Circulatory system 3

What is refractoriness?

inability of more than enough stimulus to elicit an action potential

When does refractoriness occur?

occurs for some time after a previous cardiac AP.

What does refractoriness give rise to?

ARP/ERP and relative refractory periods RRP

What is the purpose of refractoriness?

protect against premature excitation.

time for recovery of excitability

What is automaticity?

ability of cardiac cells to initiate/ fire APs spontaneously

What are the 3 normal cardiac automatic/pacemaker cells?

SA node, AV node and specialised conducting tissue (His-Purkinje system)

In what way does the heart beat?

spontaneously and rhythmically.

What is the natural pattern of excitation triggered by?

spread of action potentials across muscle cell membranes

In what order are action potentials initiated and conducted ?

cyclically initiated and conducted in orderly sequence through the heart.

What is orderly sequence that action potentials are conducted through the heart?

SA node —> atria —> AV node —> bundle of His—> Purkinje fibres —> ventricles

Why is there an AV conduction delay?

allows ventricles to relax while the atria contract

How are electrical currents generated by the cardiac muscle?

during depolarisation and repolarisation, conducted through body fluid and into tissues around the heart

How is electrical activity of the heart measure?

electrocardiograms, ECG/EKG

What is an ECG?

summation of overall spread of electrical activity throughout the heart during depolarisation and repolarisation.

Where do the limb leads go for an ECG?

Lead I- right arm to left arm

Lead II- right arm to left leg

Lead III- left arm to left leg

What are the three distinct waveforms of a normal ECG and what do they represent?

• P wave represents atrial depolarisation

• QRS complex represents ventricular depolarisation

• T wave represents ventricular repolarisation

What are the basic functional units of the heart pump?

cardiac muscle fibres.

What is formed when cardiac muscle cells interconnect?

branching fibres

What are intercalated discs?

Adjacent cells joined end to end at specialized structures

What 2 types of membrane junctions are present in an intercalated disc?

desmosomes and gap junctions

Describe the full process of excitation contraction coupling.

• action potential in cardiac cell

• travels down T tubules

• Ca2+ enters from ECF, and from sarcoplasmic reticulum

• increased cytosolic Ca2+

• troponin-tropomyosin complex in thin filaments pulled aside

• cross bridge cycling between thick and thin filaments

• thin filaments slide inward between thick filaments

• contraction

What is systole?

phase of ventricular contraction and emptying. isovolumetric contraction and ejection periods

What is diastole?

phase of ventricular relaxation and filling. isovolumetric relaxation and filling periods.

Describe the 5 stages of the cardiac cycle.

• late diastole- both chambers relaxed and ventricles fill passively.

• atrial systole- atrial contraction forces some blood into ventricles

• isovolumic ventricular contraction- ventricles contract and close AV valves.

• ventricular ejection- ventricular pressure rises and open SL valves, blood ejected

• isovolumic ventricular relaxation- pressure falls and blood flows back, SL valves close.