Updated: 2/1/2018

Muscle Conduction to Contraction

Review Topic
  • 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
  • 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)
Lab Values
Blood, Plasma, Serum Reference Range
ALT 8-20 U/L
Amylase, serum 25-125 U/L
AST 8-20 U/L
Bilirubin, serum (adult) Total // Direct 0.1-1.0 mg/dL // 0.0-0.3 mg/dL
Calcium, serum (Ca2+) 8.4-10.2 mg/dL
Cholesterol, serum Rec: < 200 mg/dL
Cortisol, serum 0800 h: 5-23 μg/dL //1600 h:
3-15 μg/dL
2000 h: ≤ 50% of 0800 h
Creatine kinase, serum Male: 25-90 U/L
Female: 10-70 U/L
Creatinine, serum 0.6-1.2 mg/dL
Electrolytes, serum  
Sodium (Na+) 136-145 mEq/L
Chloride (Cl-) 95-105 mEq/L
Potassium (K+) 3.5-5.0 mEq/L
Bicarbonate (HCO3-) 22-28 mEq/L
Magnesium (Mg2+) 1.5-2.0 mEq/L
Estriol, total, serum (in pregnancy)  
24-28 wks // 32-36 wks 30-170 ng/mL // 60-280 ng/mL
28-32 wk // 36-40 wks 40-220 ng/mL // 80-350 ng/mL
Ferritin, serum Male: 15-200 ng/mL
Female: 12-150 ng/mL
Follicle-stimulating hormone, serum/plasma Male: 4-25 mIU/mL
Female: premenopause: 4-30 mIU/mL
midcycle peak: 10-90 mIU/mL
postmenopause: 40-250
pH 7.35-7.45
PCO2 33-45 mmHg
PO2 75-105 mmHg
Glucose, serum Fasting: 70-110 mg/dL
2-h postprandial:<120 mg/dL
Growth hormone - arginine stimulation Fasting: <5 ng/mL
Provocative stimuli: > 7ng/mL
Immunoglobulins, serum  
IgA 76-390 mg/dL
IgE 0-380 IU/mL
IgG 650-1500 mg/dL
IgM 40-345 mg/dL
Iron 50-170 μg/dL
Lactate dehydrogenase, serum 45-90 U/L
Luteinizing hormone, serum/plasma Male: 6-23 mIU/mL
Female: follicular phase: 5-30 mIU/mL
midcycle: 75-150 mIU/mL
postmenopause 30-200 mIU/mL
Osmolality, serum 275-295 mOsmol/kd H2O
Parathyroid hormone, serume, N-terminal 230-630 pg/mL
Phosphatase (alkaline), serum (p-NPP at 30° C) 20-70 U/L
Phosphorus (inorganic), serum 3.0-4.5 mg/dL
Prolactin, serum (hPRL) < 20 ng/mL
Proteins, serum  
Total (recumbent) 6.0-7.8 g/dL
Albumin 3.5-5.5 g/dL
Globulin 2.3-3.5 g/dL
Thyroid-stimulating hormone, serum or plasma .5-5.0 μU/mL
Thyroidal iodine (123I) uptake 8%-30% of administered dose/24h
Thyroxine (T4), serum 5-12 μg/dL
Triglycerides, serum 35-160 mg/dL
Triiodothyronine (T3), serum (RIA) 115-190 ng/dL
Triiodothyronine (T3) resin uptake 25%-35%
Urea nitrogen, serum 7-18 mg/dL
Uric acid, serum 3.0-8.2 mg/dL
Hematologic Reference Range
Bleeding time 2-7 minutes
Erythrocyte count Male: 4.3-5.9 million/mm3
Female: 3.5-5.5 million mm3
Erythrocyte sedimentation rate (Westergren) Male: 0-15 mm/h
Female: 0-20 mm/h
Hematocrit Male: 41%-53%
Female: 36%-46%
Hemoglobin A1c ≤ 6 %
Hemoglobin, blood Male: 13.5-17.5 g/dL
Female: 12.0-16.0 g/dL
Hemoglobin, plasma 1-4 mg/dL
Leukocyte count and differential  
Leukocyte count 4,500-11,000/mm3
Segmented neutrophils 54%-62%
Bands 3%-5%
Eosinophils 1%-3%
Basophils 0%-0.75%
Lymphocytes 25%-33%
Monocytes 3%-7%
Mean corpuscular hemoglobin 25.4-34.6 pg/cell
Mean corpuscular hemoglobin concentration 31%-36% Hb/cell
Mean corpuscular volume 80-100 μm3
Partial thromboplastin time (activated) 25-40 seconds
Platelet count 150,000-400,000/mm3
Prothrombin time 11-15 seconds
Reticulocyte count 0.5%-1.5% of red cells
Thrombin time < 2 seconds deviation from control
Plasma Male: 25-43 mL/kg
Female: 28-45 mL/kg
Red cell Male: 20-36 mL/kg
Female: 19-31 mL/kg
Cerebrospinal Fluid Reference Range
Cell count 0-5/mm3
Chloride 118-132 mEq/L
Gamma globulin 3%-12% total proteins
Glucose 40-70 mg/dL
Pressure 70-180 mm H2O
Proteins, total < 40 mg/dL
Sweat Reference Range
Chloride 0-35 mmol/L
Calcium 100-300 mg/24 h
Chloride Varies with intake
Creatinine clearance Male: 97-137 mL/min
Female: 88-128 mL/min
Estriol, total (in pregnancy)  
30 wks 6-18 mg/24 h
35 wks 9-28 mg/24 h
40 wks 13-42 mg/24 h
17-Hydroxycorticosteroids Male: 3.0-10.0 mg/24 h
Female: 2.0-8.0 mg/24 h
17-Ketosteroids, total Male: 8-20 mg/24 h
Female: 6-15 mg/24 h
Osmolality 50-1400 mOsmol/kg H2O
Oxalate 8-40 μg/mL
Potassium Varies with diet
Proteins, total < 150 mg/24 h
Sodium Varies with diet
Uric acid Varies with diet
Body Mass Index (BMI) Adult: 19-25 kg/m2

(M1.MK.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?
Review Topic

QID: 106962

Distance from M line to the Z disc




Distance between the two Z discs




H zone




I band




A band




Select Answer to see Preferred Response


(M1.MK.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? Review Topic

QID: 106489

Section C: A Band




Section C: H Zone




Section B: I Band




Section D: I Band




Section B: A Band




Select Answer to see Preferred Response

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