Module 5: Electrical Activity of the Heart (3)

The heart is an electro-mechanical organ. What does this mean? Because the heart is an electro-mechanical organ it contains two different types of cardiac muscle cells. What are they?

Has electrical activity (generates and conducts APs in a manner similar to nerves) and its also a large muscle that mechanically contracts.

Muscle cells:

• those that are primarily contractile and do mechanical work

• those that are electrical to generate and propagate APs to contractile cells.

What are cardiac autorhythmic cells? What makes these cells unique?

specialized cardiac muscle cells that generate APs.

• instead of a flat, constant or resting membrane potential, these cells contain ion channels that cause the membrane potential to slowly depolarize until threshold is hit and action potentials are fired

• these cells may contain l_f channels that when activated let both sodium and potassium into the cell

  • activation may be due to hyperpolarization-activated cyclic nucleotide-gated channel family or a type of calcium channel

• unique too bc the upstroke of the action potential is due to another calcium channel (l-type) instead of sodium channels as seen in neurons and cardiac contractile cells.

Autorhythmic cells are localized in very specific regions of the heart. When discussing the cardiac conduction system, where are these cells located (think 4 main regions).

1. Sinoatrial Node

   • small area located in right atrial wall near opening of superior venae cavae

2. Atrioventricular Node

   • small area located in right atrium right where right atria and right ventricle come together

   • aka in the interatrial septum (centre of the heart)

3. Bundle of His

   • consists of specialized cells arising from AV node.

   • Divides into two bundle branches that travel down each side of the septum to bottom of the heart where they curve around and travel back towards the atria.

4. Purkinje Fibres

   • small fibres that branch off bundles of His and spread along inner surface of ventricles.

Several types of autorhythmic cells work together to create a unified electrical signal. What type of autorhythmic cells are considered the pacemakers of the heart? Why?

Autorhythmic cells in the sinoatrial (SA) node

• fastest rate of depolarization

• control heart rate and keep it around 70-80 bpm

• once an AP is generated in these cells, it conducts through the rest of cardiac conduction system, overriding pacemaker activity of other autorhythmic cells

• as long as the sinoatrial node is functioning fine it controls the heart rate → without pacemaker activity the heart doesn’t beat.

For efficient cardiac contraction, three criteria must be satisfied. What are they?

1. Atrial excitation and contraction should be complete before onset of ventricular contractions.

2. Excitation of cardiac muscle fibres needs to be coordianted.

3. The pairs of atria and the pairs of ventricles must be functionally coordinated.;

once the sinoatrial node fires an action potential, this wave of excitation travels throughout the atria by 2 main mechanisms. What are they?

1. gap junctions between atrial cells

2. pathways → interatrial and internodal pathways

   • act like nervous tissue (they move excitation waves faster than gap junctions alone)

** Interatrial extends from right atria to left atria and ensures wave of excitation spreads across both atria at the same time; means they’ll contract at the same time.

**internodal connects the sinoatrial node to the atrioventricular node

The muscle cells of the atria are separated from those of the ventricle by a dense region of connective tissues lying between them. How do electrical signals move from the atria to the ventricles?

AV node and bundle of His fibres are the only ways an electrical signal can move from atria to ventricle.

• even tho signal moves from SA node to the AV node very quickly, the rate of conduction slows down through the AV node aka the AV nodal delay.

Ventricles are much larger masses of muscle than atrias and they are hollow chambers. How do they spread excitation.

Due to their size and shape if they only relied on gap junctions, the top part of the heart would contract before the wave hit the bottom SO bundles of His and purkinje fibres solve and address this.

After AV nodal delay, the wave of excitation spreads down both the right and left bundles of His and purkinje fibres.

• the p fibres don’t terminate on ALL the ventricle muscle cells so gap junctions spread excitation to those cells

This just ensures both ventricles contract at the same time.

The cardiac action potential is different in shape than that of a nerve cell and SA node. What causes these differences?

different kinds of voltage-gated ion channels are found in ventricular muscle cells. The resting membrane potential for a cardiac myocyte is about -80mV.

• these cells have no pacemaker currents so their resting membrane potential is steady until the cell is excited.

Describe a Cardiac Myocyte’s action potential.

when it reaches threshold, voltage gated sodium channels open and the membrane potential rapidly depolarizes towards +50mV

this rapid depolarization activates other ion channels like the transient outward K+ channel that rapidly moves K+ out of the cell to counter the increase of sodium.

It also activates L-Type Ca2+ channels. These currents create a balance of the membrane potential where it’s neither depolarizing or repolarizing. IT’s steady, at a plateau (unlike other APs.)

Eventually transient outward K+ channels and L-type calcium currents inactivate and this lets the cells hyperpolarize and reach their resting membrane potential.

Cardiac myocytes undergo a well-defined pattern of excitation-contraction coupling, the process where an action potential triggers the myocyte to contract. Describe how this occurs.

• recall that cardiac muscles also have well-defined T-tubule systems

Action potential in cardiac contractile cells.

• during the plateau phase, L-type calcium channels (in the T-tubules) open.

Release of calcium

• opening of L-type channels allows calcium to enter the cell

Interaction with contractile apparatus (tg)

• calcium can directly interact with it.

Large release of calcium from SR (tg)

• release of calcium can interact with ryanodine receptors on SR which activate them and triggers additional release of calcium from internal stores (Calcium induced calcium release CICR)

Contraction

• influx of calcium initiates cardiac muscle contraction

• when calcium is removed from cytosol moving across plasma membrane or is pumped back into SR, the contraction ends.

Cardiac myocytes have long action potentials due to a prominent plateau phase. How does this action potential duration relate to the length of muscle contraction.

The length of the cardiac action potential prevents twitch summation

• during the plateau phase, the depolarized membrane potential keeps voltage-gated sodium channels in inactivated state, so even if another wave of excitation came, they wouldn’t open

• refractory period os long enough that most of the muscle contraction has been completed.

How does an ECG work?

12 leads- 3 limbs, 6 around heart, 3 mathematically derived

• measures electrical activity of the heart but indirectly as it is measure changes of electrical potentials that originate in the heart but must be transmitted throughout the body to surface where it can be measured.

• measures a summation of all electrical activity at any given time, which allows us to observe spread of electrical activity through the heart.

• heart generates electrical field transmitted via body fluids sensed at surface of the skin

In an ECG reocrding, ECG tracings revolved around isoelectric or 0 voltage lines. Generally depolarization is seen as upward deflections and repolarization is seen as downwards. Describe what we see on an ECG during a heart beat.

1. Initiation of the heart beat = firing of the sinoartrial node. However, it’s electrical activity is too small to be detected on surface. However, SA node triggers the atria to undergo excitation and THIS depolarization is seen as the p wave.

2. While the atria are depolarized, there no net movment of charge meaning the ECG remains flat again until the AV node delay has occured.

3. After this delay, the wave of excitation travels down the bundles of His and Purkinje fibre to depolarize the ventricles. This is seen as the QRS complex.

4. While the ventricles are depolarized, there’s not net current. Thus the ECG is flat until repolarized. this repolarization is seen as the T-wave.

5. Once they repolarize there’s no net current until SA node fires and process repeats.

Portions of the ECG are often grouped together as abnormalities tend to present themselves as changes in these segments: PR, ST, TP, QT. What do these mean?

PR= AV nodal delay

ST = when ventricles are contracting and emptying

TP = when ventricles are relaxing and filling

QT - electrical depolarization and repolarization of the ventricles

Several heart rate and rhythm abnormalities can occur. What is Tachycardia?

heart rate exceeds normal resting rate (ex. greater than 100bpm)

• this can decrease cardiac output due to reduce ventricular filling.

Causes: exercise, caffeine, electrical abnormality

Several heart rate and rhythm abnormalities can occur. What is extrasystole?

common; where the heart beats are initiated by the Purkinje fibres not the SA node

• felt as palpitations in chest

• can be sign of reduced oxygenation to heart but can also be found in healthy hearts.

here the ventricles contract before the atrias and aren’t optimally filled with blood, ultimately reducing cardiac output.

Several heart rate and rhythm abnormalities can occur. What is Ventricular Fibrillation?

heart is quivering rather than pumping due to abnormal electrical activity in ventricles

• results in cardiac arrest with loss of consciousness and no pulse

• irregular unformed QRS complex with no clear p waves

Several heart rate and rhythm abnormalities can occur. What is Complete heart block?

3rd degree AV block; condition where impulse generated at SA node doesn’t travel to the ventricles.

since the impulse is blocked the pacemaker cells in the AV node independently activate the ventricles which allows for 2 independent rhythms to be seen on an ECG

patients often experience abnormally low heart rates and blood pressure.