Lecture 19 : Anatomy & Electrical Properties of the Heart Flashcards
Describe the flow of blood through the CVS from when blood first enters the heart:
Heart:
1. Right atrium
2. Right AV (tricuspid) valve
3. Right ventricle
4. Pulmonary valve
Pulmonary circuit:
1. Pulmonary trunk
2. Pulmonary arteries
3. Capillaries of lungs
4. Pulmonary veins
Heart:
1. Left atrium
2. Left AV (bicuspid) valve
3. Left ventricle
4. Aortic valve
Systemic circuit:
1. Aorta
2. Arteries
3. Arterioles
4. Capillaries
5. Venules
6. Veins
7. Venae cavae
Name the 2 main cardiac cells:
- Contractile myocardial cells
- Nodal tissue (SA & AV node)
What are contractile myocardial cells?
- Arranged in layers, associated at the intercalated disc, and form the bulk of the atria and ventricles
- All myocardial cells contract with every beat due to gap junctions
- Filaments of actin and myosin are arranged with a striated appearance
Describe the role of gap junctions within the contractile myocardial cells:
- Low electrical resistance
- Present in the intercalated disc
- Allows the passage of ions and small molecules of up to 1KDa
What is nodal tissue?
- 1% of the cardiac cells
- Contains small round cells with no, or little, contractile protein
- Specialised for the generation and conduction of action potentials in the atria
- Gap junctions are present
Where are action potentials generated in the heart?
Sino-atrial (SA) node - pace maker of the heart
How do action potentials spread around the heart?
Spread from cell to cell by gap junctions determined by the location of intercalated discs
What structure do action potentials have to pass through to get to the ventricles?
Atrioventricular ring via AV node
How fast are APs generated at the SA node?
100-110 min-1
How fast is an AP conducted through atrial muscle?
~0.5 ms-1
How fast is an AP conducted through the AV node and what is the role of this?
Slowly ~0.05 ms-1
* Provides a delay for full depolarisation and contraction of the atria before the ventricles are depolarised
How fast are APs conducted through the bundle of His, bundle branches, and Purkinje system?
Rapidly ~5 ms-1
How fast are the APs spread at the ventricular myocardium?
~0.5 ms-1
What is the purpose of the speed of the APs from the bundle of His to the ventricular myocardium?
Allows synchronous depolarisation and contraction of all regions of the ventricles
Describe the basis of resting and action potentials:
The resting membrane potential (RMP) of nodal and myocardial cells depends upon a high resting permeability to K+
(PK )
- K+ moves out of cell making inside more negative
What is the PK and RMP of nodal tissue relative to myocardial cells?
- PK is lower in nodal tissue than myocardial cells and the RMP is less negative
- The RMP of the SA nodal cells is unstable
What occurs to PK within pacemaker cells:
PK gradually reduces, and this combined with an increasing permeability to Na+ and Ca2+, causes the gradual decrease in RMP
What is the gradual decrease in RMP in pacemaker cells called?
This slow depolarisation is called the pacemaker potential
What happens when threshold voltage is reached in a pacemaker cell?
Other Ca2+ channels open, and a relatively ‘slow’ AP occurs
- A similar AP occurs in AV nodal cells
- Other cells may spontaneously depolarise -> ectopic pacemakers/arrhythmias
Describe the action potentials within ventricular cells:
- PK is higher and hence the RMP is more negative
- Depolarization phase of AP is due to the opening of the voltage-gated Na+ channels
Describe the process of excitation contraction coupling:
- “Excitation” - depolarisation of plasma membrane
- Opening of plasma membrane, L-type Ca2+ channels in T-tubules
- Flow of Ca2+ into cytosol
- Ca2+ binds to Ca2+ receptors (ryanodine receptors) on the outside of the sarcoplasmic reticulum
- Opening Ca2+ channels intrinsic to these receptors
- Flow of Ca2+ into cytosol
- Increase cytosolic Ca2+
- Calcium triggers contraction (via troponin) - contraction lasts for as long as the plateau of the AP
- Normally about 30% maximal effect -> if more calcium can get stronger contraction
What is a refractory period?
A time between which an AP arrives at the cell to trigger contraction and the next AP arriving at the cell
What do long absolute and relative refractory periods prevent?
- Prevent re-excitation of heart muscle during most of the contraction period
- Also prevent circuitous recycling of the action potential
Does tetanus normally occur in heart muscle?
No - summation of contractions isn’t possible
What is an ECG?
A recording of potential changes at the skin surface that result from the depolarization and repolarization of the heart muscle
What are the different waves on an ECG?
P - Atrial depolarisation; precedes atrial contraction
QRS - Ventricular depolarisation; precedes ventricular contraction
T - Ventricular repolarization; ventricular relaxation
What are the electrical events causing the ECG?
- Depolarisation toward +ve electrode = +ve bump
- Depolarisation away from +ve electrode = -ve bump
- Repolarisation away from +ve electrode = +ve bump
- Shape of waves depends where you “look at” heart from
What is Einthoven’s triangle?
LA-RA = lead I
LL-RA = lead II “classic” ECG
LL-LA = lead III
What are the augmented leads?
- Lead AVL heart from left shoulder; left free wall
- Lead AVR heart from right shoulder; cavity of the heart
- Lead AVF heart from foot; inferior surface of the heart
