<s>
right|thumb|350px|Image	O
showing	O
the	O
cardiac	B-Algorithm
pacemaker	I-Algorithm
or	O
SA	O
node	O
,	O
the	O
primary	O
pacemaker	B-Device
within	O
the	O
electrical	B-Algorithm
conduction	I-Algorithm
system	I-Algorithm
of	I-Algorithm
the	I-Algorithm
heart	I-Algorithm
.	O
</s>
<s>
The	O
contraction	O
of	O
cardiac	O
muscle	O
(	O
heart	O
muscle	O
)	O
in	O
all	O
animals	O
is	O
initiated	O
by	O
electrical	O
impulses	O
known	O
as	O
action	B-Algorithm
potentials	I-Algorithm
that	O
in	O
the	O
heart	O
are	O
known	O
as	O
cardiac	B-Algorithm
action	I-Algorithm
potentials	I-Algorithm
.	O
</s>
<s>
The	O
cells	O
that	O
create	O
these	O
rhythmic	O
impulses	O
,	O
setting	O
the	O
pace	O
for	O
blood	O
pumping	O
,	O
are	O
called	O
pacemaker	B-Algorithm
cells	I-Algorithm
,	O
and	O
they	O
directly	O
control	O
the	O
heart	O
rate	O
.	O
</s>
<s>
They	O
make	O
up	O
the	O
cardiac	B-Algorithm
pacemaker	I-Algorithm
,	O
that	O
is	O
,	O
the	O
natural	B-Algorithm
pacemaker	I-Algorithm
of	O
the	O
heart	O
.	O
</s>
<s>
In	O
most	O
humans	O
,	O
the	O
highest	O
concentration	O
of	O
pacemaker	B-Algorithm
cells	I-Algorithm
is	O
in	O
the	O
sinoatrial	O
(	O
SA	O
)	O
node	O
,	O
the	O
natural	O
and	O
primary	O
pacemaker	B-Device
,	O
and	O
the	O
resultant	O
rhythm	O
is	O
a	O
sinus	O
rhythm	O
.	O
</s>
<s>
Sometimes	O
a	O
secondary	O
pacemaker	B-Device
sets	O
the	O
pace	O
,	O
if	O
the	O
SA	O
node	O
is	O
damaged	O
or	O
if	O
the	O
electrical	B-Algorithm
conduction	I-Algorithm
system	I-Algorithm
of	I-Algorithm
the	I-Algorithm
heart	I-Algorithm
has	O
problems	O
.	O
</s>
<s>
Cardiac	B-Application
arrhythmias	I-Application
can	O
cause	O
heart	B-Algorithm
block	I-Algorithm
,	O
in	O
which	O
the	O
contractions	O
lose	O
any	O
useful	O
rhythm	O
.	O
</s>
<s>
In	O
humans	O
,	O
and	O
sometimes	O
in	O
other	O
animals	O
,	O
a	O
mechanical	O
device	O
called	O
an	O
artificial	B-Device
pacemaker	I-Device
(	O
or	O
simply	O
"	O
pacemaker	B-Device
"	O
)	O
may	O
be	O
used	O
after	O
damage	O
to	O
the	O
body	O
's	O
intrinsic	O
conduction	B-Algorithm
system	I-Algorithm
to	O
produce	O
these	O
impulses	O
synthetically	O
.	O
</s>
<s>
The	O
sinoatrial	O
node	O
(	O
SA	O
node	O
)	O
is	O
the	O
primary	O
pacemaker	B-Device
of	O
the	O
heart	O
.	O
</s>
<s>
The	O
cells	O
that	O
make	O
up	O
the	O
SA	O
node	O
are	O
specialized	O
cardiomyocytes	O
known	O
as	O
pacemaker	B-Algorithm
cells	I-Algorithm
that	O
can	O
spontaneously	O
generate	O
cardiac	B-Algorithm
action	I-Algorithm
potentials	I-Algorithm
.	O
</s>
<s>
These	O
signals	O
are	O
propogated	O
through	O
the	O
heart	O
's	O
electrical	B-Algorithm
conduction	I-Algorithm
system	I-Algorithm
.	O
</s>
<s>
The	O
pacemaker	B-Algorithm
cells	I-Algorithm
are	O
connected	O
to	O
neighboring	O
contractile	O
cells	O
via	O
gap	O
junctions	O
,	O
which	O
enable	O
them	O
to	O
locally	O
depolarize	O
adjacent	O
cells	O
.	O
</s>
<s>
Gap	O
junctions	O
allow	O
the	O
passage	O
of	O
positive	O
cations	O
from	O
the	O
depolarization	O
of	O
the	O
pacemaker	B-Algorithm
cell	I-Algorithm
to	O
adjacent	O
contractile	O
cells	O
.	O
</s>
<s>
This	O
starts	O
the	O
depolarization	O
and	O
eventual	O
action	B-Algorithm
potential	I-Algorithm
in	O
contractile	O
cells	O
.	O
</s>
<s>
All	O
the	O
while	O
being	O
in	O
sync	O
with	O
the	O
pacemaker	B-Algorithm
cells	I-Algorithm
;	O
this	O
is	O
the	O
property	O
that	O
allows	O
the	O
pacemaker	B-Algorithm
cells	I-Algorithm
to	O
control	O
contraction	O
in	O
all	O
other	O
cardiomyocytes	O
.	O
</s>
<s>
Impulses	O
from	O
the	O
sinus	O
node	O
reach	O
the	O
atrioventricular	O
node	O
which	O
acts	O
as	O
the	O
secondary	O
pacemaker	B-Device
.	O
</s>
<s>
The	O
cells	O
of	O
the	O
AV	O
node	O
normally	O
discharge	O
at	O
about	O
40-60	O
beats	O
per	O
minute	O
,	O
and	O
are	O
called	O
the	O
secondary	O
pacemaker	B-Device
.	O
</s>
<s>
Further	O
down	O
the	O
electrical	O
conducting	B-Algorithm
system	I-Algorithm
of	I-Algorithm
the	I-Algorithm
heart	I-Algorithm
is	O
the	O
Bundle	B-Algorithm
of	I-Algorithm
His	I-Algorithm
.	O
</s>
<s>
The	O
left	O
and	O
right	O
bundle	O
branches	O
,	O
and	O
the	O
Purkinje	B-Algorithm
fibers	I-Algorithm
,	O
will	O
also	O
produce	O
a	O
spontaneous	O
action	B-Algorithm
potential	I-Algorithm
at	O
a	O
rate	O
of	O
30-40	O
beats	O
per	O
minute	O
,	O
so	O
if	O
the	O
SA	O
and	O
AV	O
node	O
both	O
fail	O
to	O
function	O
,	O
these	O
cells	O
can	O
become	O
pacemakers	B-Device
.	O
</s>
<s>
It	O
is	O
important	O
to	O
realize	O
that	O
these	O
cells	O
will	O
be	O
initiating	O
action	B-Algorithm
potentials	I-Algorithm
and	O
contraction	O
at	O
a	O
much	O
lower	O
rate	O
than	O
the	O
primary	O
or	O
secondary	O
pacemaker	B-Algorithm
cells	I-Algorithm
.	O
</s>
<s>
The	O
SA	O
node	O
controls	O
the	O
rate	O
of	O
contraction	O
for	O
the	O
entire	O
heart	O
muscle	O
because	O
its	O
cells	O
have	O
the	O
quickest	O
rate	O
of	O
spontaneous	O
depolarization	O
,	O
thus	O
they	O
initiate	O
action	B-Algorithm
potentials	I-Algorithm
the	O
quickest	O
.	O
</s>
<s>
The	O
action	B-Algorithm
potential	I-Algorithm
generated	O
by	O
the	O
SA	O
node	O
passes	O
down	O
the	O
electrical	B-Algorithm
conduction	I-Algorithm
system	I-Algorithm
of	I-Algorithm
the	I-Algorithm
heart	I-Algorithm
,	O
and	O
depolarizes	O
the	O
other	O
potential	O
pacemaker	B-Algorithm
cells	I-Algorithm
(	O
AV	O
node	O
)	O
to	O
initiate	O
action	B-Algorithm
potentials	I-Algorithm
before	O
these	O
other	O
cells	O
have	O
had	O
a	O
chance	O
to	O
generate	O
their	O
own	O
spontaneous	O
action	B-Algorithm
potential	I-Algorithm
,	O
thus	O
they	O
contract	O
and	O
propagate	O
electrical	O
impulses	O
to	O
the	O
pace	O
set	O
by	O
the	O
cells	O
of	O
the	O
SA	O
node	O
.	O
</s>
<s>
This	O
is	O
the	O
normal	O
conduction	B-Algorithm
of	O
electrical	O
activity	O
in	O
the	O
heart	O
.	O
</s>
<s>
There	O
are	O
3	O
main	O
stages	O
in	O
the	O
generation	O
of	O
an	O
action	B-Algorithm
potential	I-Algorithm
in	O
a	O
pacemaker	B-Algorithm
cell	I-Algorithm
.	O
</s>
<s>
The	O
key	O
to	O
the	O
rhythmic	O
firing	O
of	O
pacemaker	B-Algorithm
cells	I-Algorithm
is	O
that	O
,	O
unlike	O
other	O
neurons	O
in	O
the	O
body	O
,	O
these	O
cells	O
will	O
slowly	O
depolarize	O
by	O
themselves	O
and	O
do	O
not	O
need	O
any	O
outside	O
innervation	O
from	O
the	O
autonomic	O
nervous	O
system	O
to	O
fire	O
action	B-Algorithm
potentials	I-Algorithm
.	O
</s>
<s>
However	O
,	O
in	O
pacemaker	B-Algorithm
cells	I-Algorithm
,	O
this	O
potassium	O
permeability	O
(	O
efflux	O
)	O
decreases	O
as	O
time	O
goes	O
on	O
,	O
causing	O
a	O
slow	O
depolarization	O
.	O
</s>
<s>
In	O
addition	O
,	O
there	O
is	O
a	O
slow	O
,	O
continuous	O
inward	O
flow	O
of	O
sodium	O
,	O
called	O
the	O
"	O
funny	O
"	O
or	O
pacemaker	B-Algorithm
current	I-Algorithm
.	O
</s>
<s>
These	O
two	O
relative	O
ion	O
concentration	O
changes	O
slowly	O
depolarize	O
(	O
make	O
more	O
positive	O
)	O
the	O
inside	O
membrane	O
potential	O
(	O
voltage	O
)	O
of	O
the	O
cell	O
,	O
giving	O
these	O
cells	O
their	O
pacemaker	B-Device
potential	O
.	O
</s>
<s>
When	O
the	O
membrane	O
potential	O
gets	O
depolarized	O
to	O
about	O
-40mV	O
it	O
has	O
reached	O
threshold	O
(	O
cells	O
enter	O
phase	O
0	O
)	O
,	O
allowing	O
an	O
action	B-Algorithm
potential	I-Algorithm
to	O
be	O
generated	O
.	O
</s>
<s>
Though	O
much	O
faster	O
than	O
the	O
depolarization	O
of	O
phase	O
4	O
,	O
the	O
upstroke	O
in	O
a	O
pacemaker	B-Algorithm
cell	I-Algorithm
is	O
slow	O
compared	O
to	O
that	O
in	O
an	O
axon	B-Algorithm
.	O
</s>
<s>
(	O
The	O
funny	B-Algorithm
current	I-Algorithm
also	O
increases	O
)	O
.	O
</s>
<s>
This	O
calcium	O
influx	O
produces	O
the	O
rising	O
phase	O
of	O
the	O
action	B-Algorithm
potential	I-Algorithm
,	O
which	O
results	O
in	O
the	O
reversal	O
of	O
membrane	O
potential	O
to	O
a	O
peak	O
of	O
about	O
+10mV	O
.	O
</s>
<s>
In	O
heart	B-Device
pacemaker	I-Device
cells	O
,	O
phase	O
0	O
depends	O
on	O
the	O
activation	O
of	O
L-type	O
calcium	O
channels	O
instead	O
of	O
the	O
activation	O
of	O
voltage-gated	O
fast	O
sodium	O
channels	O
,	O
which	O
are	O
responsible	O
for	O
initiating	O
action	B-Algorithm
potentials	I-Algorithm
in	O
contractile	O
(	O
non-pacemaker	O
)	O
cells	O
.	O
</s>
<s>
For	O
this	O
reason	O
,	O
the	O
pacemaker	B-Device
action	B-Algorithm
potential	I-Algorithm
rising	O
phase	O
slope	O
is	O
more	O
gradual	O
than	O
that	O
of	O
the	O
contractile	O
cell	O
(	O
image	O
2	O
)	O
.	O
</s>
<s>
Restoring	O
these	O
ion	O
concentrations	O
is	O
vital	O
because	O
it	O
enables	O
the	O
cell	O
to	O
reset	O
itself	O
and	O
enables	O
it	O
to	O
repeat	O
the	O
process	O
of	O
spontaneous	O
depolarization	O
leading	O
to	O
activation	O
of	O
an	O
action	B-Algorithm
potential	I-Algorithm
.	O
</s>
<s>
If	O
the	O
SA	O
node	O
does	O
not	O
function	O
,	O
or	O
the	O
impulse	O
generated	O
in	O
the	O
SA	B-Algorithm
node	I-Algorithm
is	I-Algorithm
blocked	I-Algorithm
before	O
it	O
travels	O
down	O
the	O
electrical	B-Algorithm
conduction	I-Algorithm
system	I-Algorithm
,	O
a	O
group	O
of	O
cells	O
further	O
down	O
the	O
heart	O
will	O
become	O
its	O
pacemaker	B-Device
.	O
</s>
<s>
If	O
the	O
AV	O
node	O
also	O
fails	O
,	O
Purkinje	B-Algorithm
fibers	I-Algorithm
are	O
occasionally	O
capable	O
of	O
acting	O
as	O
the	O
default	O
or	O
"	O
escape	O
"	O
pacemaker	B-Device
.	O
</s>
<s>
An	O
ectopic	B-Algorithm
pacemaker	I-Algorithm
also	O
known	O
as	O
an	O
ectopic	B-Algorithm
focus	I-Algorithm
or	O
ectopic	B-Algorithm
foci	I-Algorithm
,	O
is	O
an	O
excitable	O
group	O
of	O
cells	O
that	O
causes	O
a	O
premature	O
heart	O
beat	O
outside	O
the	O
normally	O
functioning	O
SA	O
node	O
of	O
the	O
heart	O
.	O
</s>
<s>
It	O
is	O
thus	O
a	O
cardiac	B-Algorithm
pacemaker	I-Algorithm
that	O
is	O
ectopic	O
,	O
producing	O
an	O
ectopic	O
beat	O
.	O
</s>
<s>
If	O
chronic	O
this	O
can	O
result	O
in	O
arhythmias	B-Application
such	O
as	O
tachycardia	B-Application
,	O
bradycardia	B-Algorithm
,	O
or	O
ventricular	O
fibrillation	O
.	O
</s>
<s>
An	O
artificial	B-Device
pacemaker	I-Device
may	O
be	O
used	O
to	O
counter	O
this	O
.	O
</s>
<s>
An	O
artificial	B-Device
cardiac	I-Device
pacemaker	I-Device
(	O
or	O
artificial	B-Device
pacemaker	I-Device
,	O
so	O
as	O
not	O
to	O
be	O
confused	O
with	O
the	O
natural	O
cardiac	B-Algorithm
pacemaker	I-Algorithm
)	O
or	O
just	O
pacemaker	B-Device
is	O
an	O
implanted	O
medical	O
device	O
that	O
generates	O
electrical	O
impulses	O
delivered	O
by	O
electrodes	O
to	O
the	O
chambers	O
of	O
the	O
heart	O
either	O
the	O
upper	O
atria	O
,	O
or	O
lower	O
ventricles	O
to	O
cause	O
the	O
targeted	O
chambers	O
to	O
contract	O
and	O
pump	O
blood	O
.	O
</s>
<s>
By	O
doing	O
so	O
,	O
the	O
artificial	B-Device
pacemaker	I-Device
takes	O
over	O
from	O
the	O
primary	O
SA	O
node	O
pacemaker	B-Device
to	O
regulate	O
the	O
function	O
of	O
the	O
heart	O
's	O
electrical	B-Algorithm
conduction	I-Algorithm
system	I-Algorithm
.	O
</s>
