Electrical activity of the heart

Describe the structure of cardiolomyoctes

• elongated, cylindrical, striated cells

• Have a single nucleus and many mitochondria

• Have gap junctions: allow depolarising currents to from from one cell to another

• Desmosomes + gap junctions form intercalated discs

Name all the hearts nodal cells

• SA node

• AV node

• Bundle of his

• L + R bundle branches

• Purkinje fibres

** generate spontaneous action potentials (slow response)

Structure of contractile cells

Have actin, myosin, troponin, tropomyosin

Have sarcoplasmic reticulum

Fast response action potentials

Where is the SA node located and what does it do?

Inferior to the superior vena cava

Pacemaker of the heart → sets sinus rhythm (60-80bpm)

What does the Bachmann’s bundle do?

Pass signals from the SA node to the L atrium

Where is the AV node located and what does it do?

Inferior to the pulmonary trunk

Gives time for atria to contract so the ventricles can fill

What does the bundle of His (AV bundle) do?

Carries action potentials from AV node to bundle branches

What does the R bundle branch do?

Carries action potential to the right myocardium

What does the L bundle branch do?

Carries action potentials to L myocardium

What does Purkinje fibres do?

Carry action potentials to ventricular muscle

What happens in nodal cells?

1. Phase 4(diastole): funny Na+ channels open + T type Ca2+ channels open → reaches threshold (-40mV)

2. Phase 0 (upstroke): L-type Ca2+ channels open → +10mV → Ions move to contractile cells via gap junctions

3. Phase 3 (final repolarisation): L-type Ca2+ channels inactivate. K

4. + channels activate and K+ leaves the cell → cell depolarises

Is the resting membrane potential of contractile cells?

-90mV

How do contractile cells generate an action potential?

1. In flow of positive ions: -90mV → -70mV

2. Phase O (upstroke): voltage gated Na+ channels open: -70 → +20

3. Phase 1 (initial repolarisation): Na+ channels close. K+ channels open → K+ moves out → 0mV

4. Phase 2 (plateau): K+ moves out and Ca2+ moves in (Sam’s amount of ions in and out)

5. Phase 3 (final repolarisation): L-type Ca2+ channels close. Ca2+ is taken back to SR by sodium-calcium exchanger and calcium proton ATPase pumps. K+ channels open and K+ exits the cell

6. Phase 4 (RMP): little movement of ions

Why do contractile cells have phase 1 and 2?

Excitation-contraction coupling

• Ca2+ activate ryanidine receptors (RYR) in the SR

• RYR opens up Ca2+ channels so more move out of the SR into the cytoplasm

• Ca2+ bins to tropocollagen. → change shape of tropmyosin → moves tropmyosin away → myosin head interacts wirh actin → more cross bridges → more contraction

What is it called when the cells all contract at the same time?

Functional syncytium