Physiology
Cardiovascular
In ventricular myocytes, the plateau phase of the action potential comes about through which of the following:
Answer:
After the intial upstroke of the action potential, Na+ channels and currents rapidly inactivate, but in cardiac myocytes, the initial depolarisation activates voltage-gated Ca2+ channels (slow L-type channels, threshold approximately - 45 mV) through which Ca2+ floods into the cell. The resulting influx of Ca2+ prevents the cell from repolarising and causes a plateau phase, that is maintained for about 250 ms until the L-type channels inactivate. The cardiac AP is thus much longer than that in nerve or skeletal muscle.Ventricular Myocyte Action Potential
Physiology / Cardiovascular / Cardiac Cycle
Last Updated: 21st April 2019
The resting potential of ventricular myocytes is about -90 mV. An action potential (AP) is initiated when the myocyte is depolarised to a threshold potential of about -65 mV, as a result of transmission from an adjacent myocyte via gap junctions.
Depolarisation
Fast voltage-gated Na+ channels are activated and a Na+ influx depolarises the membrane rapidly to about +30 mV. This initial depolarisation is similar to that in nerve and skeletal muscle, and assists the transmission to the next myocyte.
Na+ channels and currents rapidly inactivate, but in cardiac myocytes, the initial depolarisation activates voltage-gated Ca2+ channels (slow L-type channels, threshold approximately - 45 mV) through which Ca2+ floods into the cell. The resulting influx of Ca2+ prevents the cell from repolarising and causes a plateau phase, that is maintained for about 250 ms until the L-type channels inactivate. The cardiac AP is thus much longer than that in nerve or skeletal muscle.
Repolarisation
Repolarisation occurs due to activation of voltage-gated K+ rectifier channels and a K+ efflux. As the AP lasts almost as long as contraction, its refractory period prevents another AP being initiated until the muscle relaxes, thus cardiac muscle cannot exhibit tetanus.
![Image modified by FRCEM Success. [CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/) or GFDL (http://www.gnu.org/copyleft/fdl.html)], via Wikimedia Commons](https://primary-cdn.frcemsuccess.com/wp-content/uploads/2017/05/AP.gif)
Ventricular Muscle Action Potential. (Image modified by FRCEM Success. Original by Unknown [CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/), via Wikimedia Commons)
Other Cardiac Action Potentials
Atrial myocytes have a similar but more triangular AP compared to the ventricles (less plateau). Purkinje fibres in the conduction system are also similar to ventricular myocytes, but have a spike at the peak of the upstroke reflecting a larger Na+ current that contributes to their fast conduction velocity.
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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 |
