NERVOUS REGULATION OF HEART

Dr. Ayisha Qureshi Professor

MBBS, MPhil

Learning Objectives

By the end of the lecture, you should be able to: • Differentiate between tachycardia and bradycardia. • Name the 2 divisions of Autonomic nervous system. • Explain the parasympathetic and sympathetic supply to the

various parts of the heart. • Describe the effects of parasympathetic and sympathetic

supply on the heart. • Explain the effects of cutting the vagus nerve on heart.

Normal heart Rate

Normal heart rate is 72/minute. (It ranges between 60 and 80 per minute.)

TACHYCARDIAC (Increased HR more

than 100 bpm)

BRADYCARDIA (Decreased HR less

than 60 bpm)

VOCABULARY• CHRONOTROPIC EFFECT

• IONOTROPIC EFFECT

• LUSITROPIC EFFECT

• DROMOTROPIC EFFECT

• BATHMOTROPIC EFFECT

MEANING• Heart rate

• Force of Contraction

• Rate of relaxation

• Rate of Conduction of Impulse

• Increased Excitability of Heart

Heart rate is initiated by auto rhythmic cells in the SA node, but it is regulated by the Autonomic Nervous System through its 2 branches: Parasympathetic &

Sympathetic Nervous System.

Parasympathetic activity slows the heart rate,while

sympathetic activity speeds it up.

Both Parasympathetic & Sympathetic nervous systems have a reciprocal relationship.

Divisions of ANSSYMPATHETIC NERVOUS

SYSTEM

• Fight, flight or Fright.• Activated during exercise,

excitement and emergencies.

PARASYMPATHETIC NERVOUS SYSTEM

• Rest and Digest. • Concerned with conserving

energy.

MEDULLA OBLONGATA CONTAINS THE NUCLEI FOR ANS.

NERVOUS CONTROL OF THE HEARTPARASYMPATHETIC NERVE SUPPLY

• The Parasympathetic Supply to the heart is carried by the 10th

cranial nerve called the VAGUS NERVE.

• The vagal fibers reaching the heart arise from 2 nuclei in the medulla oblongata: Nucleus Ambigus & Dorsal Nucleus of Vagus.

• Collectively, they are called the CARDIOINHIBITORY CENTER.

• Neurotransmitter released: ACETYLCHOLINE

• Receptors: MUSCARINIC (M) RECEPTORS.

• Effects on the Heart: CARDIOINHIBITORY

SYMPATHETIC NERVE SUPPLY

• The Sympathetic supply arises from the Nucleus of TractusSolitarius in the Medulla Oblongata, from a group of cells called CARDIOEXCITATORY CENTER.

• Neurotransmitter released: NOREPINEPHRINE.

• Receptors: BETA- ADRENERGIC RECEPTORS.

• Effect on Heart: CARDIOEXCITATORY.

Supply by Parasympathetic & Sympathetic nervous system to the Heart is Different!

SYMPATHETIC NERVOUS SYSTEM

• Increases the heart rate (from the 70 beats/min up to 180 to 200 and, rarely, even 250 beats/min).

• Increases the force of heart contraction to as much as double the normal, thereby increasing the volume of blood pumped and increasing the ejection pressure.

This effect is brought about in 2 ways:1. Increases the rate of sinus nodal

discharge,2. Increases the rate of conduction &

excitability in all portions of heart,3. Increase the force of contraction

of all cardiac muscle. NOTE: Major supply is to the ventricles of the heart which are the power pumps.

PARASYMPATHETIC NERVOUS SYSTEM

• Decreases the Heart Rate.• Decreases the strength of

contraction by 20 to 30 percent • The great decrease in heart rate

combined with a slight decrease in heart contraction strength can decrease ventricular pumping 50 % or more.

These effects are brought about in 2 ways: 1. Decreases the rate of rhythm of

Sinus node,2. Decreases the excitability of the

AV node.NOTE: Major supply is to the SA node & AV node.

What Happens when you CUT ONE VAGUS NERVE?

What Happens when you CUT ONE VAGUS NERVE?

If one Vagus nerve is cut, it results in an increase in the Heart rate.

What Happens when you CUT BOTH VAGUS NERVES?

What Happens when you CUT BOTH VAGUS NERVES?

If both Vagus nerves are cut, then there is an even greater increase in

the heart rate.

This shows that the heart is under constant inhibitory control of the Vagus Nerves. This

is called the VAGAL TONE.

The vagal tone can also be abolished when a parasympatholytic drug, ATROPINE, is administered which specifically blocks the Muscarinic

Receptors. However, certain procedures increase vagal stimulation of the heart

and can be employed clinically: 1. Carotid sinus Massage

2. Pressure on the eyeball

CAROTID SINUS MASSAGE

What happens when you stimulate the Right Vagus Nerve?

When Right Vagal Nerve is stimulated, a phenomenon called Vagal Escape occurs.Intense stimulation of Right Vagus Nerve (it supplies the SA node)

↓Cardiac standstill (Cardiac Asystole)

↓After 5 to 20 seconds, heart resumes activity even if vagal stimulation is

continued. ↓

Because the Pacemaker shifts from SA node to the AV Bundle.↓

Vagal escape refers to escape of ventricles from the inhibitory effect of vagal stimulation.

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

Effect of Vagal stimulation on Heart

CAUSE FOR VAGAL ESCAPE

Stimulation of right vagus stops the 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 heart beat is continued for some time

(by vagal stimulation), ↓

Another part of the conduction system becomes pacemaker and starts producing impulses.

↓This is called Vagal Escape.

↓Thus, vagal escape indicates escape of ventricles from the effect of the SA

node.

What happens when you stimulate the Left Vagus Nerve?

When Left Vagus Nerve is stimulated, a Heart Block occurs.

Left vagus supplies mainly the AV node. ↓

Stimulation of left vagus with a weak stimulus causes a slight reduction in rate of ventricular contraction.

(Because of inhibition of AV node, some of the impulses from SA node are not conducted to ventricles. This is called the Partial heart block. )

↓The 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 the complete inhibition of AV node.)↓

The prolongation of stimulation causes Idioventricular Rhythm, in which the ventricles take over the pacemaker activity. The rate (Rate: 30 – 45 bpm) is

different from the rate of atrial contraction.