Block IV: Cardiac muscle

Question 1

What are the histological characteristics of cardiac muscle?

Answer 1

1. Striated = sarcomeres
2. Intermediate diameter fibers (5 -10 nuclear diameters across)
3. Fiber = multiple cells end to end. Not a physical syncytium
4. Branching fibers
5. Single nuclei per cell (occasionally binucleate)
6. Centrally located nuclei
7. Presence of intercalated discs
8. Many very large mitochondria
9. Contraction occurs by spread of electrical depolarization from cell to cell by gap junctions at intercalated disc

Question 2

identify

Answer 2

Medium magnification micrograph of cardiac muscle showing
the striations, centrally located nuclei, single nucleus per cell,
intercalated discs, and branching fibers

Question 3

identify

Answer 3

Longitudinaly sectioned cardiac muscle

Question 4

identify

Answer 4

cross-sectioned cardiac muscle

Question 5

WHere are intercalated discs found?

Answer 5

Characteristic of cardiac muscle - not found skeletal or
smooth muscle - step like appearance. Are junctional areas at ends
of cells

Question 6

Function of intercalated discs?

Answer 6

Play role of (in position of) Zband of terminal sarcomere – serves for actin thin filament attachment at end of muscle cell

Question 7

What are the 3 types of inetrcellular junctions in intercalated discs?

Answer 7

Fascia adherens, desmosomes, and gap
junctions (nexus)

Gap junction = electrical coupling of cells = wave of synchronized contraction in heart

Question 8

identify

Answer 8

intercalated disc

Question 9

identify

Answer 9

intercalated disc: fascia adherens

Question 10

identify

Answer 10

gap junction (

Question 11

identify

Answer 11

macula adherens (desmosome and gap junction)

Question 12

identify

Answer 12

gap junction

Question 13

identify

Answer 13

EM of longitudinally sectioned cardiac muscle showing the large number of mitochondria

Question 14

why are cardiac mitochondria important?

Answer 14

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.

Question 15

What type of phosphorylation does cardiac mitochondria use?

Answer 15

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

Question 16

How is the sarcoplasmic reticulum in cardiac muscle?

Answer 16

less developed than skeletal muscle. No continuous terminal cisterna.

Question 17

Where are T-tubules found in cardiac muscle?

Answer 17

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

Question 18

How are triads in cardiac muscle?

Answer 18

No triads. Instead we have diads in ventricle composed of a Ttubule + 1 flattened sac of SR

Question 19

identify

Answer 19

EM of a dyad junction in cardiac ventricular muscle cell

Question 20

Where do we only have diads?

Answer 20

in ventricular fiber

Question 21

What are the channels in diads and its fucntion?

Answer 21

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

Question 22

What are other Ca+2 release sites?

Answer 22

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

Question 23

WHat makes calcium return to the SR of cardiac muscle?

Answer 23

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

Question 24

WHat contracts the ventricle?

Answer 24

Purkinje fibers bring impulse (depolarization)

Question 25

What happens upon depolarization for contraction of ventricle?

Answer 25

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.

Question 26

How is calcium release in atria?

Answer 26

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

Question 27

Explain esxcitation contraction coupling in skeletal vs cardiac muscle

Answer 27

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