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Physiology

Basic Cellular

Question 92 of 180

Regarding the sarcomere, which of the following best describes the A-band:

Answer:

The A-band extends along the whole length of the myosin filament. The A-band therefore does not shorten in muscle contraction, as the myosin filament does not shorten itself (rather the sarcomere shortens as the myosin and actin filaments slide over one another).

Sarcomere

Physiology / Basic Cellular / Skeletal Muscle Physiology

Last Updated: 29th July 2024

The sarcomere is the functional unit of the muscle.

Each muscle fibre is divided at regular intervals along its length into sarcomeres separated by Z-lines.

The I-band extends from either side of the Z-line to the start of the thick myosin filament (i.e. it is the zone of thin actin filaments that is not superimposed by thick myosin filaments).

The A-band extends along the whole length of the myosin filament. The A-band therefore does not shorten in muscle contraction, as the myosin filament does not shorten itself (rather the sarcomere shortens as the myosin and actin filaments slide over one another).

The H-zone is at the centre of the sarcomere ending at the start of the actin filaments (i.e. it is the zone of thick myosin filaments that is not superimposed by thin actin filaments).

The M-line is a disc of filaments in the middle of the H-zone that holds the myosin filament in position so that each one is surrounded by six actin filaments.

Structure of the Sarcomere. (Image by OpenStax [CC BY 4.0 , via Wikimedia Commons)

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  • Biochemistry
  • Blood Gases
  • Haematology
Biochemistry Normal Value
Sodium 135 – 145 mmol/l
Potassium 3.0 – 4.5 mmol/l
Urea 2.5 – 7.5 mmol/l
Glucose 3.5 – 5.0 mmol/l
Creatinine 35 – 135 μmol/l
Alanine Aminotransferase (ALT) 5 – 35 U/l
Gamma-glutamyl Transferase (GGT) < 65 U/l
Alkaline Phosphatase (ALP) 30 – 135 U/l
Aspartate Aminotransferase (AST) < 40 U/l
Total Protein 60 – 80 g/l
Albumin 35 – 50 g/l
Globulin 2.4 – 3.5 g/dl
Amylase < 70 U/l
Total Bilirubin 3 – 17 μmol/l
Calcium 2.1 – 2.5 mmol/l
Chloride 95 – 105 mmol/l
Phosphate 0.8 – 1.4 mmol/l
Haematology Normal Value
Haemoglobin 11.5 – 16.6 g/dl
White Blood Cells 4.0 – 11.0 x 109/l
Platelets 150 – 450 x 109/l
MCV 80 – 96 fl
MCHC 32 – 36 g/dl
Neutrophils 2.0 – 7.5 x 109/l
Lymphocytes 1.5 – 4.0 x 109/l
Monocytes 0.3 – 1.0 x 109/l
Eosinophils 0.1 – 0.5 x 109/l
Basophils < 0.2 x 109/l
Reticulocytes < 2%
Haematocrit 0.35 – 0.49
Red Cell Distribution Width 11 – 15%
Blood Gases Normal Value
pH 7.35 – 7.45
pO2 11 – 14 kPa
pCO2 4.5 – 6.0 kPa
Base Excess -2 – +2 mmol/l
Bicarbonate 24 – 30 mmol/l
Lactate < 2 mmol/l

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