3.6 - Cardiac Muscle

Question 1

What do “myo” and “cardiac” mean literally?

Answer 1

myo = muscle
cardiac = heart

Question 2

Cardiac muscle cells are called:

Answer 2

myocardial cells

Question 3

TRUE OR FALSE:

Myocardial cells are not striated

Answer 3

FALSE

Question 4

Are myocardial cells organized in sarcomeres?

Answer 4

Yes

Question 5

How do cardiac muscle cells differ from skeletal muscle cells?

Answer 5

• much smaller with single nucleus with about 1/3 cell volume occupied by mitochondria
• T tubules are larger and branched
• SR is smaller

Question 6

How are adjacent cells joined?

Answer 6

intercalated discs with desmosomes

Question 7

What do desmosomes do?

Answer 7

transmit force from one cell to another

Question 8

what are auto rhythmic cells and what do they do?

Answer 8

• electrical conducting system of the heart
• initiate the heart beat, spread electrical excitation rapidly throughout the heart
• connected to other cardiac cells via gap junction

Question 9

steps of cardiac muscle contraction:

Answer 9

• action potential enters from adjacent cell
• voltage gated Ca2+ channels open, Ca2+ enters cell
• Ca2+ induces Ca2+ release through RyR channels on SR (90% of Ca2+ needed for contraction)
• local release causes Ca2+ spark
• summed Ca2+ sparks create Ca2+ signal
• Ca2+ ions bind to troponin to initiate contraction

Question 10

What type of contraction is exhibited by cardiac cells? Explain

Answer 10

graded contraction;-

• force generated is proportional to number of active crossbridges
• number of active crossbridges is proportional to cytosolic Ca2+ concentrations

Question 11

What factors influence cardiac muscle contraction force?

Answer 11

i) changes in [Ca2+] :
- regulated by epinephrine and norepinephrine –> bind to and activate b1 Adrenergic receptors
- binding activates cAMP second messenger signalling pathway which leads to:
phosphorylation of voltage gated Ca2+ channels, increasing probability of channel to open to Ca2+
phosphorylation of phospholamban
- leads to increase SR Ca2+ - atpASE activity –> increase SR Ca2+
ii) Sarcomere length:
- tension generated is proportional to length of muscle fibre
- due to degree of overlap between actin and myosin
- stretching myocardial muscle cell may allow more ca2+ to enter through cell membrane ca2+ channels

Question 12

The cardiac muscle is an excitable, this means it can

Answer 12

generate action potentials

Question 13

list the major sequence of events:

Answer 13

phase 4: resting membrane potential (around 90mV)
phase 0: depolarization: AP opens voltage gated Na+ channels, causing a rapid increase in membrane Na+ permeability (close again)
phase 1: initial repolarization: open fast k+ channels allow initial repol
phase 2: plateau: initial depolarization triggered voltage gated ca2+ channels to slowly open, causing increase in Ca2+ permeability and fast K+ channels close
phase 3: rapid repolarization: ca2+ channels close and slow voltage gated k+ channels open (initial depolarization was the trigger), resting stage ion permeability is restored (phase 4)

Question 14

The sustained depolarization is due to what?

Answer 14

slow opening of voltage gated Ca2+ channels

Question 15

What are the results of sustained depolarization?

Answer 15

typical AP in cardiac muscle takes longer (around 200 msec) vs. 1-5 msec in neuron or skeletal muscle

Question 16

Why is it important for cardiac muscles to have longer AP?

Answer 16

allows the heart to relax between contractions and prevents tetanus as it has to last for a while

Question 17

Why don’t cardiac muscle cells undergo summation and tetanus?

Answer 17

because of the longer refractory period, the cell has finished contracting before the next action potential