Biomedical Engineering Reference
In-Depth Information
potential from approximately negative 60 mV to approximately negative 40 mV. Once the
membrane potential reaches approximately negative 40 mV (the SA node action potential
threshold), the slow calcium channels and the fast sodium channels activate, allowing
both calcium and sodium ions to pass rapidly into the SA node cells. Once this occurs,
there is a rapid depolarization of the SA node cells, which leads to the action potential
impulse throughout the atrial syncytium. Again, the cells that compose the SA node are
directly connected with the neighboring atrial myocytes through extensive gap junction
connections. The SA node cells also contain potassium channels, allowing for the efflux of
potassium out of the cells, to bring the membrane potential back to approximately nega-
tive 60 mV. The constant sodium leak is counterbalanced by the presence of these potas-
sium channels and the inactivity of the calcium and fast sodium channels (after the
channels close). Both of the potassium and the calcium channels act at the same time to
prevent the SA node cells from continually experiencing a positive resting potential (near
the sodium equilibrium potential), due to the continuous influx of sodium ions. Once the
SA node has become depolarized, the action potential signal passes to neighboring myo-
cytes through gap junctions, allowing for atria contraction. The conduction speed through
the atria proceeds at approximately 0.3 m/sec. The entire process of sodium leak into the
SA node, depolarization of the SA node, and repolarization of the SA node occurs approxi-
mately once every second during normal resting conditions.
The SA node is also directly coupled to an interatrial band and three internodal pathways.
The interatrial band connects the left and right atria, so that they can contract at approxi-
mately the same time. Remember that atrial contraction is more passive and the atria act as
primer pumps for the ventricles. The interatrial band passes through the right atrium wall
into the left atrium and is composed of specialized conducting cells that pass the depolariza-
tion signal to the left atrium at a rate of at least 1 m/s. The three internodal bands initiate at
the SA node, follow along the right atrium wall (along the anterior, middle, and posterior
sides) and terminate at the atrioventricular (AV) node. These fibers rapidly conduct the
action potential signal (less than 0.04 seconds after they become activated by the action
potential) to the AV node, which is responsible for instigating ventricle contraction.
The AV node is located in the wall of the right atrium and is primarily responsible for
delaying the action potential signal from entering the ventricles. This allows both of the
atria to fully prime the ventricles prior to ventricular contraction. The signal is delayed in
the AV node for approximately 0.09 seconds before the action potential signal is allowed
to pass through the penetrating portion of the AV node (total of approximately 0.13 sec-
onds after SA node depolarization). The penetrating portion of the AV node passes
directly through the highly resistant fibrous tissue that separates the atria and the ventri-
cles. After passing through the fibrous tissue, there is another delay of approximately
0.03 seconds, in which time the action potential signal passes into the Purkinje fiber system
to allow for rapid and synchronized depolarization of the ventricular muscle cells. The
cause of this slowed transmission in the AV node system is due to a decreased number of
gap junctions between these cells. This effectively increases the resistance to action poten-
tial transmission as compared to the normal cardiac myocytes.
The Purkinje fibers act as a direct coupling between the AV node system and the ventri-
cles. These fibers are very large and function to transmit the action potential signal very
rapidly at speeds of up to 4 m/s. Recall that with an increase in fiber diameter there is a
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