Lecture 11 - Cardiovascular System - Cardiac Physiology

What is cardiac muscle?

Heart muscle.

Describe the features of cardiac muscle. (4)

- Striated.
- Branched.
- Single nucleus.
- Joined end to end.

Cardiac muscle is joined end to end. What features allow this to happen?

- Intercalated discs that connect muscles with the help of desmosomes.
- Gap junctions that allow electrical current transmission.

What are desmosomes?

A type of junction in which cells are fastened together in strong sheets (like rivets).

What are gap junctions?

Small tunnels that connect cells, facilitating the movement of small molecules and ions between the cells.

The way cardiac muscles interact (joined end to end) allow what?

- Functional syncytium where individual chambers act as a single coordinated unit.
- All or none contraction pattern (no graded responses).

Why is the all or none contraction of the heart important?

To ensure that equal amounts of blood are being sent out and brought in.

Cardiac muscle relys on what type of metabolism?

Aerobic (oxygen-requiring).

25% of the volume of cardiac muscle is what?

Mitochondria.

In order for aerobic metabolism to take place in heart muscle, there must be an abundance of oxygen. How does the muscle keep oxygen?

Using myoglobin.

What is myoglobin?

This is a protein found in muscles and it is very similar to hemoglobin. It is an oxygen-binding compound.

Cardiac muscle has a less extensive sacroplasmic reticulum compared to skeletal muscle, meaning what?

It has a smaller intracellular Ca2+ reserve.

How is calcium used in the sacroplasmic reticulum of cardiac muscle?

It is not used in the same way as skeletal muscle. The sacroplasmic reticulum holds calcium and will only release it when there is calcium coming into the tissue from outside the cell. This will then trigger the sacroplasmic reticulum to release the rest of the Ca2+, if needed.

Does the heart contract with or without the nervous system?

Without. However, rhythm can be altered by the autonomic nervous system (ANS).

What are the two types of electrical events of the heart?

The intrinsic conduction system and the extrinsic innervation of the heart.

What is the intrinsic conduction system?

Heart muscle cells contract, without nerve impulses, in a regular, continuous way. It sets the basic rhythm of the heart. This comes from inside the heart.

What is the extrinsic innervation of the heart?

Heartbeat modified by ANS via cardiac centers in medulla oblongata (ANS). This comes from outside the heart.

The intrinsic conduction system consists of what?

Cardiac pacemaker cells.

What are cardiac pacemaker cells?

Non-contractile cardiac muscle cells.

What is the job of the pacemaker cells?

It produces the electrical impulses that cause your heart to beat.

What are pacemaker potentials?

Unstable resting potential that leads to an action potential. This is due to "funny" Na+ leak channels (channels that stay open at rest).

The action potential of a pacemaker cell is caused by what type of channels?

Voltage-gated Ca 2+ channels.

True or False: During the unstable resting potential of a pacemaker cell, there is some Na+ that is entering the cell, thus making it positive. Once it hit threshold, an abundance of Ca2+ enters the cell, thus causing depolarization and an action potential. An outflux of K+ will then bring down the "positivity" within the cell and return it back to the unstable resting potential

True.

Does calcium cause depolarization in only non-contractile muscle cells?

Yes.

What is the sinoatrial (SA) node?

It is a structure in the right atrium that tells the heart to "beat" or squeeze by sending out an electrical signal.

Action potentials are conducted throughout the heart via?

Gap junctions and conducting system.

The action potentials of the pacemakers are responsible for what?

Heart beats.

The intrinsic conduction sequence consists of four landmarks where the impulse passes through the pacemaker cell. What is the order?

1. Sinoatrail (SA) node.
2. Atrioventricular (AV) node.
3. Atrioventricular (AV) bundle (bundle of His)
4. Purkinje fibres (subendocardial conducting network).

Describe the first landmark, the sinoatrial (SA) node for the impulses of the pacemaker cell.

It is located in the right atrium and is the primary pacemaker for the heart. It causes an impulse that spreads over both atria.

Describe the second landmark, the atrioventricular (AV) node for the impulses of the pacemaker cell.

It is located in the interatrial septum and is the electrical connection to the ventricle. It delays firing slightly to allow atria to finish contracting before ventricles contracts.

Why is it important that atria contract before ventricles?

To ensure that there is blood available in the ventricles for contraction.

Describe the third landmark, the atrioventricular (AV) bundle (bundle of His) for the impulses of the pacemaker cell.

This conducts impulses into ventricles from the AV node. It has a right and left bundle branches that conduct impulses down the interventricular septum.

Describe the forth landmark, the purkinje fibres (subendocardial conducting network) for the impulses of the pacemaker cell.

This penetrates throughout ventricular walls, distributing impulses through the ventricles. This allows ventricles to contract, beginning at the apex, and moving towards atria.

Why is it important that contraction begins at the apex (bottom) of the ventricle?

To ensure that the blood is being pushed up and out. If the contraction had occurred at the top of the ventricle, blood would collect at the bottom and bulge since there is no "exit".

Are there gap junctions between atria and ventricles?

No.

What are pacemakers?

Electrical devices used to correct irregularities in the heart rate.

Defects in the intrinsic conduction system can cause what?

Uncoordinated atrial and ventricular contractions.

What is arrhythmia?

Irregular heart beat/rhythm.

What is fibrillation?

Rapid, irregular contractions. This causes the heart to become useless for pumping blood, thus causing circulation to cease and can lead to brain death.

What is the treatment for fibrillation?

Defibrillation.

What is defibrillation?

The delivery of an electrical shock that may help re-establish an effective rhythm. It disrupts the chaotic twitching and resets the heart to regular, normal depolarizations.

Describe the autonomic regulation of the heart.

The autonomic nervous system (ANS) also plays a role in heart rate as it modifies it and forces contraction through cardiac centres in the medulla oblongata.

In what situation is the ANS vital in controlling heart rate?

Whilst exercising. Your body requires more oxygen to supply muscles, and so the brain signals the medulla oblongata to increase/change the "resting" heart rate.

There are two ways heart rate can be controlled by the ANS, through increasing and decreasing the rate. What are the centres involved in each called?

To increase heart rate, the cardiacceleratory centre is involved. To decrease heart rate, the cardioinhibitory centre is involved.

Describe how the carioacceleratory centre works.

It projects to sympathetic neurons throughout the heart. This increases both the heart rate and contractile force.

Describe how the cardioinhibitory centre works.

It sends impulses to parasympathetic dorsal vagus nucleus in the medulla oblongata where it will then stimulate the vagus nerve to the heart. This slows heart rate but has LITTLE to NO effect on the contraction force.

Where are both the cardioacceleratory and cardioinhibitory centres found?

In the medulla oblongata.

How is the heart rate adjusted using the sympathetic branch?

1. Adrenergic neurons release norepinephrine/noradrenaline.
2. Adrenal medulla releases epinephrine.
3. B1 adrenergic receptors in the heart (G-protein coupled receptors) are activated by the sympathetic system using both epinephrine and norepinephrine.
4. cAMP-mediated signalling pathway affects ion channels, thus causing Na+ and Ca2+ influx, resulting in depolarization.
5. Enhanced "slow" Ca2+ channels allow for an increase in Ca2+, thus increasing the contractile strength.

B1 adrenergic receptors are what kind of receptors?

Excitatory receptors.

How is heart rate adjusted using the parasympathetic branch?

1. Cholinergic neurons release ACh.
2. ACh binds to M2 muscarinic cholinergic receptors in the heart.
3. Triggers K+ channels to open, Na+ channels to close. This hyperpolarizes the cell due to it becoming negative (K+ leaving \= increased efflux) and the Ca2+ slowly getting in causing a slower depolarization of the pacemaker potential (Ca2+ not being able to enter \= decreased influx).

What does cholinergic mean?

This refers to the fact that a receptor uses acetylcholine almost exclusively as their primary neurotransmitter.

What are the five phases of contractile cardiac muscle cells? (action potential)

1. Rapid depolarization.
2. Brief repolarization.
3. Plateau phase.
4. Rapid repolarization.
5. At rest.

Describe a contractile cardiac muscle cell during rapid depolarization.

Fast voltage-gated Na+ channels open.

Describe the brief repolarization of contractile cardiac muscle cells.

Transient K+ channels open.

Describe the plateau phase of contractile cardiac muscle cells.

- Only found in contractile cells.
- Has slow Ca2+ channels that open and decrease K+ permeability.

Describe the rapid repolarization phase of contractile cardiac muscle cells.

Innactivation of the Ca2+ channels and delayed activation of "normal" voltage-gated K+ channels.

What is the difference between noncontractile cardiac muscle cells and contractile cardiac muscle cells in terms of their channels?

Non-contractile uses calcium, whereas contractile uses sodium.

Describe contracticle cardiac muscle cells at rest.

Voltage-gated K+ channels close, and leaky K+ channels open to allow the cycle to continue again.

Contracticle cardiac muscle cells have no pacemaker potential, meaning what?

That they require a trigger from the conducting cells (pacemaker cell) located somewhere else in the body.

In contractile cardiac muscle cells, the action potential is (shortens/prolonged) relative to skeletal muscle.

Prolonged.

Why are action potentials in cardiac muscle cells more prolonged?

This is due to the more significant role of Ca2+ entry from the extracellular fluid.

Slow contraction of the heart is sustained to ensure what?

All of the blood is squeezed out of the heart.

The heart has a long refractory period. Why?

- The heart can be refilled with blood before contracting again.
- Prevents tetanus of the heart because their is no way you can stimulate another action potential UNTIL the contraction is over.

Why would tetanus be bad in the heart?

It cause cause fatigue of the heart muscle and this lead it to halt.

What does ECG stand for?

Electrocardiogram.

Is ECG and EKG the same thing?

Yes.

What is an ECG?

A summation of membrane potential changes (depolarization and repolarization) across all heart cells.

Is an ECG a direct recording of the actual electrical activity of the heart?

No.

A typical ECG has three deflections, what are they?

- P wave
- QRS complex
- T wave

What is a p wave?

It indicated depolarization of the atria.

What is the QRS complex?

Ventricular depolarization.

What is a T wave?

Ventricular repolarization.

How many different phases are there in a ECG?

6.

Describe phase 1 of an ECG.

Atrial depolarization occurs. This is initiated by the SA node and causes the P wave.

Describe phase 2 of an ECG.

With atrial depolarization complete, the impulse is delayed at the AV node.

Describe the phase 3 of an ECG.

Ventricular depolarization begins at the apex. This causes the QRS complex. Additionally, atrial repolarization occurs.

Describe phase 4 of an ECG.

Ventricular depolarization is complete.

Describe phase 5 of an ECG.

Ventricular repolarization begins at the apex, causing the T wave.

Describe phase 6 of an ECG.

Ventricular repolarization is compete.

What are the mechanical events of the heart?

- Cardiac cycle
- Systole
- Diastole

What is the cardiac cycle?

A series of mechanical events, pressure and volume changes, in the heart during ONE heartbeat.

What is systole?

The contractile phase of the cardiac cycle.

What is diastole?

The relaxation phase of the cardiac cycle.

Atrial systole and diastole are followed by?

Ventricular systole and diastole.

When do ventricles fill?

During mid-to-late ventricle diastole.

Describe the filling of blood that occurs during mid-late ventricular diastole.

- AV valves open and semilunar valves close.
- Blood flows (~80%) passively into ventricles.

When do the atria contract?

During the end of the ventricular diastole.

Describe the contraction of atria that occurs during the end of the ventricular diastole.

- Propels final blood volume (remaining 20%) into ventricles.

What is the end diastolic volume?

The volume of blood in each ventricle at the end of ventricular diastole. (after the atrium contracts, but before the ventricle contracts).

What is ventricular systole?

Where the atria relax and the ventricles contract.

What is isovolumetric contraction?

The first phase of ventricular systole. All the valves are closed at this point because the volume of the ventricle is not changing.

What is ventricular ejection?

Where the ventricular pressure exceeds that of the atrial pressure. This forces semilunar valves open to eject blood into the arteries.

What is the end systolic volume?

The volume of blood remaining in each ventricle after systole. This prevents the atria from having more pressure than the ventricles.

When does isovolumetric relaxation occur?

During early diastole.

What is isovolumetric relaxation?

A brief period in early diastole when all valves are closed.

How does isovolumetric relaxation occur?

Results in a drop in ventricular pressure which causes the closing of semilunar valves before the opening of AV valves. This prevents backflow.

Is there a change in volume during isovolumetric relaxation?

No.

What is the dicrotic notch?

A brief rise in blood pressure as blood rebounds off semilunar valves due to the backflow of blood in the aorta and pulmonary trunk.

When atrial pressure exceeds ventricular pressure, what happens?

AV valves open and the cycle begins again.

What are the normal sounds of the heart?

Lub (S1) and Dub (S2)