Myocardial cells

Question 1

Components of sarcomere

Answer 1

region between 2 Z line
* Thick myosin filaments: A band
* Thin actin finalments: I band
o Z line links 2 thin actin filaments

Question 2

Subcellular organelles

Answer 2

sarcolemma (¢ membrane)
T tubule: extensions of the sarcolemma => Communicate freely with extra¢ space
* Numerous myofibrils connected in series => contractile elements = 50% of myocyte mass
o Composed of longitudinally repeating sarcomeres

• Centrally located nuclei
• Numerous mitochondria
• Complex sarcoplasmic reticulum
• o Communicate freely with extra¢ space

Question 3

Various lines in sarcomere

Answer 3

Zline: contractile unit, line btwn 2 sarcomere

A band: thick + thin filaments

H band: thick filaments

I band: thin filaments

M line: center of sarcomere

Question 4

Interskeletal support of cardiomyocytes

Answer 4

Cytoskeleton:
* Include proteins that connect
o Contractile system + sarcolemma
o Sarcomeres to each other
o ¢ to extra¢ structures
* Provide spatial stability + transmit lateral forces from Z line => sarcolemma + extra¢ matrix

Question 5

Cytoskeletal proteins

Answer 5

Proteins
* Desmin filaments:
Major intermediate filament
Z bands => sarcolemma via costameres
Z line to each other => actin filaments
Stabilizes sarcomere during contraction

• Vinculin: Z line => talin + integrin complex of costamere
• Integrin: transmembrane protein w ¢ surface adhesion R
  Internal talin binding
  External binding to collagen, laminin, fibronectin
• Talin: w integrin in ¢ to matrix jcts
  Links integrin to vinculin
• Alpha-actinin molecules: Z bands
  Links tails of actin fibers at Z line
• Costameres: subsarcolemmal structures => physical stabilizing function + external communication
  o Formed by spectrins
  o Link sarcomere => extra¢ matrix
  o Cystoskeletal actin + intermediate filaments attach costamere => sarcomere

Question 6

Dystroglycan complex

Answer 6

• Transmit lateral force from sarcomere => extra¢ laminin
• Dystrophin = core component
  o Link cytosolic actin => dystroglycan in sarcolemma
  o Link other components of costamere: vinculin, spectrin

Question 7

Cardiomyocyte junctions

Answer 7

Intercalated disk
* Longitudinally to provide lateral contact
* Function:
o Anchor actin filaments
o Bind ¢ to each other
o Communicate btw ¢

Question 8

Components of intercalated discs

Answer 8

Gap/nexux jct
Fascia adherens
Desmosomes

Question 9

Gap/nexus jcts

Answer 9

longitudinal part = approx. 5% of space btwn 2 ¢
* Rapid electrical communication btwn ¢ => unified syncytium
o Mainly longitudinal in ventricles, can also occur laterally mainly in atrial ¢
* Microchannels allowing physical communication of the cytosol
o Transport small molecules/ions = low resistance electrical pathways

Question 10

Gap jct proteins

Answer 10

6 connexins = connexon = 1 hemichannel
* Sensitive to abnormal incr in [Ca2+] (ie ischemia) => close and seal off the damaged ¢

Gap junction channel = 2 connexon, 1 from each ¢

Close association to Na+ channels

Connexin 43: major isoform in ventricles
* Short ½ life: 1 to 5h => active participation in ¢ responses

Question 11

Fascia adherens: proteins

Answer 11

• Terminal anchor of actin filaments
• Associated proteins:
  o N-cadherin: important structural protein
   Links 2 adjacent myocytes
  o Catenin
  o Vinculin: links A-catenin to N-cadherin
  o A-actinin

Question 12

Desmosome

Answer 12

transverse part
* Integrators of mechanical integrity
* Insertion sites for desmin-containing intermediate filaments
o Trans¢ cytoskeletal network => longitudinal force transmission
* Attached proteins: desmoplakins and desmosomal cadherins