Electrical Events of the Cardiac Cycle Flashcards
What is the cardiac cycle controlled by?
Nervous system but can function without:
99% cardiac cells - contractile
1% are capable of spontaneously firing - autorhythmic
Where are spontaneously firing cells located?
- Sinoatrial node - right atrial wall near opening of SVC
- atrioventricular node - base of right atrium near septum above AV junction
- Bundle of His (right and left), Purkinje fibres
What dictates the rate/rhythm of the heart?
Spontaneous discharge rate of the fastest firing cells - SA node is natural pacemaker
Spontaneous discharge rate of SA node
70-80 action potentials/min
What dictates the rate of the heart if the SA node is affected?
AV node
40-60 action potentials/min
Resting potential of SA node cell
-60mv
What happens at the resting potential in a SA node cell
Na+ channels open, increased influx of Na+ (funny current)
Ca2+ channels open, increased Ca2+ influx
K+ channels close, decreased K+ efflux (retain K+ and +ve charge)
Therefore charge becomes less negative, reaching threshold potential
Threshold potential of SA node cell
-40mv
What happens in a SA node cell once threshold potential is reached?
Increased Ca2+ influx, until reaching 0 mv
What happens in SA node cell once 0mv is reached?
Na+ channels close
K+ channels open, K+ efflux
Charge decreases back to resting potential
Spread of excitation through the heart
SA node
Rapidly through atria (around 1m/s)
AV node (slowly conducting, 0.05m/s, delay)
Rapidly through bundle of His and down the branches and Purkinje fibres (around 1-4m/s)
Through ventricular muscle cells
Why is excitation spread rapidly through the heart?
Allows heart to be an effective pump and can act as a single unit/syncytium
Entire contraction of myocardium
What allows the rapid spread of excitation through the heart?
Intercalated discs (gap junctions) between fibres They provide low resistance pathways
What does the coordination of the spread of excitation mean?
Atrial excitation and contraction are complete before ventricular contraction due to AV delay - enables efficient emptying of blood from atria to ventricles
Ventricular excitation occurs synchronously due to rapid spread down septum and through Purkinje fibres - enables ventricles to contract as co-ordinated units and can expel blood effectively
Resting membrane potential of ventricular (contractile) cell
-90mv
What causes rapid depolarisation of contractile cells?
Opening of Na+ channels causes large and rapid Na+ influx to around 20mv
What occurs in contractile cells after reaching 20mv?
K+ channels open resulting in efflux, decreasing charge slightly
Ca2+ influx causes a plateau, the channels begin to close and K+ efflux is dominant, resulting in repolarisation of the cell
What does excitation-contraction coupling require?
movement of Ca2+ ions
Describe how the movement of Ca2+ causes systole
influx of Ca2+ during action potential
triggers release of further Ca2+ from sarcoplasmic reticulum
free Ca2+ activates contraction of myocardial fibres
What does the amount of Ca2+ determine in the excitation-contraction coupling?
cross-bridge cycling and force of contraction
Describe how the movement of Ca2+ causes diastole
uptake of Ca2+ by sarcoplasmic reticulum and extrusion of Ca2+ by Na+/Ca2+ exhange and outward Ca2+ pump
lowers free Ca2+ allowing relaxation
What is tetanus?
sustained, fused contracton
What protects the heart from tetanus occuring?
plateau phase of AP provides long refractory period (without pump action)
additional stimuli will not cause tetanus
Direction of ventricular excitation/contraction
endocardium to epicardium (inside to outside)
apex to base (bottom to top)
Apart from pacemakers, what can influence the rate of the heart?
nerves
How do nerves influence the rate of the heart?
activation of sympathetic nerves increases heart rate by activation of beta 1 adrenoceptors in SA node
Consequence of activation of beta 1 adrenoceptors in SA node
increases slope fo pacemaker by increasing funny current (Na+ influx) and influx of Ca2+
therefore decreased time to reach threshold potential and faster reaching -60mv again (resting membrane potential)
How can the slope of pacemaker be decreased?
activation of parasympathetic nerves decreases heart rate by activation of M2 muscarinic receptors in SA noce
vagus nerve
What nerve dominates at rest?
vagus
cuts off at exercise
Effect of activation of vagus nerve on AP
increases K+ permeability to hyperpolarise membrane potential (RMP more negative, -80mv)
decreases slop of pacemaker potential by decreasing funny current and current of Ca2+
