Physiology
Cardiovascular
Regarding the sinoatrial node (SAN), which of the following statements is CORRECT:
Answer:
The resting potential of the SAN is about - 60 mV, and it decays steadily with time until it reaches a threshold potential of about - 40 mV, when an action potential is initiated. The upstroke of the AP is slow, as it is not due to activation of fast Na+ channels like cardiac myocytes, but instead slow L-type Ca2+ channels; the SA node contains no functional fast Na+ channels. The rate of decay of the SAN resting potential determines the rate of AP and therefore of heart rate, it is therefore called the pacemaker potential. Factors that affect this rate of decay, and thus the heart rate are called chronotropes.Sinoatrial Node Action Potential
Physiology / Cardiovascular / Cardiac Cycle
Last Updated: 26th July 2024
Pacemaker Potential
The action potential (AP) of the sinoatrial node (SAN) differs from that in ventricular muscle.
The resting potential of the SAN is about - 60 mV, and it decays steadily with time until it reaches a threshold potential of about - 40 mV, when an action potential is initiated.
The upstroke of the AP is slow, as it is not due to activation of fast Na+ channels like cardiac myocytes, but instead slow L-type Ca2+ channels; the SA node contains no functional fast Na+ channels. The slow upstroke means that conduction between nodal myocytes is slow, which is particularly important at the atrioventricular node (which has a similar AP to the SAN).
The rate of decay of the SAN resting potential determines the rate of AP and therefore of heart rate; it is therefore called the pacemaker potential. The pacemaker potential decays because of a slowly reducing outward K+ current set against a slow inward Na+ leak through slow Na+ channels (and to a lesser extent, a slow inward Ca2+ leak through T-type Ca2+ channels). Factors that affect these currents alter the rate of decay and the time to reach threshold and thus heart rate and are called chronotropic agents.
Sinoatrial Node Action Potential. (Image by OpenStax College [CC BY 3.0 , via Wikimedia Commons)
Chronotropic Agents
Noradrenaline (the sympathetic neurotransmitter) is a positive chronotrope and causes a faster rate of decay and thus heart rate whereas acetylcholine (the parasympathetic neurotransmitter) is a negative chronotrope and lengthens the time to reach threshold and decreases heart rate.
Other Cardiac Action Potentials
Other atrial cells, the AV node, the bundle of His and Purkinje system may also exhibit decaying resting potentials that can act as pacemakers. However the SAN is normally fastest and predominates - this is called overdrive suppression.
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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 |
