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

Download MBBS (Bachelor of Medicine, Bachelor of Surgery) 1st Year, 2nd Year, 3rd Year and Final year Physiology 64 Properties of Cardiac Muscle Part 1 PPT-Powerpoint Presentations and lecture notes

Properties of Cardiac muscle

(Part 1)

Properties of cardiac muscle
Properties of the cardiac muscle:

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

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

a propagated action potential

I . Conductivity: The ability to conduct excitation through the

cardiac tissue

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

stimulation

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

continuously and regularly without external stimulation

Refractory Period

Excitability changes during the action potential


Phases of action potential in ventricles/SA node)

1

2

Phase 0 = depolarization

Phase 1 = early repolarization

Phase 2 = Plateau potential

Phase 3 = repolarization

3

Phase 4 = resting state of polarization

0

SA node

4

0

~ 200 msec

3

ARP

RRP

4

ARP = absolute refractory period

RRP = relative refractory period

Relative Permeabilities During Action

Potential
Pacemaker Potential

Channels responsible for pacemaker

potential
Pacemaker of the heart: SA- node

? Small, ellipsoid strip about

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

? Located in the superior

postero-lateral wall of the
right atrium immediately
below and slightly lateral to
the opening of the superior
venacava

Pacemaker of the heart: SA- node

? Contains the P- cells, which

are actual pacemaker cells

? Unstable RMP and

spontaneous depolarization

? Has the fastest rhythm: 90

impulses/min


Rate of generation of AP at different sites of the heart

RATE

SITE

(impulses/min)

100

SA node

40 - 60

AV node

20 - 35

AV bundle, bundle branches,&

Purkinje fibres

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

do not establish the fundamental rhythm

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

Action potential in different regions of the heart


Effect of ANS

Conductivity:

? Inter-nodal Pathway

? Anterior internodal

bundle of Bachman

? Middle internodal

bundle of Wenkebach

? Posterior internodal

bundle of Thoral


Conduction velocities in different regions of heart

Region of heart

Speed of spread

Atria

0.5 m/sec

Inter-nodal fibers

1 m/sec

AV node

0.05 m/sec

Bundle of His

2 m/sec

Purkinje fibers

4 m/sec

Ventricles

1 m/sec

Due to differences in gap junction properties

Time of impulse arrival at different regions of the heart

Regions

Time

SA node

0.00 sec

AV node

0.03 sec

Bundle of His

0.12 sec

Ventricular septum 0.16 sec

Endocardium

0.17 - 0.19 sec

Epicardium

0.21 ? 0.22 sec


Exercises
What is the normal total delay of the

cardiac impulse in the A-V node and

the A-V bundle system?

Conduction velocity is least in

............and maximum in ..........
Channels involved in prepotential

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

Why cardiac muscle can not be

tetanized ?