Block IV: Cardiac muscle Flashcards
What are the histological characteristics of cardiac muscle?
- Striated = sarcomeres
- Intermediate diameter fibers (5 -10 nuclear diameters across)
- Fiber = multiple cells end to end. Not a physical syncytium
- Branching fibers
- Single nuclei per cell (occasionally binucleate)
- Centrally located nuclei
- Presence of intercalated discs
- Many very large mitochondria
- Contraction occurs by spread of electrical depolarization from cell to cell by gap junctions at intercalated disc
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Medium magnification micrograph of cardiac muscle showing
the striations, centrally located nuclei, single nucleus per cell,
intercalated discs, and branching fibers
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Longitudinaly sectioned cardiac muscle
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cross-sectioned cardiac muscle
WHere are intercalated discs found?
Characteristic of cardiac muscle - not found skeletal or
smooth muscle - step like appearance. Are junctional areas at ends
of cells
Function of intercalated discs?
Play role of (in position of) Zband of terminal sarcomere – serves for actin thin filament attachment at end of muscle cell
What are the 3 types of inetrcellular junctions in intercalated discs?
Fascia adherens, desmosomes, and gap
junctions (nexus)
Gap junction = electrical coupling of cells = wave of synchronized contraction in heart
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intercalated disc
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intercalated disc: fascia adherens
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gap junction (
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macula adherens (desmosome and gap junction)
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gap junction
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EM of longitudinally sectioned cardiac muscle showing the large number of mitochondria
why are cardiac mitochondria important?
Note that mitochondria are very large and present in very large numbers in cardiac muscle. This is characteristic and correlates with the high energy requirements of cardiac muscle.
What type of phosphorylation does cardiac mitochondria use?
These mitochondria do use aerobic oxidative phosphorylation
to produce high-energy compounds such as ATP, thus the extreme need of cardiac muscle to have oxygen present and their damage during ischemia when oxygen is limited
How is the sarcoplasmic reticulum in cardiac muscle?
less developed than skeletal muscle. No continuous terminal cisterna.
Where are T-tubules found in cardiac muscle?
T-tubules not typically found in atrial muscle. Found in ventricle, but wider in width (larger lumen) than skeletal.
T-tubules in ventricle lie at the position of the Z-line, not the AI junction seen in skeletal muscle
How are triads in cardiac muscle?
No triads. Instead we have diads in ventricle composed of a Ttubule + 1 flattened sac of SR
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EM of a dyad junction in cardiac ventricular muscle cell
Where do we only have diads?
in ventricular fiber
What are the channels in diads and its fucntion?
voltage sensitive L-type Ca+2 channels in the T-tubule and ryanodine 2 receptor calcium channels in the membrane of terminal cisterna. Note that the T-tubule and terminal cisterna sac are not in direct contact
Are the main sites of calcium release for contraction in the ventricle
What are other Ca+2 release sites?
Junctional contacts between the plasma membrane itself and terminal cisterna sacs of the SR. These appear similar to the diads in function
Corbular sarcoplasmic reticulum: These are regions of tubular SR near the Z-line, characterized by expanded bouton- like saccules of the tubular SR which contain
ryanodine 2 calcium channels which may open to release calcium when cytoplasmic calcium levels rise
WHat makes calcium return to the SR of cardiac muscle?
A smooth endoplasmic reticulum calcium ATPase (SERCA2a isoform) is also present in cardiac muscle and acts to pump calcium back into the SR between contractions
It is associated with and regulated by another SR protein called phospholamban which depending on its phosphorylation state can either inhibit or open the SERCA pump channels
WHat contracts the ventricle?
Purkinje fibers bring impulse (depolarization)
What happens upon depolarization for contraction of ventricle?
voltage-sensor proteins in the T-tubule membrane act as open L-type calcium channels releasing calcium into the sarcoplasm.
This in turn causes the ryanodine receptors (RyR2) in the adjacent sacs of SR to release additional calcium into the cytoplasm.
Note that physical interaction of t-tubule voltage sensor proteins and ryanodine-2 receptors not required.
How is calcium release in atria?
Since there are no T-tubules and no dyads in the atrial fibers, the calcium release is by the sarcoplasmic reticulum terminal cisterna adjacent to the plasma membrane.
the ryanodine-2 receptor calcium channels in the saccules of the corbular SR in response to an increase in the cytoplasmic calcium levels will release additional calcium into the cytoplasm
Explain esxcitation contraction coupling in skeletal vs cardiac muscle
Note that in skeletal muscle, the voltage sensor protein in the ttubule (DHSR) does not act as a major calcium channel, but through a conformational change or other direct interaction with the RyR1 causes RyR1 to open as a calcium channel to release
calcium from terminal cisterna of SR
In contrast in cardiac muscle the voltage sensor proteins in the Ttubule or in the plasma membrane in response to depolarization act as calcium channels allowing some calcium to enter from the T-tubule lumen or plasma membrane. This
increase in calcium then causes the RyR2 proteins in the SR terminal cisterna membrane or in the corbular SR to open allowing a large increase in calcium.
Physical interaction of the voltage sensor and the RyR2 is not required
