.

o Myosin head acts as an ATPase enzyme. This property allows the head to cleave ATP and use the energy derived to energize the contraction process.