Electrical Activity of the Heart

Question 1

What can act as a large calcium store?

Answer 1

Sarcoplasmic reticulum

Question 2

What does calcium bind to?

Answer 2

Troponin

Question 3

Explain why cardiac muscle can act as a functional syncytium?

Answer 3

Cardiac muscle can function like one massive cell despite being composed of many different cells

Question 4

State 3 ways in which cells are linked together.

Answer 4

• Desmosomes
• Intercalated discs
• Gap junctions

Question 5

Desmosomes provide what type of connection?

Answer 5

Physical connection

Question 6

Gap junctions provide what type of connection?

Answer 6

Electrical connection

Question 7

_____ muscle can exhibit tetanus while there is no tetanus in _____ muscle

Answer 7

Skeletal

Cardiac

Question 8

Compare the length of the action potential of cardiac muscle with that of skeletal muscle.

Answer 8

Cardiac - 250 ms (longer because more calcium is released)

Skeletal - 2 ms (shorter)

Question 9

State why cardiac muscle cannot exhibit tetanic contraction.

Answer 9

Cardiac muscle has a long refractory period (ensures heart fully relaxes and prevents tetanic contraction)

Question 10

Why do you not want tetanic muscle contraction in your heart?

Answer 10

It would mean your heart would beat once and there would not be another one causing ASYSTOLE (cardiac arrest).

Question 11

What forms the intercalated discs?

Answer 11

Desmosomes and gap junctions

Question 12

Calcium saturation troponin

Answer 12

LATER

Question 13

What do cells which have unstable resting membrane potentials act as?

Answer 13

Pacemaker cells (these produce the electrical impulses that cause your heart to beat)

Question 14

Graph of non - pacemaker vs pacemaker potential:

Answer 14

INSERT HERE

Question 15

What is responsible for the resting membrane potential of non-pacemaker action potentials?

Answer 15

The permeability of potassium ions (PK+)

Question 16

What is responsible for the initial depolarisation in non-pacemaker action potentials?

Answer 16

Increase in permeability to sodium ions (PNa+)

Question 17

What is responsible for the plateau in non-pacemaker action potentials?

Answer 17

Increase in the permeability to calcium ions (PCa2+, L-type)
And a decrease in the permeability to potassium ions (PK+)

Question 18

What is responsible for the repolarisation in non-pacemaker action potentials?

Answer 18

Decrease in the permeability to calcium ions (PCa2+)

And an increase in the permeability to potassium ions (PK+)

Question 19

What is responsible for the action potential in pacemaker action potentials?

Answer 19

Increase in the permeability to calcium ions (PCa2+, L-type)

Question 20

What is the pacemaker potential (pre-potential) due to?

Answer 20

• Gradual decrease in the permeability to potassium
• Early increase in the permeability to sodium
• Late increase in the permeability to calcium (T-type)

Question 21

State 7 modulators of electrical activity.

Answer 21

• Sympathetic and parasympathetic systems
• Temperature
• Drugs
• Hyperkalemia
• Hypokalemia
• Hypercalcemia
• Hypocalcemia

Question 22

Drugs: What do calcium channel blockers do to modulate electrical activity?

Answer 22

Decrease force of contraction

Question 23

Drugs: What do cardiac glycosides do to modulate electrical activity?

Answer 23

Increase force of contraction

Question 24

How does temperature modulate electrical activity?

Answer 24

Increases the heart rate by 10 beats/min/degree

Question 25

How does hyperkalcemia modulate electrical activity?

Answer 25

Fibrillation and heart block

Question 26

How does hypokalaemia modulate electrical activity?

Answer 26

Fibrillation and heart block (anomalous)

Question 27

How does hypercalcemia modulate electrical activity?

Answer 27

Increased heart rate AND force of contraction

Question 28

How does hypocalcemia modulate electrical activity?

Answer 28

Decreased heart rate AND force of contraction

Question 29

List the sequence of events occurring during excitation-contraction coupling in cardiac muscle. (8)

Answer 29

- An action potential enters from adjacent cell - Voltage-gated calcium channels open and calcium enter the cell - Calcium induces calcium release form the sarcoplasmic reticulum through ryanodine receptor channels, which causes calcium to spark - Summed calcium sparks create a calcium signal - Calcium ions then bind to troponin, which cause tropomyosin to detach from the actin filament, exposing the actin active sites - Contraction occurs, myosin cross bridges alternatively attach to actin and detach, pulling the actin filaments towards the centre of the sarcomere, release of ATP hydrolysis powers the cycling process - Removal of calcium by active transport into the sarcoplasmic reticulum after the action potential ends - Tropomyosin blockage restored on active sites, contraction ends and the muscle fibre relaxes