Initiation and propagation of the heart beat

Due to myogenic property of the heart

• beat without neurl input

• can perform experiments of dead frog

• but must be recently deceased

Frog heart anatomy

• two atria

• single ventricle

• sinus venosus

Even though has a single ventricle

• loosely compartmentalised→ keeps oxygenated blood separate

Sinus venosus

• contains pacemaker region

  • equivalent to SAN in mammals

• Antechamber created by the large veins entering the back of the heart

AVN in the frog?

• no

but

• there is slowly conducting pathways from the atrium to the venticle through the fibrous ring

Purkinje fibres?

• none

Care of the heart during the epxeriments

1. heart moist

   • add Ringer’s solution (pseudophysiological extracellular solution)

2. Heart pulled out of the chest cavity and attached to mechanical transducer for recordings

   • BUT THIS MEANS→ atria must pump blood upwards against gravity

   • THEREFORE: helpful to periodically discounnet pin from electrode and allow heart to lie back in chest cavity

Powerlab connections

• simultaneously record the contraction of the heart and electrical activity of the heart via the pin electrode

1. Mechanical activity → redcord at input 1

2. Electrical acitivty→ record at input 2

3. Also electrical activity from the probe→ input 3 through bridge amplifier

   • used to stimulate probe electrodes connected to the output for this bit?

Recording electrical and mechanical activity using the pin electrode: Experiment 1

Record electrical acitivty of heart

• using pin electrode through tip of ventricle

simultaneously

• record contraction of heart→ connect pin electrode to mechanical transducer

  • using long piece of thread

Experiment 1: Electrical recordings

Because the hook has penetrated some of the cells of the ventricle:

• records quasi-intracellular electrical potential→ (injury potential)

also

• partly record extracellular potentials generated by rest of heart

Picture: shows early recordings

Description of recording

1. largest electrical event→ ventricle

   • coz where pins are and venticle is a large mass of tissue

2. Plataeu→ looks similar to what expect if record cardiac AP from single cells with intracellular potential

Recording with time

1. Lose the plateau as cells die off

   • Recodring becomes more extracellular in form

but

2. Ventrical de and re- polarisation are still obvisous as dominant electrical events

3. Small event before ventricular dep→ depolarisation of atria

Repolarisation of atria?

→ masked by depolarisation of ventricle

Experiment 1: Mechanical recordings

• seen separately to the elctrical→ so can be compared

  • see temporal relationship

Typical mechanical recording

• Atrial contraction→ associated with small downward deflection of lever arm of mechanical transducer

• Ventricular contraction→ associated with much larger downward deflection of the arm

Why may the recording be a bit off?

1. ventricle will not fully relax before the atria contracts

   • → masks the initial contraction phase of atria

   • more common with larger hearts

Important to interpret records carefully

Although the frog’s heart electrical activity is myogenic

Still affected by

1. Sympathetic stimulation

→ via cardioaccelerator nerves

2. parasympathetic

→ via vagus nerve

3. Temperature

Experiment 2: Parasympathetic effects easily observed by

• applying ACh directly to sinus venosus

  • → EFFECT→ bradycardia

can also observed sympathetic effects with noradrenaline

How does ACh exert these effects

Reducing the rate of spontaneous depolarisation of the pacemaker cells in sinus venosus

1. activate K+ channels

2. reduce number of open Ca2+ channels (and Na+ channels probs)

3. membrane potential held closer to K+ equilibirium potential

4. If enough K+ channels activated→ pacemaker potential may neer reach threshold

5. will not initiate AP

6. heart stop beating
   

What noradrenaline does

1. increase the slope of pacemaker potential

2. threshold is reached more rapidly

3. heart rate increases

4. Tachycardia

How NA works to increase slope

1. opens Na+ and Ca2+ channels

2. inhibit K+ channels

Experiment 3: effects of temperature

• irrigate heart with warm or cold Ringer’s solution

• will be able to observe reversibility of response too as it returns to room temp

  • measure:

    • interval between beats

    • duration of ventricular AP

    • interval between the start of the atrial and ventricular APs

Probe electrode for stimulating the heart experiment

• adjust probe to record extracellularly from specific places in the heart

  • → to identify different components of the pin electrode recording

• Probe set up at input 3 via bridge amplifier

1. Recording from the atria

• largest deflection→ electrical activity of atria

• compare events recorded from the pin electrode with probe

2. Recording from sinus venosus: how set up

• BLACK→ abdominal viscera

• RED→ sinus venosus

• must increase sensitivity of probe→ electrical signal is very small

2. Result

• See a small deflection BEFORE the atrial deflection

figure:

• compare the lectrical timing of the electrical activity of the heart

  • recorded from the pin and probe electrodes

OVERALL: establish order at which the electrical activity of the different chambers occurs

( sinus venous→ atria→ ventricles)

Experiment 4?: Stimulation of the heart

Use to get further evidence for localising actual pacemaker region

Principle of experiment:

• if stimulate part of heart that is being driven elsewhere→ you may get abnormal premature contraction

  • but

  • → basic cycle is not disturbed

  • MEANING: pacemaker continues to output normally

• HOWEVER: beat may be absent→ as a result of normal beat attempting to stimulate ventricle during its refractory phase

  • → ‘extra systole’ and ‘Compensatory phase’

Why does the compensatory pause arise

• as a result of the long refractory period of cardiac tissue

THEREFORE: a slower look at the refractory period is in order

Reminder of the refractory period

Picture→ recorded with intracellular electrode in myocyte

• point where cell cannot fire a second AP

  • due to inactivation of voltage-ativated Na+ channels

  • lasts for more than 100ms

There are two refractory periods

1. Absolute refractory period (‘effective’)

   • not possible to stimulate an AP

   • due to Na+ channel inactivation

2. Relative refractory period

   • repolarisation phase

   • AP may be initiated BUT stimulus is greater than that required to initiate an AP

   • WHY?: result of incomplete recovery of inactive Na+ channels

   • cell is in a state of reduced excitability

As the cell continues to repolarise

• Na+ channel inactivation is removed

• normal electrical input can induce an AP

THEREFORE: if we stimulate a cell that is being driven by electrical events from elsewhere

• it is then possible that the arrival of the pacemaker induced event may occur at a time when the cells are refractory

  • → resulting in no AP generation (so get a missing beat)

→ This is what underlies the compensatory pause folloing electrical stimulation of the ventricle

• leading to an extra systole

This is observed as

• normal beat missing

but

• pacemaker is firing at its normal point in time

BUT: if we stimulate the pacemaker: result

• reset the rhythm of the heart

Observation

• displacemnet of the normal beat in time

OVERALL the stimulating experiment is used to

• Find which specifica areas are the pacemakers

  • if a pacemaker→ will just reset the cycle

  • if not a pacemaker→ will miss a beat coz it is in refractory period