Electrical activity of the Heart

Question 1

Blood flows through the heart in defined pattern

Answer 1

To achieve this pattern of flow, contraction of the heart must occur
Sequentially: first the atria, then the ventricles
In a specific direction: atria downwards, ventricles upwards

Question 2

how is this all connected

Answer 2

1. inter-connected muscle cells 2. ‘self-excitation’

3. conduction system

Question 3

Cardiac muscle cells

Answer 3

Heart walls are composed of spirally arranged cardiac muscle fibres
Three layers:
• inner layer, endothelium, lines the heart
• middle layer, myocardium, cardiac muscle
• external layer, epicardium, covers the hear

Question 4

Cardiac vs Skeletal muscle cells

similarities

Answer 4

• Striated appearance: same arrangement of thick/thin filaments
• Same contractile mechanism: actin, myosin, crossbridges
• Similar t-tubule system (although cardiac are bigger)
• Similar sarcoplasmic reticulum system
• Action potentials do not summate

Question 5

Cardiac vs Skeletal muscle cells

differences

Answer 5

• Contraction is involuntary
• Smaller cells (100 μm long)
• Cells connected via intercalated disks
• Entire heart muscle contracts in a coordinated fashion: “syncytia”
• SR provides 80% of calcium for muscle contraction, remainder from ECF
• The cardiac muscle AP lasts 200-300 msecs, compared with 2-3 msecs in skeletal muscle
• AP propagation slower: Cardiac 0.05- 0.5 m/sec vs Skeletal 3-5 m/sec
• AP refractory period much longer: 200-300msecs

Question 6

WHAT CAN SKELETAL MUSCLE DO THAT CARDIAC MUSCLE CAN’T DO

Answer 6

• The cardiac muscle AP lasts 200-300 msecs, compared with 2-3 msecs in skeletal muscle
• AP refractory period much longer: 200-300msecs

HEARTS CAN’T GO INTO TETNIS

Question 7

Cardiac muscle cells 2

Answer 7

Cardiac muscle fibres are interconnected by intercalated discs to form ‘functional’ syncytia

Question 8

Cardiac muscle cells 3

Answer 8

• Intercalated discs contain desmesomes and gap junctions
• Desmesomes holds cells together
• Gap junctions allow action potentials to spread to adjacent cells
• Cardiac muscle cells all act together as one = functional syncytia

Question 9

Electrical activity in the heart

Answer 9

The heart is ‘self-excitable’, initiating its own rhythmic
contractions. The heart contains:
Contractile cells, 99% of the cardiac muscle cells, who do the mechanical work
Autorhythmic cells initiate the action potentials which spread across the heart

• THEIR APS DIFFER

Question 10

Electrical activity in the heart

- 2

Answer 10

Cardiac autorhythmic cells are pacemakers: Their membrane potential slowly depolarizes
between action potentials, drifting to threshold
This cyclically initiates APs that spread throughout the heart to trigger rhythmic contractions

Question 11

AP IN autorhythmic cells

Answer 11

1. Slow depolarisation
   Na+ permeability increases, K + permeability decreases
   = Na+ in
2. Slow depolarisation (cont) Ca+ permeability increases, Na+ permeability decreases
   = Ca+ in
3. Fast depolarisation transient Ca+ channels close, long-lasting Ca+ channels
   open = more Ca+ in
4. Repolarisation
   K+ permeability increases, Ca+ permeability decreases
   = K+ out

Question 12

Action potentials in contractile cells

Answer 12

The action potential of cardiac contractile cells shows a plateau phase
Due primarily to activation of slow L-type Ca2+ channels
Ensures adequate ejection time

Question 13

Action potentials = contraction

Answer 13

Plateau phase + long refractory period prevents summation or tetanus in cardiac muscle cells