Updated: 2/1/2018

Muscle Conduction to Contraction

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Introduction
  • Skeletal muscle cells are composed of muscle fascicles
    • muscle fascicles are composed of multi-nucleated muscle fibers
      • muscle fibers are composed of myofibrils
        • myofibrils contain sarcomeres, where actin and myosin filaments slide and produce contractions
  • Muscle cell transverse tubules (T-tubules), invaginations of the plasma membrane, are juxtaposed with the sarcoplasmic reticulum’s terminal cisternae to contract
    • T-tubules are responsible for conducting the action potential to the cisternae of the sarcoplasmic reticulum
    • skeletal muscle triad
      • 1 T-tubule and 2 terminal cisternae
    • cardiac muscle dyad
      • 1 T-tubule and 1 terminal cisterna
  • Sliding filament theory
    • sliding of thick and thin filaments form a contraction
    • thick filaments
      • composed of protein myosin
      • anchored to M line
    • thin filaments
      • composed of actin, tropomyosin, and troponin
        • actin contains binding sites for myosin
        • tropomyosin covers these binding site grooves
      • anchored to Z line

Excitation-Contraction Coupling

  • Action potential depolarizes plasma membrane
    • presynaptic voltage-gated Ca2+ channels open and neurotransmitters are released from the neuron
    • post-synaptic ligand is bound, causing depolarization of the motor end plate of the muscle
  • Depolarization spreads to the T-tubule
    • depolarization causes a conformational change in the voltage-sensitive dihydropyridine receptor
    • this causes a conformational change in the ryanodine receptor (also a calcium channel protein)
    • Ca2+ is released from the sarcoplasmic reticulum
      • this initiates the contraction
Contraction
  • Free Ca2+ binds to troponin C
    • this causes a conformational change and moves tropomyosin out of a myosin-binding groove
  • Myosin releases ADP and Pi, causing displacement of myosin on actin filament
    • this produces a power stroke and contraction shortens H and I bands, resulting in shortening between the Z lines (HIZ shrinkage)
    • A band remains the same length
  • Myosin head is detached from the actin filament because of binding of a new ATP molecule
  • Hydrolysis of ATP to ADP causes myosin head to adopt a high-energy position, in preparation for the next contraction
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Questions (2)

(M1.MK.15.6003) Upon depolarization of the muscle cell plasma membrane, dihydropyridine receptors in the T tubules are activated. Similarly, ryanodine receptors in the sarcoplasmic reticulum (SR) are activated leading to the release of calcium from the SR. This transient increase in calcium results in muscle contraction. Which of the following regions (Figure A) does not shorten during a muscle contraction?
Tested Concept

QID: 106962
FIGURES:
1

Distance from M line to the Z disc

0%

(0/134)

2

Distance between the two Z discs

8%

(11/134)

3

H zone

6%

(8/134)

4

I band

6%

(8/134)

5

A band

78%

(104/134)

M 1 E

Select Answer to see Preferred Response

(M1.MK.15.70) A scientist is studying the properties of muscle tissue. She is able to examine a single sarcomere within a myocyte, as demonstrated in Figure A. She then artificially induces the sarcomere to contract. When this occurs, which of the following sections (as identified by the correct letter in Figure A and correct name) will not decrease in length? Tested Concept

QID: 106489
FIGURES:
1

Section C: A Band

15%

(19/127)

2

Section C: H Zone

10%

(13/127)

3

Section B: I Band

12%

(15/127)

4

Section D: I Band

4%

(5/127)

5

Section B: A Band

57%

(72/127)

M 1 D

Select Answer to see Preferred Response

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