6. Pacemaker of the heart
Unstable resting potential
◦ Exhibits automaticity
◦ AV node & His-Purkinje system are latent pacemakers
Phase 1 & 2 are not present in pacemaker action
potentials
10. Intrinsic ability of cardiac muscle
Also called ‘inotropism’ or ‘inotropy’
Related to the intracellular [Ca2+]
Inotropic agents
◦ positive: increase contractility
◦ negative: decrease contractility
11. Chronotropy
◦ rate of contraction
◦ also affected by intracellular [Ca2+]
Dromotropy
◦ rate of impulse conduction
◦ noted particularly at AV node
14. Ca2+ entry through L-type channels in T tubules
triggers larger release of Ca2+ from sarcoplasmic
reticulum
◦ Ca2+ induced Ca2+ release leads to cross-bridge
cycling and contraction
18. Interconnected by intercalated discs and
form functional syncytia
Withinintercalated discs – two kinds of
membrane junctions
◦ Desmosomes
◦ Gap junctions
19.
20. Exhibit branching
Adjacent cardiac cells are
joined end to end by
specialized structures known
as intercalated discs
Within intercalated discs there
are two types of junctions
◦ Desmosomes
◦ Gap junctions..allow action
potential to spread from one
cell to adjacent cells.
Heart function as syncytium
when one cardiac cell
undergoes an action potential,
the electrical impulse spreads
to all other cells that are joined
by gap junctions so they
become excited and contract
as a single functional
syncytium.
Atrial syncytium and ventricular
syncytium
21. Autorhythmicity: The ability to initiate a heart beat
continuously and regularly without external
stimulation
Excitability: The ability to respond to a stimulus of
adequate strength and duration (i.e. threshold or
more) by generating a propagated action potential
Conductivity: The ability to conduct excitation
through the cardiac tissue
Contractility: The ability to contract in response to
stimulation
22. Definition: the ability of the heart to initiate its beat
continuously and regularly without external
stimulation
myogenic (independent of nerve supply)
due to the specialized excitatory & conductive
system of the heart
↓
intrinsic ability of self-excitation
(waves of depolarization)
↓
cardiac impulses
23. Autorythmic cells do not
have stable resting
membrane potential
(RMP)
Natural leakiness to Na &
Ca→ spontaneous and
gradual depolarization
Unstable resting
membrane potential (=
pacemaker potential)
Gradual depolarization
reaches threshold (-40
mv) → spontaneous AP
generation
24. SITE SA node acts as heart
RATE(Time/minu pacemaker because it has the
te) fastest rate of generating action
SA Node 100 potential
Nerve impulses from
AV Node 40-60
autonomic nervous system and
hormones modify the timing and
strength of each heart beat but do
AV bundle, bundle 20-35 not establish the fundamental
branches,& Purkinje rhythm.
fibres
25. Definition: The ability of cardiac muscle to
respond to a stimulus of adequate strength &
duration by generating an AP
AP initiated by SA node→travels along conductive
pathway→ excites atrial & ventricular muscle fibres
26.
27.
28. Long refractory period (250
msec) compared to skeletal
muscle (3msec)
During this period membrane is
refractory to further stimulation
until contraction is over.
It lasts longer than muscle
contraction, prevents tetanus
Gives time to heart to relax after
each contraction, prevent
fatigue
It allows time for the heart
chambers to fill during diastole
before next contraction
AP in skeletal muscle : 1-5 msec
AP in cardiac muscle :200 -300 msec
29. Definition: ability of cardiac muscle to contract
in response to stimulation
31. Definition: property by which excitation is
conducted through the cardiac tissue
32.
33. Cardiac impulse originates at SA node
Action potential spreads throughout right and left atria
Impulse passes from atria into ventricles through AV
node (only point of electrical contact between chambers)
Action potential briefly delayed at AV node (ensures
atrial contraction precedes ventricular contraction to
allow complete ventricular filling)
Impulse travels rapidly down interventricular septum by
means of bundle of His
Impulse rapidly disperses throughout myocardium by
means of Purkinje fibers
Rest of ventricular cells activated by cell-to-cell spread of
impulse through gap junctions
