electrical activity of the heart

How does blood flow through the heart + how is it coordinated? 

Pattern of blood flow in heart: 

Sequence: Atria contraction downwards → ventricle contraction upwards 

Coordination contributors: 

1. Interconnected muscle cells 

2. Self excitation 

3. Conduction system

How are cardiac muscle cells arranged ? 

Spiral pattern + 3 layers ( from innermost to outermost) 

1. Endocardium 

2. Myocardium 

3. Epicardium 

What are the similarities of cardiac + skeletal muscle cells ?

 

Similarities	Cardiac muscle cells	Skeletal muscle cells
Appearance	Striated appearance
Contractile mechanism	Actin + myosin cross bridges
Other components	T tubule + sarcoplasmic system
Summation of action potentials	X

What are the differences of cardiac + skeletal muscle cells ?

Differences	Cardiac muscle cells	Skeletal muscle
Control of contraction	Involuntary	Voluntary + involuntary
Presence of intercalated discs	Y → contains desmosomes + gap junctions	N
Coordination	Syncytia → coordinated contraction	Contraction coordinated by nervous system
Duration of action potentia	Longer	Shorter
Refractory periods	Longer	shorter

→ cardiac cells X go into tetanus bc X summation of contraction 

*Functional syncytia: a group of cells working in a coordinated fashion even though individual components are maintained 

What is the basis of action potential generation in cardiac cells

different membrane potentials of contractile cells + autorhythmic cells 

→ autorhythmic cells have membrane potentials that slowly depolarises bwt action potentials + drifts to threshold → cyclic initiation of APs

Sequence of events in action potential generation of cardiac cells

Slow depolarisation —> fast depolarisation —> repolarisation

ionic process in cardiac cells in slow depolarisation

Opening of funny channels simultaneously: 

1. Na+ permeability ↑

2. K+ permeability ↓

→ Na+ input + K+ output drops

Opening of transient T-type Ca2+ channels  

3. Ca2+ permeability ↑

4. Na+ permeability ↓

→ Ca2+ input 

→ reach action potential threshold

ionic process in cardiac cells in fast depolarisation

Transient Ca2+ channels close  Long lasting Ca2+ channels open  → increase in Ca2+ input → membrane potential increases to peak

ionic process in cardiac cells in repolarisation

1. Ca2+ channels close –. Ca2+ permeability drops 

2. K+ channels open → K+ output 

→ membrane potential decreases

Why is there a plateau phase in action potential of contractile cells? 

Activation of slow L-type Ca2+ channels → ensures adequate ejection time + prevents summation/ tetanus in cardiac muscle cells 

What is the purpose of synchronised sequence of cardiac contraction + criteria

Purpose: ensures optimum ejection of blood by coordinated sequence + direction of cardiac muscle contraction

Criteria: 

1. Atrial contraction + excitation complete before ventricular contraction begins 

2. Excitation of cardiac muscles fibres coordinated → atrium + ventricle contraction as a unit 

3. Pairs of atria + pairs of ventricles needs to be coordinated for simultaneous contraction 

Components of cardiac conduction system + location

Components of cardiac conduction system	Location
Sinoatrial node	In right atria close to the opening of superior vena cava
Atrioventricular node	Base of right atrium near septum
Bundle of His	Originate: AV node → interventricular septum
Purkine fibres	terminal fibres of bundle of His → ventricular myocardium

How is cardiac excitation spread

1. AP starts in SA node → spreads to right  + left atria 

2. AP can only pass  from atria into ventricles through AV node w/ delay 

Function of delay: allow atrial contraction to complete ventricular filling before ventricular contraction 

3. AP travels rapidly down interventricular septum via Bundle of His → rapidly spread to myocardium through Purkinje fibres 

Action from ventricles: contracting from base upwards 

4. AP moves through gap junctions → activates remaining ventricular cells 

→ completion of ventricular contraction

What is an ECG

Definition of ECG: sum of all electrical activity as it spreads through the heart 

How to read an electrocardiogram? 

Components of ECG	Representation	How it is made
P wave	Atrial depolarisation	SA node fires → Atrial depolarisation
PR segment	AV node delay
QRS complex	Ventricular depolarization	Ventricular depolarization ( atria repolarization as well)
ST segment		Ventricles contracting + emptying
T wave	Ventricular repolarization
TP interval		Ventricles relaxing + filling

How is ECG measured

By comparing signals bwt signal electrodes to produce leads : a graphical representation of electrical activity of the heart from different angles

Different types of leads: bipolar limb/ unipolar leads/ chest leads