Biology Reference
In-Depth Information
cardiac conduction system, and the cardiac ventricles. Each of these
cells differs in its structure as well as in the type and expression of
membrane currents and transporters shaping its electrical excitability.
A review of each of these cell types is beyond the scope of this topic
(see ref. [1]). Instead, we focus on a review of properties of the cardiac
ventricular myoycte, as these myocytes are the most abundant in the
heart and their properties are of fundamental importance to electrical
conduction during both health and disease.
Structure of the Ventricular Myocyte
Structural organization of the ventricular myocyte is shown in figure 9.1.
Ventricular myocytes are long (~120
m)
muscle cells bounded by a cell membrane known as the sarcolemma.
The basic unit of contraction in the ventricular myocyte is the sarcomere
(figure 9.1a). Individual sarcomeres are approximately 2.0
µ
m), thin (diameter ~25
µ
m in length
and are bounded on both ends by the Z-disks. The H-zone is
the region of the thick filament that contains only myosin tails and not
the head regions. While the cardiac sarcomere is similar in many
respects to that of skeletal muscle, some important differences exist.
Specifically, cardiac muscle cannot be extended to lengths above
2.3
µ
m and hence always operates on the ascending limb of the
force-sarcomere length relationship. The A-band is the region spanned
by the length of the thick filaments. The shaded region in figure 9.1a
represents the region of overlap of thick and thin filaments. Muscle
contraction is accomplished by the sliding motion of the thick and thin
filaments relative to one another in this region in response to ele-
vated levels of intracellular Ca 2+
µ
and adenosine triphosphate (ATP)
hydrolysis.
Figure 9.1a also shows that sarcomeres are bounded on each end
by the T-tubules [2]. T-tubules are cylindrical invaginations of the
sarcolemma that extend into the myocyte (figure 9.1b), approaching
an organelle known as the sarcoplasmic reticulum (SR). The SR is
composed of two components known as junctional SR (JSR) and net-
work SR (NSR). The NSR is a lumenal organelle extending throughout
the myocyte. NSR membrane contains a high concentration of
the SR Ca 2+ -ATPase (SERCA2a) pump, which transports Ca 2+ from the
cytosol into the lumen of the NSR. The JSR is that portion of the SR
most closely approximating the T-tubules. The close proximity of these
two structures (they are separated by a distance of ~10 nm) forms a
restricted region commonly referred to as the dyad with an approxi-
mate diameter of 400 nm. Ca 2+ -sensitive Ca 2+ -release channels (known
as ryanodine receptors or RyR) are located in the dyadic region of the
JSR membrane. In addition, sarcolemmal L-type Ca 2+ channels (LCCs)
are located preferentially within the dyadic region of the T-tubules,
where they are in close apposition to the RyR. It has been estimated
Search WWH ::




Custom Search