<s>
In	O
mammals	O
,	O
cardiac	O
electrical	O
activity	O
originates	O
from	O
specialized	O
myocytes	O
of	O
the	O
sinoatrial	O
node	O
(	O
SAN	O
)	O
which	O
generate	O
spontaneous	O
and	O
rhythmic	O
action	B-Algorithm
potentials	I-Algorithm
(	O
AP	O
)	O
.	O
</s>
<s>
The	O
voltage	O
region	O
encompassed	O
by	O
this	O
transition	O
is	O
commonly	O
known	O
as	O
pacemaker	O
phase	O
,	O
or	O
slow	O
diastolic	B-Algorithm
depolarization	I-Algorithm
or	O
phase	O
4	O
.	O
</s>
<s>
The	O
duration	O
of	O
this	O
slow	O
diastolic	B-Algorithm
depolarization	I-Algorithm
(	O
pacemaker	O
phase	O
)	O
thus	O
governs	O
the	O
cardiac	O
chronotropism	O
.	O
</s>
<s>
Evidence	O
in	O
support	O
of	O
the	O
active	O
presence	O
of	O
K+	O
,	O
Ca2+	O
,	O
Na+	O
channels	O
and	O
Na+	O
/K	O
+	O
exchanger	O
during	O
the	O
pacemaker	O
phase	O
have	O
been	O
variously	O
reported	O
in	O
the	O
literature	O
,	O
but	O
several	O
indications	O
point	O
to	O
the	O
funny	B-Algorithm
current	I-Algorithm
(	O
If	O
)	O
as	O
one	O
of	O
the	O
most	O
important	O
.	O
</s>
<s>
There	O
is	O
now	O
substantial	O
evidence	O
that	O
also	O
sarcoplasmic	O
reticulum	O
(	O
SR	O
)	O
Ca2+	O
-transients	O
participate	O
in	O
the	O
generation	O
of	O
the	O
diastolic	B-Algorithm
depolarization	I-Algorithm
via	O
a	O
process	O
involving	O
the	O
Na	O
–	O
Ca	O
exchanger	O
.	O
</s>
