Normal heart rate is 72/minute.

It ranges between 60 and 80 per minute.

Tachycardia means increase in heart rate above 100 per minute.

Bradycardia means decrease in heart rate below 60 per minute.

Conditions When Tachycardia Occurs

Conditions When Bradycardia Occurs

Heart rate is maintained within normal range constantly.

It is subjected to variation during conditions such as exercise, emotion, etc.

However, under physiological conditions, the altered heart rate is quickly brought back to normal.

It is because of perfectly tuned regulatory mechanism in the body.

Components of Nervous Mechanism Regulating Heart Rate

1. Vasomotor center
2. Motor (efferent) nerve fibers to the heart
3. Sensory (afferent) nerve fibers from the heart

Vasomotor center is a nervous center that regulates the heart rate. It also regulates blood pressure.

Earlier it was called the cardiac center.

Situation of Vasomotor Center

Vasomotor center is bilaterally situated in the reticular formation of medulla oblongata and lower part of pons.

Areas of Vasomotor Center

1. Vasoconstrictor area
2. Vasodilator area
3. Sensory area

Vasoconstrictor Area

Situation

Vasoconstrictor area is situated in reticular formation of medulla of brain in the floor of IV ventricle and it forms lateral portion of vasomotor center.

Vasoconstrictor area is otherwise known as pressor area or cardioaccelerator center.

Functions

This area increases the heart rate by sending accelerator impulses to heart through sympathetic nerves.

It also causes constriction of blood vessels.

Control of Vasoconstrictor Area

Vasoconstrictor area is under the control of cerebral cortex and hypothalamus.

Vasodilator Area

Situation

Vasodilator area is also situated in reticular formation of medulla oblongata in the floor of IV ventricle.

It forms the medial portion of vasomotor center.

It is also called cardio depressor area or cardioinhibitory center.

Functions

Vasodilator area decreases the heart rate by sending inhibitory impulses to the heart through vagus nerve.

It also causes dilation of blood vessels.

Control of Vasodilator Area

Vasodilator area is under the control of cerebral cortex and hypothalamus.

It is also controlled by impulses from baroreceptors, chemoreceptors and other sensory impulses via afferent nerves.

Sensory Area

Situation

Sensory area is in the nucleus of tractus solitarius in medulla oblongata.

It forms the posterior part of vasomotor center.

Functions

Sensory area receives sensory fibers via the glossopharyngeal nerve and vagus nerve, particularly from baroreceptors.

In turn, this area affects the vasoconstrictor and vasodilator areas.

Heart receives efferent nerves from both divisions of the autonomic nervous system.

Parasympathetic fibers arise from the medulla oblongata and pass through the vagus nerve.

Origin of Parasympathetic Nerve Fibers

Parasympathetic nerve fibers supplying heart arise from the dorsal nucleus of vagus situated in the floor of fourth ventricle in medulla oblongata.

Distribution of Parasympathetic Nerve Fibers

Preganglionic parasympathetic nerve fibers from dorsal nucleus of vagus reach the heart and terminate on postganglionic neurons.

Postganglionic fibers from these neurons innervate heart muscle.

• Most of the fibers from right vagus terminate in the SA node.
• Remaining fibers supply atrial muscles and AV node.
• Most of the fibers from left vagus supply the AV node.
• Few fibers supply atrial muscle and SA node.
• Ventricles do not receive the vagus nerve supply.
• Few fibers are identified in the bases of ventricles, but the functions of these nerve fibers are not known.

Functions of Parasympathetic Nerve

Vagus nerve is cardioinhibitory in function and carries inhibitory impulses from vasodilator area to heart.

The impulses decrease rate and force of contraction.

Vagal Tone

Vagal tone is a continuous stream of inhibitory impulses from vasodilator area to heart via vagus nerve.

Heart is kept under control because of vagal tone.

Impulses from vasodilator area reach the heart and exert inhibitory effect on heart.

Heart rate is inversely proportional to vagal tone.

In experimental animals, removal of vagal input by sectioning the vagus increases the heart rate.

This proves the existence of vagal tone.

Under resting conditions, vagal tone dominates sympathetic tone.

Vagal tone is also called cardioinhibitory tone or parasympathetic tone.

Effects of Stimulation of Right Vagus Nerve

Vagal Escape

Right vagus supplies mainly SA node.

Stimulation of right vagus in experimental animals such as dog, with a weak stimulus causes reduction in heart rate and force of contraction.

Stimulation with strong stimulus causes stoppage of heart due to inhibition of SA node.

If the stimulus is continued for some time, the ventricles start beating, but the rate of contraction is slower than before.

This is because of vagal escape.

Definition of Vagal Escape

Vagal escape is the escape of ventricles from inhibitory effect of vagal stimulation.

If stimulation of vagus nerve is stopped, heart starts beating normally.

Causes for Vagal Escape

Stimulation of right vagus stops heartbeat due to inhibition of SA node and atria.

However, ventricles are not supplied by vagus. So, the ventricles are not inhibited by vagal stimulation.

Because of this, when stoppage of heartbeat is continued for some time by vagal stimulation, a part of ventricular musculature becomes pacemaker and starts producing impulses.

It results in contraction of ventricles, which is called vagal escape.

Vagal escape includes only ventricular contractions.

Rhythmicity of ventricular muscle is less and it is about 20 per minute.

Effects of Stimulation of Left Vagus Nerve

Heart Block

Left vagus supplies mainly the AV node.

Stimulation of left vagus in dog with a weak stimulus causes inhibition of AV node.

Because of inhibition of AV node, some of the impulses from SA node are not conducted to ventricles.

This is called partial heart block.

Ratio between atrial contraction and ventricular contraction is 2:1, 3:1 or 4:1, depending upon the strength of stimulus.

Stimulation of left vagus with strong stimulus causes stoppage of ventricular contraction, which is called complete heart block.

This is because of complete inhibition of AV node.

Prolongation of stimulation causes idioventricular rhythm, which is different from the rhythm of atrial contraction.

Mode of Action of Vagus Nerve

Vagus nerve acts on heart by secreting acetylcholine.

Origin

Preganglionic fibers of sympathetic nerves to heart arise from the lateral gray horns of the first 4 thoracic segments (T1 to T4) of spinal cord.

Course and Distribution of Sympathetic Nerve Fibers

Preganglionic fibers reach the superior, middle and inferior cervical sympathetic ganglia situated in sympathetic chain.

Inferior cervical sympathetic ganglion fuses with first thoracic sympathetic ganglion forming the stellate ganglion.

From these ganglia, the postganglionic fibers arise.

Postganglionic fibers form superior, middle and inferior cervical sympathetic nerves.

Nerves Formed by Sympathetic Postganglionic Fibers

1. Superior cervical sympathetic nerve: It innervates larger arteries and base of the heart.
2. Middle cervical sympathetic nerve: It supplies the rest of the heart.
3. Inferior cervical sympathetic nerve: It serves as sensory (afferent) nerve from the heart.

Functions of Sympathetic Nerves

Sympathetic nerves are cardioaccelerator in function and carry cardioaccelerator impulses from vasoconstrictor area to the heart.

The impulses increase the rate and force of contraction of heart.

Sympathetic Tone

Sympathetic tone or cardioaccelerator tone is the continuous stream of impulses produced by the vasoconstrictor area.

The impulses pass through sympathetic nerves and accelerate the heart rate.

Under normal conditions, vagal tone is dominant over sympathetic tone.

Whenever vagal tone is reduced or abolished, the sympathetic tone becomes more powerful.

Effects of Stimulation of Sympathetic Nerves

Stimulation of sympathetic nerves increases heart rate and force of contraction of heart.

Effect depends upon the strength of stimulus.

Mode of Action of Sympathetic Nerves

Sympathetic nerves increase heart rate by secreting noradrenaline.

Afferent sensory nerve fibers from the heart pass through the inferior cervical sympathetic nerve.

These nerve fibers carry sensations of stretch and pain from the heart to the brain via spinal cord.

Vasomotor center regulates the cardiac activity by receiving impulses from different sources in the body.

After receiving the impulses from different sources, vasodilator area alters the vagal tone and modulates the activities of heart.

1. Impulses from Higher Centers

Vasomotor center is mainly controlled by impulses from higher centers in brain.

Cerebral Cortex

Area 13 in cerebral cortex is concerned with emotional reactions of the body.

During emotional conditions, this area sends inhibitory impulses to the vasodilator area.

This causes reduction in vagal tone leading to cardioacceleration.

Hypothalamus

Hypothalamus influences the heart rate via vasomotor center.

• Stimulation of posterior and lateral hypothalamic nuclei causes tachycardia.
• Stimulation of anterior nuclei causes bradycardia.

2. Impulses from Respiratory Centers

During forced breathing, heart rate increases during inspiration and decreases during expiration.

It is called respiratory sinus arrhythmia.

This is common in some children and in some adults even during quiet breathing.

3. Impulses from Baroreceptors: Marey’s Reflex

Baroreceptors or pressoreceptors are the receptors which give response to change in blood pressure.

Situation of Baroreceptors

Baroreceptors are of two types:

1. Carotid baroreceptors
2. Aortic baroreceptors

Carotid baroreceptors are situated in the carotid sinus, which is present in the wall of internal carotid artery near the bifurcation of common carotid artery.

Aortic baroreceptors are situated in the wall of arch of aorta.

Nerve Supply to Baroreceptors

• Carotid baroreceptors are supplied by Hering’s nerve, a branch of glossopharyngeal (IX cranial) nerve.
• Aortic baroreceptors are supplied by aortic nerve, a branch of vagus (X cranial) nerve.

Nerve fibers from baroreceptors reach the nucleus of tractus solitarius situated adjacent to vasomotor center.

Functions of Baroreceptors: Marey’s Reflex

Baroreceptors regulate the heart rate through a reflex called Marey’s reflex.

Stimulus for this reflex is increased blood pressure.

Marey’s reflex is a cardioinhibitory reflex which decreases heart rate when blood pressure increases.

Whenever blood pressure increases, the carotid baroreceptors are stimulated and afferent impulses are sent to nucleus of tractus solitarius through Hering’s nerve and aortic nerve.

The nucleus of tractus solitarius stimulates vasodilator area, which in turn increases vagal tone leading to decrease in heart rate.

When pressure is less, the baroreceptors are not stimulated. So, no impulses go to the nucleus of tractus solitarius and heart rate is not decreased.

Thus, heart rate is inversely proportional to blood pressure.

Marey’s Law

According to Marey’s law, the pulse rate, which represents heart rate, is inversely proportional to blood pressure.

Baroreceptors produce Marey’s reflex only during resting conditions.

So, in many conditions such as exercise, there is an increase in both blood pressure and heart rate.

4. Impulses from Chemoreceptors

Chemoreceptors are the receptors giving response to change in chemical constituents of blood, particularly oxygen, carbon dioxide and hydrogen ion concentration.

Situation of Chemoreceptors

Peripheral chemoreceptors are situated in the carotid body and aortic body adjacent to baroreceptors.

Nerve Supply to Chemoreceptors

• Chemoreceptors in carotid body are supplied by Hering’s nerve.
• Chemoreceptors in aortic body are supplied by the aortic nerve.

Functions of Chemoreceptors

Whenever there is hypoxia, hypercapnia and increased hydrogen ion concentration in the blood, the chemoreceptors are stimulated and inhibitory impulses are sent to vasodilator area.

Vagal tone decreases and heart rate increases.

Chemoreceptors play a major role in maintaining respiration than the heart rate.

Sinoaortic Mechanism and Buffer Nerves

Sinoaortic mechanism is the mechanism of baroreceptors and chemoreceptors in carotid and aortic regions which regulates heart rate, blood pressure and respiration.

Nerves from these receptors are called buffer nerves.

5. Impulses from Right Atrium: Bainbridge Reflex

Bainbridge reflex is a cardioaccelerator reflex that increases the heart rate when venous return is increased.

Since this reflex arises from right atrium, it is also called right atrial reflex.

Some stretch receptors are present in the wall of right atrium.

When venous return increases, the right atrium is distended.

Right atrial distention stimulates the stretch receptors.

Stretch receptors, in turn, send inhibitory impulses through inferior cervical sympathetic nerve to vasodilator area of vasomotor center.

Vasodilator area is inhibited resulting in decrease in vagal tone and increase in heart rate.

6. Impulses from Other Afferent Nerves

Stimulation of sensory nerves produces varying effects.

Examples

1. Stimulation of receptors in nasal mucous membrane causes bradycardia. Impulses from nasal mucous membrane pass via the branches of V cranial nerve and decrease the heart rate.
2. Most of the painful stimuli cause tachycardia and some cause bradycardia. The impulses are transmitted via pain nerve fibers.

7. Bezold-Jarisch Reflex

Bezold-Jarisch reflex is the reflex characterized by bradycardia and hypotension, caused by stimulation of chemoreceptors present in the wall of left ventricle by substances such as alkaloids.

It is also called coronary chemoreflex.

Vagal fibers form the afferent and efferent pathways of this reflex.

Bezold-Jarisch reflex is a pathological reflex and it does not occur in physiological conditions.

Conditions When Bezold-Jarisch Reflex Occurs

1. Myocardial infarction
2. Administration of thrombolytic agents
3. Hemorrhage
4. Aortic stenosis
5. Syncope

Overall Relationship