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Regulation of Heart Rate, Rhythm & Contractility

Heart is innervated by

both divisions of autonomic nervous system (ANS)

i.e. sympathetic &

parasympathetic.

• Nervous stimulation is not required to initiate cardiac impulse.

ANS Can modify rate, rhythm & force of contraction.

ANS Can modify rate, rhythm & force of contraction.

Parasympathetic nerves to heart:

– Through Rt Vagus (to SA node) & Lt Vagus (to AV node) Nerves

Main supply to SA & AV nodes but less to Atria & very little to ventricles

Vagal fibers (Cholinergic) are endocardial

Sympathetic nerves to heart:

– Through Rt & Lt Stellate ganglions

– SA node, AV node & Ventricular muscles

Noradrenergic fibers are epicardial

Reciprocal Inhibition by Sympathetic & Parasympathetic

innervations to heart:

Ach acts presynaptically to inhibit NE release from sympatheic nerves

Neuropeptide-Y from noradrenergic endings inhibits Ach release

Role of Autonomic Nerves

• Parasympathetic:

– Controls heart action in quiet, relaxed situations;

when body is not demanding enhanced CO.

– Rt Vagus —– Slows heart ( ↓ Heart Rate)

– Lt Vagus —– Slows AV conduction (↑ AV nodal delay)

• Weak to moderate stimulation: ↓ heart pumping by 50%

• Strong stimulation abolish spontaneous discharge; ventricular escape

Mechanism of Action of Ach:

• Mediated by M2 muscarinic receptors via βγ subunit of G protein &

opens special K+ channels i.e. ↑ Pk+ (K+ conductance)

(Hyper polarization)

M2 receptors decrease cAMP in cells, slows opening of Ca++ channels,

decreases firing rate (↓Heart Rate)

Sympathetic:

– Controls heart in emergency or exercise

– Rt stellate ganglion → SA node → Accelerates heart through pacemaker

– Lt stellate ganglion → ↓AV nodal conduction time& refractoriness → →

→ ↓ AV nodal delay → → Accelerates conduction

• Strong stimulation → ↑ overall activity of heart

• H.Rate 3 x times

• Strength 2 x times

Mechanism: NE binds β1 receptors → ↑ intracellular cAMP → →

→ opens L-type Ca++ channels → membrane potential

fall rapidly (depolarization)

Effects: ↑ Rate of Discharge (Chronotropic)

↑ Rate of conduction (Dromotropic)

↑ Level of excitability (Bathmotropic)

↑ Force of contraction (Inotropic)

Regulation of Heart Pumping

‘Regulation of heart rate and contractility’

• At Rest heart pumps 4 – 6 L of blood per min

• During strenuous exercise : 4 -7 folds increase.

Regulating Mechanisms:

1. Intrinsic cardiac regulation;

“Frank- Starling’s Mechanism”

2. Role of ANS;

• Sympathetic

• Parasympathetic (Vagus)

3. Other mechanisms;

• Hormones; Catecholamines; Epi & NE

Thyroxin

• Ions; K+

Ca++

• Temperature; Fever (10 beats/min/1F rise)

Hypothermia (60-70 F)

• Age

• Gender

• Physical Fitness “Athletes Heart”

Frank – Starling’s Mechanism

• Intrinsic ability of heart to adapt to the changing volumes EDV

i.e. greater the heart muscle is stretched during filling (EDV), the

greater will be the force of contraction and greater the amount of

blood pumped out.

Definition:

“Within physiological limits, the heart pumps all the blood that

comes to it without allowing excessive damming of blood in veins”

• Mechanism:

↑ EDV cardiac muscle stretched so ↑force of contraction

Optimal degree of interdigitation of Actin & Myosin filaments

↑Venous return leads to stretching of Rt atrium ↑H.Rate by 10-20%

Activation of SA node (Bain- Bridge Reflex)


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