Mechanism of change in heart rate by autonomic nervous system (chronotropy)

Sympathetic and parasympathetic stimulation affect the nodal action potential to change the heart rate

Sympathetic stimulation increases heart rate

Stimulation of sympathetic system releases noradrenaline which binds with beta 1 receptor. Due to this G alpha S gets activated and it causes activation of adenyl cyclase which in turn leads to formation of cAMP. The HCN channels which are responsible for prepotential.. their opening and closing is affected by the concentration cAMP inside the cells. When cAMP increases, the probability of opening of these channels increases. So, in short sympathetic activation leads to more opening of HCN channels, causing more entry of sodium inside the cells leading to faster change in the potential towards threshold. So there is a change in the slope of the prepotential which becomes steeper and due to this SA node generates more number of impulses in the same time and hence it increases heart rate.

Parasympathetic stimulation decreases heart rate

Acetylcholine released from vagus causes activation of G i type of proteins which inhibits adenyl cyclase leading to decreased production of cAMP. Thus is leads to decreased opening of HCN channels and also T type channels. Hence the slope of pre-potential decreases leading to decrease in the number of impulses generated and hence decreased heart rate. Parasympathetic activation has one more effect. It delays the closure of these potassium channel which are responsible for phase 3. So more potassium moves out which leads to hyperpolarization. that is more negative maximum diastolic potential.


 

Pacemaker potential I SA node action potential I Cardiovascular physiology

In this video, we differentiate between SA node action potential and ventricular action potential. 

 SA node action potential is responsible for automaticity of heart. It consists of 3 phases : phase 4, phase 0 and phase 3.

Phase 4 is due to: Hyperpolarization gated cyclic nucleotide channels (HCN channels) and T type voltage gated calcium channels.

Phase 0 is due to opening of L type voltage gated calcium channels and entry of calcium inside the cells

Phase 3 is due toopening of voltage gated potassium channels and exit of potassium from the cells.

(Where are phases 1 and 2 ?) 😮

Phase 4 is also known as prepotential or pacemaker potential. The slope of the phase 4 determines the number of impulses which are generated in 1 minue. Steeper slope means more impulses, less steep slope means less impulses.

 

Waves, 3 segments and 5 intervals in ECG (EKG) | ECG basics

ECG Waves, 3 segments and 5 intervals in ECG (EKG) | ECG basics 

ECG waves: P waves, QRS complex, T wave

Segments: Do not include waves (PR segment, ST segment and TP segment )

Do you know the importance of TP segment ? It is used to find whether ST segment elevation or depression is present or not..

Intervals: PR interval, ST interval, QT interval, RR interval (used to calculate heart rate in case of sinus rhythm), P-P interval (used to calculate atrial rate in case sinus rhythm is not present)

Waves, segments, intervals of ECG | How to read an ECG

Watch waves, segments and intervals of ECG..

Do you know the significance of TP segment?

Well for that matter how many waves and intervals of ECG are you aware of ?

Watch this video to learn the physiological correlates of ECG waves, segments and intervals and try to answer these questions...