Cardiac Muscle

Question 1

Where is cardiac muscle located and properties?

Answer 1

Found only in the heart

- Combines properties of both skeletal and smooth muscle

Question 2

Properties of cardiac muscle

Answer 2

Has a striated appearance (like skeletal muscle)

Relatively small and generally contain a single nucleus

Adjacent cells are joined end to end at
structures called intercalated discs

Question 3

What is the arrangement of actin and myosin called? Which one is thick and thin?

Answer 3

regular array of thick (myosin) and thin (actin) filaments - known as myofibrils

Question 4

What is myosin?

Describe it

Answer 4

Forms majority of thick filament
Has globular head which binds to actin
Has an actin and ATP binding site

Question 5

What is actin?

What does it contain?

Answer 5

Forms majority of thin filament

The thin filament is composed mainly of actin, but also of troponin & tropomyosin

Question 6

What does tropomyosin do?

Answer 6

elongated molecule that occupies the grooves between the two actin strands, overlies MYOSIN binding sites on actin

Blocks myosin binding to actin

Question 7

What does troponin do?

Answer 7

Protein that changes shape when Ca2+ binds to it, when it does it changes shape in doing so pushes the tropomyosin EXPOSING myosin binding sites on actin enabling contraction to occur

Question 8

What happens in myocytic contraction?

Answer 8

Action potential causes waves of depolarization across myocardium ; induce Na+ influx

Plateau phase- Ca2+ causes release of Ca2+ from sarcoplamic reticulum by ryanodine 2 receptors - calcium induced calcium release

Ca2+ binds to troponin c
Changes shape, removes tropomyosin from myosin head, actin myosin bridges form

Question 9

What happens after actin myosin bridges form?

Answer 9

ATP is needed to:
Break cross bridges, so myosin can move along/relax
Return Ca2+ to sarcoplasmic reticulum

Question 10

What happens in excitation contraction coupling in contractile cells? What’s the phases?

Answer 10

4, 0, 1, 2, 3, 4

Question 11

What happens in phase 4, what is the range in mV?

Answer 11

Resting membrane potential

-90 - -70

Question 12

What happens in phase 0, what are the ranges?

Answer 12

Rapid depolarization 
Exceeds threshold value (-70)
Fast Na+ channels open ; increased Na+ influx
Potential difference increase to +20
-70 to +20

Question 13

What happens in phase 1, what is the range?

Answer 13

Partial repolarisation
Fast Na+ channels close
Transient K+ channels open - K+ moves out
+20 to 0

Question 14

What happens phase 2, what are the ranges?

Answer 14

Plateau phase
Ca2+ L type channels open at the same rate as transient K+
Ca2+ in, K+ out
0 to 0

Question 15

What happens in phase 3 and ranges?

Answer 15

Repolarisation
Ca2+ L type channels close
K+ rectifying channels open 
More K+ out
0 to -90

Question 16

What ions want to enter or exit the cell?

Answer 16

Na+, enter
K+, exit
Ca2+, enter

Question 17

What is there no of and why?

Answer 17

No hyperpolarisation
As contractions are autorythmic and continuous
Need to be consistent

Question 18

What are the 2 refractory periods?

Answer 18

Absolute

Relative

Question 19

Absolute refractory period?
When is it?
What happens?

Answer 19

Phase 1 & 2

No further action potential

Question 20

Relative refractory period
What phases?
What does it do?

Answer 20

Phase 3

Can generate action potential but needs big stimulus for it

Question 21

Nodal cell depolarization

What happens?

Answer 21

-60 to -40
T type Ca2+ channels open, small calcium influx

-40 (threshold) to +10
L type Ca2+ channels open, bigger calcium influx

+10 to -60
Voltage gated K+ channels open, K+ out

Question 22

Are cycles in nodal cells simpler or more complex and why?

Answer 22

Much simpler

Sinus rhythm generation needs to be shorter than contractile mechanism (impulse generators are faster than contraction)