Exam 1: Lecture 5

Question 1

Can you diagram the organization of actin and myosin in a muscle fiber as shown in figure 6-3?

Answer 1

Yes

Question 2

In order from smallest to largest, describe the components + connective tissue of the hierarchical structure of skeletal muscle

Answer 2

Actin and myosin form a sarcomere called MYOFILAMENT, a chain of sarcomeres forms the MYOFIBRIL, a bundle of myofibrils forms the MYOFIBER or muscle cell which is surrounded by endomysium, a bundle of myofibers forms the FASCICLE which is surrounded by the perimysium, and a bundle of fascicles forms the MUSCLE which is surrounded by the epimysium

Question 3

Describe the structural and molecular characteristics of myofibrils functional component (sarcomere)

Hint: Visualize the structure while answering

Answer 3

The sarcomere is comprised of the sarcolemma, 2 T-tubules, and the sarcoplasmic reticulum. There are 4 regions: the Z-disc at either end anchoring actin, I-Bands adjacent to z-discs composed of actin, A-Bands composed of actin and myosin in the middle, H-Bands composed entirely of myosin within the a-band

Question 4

Describe the changes in the banding pattern during muscle contraction (leave out description of M-line)

Answer 4

When the muscle contracts the I bands become narrower in width, the A-band stays the same and the H-band becomes narrower

Question 5

Generally explain the sliding filaments mechanism events

Answer 5

An initial action potential causes a series of channels to open sending signals that cause another action potential on the sarcolemma. This triggers an increase of calcium in the cytosol and triggers the sliding filament mechanism. Myosin and actin to interact, energized by the hydrolysis of ATP via ATP-dependent calcium pump. This repeats until SR brings the calcium ions back within it.

Question 6

Describe the events that occur between myosin and actin filaments during the muscle contraction; start after calcium enters the cytosol and binds to troponin

Answer 6

Troponin undergoes a conformational change causing tropomyosin to expose the myosin binding sites on actin. The myosin head binds to actin and realeases an phosphate, and pulls along the actin releasing ADP, a second ATP binds to myosin causing it to release the actin. This process continues to repeat until the calcium is removed from the cell.

Question 7

Describe the role of T-Tubules and the sarcoplasmic reticulum in muscle contraction

Answer 7

T-tubules or transverse tubulues lie on top of the SR. During contractions, the channels along the T-Tubules are stimulated by the action potential and cause the SR, which houses the calcium ions, to release the calcium ions into the cytosol leading to the activation of the sliding filament mechanism

Question 8

Compare preload to afterload and include the results of each

Answer 8

Preload is the load on a muscle before it contracts, that results in a stretching of a sarcomere and generates a directly proportional passive tension. Afterload is the load the muscle works against that produces an active tension

Question 9

Differentiate between active and passive tension

Answer 9

Passive tension is produced by the preload and is the force the muscle uses to resist the load; active tension is produced by afterload;
passive tension + active tension = Total tension

Question 10

Explain what is meant by cross-bridge cycling and describe the role of ATP. Include how cross bridge cycling starts and ends

Answer 10

Cross bridge cycling is the cycle of the myosin head binding to actin; ATP is used to break the link with actin and recocks the myosin head; Cross bridging starts with influx of calcium and doesn’t end until calcium leaves the environment or ATP is depleted

Question 11

Describe the muscle length-tension relation relative to changes in sarcomere length and explain in terms of actin and myosin relationship

1. Actin filaments overlap
2. Actin filaments touch
3. ACtin filaments overlap the myosin cross bridge
4. Actin filament do not overlap

Answer 11

1. Tension is zero and sarcomere <1.65um
2. Tesion is max and sarcomere 1.65um
3. Tension is max and sarcomere 2.2um
4. Tension is zero and sarcomere 3.5um

Question 12

Describe where ATP is required during muscle contraction

Answer 12

• The majority: sliding filament mechanism;
• pumping calcium ions from sarcoplasm back into the SR
• pumping sodium and potassium through the sarcolemma to reestablish resting potential

Question 13

List the sources for rephosphorylation during muscle contraction and the relative significance of each of these sources

Answer 13

Phosphocreatine: Releases energy quickly but provides enough energy for 5-8 seconds of contraction with ATP+ phosphocreatine
Glycolysis: Sustains muscle contractions and causes lactic acid build up
Oxidative metabloism: provides 95% of long term contraction energy

Question 14

Compare isotonic and isometric contractions and include examples of each

Answer 14

Isotonic is an increase in tension without a change in length, and isotonic contractions are when the muscle lengthens via eccentric contraction, or the muscle shortens in a concentric contraction

Question 15

Compare the characteristics of fast fibers and slow fibers

Answer 15

Fast fibers (ex. gastrocnemius) have little endurance; are white and have less mitochondria and is thus used for anaerobic respiration relying on glycolysis and has high concentrations of ATPase, and low concetration of myoglobin; Slow twitch have the most endurance (ex. soleus); muscles have more mitochondria and are used for aerobic respiration, has high concentration of myoglobin and small concentration of ATPase
Slow twitch: posture
Fast twitch: quick movements

Question 16

Define motor unit

Answer 16

Neuron + myofiber (chain of sarcomeres)

Question 17

Define summation and tetany

Answer 17

Summation: the increase of muscle tension due to addition of electrical events
Tetany: when the muscle remains at maximal contraction due to the high frequency of electrical events without rest

Question 18

Explain the mechanism behind summation and tetany

Answer 18

Before the muscle can return to rest, new electrical events (stimulation) cause spikes that increase the total amount of calcium ion concentration in the cytosol; this increasing concentration increases the rate of cross bridge cycling and increases the muscle tension causing summation and eventually leads to tetany when the muscle is maximally contraction

Question 19

If the number of myofibers cannot be increased after birth, how does the muscle get bigger?

Answer 19

The number of myofibrils can be increased which can lead to the mass of a myofiber can increase

Question 20

What replaces lost muscle tissue?

Answer 20

Scar tissue made of fibrous connective tissue

Question 21

What replaces lost muscle tissue?

Answer 21

Scar tissue made of fibrous connective tissue

Question 22

List and describe 3 types of lever systems as they apply to skeletal muscles

Answer 22

First-class is where the fulcrum is in between the  inforce and outforce
Second class is when the outforce or resistance is in the middle 
Third class is when the inforce is in the middle

Question 23

What is the in-force and what is the out-force?

Answer 23

The inforce is the muscle attachment location where effort occurs and the out-force is the location of resistance to the force on the lever arm

Question 24

Describe a first class lever in terms of the direction of in-force and out-force and provide an example

Answer 24

the inforce and outforce move in opposite directions like on a seesaw

Question 25

Describe a second class lever in terms of the direction of in-force and out-force and provide an example

Answer 25

the inforce and outforce are on the same side of the fulcrum and the outforce is in the middle like raising the body on the ball of the foot

Question 26

Describe a third class lever in terms of the direction of in-force and out-force and provide an example

Answer 26

the inforce and outforce move in the same direction and are on the same side of the fulcrum, but the inforce is in the middle like lifting a weight in the palm of your hand