00:01
Well, let’s look at the
conducting system of the heart.
00:06
The conducting system of the heart consists
of the sinoatrial node, the atrioventricular
node, and then these bundles of conducting
fibres named in various ways, but I don’t
want to get into those namings in this particular
lecture. In the middle section, labeled is
the atrioventricular bundle. It’s embedded
with connective tissue. It's cardiac muscle
cell. And on the right-hand side, you can
see the endocardium. And directly under the
endocardium if you look very carefully, often,
you find evidence of these conducting fibres
called Purkinje fibres. Now, the atrioventricular
bundle is just a large bundle of Purkinje fibres.
00:55
Those Purkinje fibres are then branched
out and head down through the myocardium,
generally, very closely associated with the
subendocardial layer. And that’s where
you could locate them histologically. Let
me stress that these conducting fibres of
the heart are not nerves. They’re specialized
cardiac muscle. So if you look at the diagram
of a heart, the blue circular structure
and the blue lines running down through the
myocardium represents these conducting fibres.
And the white circular structure up at the
top that’s actually lying right next to
the entry of the superior vena cava is
the sinoatrial node. And it has processes
Purkinje fibres or conducing fibres radiating
from that sinoatrial node. The sinoatrial
node is the pacemaker of the heart.
02:04
Cardiac muscle, when it developed, has quite an inherent
ability to contract, that contracts without
any stimulation except the impulses emanating
from the sinoatrial node. However, that sinoatrial
node can be influenced by the autonomic nervous
system. The firing rate of the impulse can
be increased or decreased by external factors,
not just nerves from the autonomic nervous
system, but by other factors as well. So you
can increase or decrease the rate of the heart
by influencing the rate of firing initiated
from that sinoatrial node. But essentially,
those conducting fibres are cardiac muscle,
but they are just specialized. And you can
tell the difference between these conducting
fibres from normal cardiac muscle. Have a
look very carefully at the central section
of the atrioventricular bundle, or even have
a look at the Purkinje fibres labeled just
under the endocardium. Both these labels or
both these fibres rather have clear types
of components within them. The cytoplasm is
less duct stained, and sometimes, it’s even
very, very pale. That’s because these cardiac
muscles are conducting fibres. They’re not
contractile. So they don’t contain all the
contractile proteins, all the contractile
factory within the cell. They don’t
need it. So they don’t take up stain because
they’re not there. It’s as simple as that.
03:54
Another way you can tell the difference between
the Purkinje fibre, particularly, and surrounding
cardiac muscle fibres is that Purkinje fibres
often have two nuclei. Remember, cardiac muscle
only has one nucleus.