conductive system of the heart rmc 2

Excitatory & Conductive System of

the HeartProf. Dr. Rashid Mahmood

Objectives • At the end of the lecture student should be able to • Describe the structure and functions of the conduction system of the heart• List the sequence of events that occur in rhythmic excitation of the heart• Explain the effects of sympathetic and parasympathetic stimulation on heart

Functions of Excitatory & Conductive System of the Heart1. Generation of rhythmical electrical impulses2. Conduction of these impulses through heart 3. Atria should contract ahead of ventricular contraction

(about 1/6th sec)4. All portions of ventricles should contract simultaneously 5. Heart should keep on contracting without extrinsic

stimulation

Specialized Excitatory & Conductive System of the Heart (in sequence)1. SA Node 2. Internodal pathways & interatrial Pathways3. Transitional fibers4. AV Node5. Bundle of His

i. Penetrating portion of AV Bundle ii. Distal portion of AV Bundle

I. Right Bundle BranchII. Left Bundle Branch

a. Left Anterior Fascicle b. Left Posterior Fascicle

6. Purkinje Fibers 7. Ventricular Muscle

1. S-A Node (sinoatrial node; sinus node)

SVCLocation: Near the junction of SVC &

superior posterolateral wall of the Rt. Atrium

Size: 15mm long, 5mm wide & 1 mm thick Diameter of the fibers = 2-7 um

Structure: Mainly comprised of P-Cells (Stellate cells) & few myofilaments.

P =Premature, Pale looking , Pace maker cells

Action Potential: RMP = -55 to -60 mvThreshold level = -40 mv

SELF-Excitability of S-A Node

Due to presence of Ca++Na+ (T) Channels or Na+ leak Channels showing Diastolic Depolarization

(“Funny Channels” leaky to both Na+ and K+) (Funny Current)

Rate of Generation of A.P = 60-80 /min

HCN: hyperpolarization-activated cyclic nucleotide-gated channels

Mechanism of Sinus Nodal Rhythmicity

1. Natural leakiness to Ca++ (T-Type) & Na+

(Slow)[“Funny channels, leaky

to Na+ & K+]

2. Voltage-gated fast Ca++

(L-Type) (& Na+) Channels

3. Closing of fast Ca++ & Opening of K+ Channels

4. K+ Channels remain open (Hyperpolarization)

5. K+ Channels close (& Natural leakiness

2. Internodal pathways & interatrial Pathways (Specialiazed Conduction fibers with fast speed)

Anterior (Bachman)

Middle (Wenchebach)

Posterior (Thorel)

3. A-V Node • Location: Near the lower part of the inter-atrial Septum Posteriorly in the Rt. Atrium.

• Three Functional Regions: • A-V Junction, • Nodal Region,

• Nodal-His Region

3. A-V nodal Delay • A-V nodal Delay = 0.09 Sec• Mechanism of Delay:

• Fiber Diameter is small & less number of gap Junction in the fibers -> Slow Conduction – Decremental Conduction

• Advantage of A-V nodal delay?

• Inherent Rate of Generation of A.P = 40-60/min

4. Bundle of His (AV Bundle)AV Node

Penetrating portion of AV Bundle

Distal portion of AV Bundle

Right Bundle Branch Left Bundle Branch

Left Anterior Fascicle Left Posterior Fascicle

Purkinje Fibers

Time taken for the impulse to travel through different parts of conduction system

• Atria to AV Node: 0.03 second• AV Nodal Delay: 0.09 second • AV Node to Ventricular Muscles:

0.04 second (main delay in penetrating AV Bundle)

• Total time taken from SA Node to Ventricular Muscles:

• 0.03+ 0.09+ 0.04= 0.16 second

Purkinje Fibers• Largest fibers in the

Heart arising from the Bundle Branches

9-18 um in Diameter• High No of Gap Junction

( More Permeability)• Function – Rapid

conduction of impulse Throughout the Heart

• Discharge rate: 15-40/ min

Spread of impulse from Purkinje fibers into Ventricular muscle

Interventricular Septum

Apex of the heart

Base of the heart

Control of Excitation and Conduction in the Heart

• The Sinus Node as the Pacemaker of the Heart• Reason:• Discharge rate of the sinus node is faster than the natural self-

excitatory discharge rate of either the A-V node or the Purkinje fibers

Abnormal Pacemakers—“Ectopic” Pacemaker

• some other part of the heart develops a rhythmical discharge rateabnormal sequence of contraction of the different parts of the

heart weakness of heart pumping.

• Causes:1. Development of excessive excitability and becoming the

pacemaker2. Blockage of transmission of the cardiac impulse from the sinus

node to the other parts of the heartAtria continue to beat at the normal rate of rhythm of the sinus node

Stokes-Adams syndrome• After sudden A-V bundle block, the Purkinje system does not begin

to emit its intrinsic rhythmical impulses until 5 to 20 seconds• person faints after the first 4 to 5 seconds because of lack of blood

flow to the brain

Autonomic Control of conducted system of heart

• Parasympathetic nerves (Vagi) mainly supply to SA and AV nodes, and to a lesser to the muscles of the atria, and very little directly to the ventricular muscle

• Sympathetic nerves supply to all parts of the Heart, especially to ventricular muscle

Effects of Parasympathetic Stimulation1. ↓ Rate of rhythm of SA

Node→↓ Heart Rate 2. ↓ Excitability of AV Junctional fibers→ ↓ Conduction Velocity 3. ↓ Force of contraction ( By increasing membrane

permeability to K+ ions)

Ventricular Escape • Strong parasympathetic

stimulation may completely block transmission of Cardiac impulse through AV Node

• Ventricles may stop beating for 5-20 seconds

• Purkinje fibers develop a rhythm of its own @ 15-40 beats per minute

Mechanism of Vagal Effects • ↑ Permeability to K+ ions →

Hyperpolarization

Effects of Sympathetic Stimulation• .1.↑ Rate of rhythm of SA Node →

↑ Heart Rate (Chronotropic Effect)

2. ↑ Excitability of AV Junctional fibers (Bathmotropic Effect)

3.↑ Conduction Velocity (Dromotropic Effect)

4.↑ .Force of Contraction (Inotropic effect)

Mechanism of Sympathetic Effects

• Neurotransmitters: Norepinephrine

• Receptors: • Beta-1 adrenergic

receptors • ↑ permeability to

sodium and calcium ions

Summary slides

Specialized Excitatory & Conductive System of the Heart (with their velocities)

1. SA Node (0.05 m/s)2. Internodal pathways & interatrial Pathways (1 m/s)3. Transitional fibers4. AV Node (0.01-0.1 m/s; average: 0.05 m/s)5. Bundle of His (1 m/s)

i. Penetrating portion of AV Bundle ii. Distal portion of AV Bundle

I. Right Bundle BranchII. Left Bundle Branch

a. Left Anterior Fascicle b. Left Posterior Fascicle

6. Purkinje Fibers (1.5 – 4.0 m/s; mainly 4 m/s)7. Ventricular Muscle (0.3-1 m/s)

Take home points • Contractions in heart are controlled via well-regulated electrical signals

that originates in pacemaker cells in the sinoatrial (SA) node and are passed via internodal and atrial pathways to atrioventrical (AV) node, the bundle of His, the Purkinje system, and to all parts of the ventricle.

• Compared to physical myocytes, pacemaker cells have a slightly different sequence of events. After repolarization to the resting potential, there is a slow depolarization that occurs due to a channel that can pass both Na+ and K+. As this “funny” current continues to depolarize the cell, Ca++ channels are activated to rapidly depolarize cell. The hyperpolarization phase is dominated by K+ current

• Sympathetic stimulation increases heart rate, force of contraction, excitability and velocity of conduction while Parasympathetic stimulation has opposite effects.

Learning resources•Guyton and Hall (Text book of physiology), 13th edition•Ganong (Text book of physiology), 24th Edition

© Prof. Dr. Rashid Mahmood 31

Assessment Questions/ Activity1. Draw the diagram of excitatory and conductive system of

heart 2. What are the effects of sympathetic and parasympathetic

stimulation on heart?3. List the functions of Excitatory & Conductive System of the

Heart

© Prof. Dr. Rashid Mahmood 32