Physiology 1- Origin and Conduction of a Cardiac Impulse Flashcards
The electrical signals which control the heart are generated where?
Within the pacemaker cells of the sinoatrial node
What is autorhythmicity?
No external signals are needed for the heart to beat
Where is the SA node?
Right atrium, close to where the SVC enters
A heart beat controlled by the SA node is said to be what?
In sinus rhythm
What type of potential do SA node pacemaker cells NOT have, and what type of potential DO they have?
Don’t have a resting membrane potential
Do have a spontaneous pacemaker potential
What does the spontaneous membrane potential of pacemaker cells do?
Take the membrane potential to a threshold which is required to form an AP
What causes depolarisation in the pacemaker cells?
Activation of L-type Ca++ channels and Ca++ influx
What causes hyperpolarisation in pacemaker cells?
Inactivation of Ca++ channels and activation of K+ channels causing K+ efflux
What is the pacemaker potential slope (just before depolarisation) in pacemaker cells caused by?
K+ efflux
Ca++ influx
Na+/K+ influx (funny current)
How can cardiac excitation spread from the SA node to the AV node?
Gap junctions or intermodal pathways
How can cardiac excitation spread from the SA node, through both atria and within ventricles?
Cell to cell conduction via gap junctions
What happens at the AV node and why?
Impulses get delayed in order to prevent the atria and ventricles contracting at the same time
Where is the AV node located?
The base of the right atrium, just above the atrio-ventricular junction
Where do impulses go after passing through the AV node?
Through the bundle of His and its left and right branches, then through the Purkinje fibres to spread throughout the ventricles
What is the resting membrane potential in cardiac myocytes before they are excited?
-90mV
What causes depolarisation in myocytes?
Fast Na+ influx
What causes phase 1 of the AP in myocytes?
Closure of Na+ channels and K+ efflux
What causes phase 2 (plateau phase) of the AP in myocytes?
Ca++ influx
What cause phase 3 (hyperpolarisation) of the AP in myocytes?
Closure of Ca++ channels and K+ efflux
What is phase 4 of the AP in myocytes?
Resting membrane potential
The heart rate is primarily influenced by what?
Autonomic nervous system- parasympathetic decreases and sympathetic increases
What is the main parasympathetic control to the heart?
Vagus nerve
What does the vagus nerve do?
Continuously influences the SA node- dominates the heart under resting condition
What is bradycardia?
< 60 bpm
What is tachycardia?
> 100 bpm
How does vagal stimulation slow the heart rate?
Through increasing AV node conduction delay
What is the neurotransmitter for the parasympathetics to the heart and what receptors does this act on?
Acetylcholine acting on M2 muscarinic receptors
What is a competitive inhibitor of acetylcholine and when is it used?
Atropine- used to speed up the heart rate in severe bradycardia
What is a chronotropic effect?
Something that affects the heart rate- can be positive or negative
What does vagal stimulation on pacemaker cells do to the pacemaker potential?
Decreases the slope
How do sympathetic nerves increase the heart rate?
Decrease the AV nodal delay and increase the force of contraction
What is the neurotransmitter for the sympathetics to the heart and what receptors does this act on?
Noradrenaline acting on beta1 adrenoceptors
What does sympathetic control increasing the heart rate do to the pacemaker potential?
Increases the slope
What do parasympathetic and sympathetic systems do to the ions involved in action potentials?
Sympathetic stimulation decreases K+ efflux and increases Na+/Ca++ influx, parasympathetic does the opposite
