Origin and Conduction of Cardiac Impulse Flashcards
Excitation of the heart normally originates in the ______ cells in the ________ node
pacemaker
sino-atrial node
The cluster of the specialised pacemaker cells in the _____ initiate the heartbeat
SA node
Where is the Sino-atrial node located?
The upper RIGHT ATRIUM close to where the Superior Vena Cava enters the right atrium
A heart controlled by the Sino-Atrial node is said to be in ______
sinus rhythm
The cells in the SA node _____ have a stable resting membrane potential
do not
The cells in the SA NODE exhibit ________ _______ ______
spontaneous pacemaker potential
The spontaneous pacemaker potential takes the membrane to a threshold to generate an _____ ______ in the SA nodal cells.
Action potential
In the pacemaker cells the permeability to ____ does not remain constant between action potentials
K+ ions
The pacemaker potential (i.e. the slow depolarisation of membrane potential to a threshold is due to: (3)
Decrease in K+ efflux Na+ and K+ influx (the funny current) Transient Ca++ influx (T-type Ca++ channels)
during the rising phase of pacemaker action potential (depolarisation) what happens on a chemical level?
activation of long lasting (L-TYPE Ca++ CHANNELS) Resulting in Ca++ influx
Repolarisation (falling phase on pacemaker action potential) is caused by…
Inactivation of L type Ca++ channels Activation of K+ CHANNELS Resulting in K+ EFFLUX
How does cardiac excitation normally spread across the heart?
From the SA node to AV node via intranodal tracts to LA and AV node AV node - Bundle of His - Purkinje fibers
Desmosomes - what is their function?
Desmosomes form links between cells, and provide a connection between intermediate filaments of the cell cytoskeletons of adjacent cells. This structure gives strength to tissues.
Gap junctions allow _____
Cell-to-cell current flow via gap junctions
Where is the AV node located
Located at the base of the right atrium; just above the junction of the atria and ventricles
What makes the AV node have slow conduction velocity?
The AV node cells are small in diameter and has slow conduction velocity
What is specific to the AV node?
Is the only point of electrical contact between atria and ventricles The AV node cells are small in diameter and has slow conduction velocity
Why is conduction delayed in the AV Node?
To allow atrial systole (contraction) to precede ventricular systole
Action potential on atrial and ventricular myocytes graph
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
The plateau phase of action potential is phase ___ and it is a unique characteristic of contractile muscle cells, the plateau phase is mainly due to ______
2 Influx of Ca++ through L-type Ca++ channels
The falling phase of action potential (repolarisation) is phase ___ , the falling phase is caused by ______
3 inactivation of Ca++ channels and activation of K+ channels Resulting in K+ EFFLUX
What changes Heart rate?
Autonomic Nervous system Sympathetic stimulation increases heart rate Parasympathetic stimulation decreases heart rate - The VAGUS NERVE (PARASYMPATHETIC supply to the heart) exerts a CONTINUOS influence on the SA node under resting conditions - VAGAL TONE DONIMATES under normal resting conditions - Vagal tone SLOWS the INTRINSIC HEART RATE from ~100bpm to produce a normal resting heart rate of ~70 bpm
Define Bradychardia
Heart rate of <60
Define Tachycardia
Heart rate of >100
Parasympathetic supply of the heart: What nerve supplies the SA and AV node
Vagus
What effect does vagal stimulation have on the heart?
Vagal stimulation slows heart rate and increases AV nodal delay (increasing the length of each cardiac cycle)
Describe what happens in each phase
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
