CVS Physiology Lecture # 19 Study Notes: Autonomic Control of Nodal Action Potential

INNERVATION OF THE HEART

• PARASYMPATHETIC NERVOUS SYSTEM (PANS)

The right and left Vagus nerve (CN10) supply the SA node and the AV node respectively. Apart from the two nodes, the Vagus nerve also innervates the atrial muscles and the AV bundle. Parasympathetic supply to the ventricular muscles is very sparse as only a few vagal branches innervate the ventricles. Therefore, it is safe to say that the Vagus supply is limited to the two nodes and the atrial muscles only. PANS causes a marked decrease in heart rate (negative chronotropic effect) and a slight decrease in heart muscle contractility (negative inotropic effect).

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• SYMPATHETIC NERVOUS SYSTEM (SANS)

On the other hand, the sympathetic supply to the heart is global as it supplies the two nodes as well as the atrial and ventricular muscles. SANS increases the heart rate and the contractility of the myocardium i.e. positive chronotropic and inotropic effect.

RATE OF DISCHARGE OF NODAL TISSUE

There is a gradual increase in the RMP of nodal tissues from -55mV to -40mV and this is referred to as the slope of depolarization. Phase 4 is followed by the Phase 0 of action potential. At -55mV, the fast sodium channels (also known as ‘funny’ channels) are in a semi-open state which allows leakage of positive ions into the nodal cells. Leakage of sodium ions causes an increase in the membrane potential in the positive direction (towards 0). The membrane potential gradually increases to            -40mV. Upon reaching this threshold potential, the sodium channels close and remain closed for the rest of the action potential as they enter a state of refractoriness.

An increase in the slope of depolarization will cause the SA node to generate action potentials at a higher rate. Flattening of the slope will result in decreased number of action potentials in a given time which greatly reduces the rate at which the heart beats.

The action of the ANS on the Phase 0 of the nodal action potential brings about changes in heart rate. SANS acts to increase the slope of depolarization so that the threshold potential (-40mV) is reached earlier than normal. This results in an increase in the heart rate. The PANS, via the vagal stimulation, causes a decrease in the slope of depolarization which causes a delay in reaching the threshold potential for spontaneous depolarization. This subsequently results in an increase in the duration of action potential, thereby decreasing the heart rate (negative chronotropic effect).

Additionally, changes in ionic conduction across the cell membrane also cause changes in the heart rate. However, the part of action potential that undergoes changes relies on the type of ions involved:

• Changes in sodium ion conduction cause a change in the slope of depolarization. This is similar to the effects of the autonomic nervous system.
• Calcium channel conduction affects the rapid upstroke or Phase 0 of depolarization.  
• The repolarization phase is dependent upon the conductance of potassium ions. Hence, any changes in potassium ion conductance will affect the time taken for the nodal tissue to repolarize.

Faster the conduction of the above mentioned ions across the cell membrane, higher would be the frequency of depolarizations and vice versa. Dromotropy is a property of the cardiac conducting system which determines the conduction velocity of impulses across the cardiac conduction system. Impulse velocity is determined by the conductance of above mentioned ions. SANS causes positive dromotropic effect, thereby increasing the conduction velocity across the conduction system of the heart. On the other hand, PANS results in a negative dromotropic effect. Negative dromotropic effect will result in decreased conduction velocity across the cardiac conduction system.

Rate changes in the SA node occur mainly via the action of autonomic nervous system. Whereas, the AV node and rest of the conduction system exhibit changes in their rate of depolarizations secondary to the movement of ions across their cell membranes.

TERMS USED TO DESCRIBE HEART RATE & CONTRACTILITY

1. Chronotropy: It refers to the rate of cardiac activity. For example, the parasympathetic nervous system causes negative chronotropy and the sympathetic nervous system causes positive chronotropy.
2. Dromotropy: This term is used to describe the velocity of conduction within the conducting fibers. Negative dromotropic agents decrease the velocity of conduction. Positive dromotropy occurs when there is an increase in the velocity of conduction.
3. Inotropy: This is a term used to describe the contractility of the cardiac musculature (ventricular muscle mainly). SANS has a positive inotropic effect whereas PANS stimulation has a mildly negative chronotropic effect which isn’t that pronounced.

MECHANISM OF ACTION OF AUTONOMIC NERVOUS SYSTEM AT MOLECULAR LEVEL

Primary neurotransmitters (Acetylcholine in PANS stimulation and adrenaline or noradrenalin in SANS stimulation) activate the G-coupled proteins present on the cell membrane of the conducting tissues. This results in the detachment of intracellularly bonded alpha subunit of the G proteins which acts on the sodium and calcium channels. The channels get activated or deactivated depending on the type of stimulation that occurs initially.