origin & conduction of cardiac impulse Flashcards
where does excitation of heart normally orginate?
in pacemaker cells in SA node
what initiates the heartbeat?
the cluster of specialise pacemaker cells in the SA node
where is sinoatrial node located?
upper right atrium close to where superior vena cava enters right atrium
what is meant when heart is in sinus rhythm?
heart controlled by sinoatrial node
how frequently do cells in SA node generate action potentials?
= they generate spontaneous pacemaker potentials
(the cells in SA node have no stable resting membrane potential)
describe the process of production of action potential in SA node?
- The spontaneous pacemaker potential takes the membrane potential to a THRESHOLD
- Every time the threshold is reached An ACTION POTENTIAL is generated
- This results in the generation of regular spontaneous ACTION POTENTIALS in the SA nodal cells
what is pacemaker potential?
slow depolarisation of membrane potential to threshold
what is pacemaker potential due to?
- funny current = a cationic current mainly due to slow Na+ influx through specific channels that open when membrane hyperpolarizes called HCN channels (hyperpolarization activation cyclic nucleotide-gated)
- decrease K+ efflux at the end of hyperpolarization period
- Ca2+ influx
what are HCN channels?
hyperpolarization activation cyclic nucleotide-gated - slow influx of Na+ through HCN channels
what happens once threshold is reached?
rising phase of action potential is caused by opening of long lasting voltage gated calcium channels which means calcium influx causing depolarization
what is the falling phase of SA nodal cell?
- inactivation of L-type Ca2+ channels
- activation of K+ channels resulting in K+ efflux
describe pathway of conduction across the heart?
- Across the atria mainly cell-to-cell conduction via gap junctions
- From SA node to AV node: mainly cell-to-cell conduction via gap junctions; but there is also some internodal pathways
- The conduction is delayed in the AV node. This allows atrial systole (contraction) to precede ventricular systole
- The Bundle of His and its branches and the network of Purkinje fibers allow rapid spread of action potential to the ventricles
what makes the upstroke of membrane potential graph for SA nodal cell
Ca2+ influx (L-type Ca2+ channels)
what makes the downstroke of membrane potential graph for SA nodal cell
K+ efflux
what areas have cell-cell spread of excitation?
from SA node →atria
from SA node →AV node
within ventricles
*all via gap junctions
where are gap junctions & desmosomes in heart?
in the intercalated discs between cells
what is the AV node?
a small bundle of specialised cardiac cells
- it’s the ONLY point of electrical contact between atria and ventricles
where is AV node located?
base of the right atrium; just above the junction of atria and ventricles
what are some characteristics of the AV node cells?
small in diameter and has slow conduction velocity
is there a difference between action potential in cardiac contractile muscle cells and action potential of SA node cells?
AP of cardiac myocytes = differ considerably from AP of SA node
in cardiac myocytes:
- phase 1 & 2 are absent (the slight decrease (when slight K+ before Ca2+) & plateau (when Ca2+ match K+) doesn’t exist)
- phase 4 is different (pacemaker potential rather than resting membrane potential)
describe phase 0 of contractile myocytes action potential?
fast Na+ influx which is upstroke from -90mV resting potential up to +20mV membrane potential
describe phase 1 of contractile myocytes action potential?
closure of Na+ channels & transient K+ efflux (makes small decrease)
describe phase 2 of contractile myocytes action potential?
mainly Ca2+ influx (L-type Ca2+ channels) = plateau phase
describe phase 3 & 4 of contractile myocytes action potential?
closure (inactivation) of Ca2+ channels and K+ efflux = rapid decrease (falling phase) of membrane potential back to phase 4 resting membrane potential
what is ECG?
record of depolarisation and repolarisation cycle of cardiac muscle obtained from skin surface
- the potential difference is compared between different electrodes, this allows cardiac electrical activity to be visualised from different angles around the heart
what does the P, QRS complex and T wave represent?
P wave = presents spread of atria depolarization
QRS complex = ventricular depolarisation
T wave = ventricular repolarisation
what does PR interval, ST segment & TP interval in ECG represent?
PR interval = largely AV node delay
ST segment = ventricular systole occurs here
TP interval = diastole occurs here
what does the vagal tone do?
- dominates under normal resting conditions
- slows the intrinsic heart rate from 100 bpm to produce a normal resting heart rate of 70 bpm
does vagus nerve influence affect SA node?
yes - the vagus nerve (parasympathetic supply to the heart) exerts a continuous influence on the SA node under resting conditions
what is normal resting heart rate?
between 60 and 100 BPM
what is bradycardia?
slow -> resting heart rate less than 60 BPM
what is tachycardia?
fast -> resting heart rate more than 100 BPM
what does vagus nerve do?
stimulation of vagus nerve slows rate of firing from SA node & increases AV node delay
what effect does parasympathetic nerve have on heart rate?
- decrease pacemaker cell Na+ and Ca2+ influx
- increase AV nodal delay
- decrease heart rate by slowed rate of firing of SA node
what effect does sympathetic nerve have on heart rate?
- increase pacemaker cell Na+ and Ca2+ influx
- decrease AV nodal delay
- increase heart rate
