conductive system of the heart rmc 2
TRANSCRIPT
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