CPR 7.03 Cardiac Muscle and Heart Histology

Question 1

Describe the general features/characteristics of cardiac muscle

Answer 1

Cardiac muscle has cross-striations and is composed of elongated, often branched cells bound to one another at structures called intercalated discs that are unique to cardiac muscle. Its contraction is involuntary, vigorous, and rhythmic. The heart consists of tightly knit bundles of cells, interwoven in spiraling layers that provide a characteristic wave of contraction that resembles wringing out of the heart ventricles.

Question 2

Describe the appearance of cardiac muscle in LM

• cytoplasm color
• fiber appearance
• nucleus
• connections

Answer 2

1. Long irregular branched cells with bands of light and dark staining.
   (a) Eosinophilic cytoplasm.
   (b) Cross-striations more easily visualized with special processing via polarized light.
2. One centrally located nucleus. (sometime bi-nucleate).
3. Bound by intercalated discs that appear as dark-staining transverse lines on LM.

Question 3

What direction do the transverse regions of intercalated discs run in cardiac muscle and what are their 2 main functions?

Answer 3

(a) Runs perpendicular to muscle filaments and act to bind cells together and prevent them from pulling apart with contraction.
(b) Contain desmosomes that confer mechanical stability and act as attachment points of intermediate filaments.
(c) Have adherent junctions which allow the attachment of thin filaments.

Question 4

What direction do the longitudinal regions of intercalated discs run? Describe their appearance and associated function.

Answer 4

(a) Runs parallel to muscle filaments.
(b) Appears less electron dense due to the presence of Gap junctions. These are sites of low electrical resistance thru which electrical synapses pass from cell to cell.

Question 5

Describe the transverse tubules appearance. Where in the heart are these primarily found?

Answer 5

1. Fingerlike invaginations of sarcolemma that course through the sarcoplasm and branch at the Z disc.
2. Mostly in ventricles and are small or absent in atria.

Question 6

What is the function of the Sarcoplasmic reticulum in cardiac muscle?

Answer 6

Calcium sequestration

Question 7

Describe the role of terminal cistern in SR of cardiac muscle.
Whatis a Dyad at the Z disc?

Answer 7

• Terminal Cisternae are expansions of the SR that come into contact with the T tubule. This is the location where calcium is released upon SR depolarization.
• Dyad at Z disc of sarcomeres consists of 1 T tubule and 1 SR terminal cisternae.

Question 8

Describe the appearance and prevalence of mitochondria in cardiac muscle.

Answer 8

Mitochondria are large, numerous, and arranged in continuous longitudinal rows.

Question 9

What are the 3 connective tissue layers of cardiac muscle?

Answer 9

Endomysium, Perimysium, and Epimysium.

Question 10

What does the endomysium of cardiac muscle consist of? What does it surround?

Answer 10

surrounds individual muscle fibers/cells. Composed of loose CT that includes fibroblasts, Reticular fibers (type III collagen fibers), and a rich capillary network

Question 11

What does the perimysium of cardiac muscle consist of? What does it surround?

Answer 11

CT septa. Surrounds fascicle. Contributes to cardiac skeleton.

Question 12

What does the epimysium of cardiac muscle consist of? What does it surround?

Answer 12

Dense irregular CT that surrounds entire muscle.

Question 13

Describe the composition of a myofibril

Answer 13

1. Consist of repeating array of Myofilaments organized into cylindrical units which are composed of 1,000’s of sarcomeres arranged end-to-end.

Question 14

What makes up thick filaments of cardiac muscle and what is their function?

Answer 14

Myosin. Overlapping “golf club” appearance function to attach to adherent junctions.

Question 15

What 3 components make up thin filaments of cardiac muscle?

Answer 15

Actin, Tropmyosin, and Troponin.

Question 16

Describe the structure, location, and function of actin in cardiac thin filaments/

Answer 16

thin and helical strand running between thick myosin filaments. Act as binding site for myosin

Question 17

Describe the structure, location, and function of tropomyosin in cardiac thin filaments

Answer 17

regulatory protein consisting of 2 polypeptide chains in a coil. Located in groove of actin strands

Question 18

Describe the structure, location, and function of troponin in cardiac thin filaments

Answer 18

regulates actin-myosin interaction. Ataches to tropomyosin and binds calcium.

Question 19

The arrangement of ——– in adjacent myofibrils causes striations in cardiac muscle

Answer 19

Sarcomere.

Question 20

Describe the appearance and components of an A band

Answer 20

Dark. Thick filaments and parts of thin filaments

Question 21

Describe the appearance and components of I bands.

Answer 21

Light. Composed only of thin filaments.

Question 22

Describe the location and function of the Z-disc and their significance in relation to the sarcomere unit

Answer 22

Dark line bisecting the I bands. TItin connects thick filaments to Z disk. α-actinin holds thin filaments to Z disk. 1 Sarcomere is from Z-disc to Z-disc

Question 23

Describe the H zone of a sarcomere

Answer 23

: The part located in center of A band that only has thick filaments

Question 24

Describe the M line of a sarcomere

Answer 24

Down middle of H zone; myomesin holds thick filaments in place at M line.

Question 25

Explain what happens during a sarcomere contraction in:

• A band
• I Band
• H Zone
• Z discs

Answer 25

1. Sarcomere shortens
2. A Band: no change because thick filaments stay the same length
3. I Band shortens.
4. H Zone: shortens as thin filaments move together, may even cover the H zone.
5. Z Discs: move closer together (towards M line)

Question 26

Describe the regenerative capacity of cardiac muscle as well as what happens to it with damage or defects.

Answer 26

A. Very little regenerative capacity beyond early childhood.
B. Defects/damage to cardiac muscle are replaced by fibroblast and CT forming myocardial scars (as in MI)

Question 27

What are the 3 main layers of the heart?

Answer 27

Endocardium, Myocardium, and Epicardium

Question 28

In what area of the heart is the endocardium thickest?

Answer 28

Atria

Question 29

Describe the appearance and function of the following layers of endocardium:
-Endothelium
Myoelastic layer
Subendocardial layer

Answer 29

(a) Endothelium: simple squamous epithelium. Inhibits clot formation.
(b) Myoelastic layer: fibrous CT and smooth muscle fibers.
(c) Subendocardial layer: loose fibroelastic CT. Merges with myocardium and has Purkinje fibers.

Question 30

In what area of the heart is the myocardium thickest?

Answer 30

Ventricles

Question 31

Describe the components and function of the epicardium

Answer 31

3. Epicardium: visceral layer of serous pericardium.
   (a) Loose CT with fat, coronary vessels and nerves.
   (b) Mesothelium: simple Squamous epithelium that produces lubricating fluid

Question 32

Cardiac valves are made comprised from what tissue layer?

What is their function?

Answer 32

Folds of endocardium with endothelial lining.

Prevent back flow

Question 33

Describe the characteristics of Purkinje fibers

Answer 33

A. Located in subendocardial layer of endocardium. Specialized for conduction. Large, pale staining cells wth large round nuclei.
B. Lots of glycogen and mitochondria. Their myofibrils are sparse and displaced to the periphery. No T tubules.