Conduction of Impulses in the heart Flashcards
What is unique about the heart’s nervous supply?
it exhibits autorhythmicity it triggers its own impulse from the SA node which will cause contraction independent of the wider nervous system
where does the excitation of the heart normally originate?
The sinoatrial SA node which is located in the right upper atrium where the superior vena cava enters the heart
when is the heart said to be in sinus rhythm?
when the heart beat is being controlled by the sinoatrial node
what is the resting membrane potential of the pacemaker cells in the SA node?
The pacemaker cells have no stable resting potential, they generate regular spontaneous pacemaker potentials
How is an action potential fired in the pacemaker cells of the SA node?
The pacemaker potential increases until the membrane potential reaches a threshold and an action potential is fired
The SA node generates
regular spontaneous action potentials
what is the pacemaker potential?
the slow depolarisation of a membrane potential to a threshold
the pacemaker potential is due to
- decrease in K+ efflux
- Na+ influx (the funny current)
- transient Ca2+ influx (T-type channels)
What causes repolarisation of the SA node?
- L-type Ca2+ channels are inactivated
- K+ channels are activated causing K+ efflux
How does cardiac excitation normally spread across the heart?
- SA node
- cell-to-cell conduction through the atria and to the AV node (although there are a few internodal pathways)
- AV node to the bundle of His
- Down the left and right fibres of the bundle to the Purkinje fibre
- Purkinje fibres deliver the excitation to the ventricles
How does cell-to-cell spread of excitation occur?
The excitation spreads between cells through the desmosomes and the gap junctions
What it the atrioventricular AV node?
- small bundle of specialised cardiac cells
- the only point of electrical conductivity between the atria and ventricles
Where is the AV node?
The base of the right atrium just above the atrioventricular junction
Why is conduction delayed in the AV node?
- the cells are small in diameter and have slow conduction velocity
- to allow atrial systole to precede ventricular systole
What is the resting membrane potential of the cardiac myocytes?
-90mV until the cell is excited
What causes depolarisation of the cardia myocytes?
fast Na+ influx
What is Phase 0 of action potential in contractile cardiac muscle cells?
The rapid rising phase of the action potential, from -90mV to +20mV
what are the phases of ventricular muscle action potential?
- Phase 0 (fast Na+ influx)
- Phase 1 (closure of Na+ channels and transient K+ efflux)
- Phase 2 (mainly Ca++ influx)
- Phase 3 (closure of Ca++ channels and K+ efflux)
- Phase 4 (resting membrane potential)
What is the Plateau Phase of Ventricular muscle action?
- a unique characteristic of contractile cardiac muscle cells
- it’s mainly due to the influx of Ca++ through L-type Ca++ channels balancing the efflux
What causes repolarisation in cardiac muscle cells?
- the inactivation of Ca++ channel and activation of K+ channels resulting in K+ efflux
What is the part of the nervous system which influences heart rate?
The autonomic nervous system
What is the effect of sympathetic stimulation on heart rate?
it increases heart rate
What is the effect of parasympathetic stimulation on heart rate?
it decreases the heart rate
What is the influence of the autonomic nervous system on heart rate under resting conditions?
- vagus nerve (parasympathetic) exerts continuous influence on the SA node under resting conditions
- vagal tone is dominant
- slows the intrinsic heart rate from ~100bpm to normal resting heart rate of ~70 bpm
What is considered normal heart rate?
Between 60 and 100 bpm
What is bradycardia?
resting heart rate less than 60bpm
What is tachycardia?
resting heart rate more than 100bpm
What effect does the vagal nerve have?
- supplies the SA and AV node
- slows heart rate
- increases AV nodal delay
What is the mechanism of the parasympathetic effect on heart rate?
- acetylcholine acting through muscarinic M2 receptors
What effect does atropine have?
It is a competitive inhibitor of acetylcholine, so is used to speed up the heart rate in extreme bradycardia
How does vagal stimulation effect the pacemaker potential?
it has a negative chronotropic effect - the slope of the potential decreases and hence it takes longer to reach the threshold
What effect does sympathetic stimulation have on the heart?
- sympathetic nerves supply SA node, AV node and the myocardium
- increases heart rate
- decreases AV nodal delay
- increases force of contraction
What is the neurotransmitter of the sympathetic supply?
noradrenaline acting on B1 adrenoreceptors
What is the effect of noradrenaline in pacemaker potential?
Positive chronotropic effect
Slope of pacemaker potential increases, threshold is reached quicker
Phases of action potential in a cardiac pacemaker myocyte
- Phase 0 (Ca++ influx) not as steep as ventricular muscle action
- Phase 3 (repolarisation)
- Phase 4 (pacemaker potential)
What is special about the refractory period of the heart?
Tetanic contraction isn’t possible each contraction of the heart is completely independent
