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Physiology

Cardiovascular

Question 151 of 180

Which of the following is not true of cardiac myocytes:

Answer:

The myocardium is composed of cardiac muscle cells called myocytes. The cells are striated due to the arrangement of the thick and thin filaments which make up the bulk of the muscle, although they are less organised than in skeletal muscle. The myocytes are small and branched, with a single nucleus and are rich in mitochondria. Adjacent cardiac myocytes are connected to each other by intercalated discs.

Cardiac Myocyte Microstructure

The myocardium is composed of cardiac muscle cells called myocytes. The cells are striated due to the arrangement of the thick and thin filaments which make up the bulk of the muscle, although they are less organised than in skeletal muscle. The myocytes are small and branched, with a single nucleus and are rich in mitochondria. The normal pumping action of the heart is dependent on the synchronised contraction of all cardiac cells.

Cardiac myocyte contraction is not dependent on an external nerve supply but instead the heart generates its own rhythm (inherent rhythmicity). The nerves innervating the heart only speed up or slow down the rhythm and can modify the force of contraction.

The synchronicity between myocytes occurs because all the adjacent cells are linked to one another at their ends by specialised gap junctions (formed of connexons), within the intercalated discs, which essentially provide a low-resistance pathway between cells. Gap junctions allow action potentials to spread rapidly from one cell to another and allows the myocardium to act as a functional syncytium. The intercalated discs also provide structural attachments (desmosomes) between myocytes to distribute force.

Although a rise in intracellular [Ca2+] initiates contraction in the same way as in skeletal muscle, the mechanisms leading to the rise in intracellular [Ca2+] are fundamentally different.

By OpenStax CNX [CC BY 3.0 , via Wikimedia Commons

Cardiac Myocyte Microstructure. (Image by OpenStax CNX [CC BY 3.0 , 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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