PHYSIOLOGICAL ANATOMY Mammalian Heart has 4 chambers 2

PHYSIOLOGICAL ANATOMY • Mammalian Heart has 4 chambers- 2 atria and 2 ventricles • Right side- pulmonarycirculation • Left side- systemiccirculation • Contraction is called systole and relaxationis called diastole

Cardiac muscle fibers Two types of muscle fibers- contractile and conducting Contractile fibers in atria and ventricles- formtwo functional syncytia due to presence of gap junctions Conducting system includes SA Node, internodal tracts, AV Node, Bundle of His, Bundle branches and purkinje fibers Conducting system hasi) less cross-striations ii) less glycogen iii) do not contract

CONDUCTING SYSTEM SA Node- Small, flattened, ellipsoid strip of specializedmuscle Size -3 x 15 x 1 mm Situation -superior lateral wall of right atrium below and lateral to opening of superior venacava Pacemaker of heart P cells- primitive cells- pale- rhythm generators

Internodal pathway- Connect SA Node and. AV Node Faster rate of conduction than Atrialmuscles Anterior-Bachman’sbundle Middle-Wenkebach’sbundle Posterior-Thorell’sbundle

AV Node Only conducting pathway between atriaand ventricles normally Situation -posterior septal wall of RA immediately behind tricuspid valve Has thinner fibers with more negative RMP & fewer gap junctions causing conduction delay Velocity of conduction 0. 05 m/sec It acts as pacemaker when SA Node isdamaged

Bundle of His- • It begins from AV Node, passesdownwards in the intraventricular septumfor 5 -15 mm • Divides into right and left bundle branches • Left branch divides into anterior and posterior fasciculus • Both divide repeatedly and lie subendocardially

Purkinje fibers- • Takes origin from terminal divisions of bundle branches • Fastest conducting • It is 1 -2 mm thick- largest conducting fiber • Passes impulses to ventricular myocytes

PROPERTIES OF CARDIACMUSCLE I)Electrical properties i) Autorhythmicity ii) Excitability iii) Conductivity II) Mechanical properties i) Contractility ii) All-or-none law iii) Staircase phenomenon iv) Refractory period

I. AUTORHYTHMICITY • All the cells of heart have an inherentability to generate impulse • SAN isthe pacemaker of heart • Rates of impulse generation SAN- 70 to 80/min AVN- 40 to 60/min Purkinje fibers- 15 to 40/min

Action potential in SAN

• MECHANISM OF SELF-EXCITATION • RMP of SAN is -60 mv (that of contractilecardiac fibers is – 90 mv) • It has a pre-potential, a spike, & arepolarization phase.

• Pre-potential or pacemaker potential • i) due to ca 2+ influx through Transient(T-type) of ca 2+channelsii) TMP changes from -60 mv towardspositivity. II) • Spike potential • i) starts at threshold potential of -40 mvii) due to opening of voltage-gated Ca 2+channels (Ltype or long-standing type)iii) potential peaks to +20 mv

• Repolarisation • i) due to closure of Ca 2+ channels andopening of K+ channels • Significance of Pre-potential • It is characteristic of tissues withautomaticitye)It is prominent in SAN and AVNf)Alterations in prepotential will alter therate of impulse generation

• • • FACTORS AFFECTING AUTORHYTHMICITY i) Autonomic nerve stimulation a) vagal stimulation decreases the slope of pre-potential and reduces the rate of impulsegenerationb) sympathetic stimulation increases the slope andincreases the impulse rateii) Temperature iii) Hormones iv) Drugs v) Ions a) K+ ↓ • increased K+ in ecf ↓ • decreased RMP ↓ • hyperpolarisation ↓ • reduced heart rate ↓ • diastolic arrest

Vagal stimulation ↓ Release of acetylcholine ↓ Ach binds to M 2 muscarinic receptor ↓ Βγ subunit of G-protein act on K+channels Reduced c. AMP ↓ ↓ Opening of K+channels Reduced Ca 2+ influx └> <˩ Decreased pre-potential slope

Sympathetic stimulation. Release of noradrenaline ↓ Binds to β 1 receptors. Increased Camp ↓ Opening of L-type Ca 2+ channels ↓ Increased Ca 2+ influx. Reduced pre-potential slope ↓ Increased rate of impulse generation

EXCITABILITY • Ability Of excitable tissues to show changein potential when stimulated • Chronaxie of cardiac muscle is 3 -30 ms

ACTION POTENTIALS INVARIOUS CARDIAC CELLS