Parasympathetic Stimulation
Vagus nerve
Primarily innervates atria, but some fibers
to ventricles also
Chemical mediator: acethycholine
Effect: slows heart rate and AV
conduction
Methods of stimulation: Valsalva
maneuver, carotid sinus pressure
Ken
Sympathetic Stimulation
Nerves arising in thoracic and lumbar
ganglia
Innervate both atria and ventricles
Chemical mediator: norepinephrine
Receptor sites: alpha, beta
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Effect of alpha Stimulation:
No effect on
heart
Peripheral
vasoconstriction
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Effect of beta Stimulation:
Increased rate and
conduction
Increased contractility
Bronchodilation
Peripheral
vasodilation
Ken
Role of Electrolytes
Cardiac function, electrical and
mechanical, influenced by electrolyte
imbalances
Major electrolytes influencing cardiac
function
Na+ Sodium
Ca++ Calcium
K+ Potassium
Ken
Role of Electrolytes
Sodium (Na +): major role in
depolarization phase of myocardial cells
Calcium (Ca ++): major role in
depolarization phase of myocardial
pacemaker cells and in myocardial
contractility
Hypercalcemia: increased myocardial contractility
Hypocalcemia: decreased myocardial contractility
and increased electrical irritability
Ken
Role of Electrolytes
Potassium (K +): major role in
repolarization phase
Hyperkalemia: decreased automaticity
and conduction
Hypokalemia: increased irritability
Potassium levels are critical to life
Hyperkalemia = Tall peaked T waves
Ken
Electrophysiology
Electrical properties of the heart
Automaticity: ability to generate an
electrical impulse without stimulation from
another source - property of pacemaker cells
Excitability: ability to respond to an
electrical stimulus -property of all myocardial
cells
Conductivity: ability to propagate an
impulse from cell to cell
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Electrical Conduction System
Allows electrical
impulses to spread
through the heart
six times faster
than through
muscle alone
Sequence of normal
electrical conduction
SA node
Internodal and
interatrial tracts
AV node
Bundle of His
Bundle branches
Purkinje fibers
Ken
Function of electrical conduction
structures
Sinoatrial (SA) node
Located in right
atrium near entrance
of superior vena cava
Usually heart's
dominant pacemaker
sa
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Internodal and interatrial tracts
Pathways that carry
impulse between SA
node and AV node
and spread it across
atrial muscle
Impulse travel time:
0.08 seconds
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Atrioventricular (AV) node:
Part of area called the "AV junctional tissue"
along with some surrounding tissue and the
non-branching portion of the Bundle of His
Responsible for creating slight delay in
conduction before sending impulse to
ventricles
Impulse travel time: 0.08-0.16 seconds
No pacemaking properties in node itself
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Bundle of His
Bundle of fibers coming off AV node,
located at top of interventricular septum
Considered part of the AV junction
Makes electrical connection between atria
and ventricles
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Bundle branches
Created by bifurcation of Bundle of His
into right and left branches
Carry electrical impulse at high velocity to
interventricular septum and each ventricle
simultaneously
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Purkinje fibers
Terminal ends of bundle branches
Network of fibers helping to spread
impulse throughout ventricular walls
Rapid impulse spread through ventricles:
0.08-0.09 seconds
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Depolarization
Process by which muscle fibers are
stimulated to contract by the
alteration of electrical charge of the
cell accomplished by changes in
electrolyte concentrations across the
cell membrane
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Depolarization at The Cellular Level
Chemical pumps in cell wall maintain
certain concentrations of electrolytes
within and outside the cell
Resting (polarized) cell normally more
electrically negative inside cell wall than
outside ( -90 millivolts (mv) in working
cells)
Ken
Depolarization at The Cellular Level
 Electrical stimulation of cell wall changes its
permeability to sodium (Na+)
 Na+ rushes into cell, causing inside to become
more positive
 Slower influx of calcium (Ca++) also causes cell to
become positive
 Muscle contraction is response to depolarization
 Depolarization wave is passed from cell to cell along
the conduction pathway to reach the muscle cells
Ken
Spontaneous diastolic
depolarization of pacemaker cells
Pacemaker cells capable of self-initiated
depolarization (automaticity)
Found throughout conduction system
except in AV node
During diastole, become less and less
negative until a certain threshold reached,
then rapidly and fully depolarize
Ken
Pacemaker Capabilities & Rates
SA node: 60-100/minute intrinsic rate
AV junctional tissue: 40-60/minute
intrinsic rate
Ventricles (bundle branches and Purkinje
fibers): 20-40/minute intrinsic rate
SA node usual pacemaker because it
discharges the fastest; pacemaker cells
below SA node normally suppressed by it
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Repolarization
Process by which cells re-establish internal
negativity and are readied for stimulation return
to resting or polarized state
Caused by rapid escape of potassium (K+) from
the cell
Proper distribution of electrolytes re-established
by cell wall pumps (Na+ pumped out of cell,
potassium pumped back into cell)
Cell returns to -90mv. internal chargerepolarized
Ken
Relationship of ECG to
electrical activity
ECG is record of electrical activity of heart
as sensed by electrodes on body surface
Gives information only about electrical
activity tells us nothing about pump
function
Isoelectric line: a flat line on the ECG
indicating absence of net electrical activity
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P wave
Rounded wave
preceding QRS;
usually upright
(positive) in Lead II
Indicates
depolarization of
atrial muscle
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QRS complex
Collective term
for three
deflections
following the
P wave
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QRS complex
Wave-first negative
deflection after P
wave
R wave-first positive
deflection after P
wave
S wave-first negative
deflection after R
wave
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QRS complex
All three waves not
always present - QRS
has many shapes
Indicates
depolarization of the
ventricular muscle
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T wave
 Rounded wave following
QRS complex; usually in
same direction as QRS
 Indicates repolarization
of ventricles
 Atrial T wave (atrial
repolarization) usually
not visible buried within
QRS complex
Ken
P-R interval
Distance between
beginning of P wave
and the beginning of
QRS complex
Indicates length of
time it takes
depolarizatin wave to
go from atria to
ventricles
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S-T segment:
Distance between the
S wave of the QRS
complex and the
beginning of the
T-wave usually in
isoelectric line
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Refractory period
 Period of time when
cells have been
depolarized and not yet
returned to polarized
state
 Heart unable to be
stimulated again
 On ECG, includes, QRS
complex and T wave
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Absolute refractory period
Time when stimulation will produce no
depolarization whatsoever
From beginning of QRS complex to apex of T
wave
Relative refractory period: time when a
sufficiently strong stimulus may produce
depolarization
Corresponds to down slope of T wave
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Nervous control of electrical
activity
Sympathetic (adrenergic) control
Effects of alpha stimulation: no direct effect
on heart
Effects of beta stimulation: increased rate,
increased conduction velocity in atria and
ventricles, increased irritability, (increased
contractility mechanical effect)
Ken
Parasympathetic (cholinergic)
control
Effects of
parasympathetic
(vagal) stimulation
Decreased firing rate
of SA node,
decreased AV
conduction, little
effect on ventricles
Ken
Ken