11.23 B

Question 1

Why does an intercalated disc appear so dark on a cardiac EM?

Answer 1

because it is very dense due to the need to stand up to a great deal of mechanical force as it conveys that force from one myocyte to another

Question 2

Sarcomeres in different cardiac myocytes connect at what general structure?

Answer 2

the zonula adherens

Question 3

The intercalated disk functionally takes the place of what sarcomere structure?

Answer 3

the Z disk

Question 4

At an intercalated disk, what are some of the conections between the two cells that are present?

Answer 4

zonula adherens, macula adherens, gap junctions

Question 5

How is mechanical force conveyed across an intercalated disk?

Answer 5

• via a zonula adherens junctional complex
• the actin of the terminal sarcomere in both cells connects to integrin which connects to the ECM
• with both cells doing this, there is a physicla connection between the two terminal sarcomeres

Question 6

Describe how the flow of ions is regulated at gap junctions?

Answer 6

it isn’t; ions are free to flow in either direction and at a rate limited only by the size of the pore

Question 7

Describe the structure of a gap junction.

Answer 7

Six connexion proteins linked together in the membrane of one cell connecting with a complex of six connexion proetins in another membrane

Question 8

What is the A band of a sarcomere?

Answer 8

the width of the sarcomere that includes the myosin tails and where they overlap the actin filaments

Question 9

What is the M line of a sarcomere?

Answer 9

the width of the sarcomere that includes only the myosin proteins in the space not overlapped by actin

Question 10

What is the I band of a sarcomere?

Answer 10

the area where there is only actin that is not overlapped with myosin

Question 11

What is tropomyosin?

Answer 11

a molecule that interferes with the interaction between actin and myosin in a sarcomere

Question 12

What is troponin C?

Answer 12

binds to calcium and then creates a conformational change in troponin I to free up the myosin-actin interaction in a sarcomere

Question 13

Generally speaking what is titan?

Answer 13

a spring like molecule that expands under mechanical force or the entrance of a water molecule and which serves as a major contributor to the elastic component of muscle

Question 14

What molecule mediates the difficulty relaxing cardiac myocytes that accompanies hypertrophy?

Answer 14

the amount of titan, which spans from the end of a myosin filament to the Z line, increases with hypertrophy and thus more force is required for relaxation

Question 15

What is a Diad?

Answer 15

a structure formed by a T-tubule contacting a single sarcoplasmic reticulum in cardiac muscle

Question 16

What is a T-tubule?

Answer 16

a structure that conducts electrical potential and triggers the release of calcium from the sarcoplasmic reticulum

Question 17

Describe the sequence of events that leads to muscle contraction.

Answer 17

• action potential travels down the T-tubule to the SR
• voltage-gated calcium channels open
• the influx of calcium opens Ryanodine channels
• even more calcium enters and binds troponin
• the actin-myosin interaction is disinhibited and the muscle contracts

Question 18

What are purkinje fibers?

Answer 18

modified cardiac myocytes with less actin and myosin that help conduct the AP from the bundle branches to the ventricular myocytes

Question 19

Histologically, how do purkinje fibers compare to cardiac myocytes?

Answer 19

purkinje fibers are larger than working cardiac muscle cells and they are separated from these cells by a heavy connective tissue layer as well as they have less actin and myosin