L8 - Electrical Acitivity of the Heart Flashcards
By which cells is electrical activity spread?
Nodal cells, conducting cells and muscle cells
Overview of how the heart is excited method
- Initiated at sino-atrial node (dominant pacemaker)
- Conduction to right atria
- Conduction to left atria and atrioventricular node
- Conduction to atrioventricular ring
- Passage through bundle of His, depolarizing septum from left to right
- Depolarization spreads upwards through the Purkinje system distributing to ventricular muscle cells
Where is the sino atrial node found?
Found in the posterior aspect of the heart
Junction between superior vena cava and right atrium
What is the speed of conduction via nodal tissue?
0.05 m/s
What is the speed of conduction via atrial myocardium
1.00 m/s
Where does electrical activity spread after the SAN?
AVN and left atrium
Where is the atrio-ventricular node found?
Found in the posterior aspect of the heart
Right side interatrial septum
What are the structural zones of the AVN?
Atrial nodal, nodal, nodal ventricular
What is the speed of conduction via atrial node to nodal?
0.05 m/s
What is the role of the AVN?
AVN delay allows atrial contraction to finish
AVN refractoriness
- Prevents excess ventricular contraction
- Increases at high heart rate
What is sub-nodal conduction?
Conduction via His - 1 m/s
Septal activation
Conduction via Purkinje fibres - 4 m/s
Conduction via ventricular muscle - 1 m/s
Spiral muscle contraction evokes a torsion
What are the two types of cardiac action potential?
Nodal cells - initiate contraction of heart
- They are able to fire action potentials without any outside stimulation
Contractile cells - atrial and ventricular muscle cannot fire action potentials without a signal
What are pacemakers cells and what is their role?
Pacemaker cells show automaticity and rhythmicity
They do not have a static resting membrane potential
- As soon as they reach -60mV they start slow depolarisation reaching threshold, causing an action potential
- Do not need a signal to stimulate this
AV node and Purkinje fibres can also be pacemakers
What are the 4 stages of a pacemaker action potential?
Pre-potential (pacemaker potential) - Decrease K+ efflux and increase Na influx (If current) Threshold - - 40 to - 50mV Depolarisation - Increase Ca2+ influx Repolarisation - Increase K+ efflux
What is an If current?
If current – activated by hyperpolarization, inactive when positive membrane potential
- Mediates slow Na influx
- Cation selective channel
What are pacemaker action potential regulated by?
Innervation, temperature and other pacemakers
What 2 things are pacemaker cells controlled by?
Parasympathetic vagal fibres (ACh)
Sympathetic (NAdr)
How do sympathetic (NAdr) fibres control pacemaker cells?
Increased prepotential slope
Increased firing rate
This increases the heart rate
How do parasympathetic vagal fibres( ACh) control pacemaker cells?
Hyperpolarisation
Decreased prepotential slope
This slows the heart rate
What is the action of ACh and NAdr on pacemaker cells?
If current regulated by cAMP
- Sympathetic stimulates increased by β1 and β2
- Parasympathetic stimulates decreased M2
Calcium clock oscillations (rhythmic alterations of sarcoplasmic reticulum Ca2+ release)
Cardiac muscle action potential characteristics?
No automaticity
Propagated and prolonged action potential with long plateau phase
- Plateau phase means muscle unable to show summation
Fast depolarisation and overshoot
Cardiac muscle action potential method?
- Voltage gated Na channels (Nav 1.5) open and Na influx depolarises the membrane
- Triggers opening of more Na channels creating positive feedback loop, increasing membrane voltage
- Na channels close when the cell depolarises and voltage reaches +30 mV
- Ca entering through slow Ca channels (Cav 1.2) prolongs depolarisation creating a plateau
a. Plateau falls slightly because of some K leakage - Ca channels close and Ca transported out of cell
- K channels open and rapid K outflow returns membrane to resting potential
