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Physiology

Cardiovascular

Question 139 of 180

What is the threshold potential of the sinoatrial node:

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.

Sinoatrial Node Action Potential

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 Naleak 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.

By OpenStax College [CC BY 3.0 , via Wikimedia Commons

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

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