Download MBBS Physiology Presentations 64 Properties of Cardiac Muscle Part 1 Lecture Notes

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Properties of cardiac muscle
Properties of the cardiac muscle:

I. Excitability: The ability to respond to a stimulus of adequate

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strength and duration (i.e. threshold or more) by generating

a propagated action potential

I . Conductivity: The ability to conduct excitation through the

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cardiac tissue

I I. Contractility: The ability to contract in response to

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stimulation

IV. Auto-Rhythmicity: The ability to initiate a heart beat

continuously and regularly without external stimulation

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Refractory Period

Excitability changes during the action potential

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Phases of action potential in ventricles/SA node)

1

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2

Phase 0 = depolarization

Phase 1 = early repolarization

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Phase 2 = Plateau potential

Phase 3 = repolarization

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3

Phase 4 = resting state of polarization

0

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SA node

4

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0

~ 200 msec

3

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ARP

RRP

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4

ARP = absolute refractory period

RRP = relative refractory period

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Relative Permeabilities During Action

Potential
Pacemaker Potential

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Channels responsible for pacemaker

potential
Pacemaker of the heart: SA- node

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? Small, ellipsoid strip about

3mm wide, 15 mm long, and
1mm thick

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? Located in the superior

postero-lateral wall of the
right atrium immediately

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below and slightly lateral to
the opening of the superior
venacava

Pacemaker of the heart: SA- node

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? Contains the P- cells, which

are actual pacemaker cells

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? Unstable RMP and

spontaneous depolarization

? Has the fastest rhythm: 90

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impulses/min


Rate of generation of AP at different sites of the heart

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RATE

SITE

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(impulses/min)

100

SA node

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40 - 60

AV node

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20 - 35

AV bundle, bundle branches,&

Purkinje fibres

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ANS and hormones modify the timing and strength of each heart beat but

do not establish the fundamental rhythm

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vagal tone controls SA- node to become 70 impulses/min

Action potential in different regions of the heart

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Effect of ANS

Conductivity:

? Inter-nodal Pathway

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? Anterior internodal

bundle of Bachman

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? Middle internodal

bundle of Wenkebach

? Posterior internodal

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bundle of Thoral


Conduction velocities in different regions of heart

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Region of heart

Speed of spread

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Atria

0.5 m/sec

Inter-nodal fibers

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1 m/sec

AV node

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0.05 m/sec

Bundle of His

2 m/sec

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Purkinje fibers

4 m/sec

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Ventricles

1 m/sec

Due to differences in gap junction properties

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Time of impulse arrival at different regions of the heart

Regions

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Time

SA node

0.00 sec

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AV node

0.03 sec

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Bundle of His

0.12 sec

Ventricular septum 0.16 sec

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Endocardium

0.17 - 0.19 sec

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Epicardium

0.21 ? 0.22 sec

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Exercises
What is the normal total delay of the

cardiac impulse in the A-V node and

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the A-V bundle system?

Conduction velocity is least in

............and maximum in ..........

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Channels involved in prepotential

are .............

Why cardiac muscle can not be

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tetanized ?