Electrical activity of the heart

Question 1

What are electrical properties of the heart?

Answer 1

• Excitation contraction coupling
• Ionic basis of the non-pacemaker potential
• Modulators of electrical activity
• The special conducting system
• The electrocardiogram

Question 2

What membrane surrounds muscle cells?

Answer 2

Sarcolemma membrane

Question 3

What are intercalated discs in cardiac cells?

Answer 3

Junctions that connect cardiomyocytes together, some of which transmit electrical impulses between cells.

Question 4

What is a sarcomere?

Answer 4

The basic unit of contractile muscle which contains myosin and actin, the two proteins that slide past one another to cause a muscle contraction.

Question 5

What is the action of desmosomes in cardiac muscle?

Answer 5

They stop separation during contraction by binding filaments, joining the cells together.

Question 6

What is the action of gap junctions in cardiac muscle?

Answer 6

allow action potentials to spread between cardiac cells by permitting the passage of ions between cells, producing depolarisation of the heart muscle.

Question 7

How long is depolarisation in skeletal muscle compared to cardiac muscle?

Answer 7

Skeletal cells = 2ms

Cardiac cells = 250ms

Question 8

Why is it important for cardiac cells to have a longer refractory period compared to skeletal muscle cells?

Answer 8

It is important because if the refractory periods were short, the heart could enter tetanic contraction/summation which is when muscle is stimulated so rapidly that it does not have a chance to relax at all between stimuli
- Fundamental in order to do not have a simultaneous contraction of atria and ventricles

Question 9

What are cardiomyocytes?

Answer 9

They are the muscle cells (myocytes) that make up the cardiac muscle (heart muscle).

Question 10

What do intercalated discs look like under microscopy?

Answer 10

Intercalated discs appear as thin lines dividing adjacent cardiac muscle cells and running perpendicular to the direction of muscle fibers.

Question 11

What is tetanus in muscle cells?

Answer 11

The prolonged contraction of a muscle caused by rapidly repeated stimuli.

• Skeletal muscle can exhibit tetanus.
• NO tetanus in cardiac muscle.

Question 12

How is contraction regulated in cardiac cells?

Answer 12

• Ca2+ entry from outside cell can regulate contraction.
• Ca2+ release does not saturate the troponin, so regulation of Ca2+ release can be used to vary the strength of contraction.

Question 13

What cells create rhythmic impulses in the heart, setting the pace for blood pumping?

Answer 13

Pacemaker cells

- They directly control the heart rate. They make up the cardiac pacemaker.

Question 14

Where are the pacemaker cells normally found in the heart?

Answer 14

The sinoatrial node
- The resultant rhythm is a sinus rhythm.

-Sometimes an ectopic pacemaker sets the pace, if the SA node is damaged

Question 15

What is a cardiac arrhythmia?

Answer 15

Heart rhythm problems (heart arrhythmias) occur when the electrical impulses that coordinate your heartbeats don’t work properly, causing your heart to beat too fast, too slow or irregularly.
- E.g. Atrial fibrillation

Question 16

What is the primary function of the sarcoplasmic reticulum?

Answer 16

To store Ca2+ ions.

• The sarcoplasmic reticulum of cardiac-muscle cells is not as well-developed as that of skeletal-muscle cells. Cardiac-muscle contraction is actin-regulated, meaning that the calcium ions come both from the sarcoplasmic reticulum (as in skeletal muscle) and from outside the cell (as in smooth muscle).

Question 17

Features of pacemaker action potentials

Answer 17

• Action potential: increase in PCa2+ (L-type).
• Pacemaker potential(=pre-potential): gradual decrease in PK+, early increase in PNa+, late increase in PCa2+ (T-type)
• Pacemaker explains autorhythmicity

Question 18

How does an action potential in cardiac cells occur?

Answer 18

Action potentials occur due to an increase in PCa2+ (L-type).

Question 19

What is autorhythmicity and what causes it?

Answer 19

It is when the cells are able to generate the action potential at a certain rate without any external stimulus due to which the heart beats continuously and rhythmically.
- The inherent leakiness of SA node fibres to Na+ ions is what causes their self-excitation.

Question 20

What are T-type calcium channels?

Answer 20

T-type calcium channels are low-voltage activated calcium channels that open during membrane depolarisation.
- T-type let in tiny amounts of calcium, which is why there is a late increase in PCa2+.

Question 21

Features of non-pacemaker action potentials

Answer 21

• Resting membrane potential: high resting PK+.
• Initial depolarisation: increase in PNa+.
• Plateau: increase in PCa2+ (L-type) and decrease in PK+.
• Repolarisation: decrease in PCa2+ and increase in PK+.

Question 22

Modulators of electrical activity

Answer 22

• Sympathetic & parasympathetic systems
• Drugs
• Temperature
• Hyperkalemia: fibrillation & heart block
• Hypokalemia: fibrillation & heart block (anomalous)
• Hypercalcemia: increased HR & force of contraction.
• Hypocalcemia: decreased HR & force of contraction.

Question 23

How can drugs affect electrical activity?

Answer 23

• Ca2+ channel blocker: decrease force of contraction.

- Cardiac glycocides: increase force of contraction.

Question 24

How can temperature affect electrical activity?

Answer 24

• Increase in temperature increases heart rate.

- increases: 10 beats/min/degrees celcius

Question 25

What serious condition can Hyperkalemia (high plasma K+) cause?

Answer 25

Ventricular fibrillation

Question 26

What is the atrioventricular node?

Answer 26

It is part of the electrical conduction system of the heart that coordinates the top of the heart. It electrically connects the atria and ventricles

Question 27

How does the atrioventricular node act as a delay box?

Answer 27

The atrioventricular node delays impulses by approximately 0.09s. This delay in the cardiac pulse is extremely important: It ensures that the atria have ejected their blood into the ventricles first before the ventricles contract.

-This also protects the ventricles from excessively fast rate response to atrial arrhythmia.

Question 28

Bundle of his and Purkinje fibres

Answer 28

The AV node tapers down into the bundle of His, which passes into the ventricular septum and divides into two bundle branches, the left and right bundles (Purkinje fibres).

Question 29

How are electrical waves recorded on an electrocardiogram?

Answer 29

• An action potential in a single myocyte evokes a very small extracellular (cf transmembrane) electrical potential
• However, lots of small extracellular electrical potentials evoked by many cells depolarising and repolarising at the same time can summate to create large extracellular electrical waves
• These can be recorded at the periphery as the electrocardiogram

Question 30

What is plateau in action potentials?

Answer 30

Phase 2 is the plateau phase of the cardiac action potential. Membrane permeability to calcium increases during this phase, maintaining depolarisation and prolonging the action potential.
- As calcium channels inactivate towards the end of the plateau phase, an inward potassium current produces re-polarisation in phase 3.