1 elect heart

8/13/2019 1 Elect Heart

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Outline

Overview of the cardiovascular system.

Review of nerve action potentials.

Action potential propagation through theheart.

ECG


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Learning Objectives

Describe the course of a cardiac impulsethrough the heart.

Understand how the Na+, K+, and Ca2+

channels function in sinoatrial and ventricularaction potentials.

Know the times a cardiac impulse appears in

each part of the heat. Know the relationship of atrial and ventricular

contraction to the ECG waves.


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Cardiovascular

SystemKeep in mind the fullsystem when studyingdetails.

What does the heartneed to do?

What signal initiatescontraction?

Must be an automaticsignal.

Rhythmical excitation of

the heart.

The normalelectrocardiogram


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Flow of Electrical

Signals in the

HeartFirst, atria contract to fillventricles.

Then, ventricles contract tosend blood to the lungs andperipheral circulation.

S-A node generates thesignal.

Signal travels throughinternodal pathways andatrial muscle (atriacontract).

A-V node and bundle delaythe signal and send it to the

ventricles.Purkinje fibres rapidly carrythe signal throughout theventricles, where it thenspreads, causingcontraction.


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Propagation of Electrical Signals in

Heart Muscle

Heart muscle is syncytial


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Cardiac Muscle

Branching cells

One or two nuclei per cell

Striated Involuntary

Medium speed contractions


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Cardiac Muscle

Found only in heart where it forms a thick layer called the

myocardium

Striated fibers that branch

Each cell usually has one centrally-located nucleus

Fibers joined by intercalated disks

IDs are composites of desmosomesand gap junctions

Allow excitation in one fiber to spread quickly to adjoining fibers

Under control of the ANS (involuntary) and endocrine system

(hormones)

Some cells are autorhythmic

Fibers spontaneouslycontract (aka Pacemaker cells)


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Cardiac Muscle Tissue


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Properties of Cardiac Muscle Fibers


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Excitation-Contraction Coupling and Relaxation of Cardiac

Muscle


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Excitation-Contraction Coupling


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How are cardiac contractions started? Cardiac conduction system

Specialized muscle cells pace the

rest of the heart; cells contain less

actin and myosin, are thin and pale

microscopically

Sinoatrial (SA) node; pace of about

65 bpm

Internodal pathways connect SA

node to atrioventricular (AV) node

AV node could act as a secondary

pacemaker; autorhythmic at about

55 bpm

Bundle of His

Left and right bundle branches Purkinje fibers; also autorhythmic at

about 45 bpm

ALL CONDUCTION FIBERS CONNECTED TO

MUSCLE FIBERS THROUGH GAP JUNCTIONS IN

THE INTERCALATED DISCS


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Action Potentials (APs)

APs are the electrical signals that we have beendiscussing.

Review nerve AP on next slide.

Should know the following:- Membrane potential

- Nernst equation

- Na+, K+, and Ca2+channels

- Na+/K+ATPase New material will be APs in the SA node and

ventricles.


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Nerve Action Potential

Note: membrane potentials are measured inside-outside. This will be important toRemember when we discuss ECGs.


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Sinoatrial Node

Pacemaker of theheart.

Flattened ellipsoid

strip of cells on the

right atrium.No contractile

filaments.

Electrically connected

to atrium.


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Sinoatrial Node

Action Potential

Phase 4: slowdepolarization due to Na+and Ca2+leak until threshold.Note fast Na+channels areinactive at -60 to -40 mV.

Phase 0: at threshold, Ca

2+

channels open.

Phase 3: As in nerves, K+channels open duringrepolarization.

Finally, note the slow riseand fall of the SA APcompared to that of thenerve AP, and the rhythmicfiring.


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AV Node and Bundle

Delays AP from reaching the ventricles, allowing the atria to empty blood into

ventricles before the ventricles contract.


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Purkinje Fibres

Receives the AP from

the AV bundle and

rapidly transmits the

impulse through theventricles.


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Impulses in

Ventricles

At the termination ofthe Purkinje fibres, theimpulse rapidly travelsthrough the ventriclemuscle fibres via gap

junctions, from theinside (endocardium) tothe outside(epicardium).

The rapid propagationof the cardiac impulsethrough the Purkinjefibres and ventricles isimportant for aneffective contraction.


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Ventricular AP

Phase 4: restingmembrane potential nearthe K+equilibrium

potential.Phase 0: depolarizingimpulse activates fast Na+channels and inactivates K+channels.

Phase 1: Transientopening of K+channels andNa+channels begin toclose.

Phase 2: Ca2+channels areopen, key difference

between nerve AP.Phase 3: repolarization,Ca2+inactivate and K+channels open.

Refractory period: Na+channels are inactive untilmembrane is repolarized.


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The refractory period is short in skeletal muscle, but very long in cardiac muscle.

This means that skeletal muscle can undergo summation and tetanus, via repeated

stimulation

Cardiac muscle CANNOT sum action potentials or contractions and cant be tetanized


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Cardiac Muscle


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Properties of Cardiac Muscle fibers


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Electrical Events

Autorhythmicity of Cellsimportant tounderstand, some cardiac drugs work at this level.


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1


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2

3

4


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Sequence of Excitation


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Modifying the Basic Rhythm: Extrinsic Inervation of the

Heart


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Autonomic nervous system modulates the frequency of depolarization of

pacemaker

Sympathetic stimulation (neurotransmitter = ); binds to b1 receptorson the SA nodal membranes

Parasympathetic stimulation (neurotransmitter = ); binds to muscarinic

receptors on nodal membranes; increases conductivity of K+ and decreases

conductivity of Ca2+

How do these neurotransmitters get these results?


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Electrocardiography (EKG)

Examines how Depolarization occurs

in the Heart


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ECG examines how depolarization events occur in the heart


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If a wavefront of depolarization

travels towards the electrode

attached to the + input terminal of

the ECG amplifier and away from the

electrode attached to the - terminal, a

positive deflection will result.

If the waveform travels away from the

+ terminal lead towards the -

terminal, a negative going deflection

will be seen. If the waveform is travelling in a

direction perpendicular to the line

joining the sites where the two leads

are placed, no deflection or a biphasic

deflection will be produced.

ECG examines how depolarization events occur in the heart


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The electrical activity of the heart

originates in the sino-atrial node. The

impulse then rapidly spreads through

the right atrium to the atrioventricular

node. (It also spreads through the

atrial muscle directly from the right

atrium to the left atrium.) Thisgenerates the P-wave

The first area of the ventricular muscle to be activated is the interventricular septum, which activates from left to right. This

generates the Q-wave

Next the bulk of the muscle of both ventricles gets activated, with the endocardial surface being activated before the epicardial

surface. This generates the R-wave

A few small areas of the ventricles are activated at a rather late stage. This generates the S-wave

Finally, the ventricular muscle repolarizes. This generates the T-wave


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Electrocardiography


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1 elect heart

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