Chapter 6 - Contraction Of Skeletal Muscle

Question 1

Thin membrane enclosing a skeletal muscle fiber

Answer 1

SARCOLEMMA

Question 2

Myofibril contains what

Answer 2

1. ) Actin (3000) filaments
   - “I” band - isotropic to polarized light
   - thin filaments
   - “light” band

2.) Myosin (1500) filaments
- thick filaments
- dark band
- A band - anisotropic to polarized light
-

Question 3

End of actin are attached to

Answer 3

Z - disk

Question 4

The portion of the myofibril that liest between two successive Z disk are called?

Answer 4

Sarcomere

Question 5

What keeps the myosin and actin filaments in place?

Answer 5

Titin (very springy)

Question 6

Intercellular fluid between myofibrils

Answer 6

Sarcoplasm

• contains: potassium, magnesium, phosphate
• large amount of mitochondria that lie parallel

Question 7

Acts as a calcium storage in the myofibrils?

Answer 7

Sarcoplasmic retinaculum

Question 8

General mechanism for muscular contraction

Answer 8

1. Action potential travels.
2. The nerve secretes ACETYLCHOLINE.
3. Acetylcholine-gated channels open.
4. Influx of large amount of sodium to the interior of the muscle membrane.
5. Action potential travels along muscle fiber.
6. Action potential depolarizes the muscle membrane causing the sarcoplasmic retinaculum to release stored calcium
7. Calcium initiate attractive forces between actin and myosin, causing them to slide along each other.
8. Calcium is pumped back into the sarcoplasmic retinaculum via Calcium membrane pump

Question 9

Location of sarcomere if the muscle is contracted.

Answer 9

Bottom

Question 10

Composition of mysosin molecules

Answer 10

1. ) Two heavy chain (wt. 200,000)
   * wrap spirally to form the TAIL of the myosin molecule
2. ) Four light chain (wt. 20,000)
   * also part of the myosin head - 2 to each head.
   * help control the function of the head during muscle contraction.

Question 11

Also acts an ATPase enzyme

Answer 11

Myosin head

Question 12

Backbone of the actin filament.

Answer 12

Double stranded F - actin protein molecule

* composed of G - actin molecule

Question 13

Location of tropomyosin in relaxed state.

Answer 13

On top of the active actin strand.

• so that attraction cannot occur between the actin and myosin filaments to cause contractions.

Question 14

Functions of each troponin.

Answer 14

1. Troponin I (strong affinity for actin)
   * clinically important in assessing myocardial infarction.
2. Troponin T (for tropomyosin)
3. Troponin C (for calcium ions)

Question 15

In the absence if troponin-tropomyosin complex (but wuth the presence of magnesium and ATP) what will happen?

Answer 15

Actin will bind instantly and strongly to myosin molecule.

Question 16

Muscle does not shorten during contraction?

Answer 16

Isometric contraction.

• when the load is greater than the force of the muscle contraction.
• 3 types
  1. Ocular muscle - duration of less than 1/50th of a second.
  2. Gastrocnemius muscle - 1/15th.
  3. Soleus muscle - 1/5th

Question 17

What is isotonic contraction?

Answer 17

When the force of the muscle contraction is greater than the load.
* shortens the muscle.

Question 18

Pathway that accounts for muscular atrophy.

Answer 18

ATP-dependent ubiquitin - proteasome pathway