Conducting system of the heart dentistry hb2 dr magdi

Conduction system of the heart Presented 2010 / 2011 by: Dr Magdi El Sersi Assistant Prof of Medical Physiology Basic Medical Sciences Department Ext. 7243 E mail: [email protected]

Copyright © 2009 Pearson Education, Inc.

Action Potentials in Cardiac Autorhythmic Cells

PLAY Interactive Physiology® Animation: Cardiovascular

System: Cardiac Action Potential

IPSlides_cardio/IP10_files/cardio/actnpot/topic1.html

IMPULSE GENERATION AND CONDUCTION SYSTEM

The heart contains special

excitatory and conductive

system each capable of

discharging at regular

intervals and of conducting

impulses

Some of cardiac

myocytes lose the ability

to contract and become

specialized, instead, for

generating action

potentials

Non contractile cells specialised regions

• Sinoatrial (SA) node

– Pacemaker

• Posterior wall of the right

atrium; inferior to entrance of

the superior vena cava

• Consists of cells that produce

impulses in a constant rhythm

This is the pacemaker that

initiates each heartbeat and

determines the heart rate.

Signals from the SA node

spread throughout the atria,

as shown by the yellow

arrows .

2. Atrioventricular node

• Atrioventricular

(AV) Node

– Located near the lower

interatrial septum, at the

junction of the atria and

ventricles

• AV Bundle (Bundle of His)

A pathway by which signals leave the AV node.

• Right and Left Bundle Branches

– Interventricular septum

• Purkinje Fibers

– Network spreads within the muscle bundles of the ventricle walls

– Denser and more elaborate in left ventricle

– Causes entire ventricular myocardium to depolarize; ventricles contract

• Atria are electrically isolated from the ventricles

– Electrical excitation can pass from the atria to the ventricles at the AV node ONLY

– Separated by a region of electrically inert connective tissue

Sinoatrial

Node

Atrioventricular

Node

Impulses normally starts in the heart’s pacemaker,

the SA node.

From there, it spreads in all directions

through the atria.

This cause the atria to contract.

When impulses reach the

AV node, it relays them by

Way of the bundle of His

and Purkinje fibers to the

ventricles causing them to

contract

The conduction velocity of

action potentials in the atrial

muscle is about 0.5 m/sec.

Firing of the SA node excites atrial

myocytes and stimulates the two atria to

contract almost simultaneously.

Normally, the only pathway

available for action potentials to

enter the ventricles is through a

specialized region of cells

(atrioventricular node, or AV

node).

The AV node is a highly

specialized conducting tissue

that slows the impulse

conduction considerably (to

about 0.05 m/sec) thereby

allowing sufficient time for

complete atrial depolarization and

contraction (systole) prior to

ventricular depolarization and

contraction.

The impulses then enter at the Bundle of

His and then follow the left and right

bundle branches along the

interventricular septum.

These specialized fibers conduct the

impulses at a very rapid velocity (about

2 m/sec).

The bundle branches then divide into an

extensive system of Purkinje fibers that

conduct the impulses at high velocity

(about 4 m/sec) throughout the

ventricles. This results in depolarization

of ventricular myocytes and ventricular

contraction.

The Cardiac Rhythm The normal heartbeat, triggered by the SA node, is called

the sinus rhythm

At rest, the adult heart rate is

usually around 70 to 80 beats per

minute (bpm).

Left to itself, the SA node would

fire more often than this, but the

vagus nerves inhibit it and hold it

down to this rate at rest.

o If the SA node is damaged, an

ectopic focus may take over the

governance of the heart rhythm.

oThe most common ectopic focus

is the AV node, which produces a

slower heartbeat of 40 to 50 bpm

called a nodal rhythm.

If neither the SA nor AV node is

functioning, other ectopic foci fire at

rates of 20 to 40 bpm.

The nodal rhythm is sufficient to

sustain life, but a rate of 20 to 40 bpm

provides too little flow to the brain to

be survivable.

This condition calls for an

artificial pacemaker.

Any abnormal cardiac rhythm

is called arrhythmia.

One cause of arrhythmia is a

heart block: the failure of

any part of the cardiac conduction

system to transmit signals,

usually as a result of disease and

degeneration of conduction

system fibers.

A bundle branch block, for

example, is due to damage to

one or both bundle branches.

Damage to the AV node

causes total heart block, in

which signals from the atria

fail to reach the ventricles and

the ventricles beat at their

own intrinsic rhythm of 20 to

40 bpm.

Physiology of the SA Node Why does the SA node spontaneously fire 70 or 80

times per minute?

Unlike skeletal muscle or

neurons, the cells of the SA node

do not have a stable resting

membrane potential.


Myocardial Contractile Cells

• Action potential of a cardiac contractile cell

4 4

0

0 100 200 300

Time (msec)

PX = Permeability to ion X

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PNa

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+20

–20

–40

–60

–80

–100

Phase Membrane channels

Na+ channels open

Na+ channels close

Ca2+ channels open; fast K+ channels close

Ca2+ channels close; slow K+ channels open

Resting potential

PNa

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0

1

2

3

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Me

mb

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0 100 200 300

Time (msec)


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Resting potential

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(mV

)

0

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2

3

4

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SA node membrane

potential starts at about -60

mV and drifts upward,

showing a gradual

depolarization called the

pacemaker potential

This is thought to result

from a slow inflow of Na+

without a compensating

outflow of K.

When the

pacemaker potential

reaches a threshold of

-40 mV, voltage-

regulated fast

calcium channels

open and Ca2+ flows

in from the

extracellular fluid

(ECF).

This produces the

rising (depolarizing)

phase of the action

potential, which

peaks slightly above

0 mV.

At that point, K+

channels open and

potassium ions leave

the cell. This makes

the inside increasingly

negative and creates

the falling

(repolarizing) phase

of the action potential.

When

repolarization is

complete, the K

channels close and

the pacemaker

potential starts over,

on its way to

producing the next

heartbeat.

Each depolarization of the SA node sets off one heartbeat.

Clinical Links • artificial pacemaker - not able to use an ultrasonic

scaler

People who wear

pacemaker can

not get ultrasonic

scaling because

the ultrasonic

sound can

interfere with the

function of the

pacemaker. DentalPath.com: Dental Cleaning

http://www.dentalpath.com/img/c_te_1l.jpg

http://www.dentalpath.com/img/c_te_2l.jpg

http://www.dentalpath.com/dp/dp_te.htm

• Tachycardia associated with anxiety And dental phobia:

Dental phobia

http://www.dentalfearcentral.org/what_is_dental_phobia.html

http://www.floss.com/dental_phobia.htm





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Conducting system of the heart dentistry hb2 dr magdi