Lecture 32: Cardiac Muscle

Question 1

What are the differences between skeletal muscle and cardiac muscle?

Answer 1

Cell length much shorter

Cell shape branced instead of cylindrical

Initiation of contraction is myogenic (involuntary) compared to neurogenic (voluntary)

Skeletal muscle cells are electrically isolated whereas cardiac muscle cells are electrically coupled

Less extensive SR

Question 2

What are intercalated discs?

Answer 2

Connection between cardiac muscle cells

Contain:
Desmosomes -> structural role preventing tears during contraction
Gap -> allows action potentials to be carried from one cell to the next (co-ordinated contraction of myocytes)

Question 3

What is difference between the action potentials in skeletal muscle and cardiac muscle?

Answer 3

Action potentials are much longer (1-2ms vs >100ms) in cardiac muscle due to plateau phase caused by L type Ca2+ channels

Question 4

What is the significance of the longer action potentials of muscle cells?

Answer 4

Prevent tetanic contraction of cardiac muscle as the contraction is spaced apart

Question 5

What are the 3 main stages of an action potential in a cardiac muscle cell?

Answer 5

1. Rapid depolarisation (fast Na+ voltage gated channels)
2. Plateau phase due to slow voltage gated Ca2+ channels
3. Repolarisation due to closing of Ca2+ channels and opening of K+ channels

Question 6

What are the 4 components of membrane proteins found in the T-tubules and SR?

Answer 6

T-tubules:
L-type voltage-gated calcium channel
Sodium/Calcium exchanger
Sodium/Potassium ATPase

SR:
Ryanodine receptor (calcium channel of SR)

Question 7

How does excitation-contraction coupling occur in cardiac muscle?

Answer 7

Depolarisation opens voltage-gated Na+ channels in sarcolemma

Depolarisation opens slow L-type Ca2+ channels in the sarcolemma

(this extra step is due to the t-tubules of cardiac muscles not being able to directly cause the opening of calcium channels in the SR)

Calcium leaves through Ca2+/Na+ exchanger

As Ca2+ enters, Ca2+ sensitive channels in the SR (RyRa) open causing Ca2+ to leave SR

Ca2+ binds to troponin causing muscle contraction to begin

Question 8

How does muscle relaxation occur in cardiac muscle?

Answer 8

For relaxation to occur Ca2+ has to leave the cytosol of the muscle fibre

Occurs via:

• SR Ca2+-ATPase
• sarcolemmal Na/Ca exchanger
• sarcolemmal Ca-ATPase
• mitochondrial Ca uniport

Question 9

How does regulation of cardiac output occur?

Answer 9

regulation of heart rate and stroke volume

Question 10

How is the rate of contraction determined?

Answer 10

The rate of contraction ie heart rate is determined by the pacemaker - Sinoatrial node (a patch of specialised cells) in the right atrium

This initial action potential is passed down to the ventricles via the atrioventricular node

Question 11

What are the steps to action potential formation in the pacemaker cells (SA+AV node)?

Answer 11

1. Pacemaker potential -> slow depolarisation
2. Depolarisation -> Ca2+ channels open causing rapid influx of Ca2+ ions
3. Repolarisation -> Ca2+ channels close and K+ channels open

Question 12

How is the heart rate regulated?

Answer 12

Decrease -> vagus (parasympathetic) nerve releases ACh

Increase -> sympathetic cardiac nerves (also increase force of contraction)

Question 13

How does the vagal nerves decrease heart rate?

Answer 13

Release ACh -> hyperpolarise the resting membrane potential causing heart rate to decrease

Question 14

How do the sympathetic nerves release noradrenaline?

Answer 14

Increase rate of spontaneous depolarisation