The cardiac cycle begins with an electrical impulse from the sinoatrial node that causes atrial contraction. This is followed by a delayed impulse from the atrioventricular node that causes ventricular contraction. The cycle involves electrical and mechanical events represented by the ECG. Cardiac function is controlled by the autonomic nervous system and hormones. Parasympathetic stimulation decreases heart rate while sympathetic stimulation increases it. Important cardiac reflexes maintain homeostasis by regulating heart rate and contractility in response to pressure and chemical sensors.

1. CARDIAC CYCLE, CONTROL
OF CARDIAC FUNCTION &
CARDIAC REFLEXES
PRESENTER - DR. PARTHASARATHI GHOSH

2. CARDIAC CYCLE
• SEQUENCE OF ELECTRICAL AND MECHANICAL
EVENTS DURING THE COURSE OF A SINGLE
HEARTBEAT
• BEGINS WITH THE INITIATION OF HEART BEAT
• CARDIAC PACEMAKER TISSUES HAVE
AUTOMATICITY AND RHYTHMICITY
• SA Node is the natural pacemaker
• it generates impulses at the highest frequency

3. Electrical Events
• Electrical events in cardiac cycle are represented by the ECG
• ECG is the result of differences in the electrical potential
generated by the heart at the sites of lead placement
• The action potential generated by the SA node, travels to Both
atrium causing atrial systole, causing P wave of ECG

4. • AV node is situated at the junction of the Atrial septum and
Ventricular septum.
• AV node is distally connected to the Bundle of HIS
• It is responsible for slow conduction
• delay between the Atrial and Ventricular contraction
occurs here
• Represented by the PR interval on ECG
Electrical Events

5. • Bundle of HIS divides into the left and right bundle branches
• terminates into the Purkinje finer system which supplies the
individual cardiomyocytes
• These fibres spread depolarisation to ventricular
myocardium
• Depicted by the QRS complex on the ECG
• depolarisation is followed by ventricular repolarisation and
depicted by T wave on ECG.
Electrical Events

6. Mechanical Events of Cardiac Cycle

8. • When ventricular chamber fills, AV valves leaflets are pushed
upwards
• Closure of AV valves, leads to start of ventricular systole depicted by
R wave on ECG
• Ventricular systole has 2 parts
• Isometric/ Isovolumic contraction
• Ventricular ejection
• Electrical impulse -> Bundle of HIS and purkinje fibres -> Ventricular
myocardium contracts -> Increased Intraventricular pressure
• If Intraventricular pressure >> PA and Aortic pressure —> Opening
of PA and Aortic valves -> Ventricular ejection (2nd part of VS)
Mechanical Events of Cardiac Cycle

9. • Ventricular ejection has 2 phases
• Rapid ejection - maximal forward flow, maximal PA
and Aortic pressure
• Reduced ejection - as the flow, PA and Aortic
pressure taper towards the end of systole
• Pressure in both ventricles reduces as blood is ejected out
Mechanical Events of Cardiac Cycle

10. • Ventricular diastole occurs with the closure of PA and Aortic valves
• VD has two phases
• Initial Isometric / Isovolumic relaxation phase
• Repolarisation of ventricular myocardial
• T wave on the ECG
• Late phase
• Rapid decrease in the intraventricular pressure till it
decreases to less than RT and LT atrial pressures
• AV valve reopens
• Cycle repeats
Mechanical Events of Cardiac Cycle

11. Control of Cardiac Function
• Two regulators - Neural and Hormonal
• Major neural component is the Autonomic Nervous
System - sympathetic and parasympathetic components
• both provide opposing input to regulate cardiac function

12. Sympathetic Nervous
System effects
• Primary neurotransmitter is Nor-Epinephrine
• it has positive chronotropic (rate), inotropic (contractility)
and lusitropic (relaxation) effects

13. Parasympathetic Nervous
System
• Major neurotransmitter is Acetylcholine
• It has more direct inhibitory effect on Atria
• Negative modulatory effect on ventricles

14. Control of cardiac function
• Both Nor-Epinephrine and Acetylcholine bind to Seven-
transmembrane-spanning G protein coupled receptors
• at rest, the heart has a tonic level of Parasympathetic
cardiac nerve firing and little (if any) Sympathetic activity
• during exercise / stress - sympathetic influence prominent

15. • Parasympathetic innervation of heart is by Vagus nerve
• Supraventricular tissue receives more intense vagal innervation
than ventricles
• Parasympathetic target neuroeffectors are muscarinic receptors
in heart - M2
• Activation of M2
• Reduces pacemaker activity
• slows AV conduction
• Directly reduces atrial contractile force
• inhibitory modulation of ventricle contractile force

16. •Sympathetic innervation is more prominent in ventricles than atria
•Nor-epinephrine stimulates Adrenergic receptors AdRs in the heart
•AdRs two types - Alpha and Beta
•both are G protein coupled receptors
•Effectors are -
•Adenylyl cyclase (AC)
•L-type calcium current (iCA)
•Phospholipase β (PLC-β)
•Intracellular signals are
•DAG - Diacetylglycerol
•Inositol 1,4,5 triphosphate IP3
•Protein Kinase C (PKC)
•cAMP
•Protein Kinase A
•Mitogen activated protein kinase MAPK

17. • Three types of β adrenoceptors are - β1, β2, β3
• β1 predominant in both atria and ventricles
• β2 substantial in atria and 20% in ventricles
• β3 in human ventricles
• Two types of ⍺ adrenoceptors - ⍺1 and ⍺2
• ⍺1 has 3 subtypes - ⍺1A, ⍺1B and ⍺1D
• ⍺1A is responsible for cardiac hypertrophy

18. Hormonal Control of Cardiac function
• Hormonal influence is either direct or indirect
• Direct hormones are those produced by the cardiac myocytes
• Indirect hormones are those produced by other tissues and
delivered to the heart
• Most hormones are plasma membrane G-protein coupled
receptors (GPCR)
• Non- GPCR include
• Natriuretic peptide receptors
• Glucocorticoid and Mineralocorticoid receptors

19. • Cardiac hormones are polypeptides secreted by cardiac tissues
• Natriuretic peptides, Aldosterone and Adrenomedullin secreted by cardiomyocytes
along with Angiotensin II
• Renin angiotensin system most important regulators of cardiovascular physiology,
cardiac growth and function.
• Angiotensin II has 2 receptors - AT1 and AT2 both in the heart
• AT1 stimulation induces Positive chronotropic and inotropic effects
• Activation of AT1 -> Cardiac hypertrophy -> heart failure and adverse remodelling
of myocardium
• AT2 mediates cell growth and proliferation of cardiomyocytes, fibroblasts
• induces growth factors aldosterone and catecholamines
• AT2 activation - counter regulatory and antiproliferative
• ACE inhibitors block AT1 receptor activty

20. • Other cardiac hormones are
• Aldosterone
• adrenomedullin
• Natriuretic peptides ANP, BNP
• angiotensin
• endothelin
• vasopressin

21. Sex Steroid Hormones
• Estradiol - 17β E2
• cardiac contractility is intense in premenopausal women than in men of
similar age group
• withdrawal of HRT in post menopausal women, significantly reduces
cardiac contractility
• Estradiol bind and act on oestrogen receptors ER⍺ and ERβ
• Estrogen has vasodilatory effect due to stimulation of vascular
endothelial nitric oxide synthase -> lower systolic BP in premenopausal
women
• Testosterone has opposite effect

22. Cardiac Reflexes
• These are fast acting reflex loops between the heart and CNS that contribute to regulation of cardiac function
and maintenance of physiologic homeostasis
• Cardiac receptors are linked to the CNS by myelinated and unmyelinated afferent fibres, that travel along the
vagus nerve.
• Intracardiac Receptors are situated in - Atrium, Ventricles, Pericardium and coronary arteries
• Extracardiac receptors located in the great vessels and carotid artery
• They are
1. Baroreceptor Reflex (carotid Sinus reflex)
2. Chemoreceptor reflex
3. Bainbridge reflex
4. Bezold-Jarisch reflex
5. Valsalva manoeuvre
6. Cushing reflex
7. Oculocardiac reflex

23. Baroreceptor Reflex
• also known as the Carotid Sinus reflex
• responsible for maintenance of arterial blood pressure.
• utilises negative feedback loop, to maintain arterial pressure
around a preset value.
• also, capable of establishing a prevailing set point for arterial
blood pressure, when the preset value has been reset
because of chronic HTN

24. Chemoreceptor Reflex
• Located in carotid bodies and aortic body
• respond to change in pH status and blood O2 tension
• PaO2 < 50mmHg or Acidosis stimulates chemoreceptors
• Impulse conducted via Nerve of Hering (branch of glossopharyngeal nerve), Vagus ->
chemosensitive area of medulla
• Response -
• Stimulation of respiratory centres, increased ventilatory drive
• Activation of parasympathetic system
• Reduction in heart rate, myocardial contractility
• Persistent hypoxia - CNS directly stimulated, increased sympathetic activity

25. Bainbridge Reflex
• Elicited by stretch receptors in the Right atrial wall and
Cavoatrial junction
• increased right sided filling pressure -> vagal afferent signals
to cardiovascular centre in medulla
• inhibits parasympathetic activity -> increases heart rate
• HR also increases by direct effect on SA node by stretching
of atrium
• Changes are dependent on the underlying HR before
stimulation

26. Bezold-Jarisch Reflex
• Responds to noxious ventricular stimuli sensed by chemoreceptors
and mechanoreceptors in the LV wall
• Induces triad of Hypotension, Bradycardia, Coronary Artery dilatation
• as it induces bradycardia, it is thought of as cardioprotective
• Vagal afferent type C fibres
• Implicated in CV conditions such as MI, thrombosis, revascularisation,
syncope
• Modulated by Natriuretic peptide receptors - ANP, BNP
• Reflex is less pronounced in patients with cardiac hypertrophy, atrial
fibrillation

27. Valsalva Maneuver
• Forced expiration against a closed glottis -> increased
intrathoracic pressure, increased CVP and decreased venous
return
• Cardiac output, blood pressure decreased after valsalva
• Baroreceptors sense this decrease -> Cause increase in HR,
myocardial contractility through sympathetic stimulation
• when glottis opens, venous return increases -> Heart
vigorously contracts -> increased SBP -> baroreceptor reflex
will stimulate parasympathetic efferent pathways to the heart

28. Cushing Reflex
• it is a result of cerebral ischemia caused by increased ICP
• at the medullary vasomotor centre induces Initial activation
of sympathetic nervous system
• increase in HR, arterial blood pressure, myocardial
contractility to improve cerebral perfusion
• reflex bradycardia will follow

29. Oculocardiac reflexes
• provoked by pressure applied on the globe of the eye / traction on
the surrounding structures
• stretch receptors located in the extra ocular muscles
• activation will send afferents through long and short ciliary nerves
which merge with ophthalmic division of trigeminal nerve at ciliary
ganglion
• trigeminal nerve -> gasserian ganglion -> increased
parasympathetic tone and subsequent bradycardia
• administer atropine or glycopyrrolate prior to surgery reduces
incidence of bradycardia

30. THANK YOU