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Feedback control & physiology > origin & conduction of cardiac impulse > Flashcards

origin & conduction of cardiac impulse Flashcards

1
Q

where does excitation of heart normally orginate?

A

in pacemaker cells in SA node

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2
Q

what initiates the heartbeat?

A

the cluster of specialise pacemaker cells in the SA node

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3
Q

where is sinoatrial node located?

A

upper right atrium close to where superior vena cava enters right atrium

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4
Q

what is meant when heart is in sinus rhythm?

A

heart controlled by sinoatrial node

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5
Q

how frequently do cells in SA node generate action potentials?

A

= they generate spontaneous pacemaker potentials
(the cells in SA node have no stable resting membrane potential)

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6
Q

describe the process of production of action potential in SA node?

A
  • 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
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7
Q

what is pacemaker potential?

A

slow depolarisation of membrane potential to threshold

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8
Q

what is pacemaker potential due to?

A
  • 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
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9
Q

what are HCN channels?

A

hyperpolarization activation cyclic nucleotide-gated - slow influx of Na+ through HCN channels

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10
Q

what happens once threshold is reached?

A

rising phase of action potential is caused by opening of long lasting voltage gated calcium channels which means calcium influx causing depolarization

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11
Q

what is the falling phase of SA nodal cell?

A
  • inactivation of L-type Ca2+ channels
  • activation of K+ channels resulting in K+ efflux
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12
Q

describe pathway of conduction across the heart?

A
  • 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
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13
Q

what makes the upstroke of membrane potential graph for SA nodal cell

A

Ca2+ influx (L-type Ca2+ channels)

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14
Q

what makes the downstroke of membrane potential graph for SA nodal cell

A

K+ efflux

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15
Q

what areas have cell-cell spread of excitation?

A

from SA node →atria

from SA node →AV node

within ventricles

*all via gap junctions

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16
Q

where are gap junctions & desmosomes in heart?

A

in the intercalated discs between cells

17
Q

what is the AV node?

A

a small bundle of specialised cardiac cells
- it’s the ONLY point of electrical contact between atria and ventricles

18
Q

where is AV node located?

A

base of the right atrium; just above the junction of atria and ventricles

19
Q

what are some characteristics of the AV node cells?

A

small in diameter and has slow conduction velocity

20
Q

is there a difference between action potential in cardiac contractile muscle cells and action potential of SA node cells?

A

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)

21
Q

describe phase 0 of contractile myocytes action potential?

A

fast Na+ influx which is upstroke from -90mV resting potential up to +20mV membrane potential

22
Q

describe phase 1 of contractile myocytes action potential?

A

closure of Na+ channels & transient K+ efflux (makes small decrease)

23
Q

describe phase 2 of contractile myocytes action potential?

A

mainly Ca2+ influx (L-type Ca2+ channels) = plateau phase

24
Q

describe phase 3 & 4 of contractile myocytes action potential?

A

closure (inactivation) of Ca2+ channels and K+ efflux = rapid decrease (falling phase) of membrane potential back to phase 4 resting membrane potential

25
Q

what is ECG?

A

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

26
Q

what does the P, QRS complex and T wave represent?

A

P wave = presents spread of atria depolarization
QRS complex = ventricular depolarisation
T wave = ventricular repolarisation

27
Q

what does PR interval, ST segment & TP interval in ECG represent?

A

PR interval = largely AV node delay

ST segment = ventricular systole occurs here

TP interval = diastole occurs here

28
Q

what does the vagal tone do?

A
  • dominates under normal resting conditions
  • slows the intrinsic heart rate from 100 bpm to produce a normal resting heart rate of 70 bpm
29
Q

does vagus nerve influence affect SA node?

A

yes - the vagus nerve (parasympathetic supply to the heart) exerts a continuous influence on the SA node under resting conditions

30
Q

what is normal resting heart rate?

A

between 60 and 100 BPM

31
Q

what is bradycardia?

A

slow -> resting heart rate less than 60 BPM

32
Q

what is tachycardia?

A

fast -> resting heart rate more than 100 BPM

33
Q

what does vagus nerve do?

A

stimulation of vagus nerve slows rate of firing from SA node & increases AV node delay

34
Q

what effect does parasympathetic nerve have on heart rate?

A
  • decrease pacemaker cell Na+ and Ca2+ influx
  • increase AV nodal delay
  • decrease heart rate by slowed rate of firing of SA node
35
Q

what effect does sympathetic nerve have on heart rate?

A
  • increase pacemaker cell Na+ and Ca2+ influx
  • decrease AV nodal delay
  • increase heart rate

Feedback control & physiology (13 decks)

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