Autonomic Control of the Heart

Question 1

What is the autonomic nervous system?

Answer 1

The neural network that control subconcious functions

Question 2

What are the divisions of the autonomic nervous system?

Answer 2

Sympathetic nervous system: Control functions under times of acute stress
Parasympatic nervous system: Control the body function during rest

Question 3

What are the cardiovascular centers?

Answer 3

Collection of neurons in the reticular formation of the medulla oblongata

Question 4

What are the components of the cardiovascular centers?

Answer 4

Vasomotor Center (divided into Cardiac Acceleration Center and Vasodilator C)
Cardiac Inhibitory Center

Question 5

Function of the cardiac acceleration center (CAC) or vasoconstrictor center

Vasomotor center component

Answer 5

Stimulation fo the CAC increases sympathetic discharge to:
1) Adrenal medulla causing Catecholamine (adrenaline and noradrenaline) release
2) Blood Vessels causing vasoconstriction

Question 6

Function of the vasodilator center (VDC)

Vasomotor center component

Answer 6

Stimulation of VDC causes generalized vasodilation through inhibition of vasoconstriction center

Question 7

Function of the Cardiac Inhibitory Center (CIC)

Answer 7

Contains dorsal motor neurons of the vagus which send inhibitory signals to the heart via the vagus nerve

Question 8

What is vagal tone?

Answer 8

Vagal stimulation of the SA node decreases the heart rate to 60-100 bpm from the typical SA node rhythm of 90-110 bpm

Under resting conditions, vagal tone predominates

Question 9

What is ventricular vagal escape?

Answer 9

A reduction in blood pressure due to muscarinic stimulation which is then compensated for stimulation from the sympathetic system to increase heart rate and thus blood pressure

Question 10

What is idioventricular rhythm?

Answer 10

A cardiac rhythm characterized by a rate of <50 beats per minute, absence of P waves and widening of the QRS complex

Question 11

Where are the sympathetic nerves derived from?

Answer 11

T1-T5

Question 12

What parts of the heart does the sympathetic nervous system system supply?

Answer 12

Sympathetic nervous system supply all part of the heart (atria, ventricles, conduction system and the coronary vessels)

Question 13

What are the effects of the sympathetic stimulation?

Answer 13

Increases all cardiac properties (Excitability, Rhythmicity , Conductivity and Contractility)
1) Increase the heart rate in young adult humans from 70 beats/min up to 180 to 200 beats/min and, rarely, even 250 beats/min
2) Increase the force of heart contraction, increasing the volume of blood pumped and the ejection pressure
3) Increase the cardiac output as much as twofold to threefold
4) Increase cardiac output, heart work and O2 consumption
5) Vasodilation of the coronary blood vessels

Question 14

Where is the parasympathetic nervous system of the heart derived from?

Answer 14

Derived from the vagus nerve

Question 15

What parts of the heart does the parasympathetic nervous system supply?

Answer 15

Parasympathetic nervous system supply part of the heart (SA Node, AV Node, atria, conduction system and the coronary vessels)
Right vagus supply SA Node, While left vagus supply AV Node

Question 16

What are the effects of parasympathetic stimulation?

Answer 16

Depression of all cardiac properties (Excitability, Rhythmicity , Conductivity and Contractility)
1) It causes –ve chronotropic effect, decrease atrial contraction (-ve inotropic), decrease atrial excitability, and decrease conductivity
2) Decrease cardiac output, heart work and O2 consumption
3) Vasoconstriction of the coronary blood vessels

Question 17

Describe how sympathetic stimulation affects heart rate

Answer 17

Post-ganglionic sympathetic fibres release norepinephrine (NE) → activate β1 adrenergic receptors located in the SA node → opens sodium (Na+) channels, “funny” current (If) → increase pacemaker potential

Question 18

Describe how parasympathetic stimulation affects heart rate

Answer 18

Post-ganglionic parasympathetic fibres release Acetylcholine (ACh) → activate Muscarinic receptors M3 located in the SA node → closes sodium (Na+) channels → decrease “funny” current (If) → decrease pacemaker potential

Question 19

Describe how sympathetic stimulation affects conductivity

Answer 19

Post-ganglionic sympathetic fibres release norepinephrine (NE) → activate β1 adrenergic receptors located in the AV node → opens (Ca++) channels → faster conduction through the AV node

Question 20

Describe how parasympathetic stimulation affects conductivity

Answer 20

Post-ganglionic parasympathetic fibres released Acetylcholine (ACh) → activate Muscarinic receptors M3 located in the AV Node → closes (Ca++) channels → Slow conduction through the AV node

Question 21

Describe how sympathetic stimulation affects contractility

Answer 21

Post-ganglionic sympathetic fibres release norepinephrine (NE) → activate β1 adrenergic receptors located in the myocyte → Open more Ca++ channels which increases the activity of the Ca++ pump and Ca++ storage in the sarcoplasmic reticulum → elevation of the plateau → increase ventricular contraction

Question 22

Describe how parasympathetic stimulation affects contractility

Answer 22

Parasympathetic nervous system does not innervate ventricular myocytes → NO direct effect on ventricular contractility

Question 23

How does the autonomic nervous system affect vascular tone?

Answer 23

The main ANS factor altering vascular tone is the sympathetic nervous system
Post-ganglionic sympathetic fibres to the blood vessels releases norepinephrine (NE) → binds strongly to α1 and α2 receptors or weakly to β2 receptors
α1 and α2 receptors → vasoconstriction of the blood vessel → increase arterial blood pressure
β2 adrenergic receptors → vasodilation of the blood vessel
Overall, the predominate effect of norepinephrine on the vasculature is vasoconstrictive → increasing vascular tone